Which building code applies to crawl spaces in Washington?

The 2021 International Residential Code with Washington amendments, which is chapter 51-51 WAC. It has been in force in every county and city since March 15, 2024. Crawl spaces are covered by Section R408, printed with Washington’s amendments in WAC 51-51-0408. Floor and duct insulation are in a separate code, the 2021 Washington State Energy Code, Residential.

Has Washington adopted the 2024 residential code?

Not as of November 11, 2026. The State Building Code Council filed a proposal on May 5, 2026, delayed its vote on October 16, 2026 and put final adoption on its October 30, 2026 agenda. The outcome of that vote is not confirmed: no minutes had been posted, and no permanent rule had been filed. The council’s planned effective date is May 3, 2027. Until a permanent rule is filed, the 2021 edition is the only residential code in force.

Can a crawl space be sealed instead of vented in Washington?

Yes, under Section R408.3, on two conditions that apply together. The ground is covered with a continuous Class I vapor retarder, sealed at the joints and sealed 6 inches up the stem wall, with a radon system installed. And the space has continuously operated mechanical exhaust at 1 cubic foot per minute for each 50 square feet of floor area, vented to the outside. A dehumidifier is not one of the conditions. Venting is the default; the sealed option is the exception.

Does a sealed crawl space in King or Snohomish County need a radon system?

As the code is written, yes. Neither county is in the code’s table of high radon potential counties, so an ordinary vented crawl space carries no radon requirement there. But the radon appendix applies to any building built with an unvented crawl space, in any county, and R408.3 itself lists a radon system as part of the first condition. How that applies when an existing crawl space is sealed is a question for the building department.

How thick does a crawl space vapor barrier have to be in Washington?

For the basic ground cover in a vented crawl space, 6-mil black polyethylene or an approved equal, lapped 6 inches at the joints and run to the foundation wall. For the reduced-vent and sealed options the code asks for a Class I vapor retarder and gives no thickness. Nothing in Section R408 requires 10, 12 or 20 mil.

Does the code require me to upgrade my existing crawl space?

Not by itself. The residential code governs new construction and work that is being done. Under Section R102.7.1, an alteration or repair has to meet the requirements for new work without the rest of the existing house having to comply, and it must not leave the house less compliant than it was. Whether a particular repair needs a permit, and what it has to meet, is decided by your city or county building department.

Is Seattle’s crawl space code different from the state’s?

Not for Section R408, on what this site has read. Seattle has adopted the 2021 International Residential Code with amendments, and its R408.1 to R408.3 carry the Washington-amendment marker and read the same as the state rule. The residential energy code is the same in every city by its own terms. Permit rules are local, and Seattle’s differ from its neighbors’.

Code Reference

Washington Crawl Space Code

What the state residential code requires under a house in King and south Snohomish counties, section by section, with the rule each statement comes from. It covers the edition in force since March 15, 2024, the radon and energy-code rules that attach to a crawl space, how Seattle’s code relates to the state’s, and where the pending 2024 edition stands. Status last checked November 11, 2026.

The 2021 edition is the code in force. The 2024 edition is proposed, not adopted.

The State Building Code Council filed its proposal for the 2024 edition on May 5, 2026. On October 16, 2026 it delayed final adoption and put the vote on its October 30, 2026 agenda. The outcome of that vote is not confirmed. When this page was checked on November 11, 2026 the council had posted no minutes of the meeting, and no permanent rule had been filed. A vote would not change what applies today in any case: the 2024 edition becomes code only when a permanent rule is filed, and the council’s planned effective date for it is May 3, 2027. Pending items on this page are marked Proposed.

The short version

Washington expects a crawl space to be vented, and allows a sealed one only with a radon system and a continuously running exhaust fan. That is Section R408 of the 2021 International Residential Code with Washington amendments, in force since March 15, 2024, and the pending 2024 edition would not change it beyond relettering the radon appendix.

What the section requires, in plain terms. A crawl space can be built either way:

Vented. The ground is covered with 6-mil black polyethylene, or an approved equal, lapped 6 inches at the joints and run to the foundation wall. The space has vent openings of at least 1 square foot for every 300 square feet of floor area, screened so no opening is larger than 1/4 inch. The vent area can drop to 1 per 1,500 where the ground cover is a Class I vapor retarder, but going below 1 per 300, or fitting operable louvers, brings a radon vent with it.
Unvented, meaning sealed or encapsulated. Two conditions, both of them. The ground is covered with a continuous Class I vapor retarder, with joints lapped 6 inches and sealed or taped and edges sealed at least 6 inches up the stem wall, and a radon system is installed. And the space is exhausted continuously by a fan at 1 cubic foot per minute for every 50 square feet of floor, vented outdoors. The section does not list a dehumidifier or conditioned supply air as a substitute for either.

These are rules for new construction and for work being done under a permit. They do not by themselves require an existing house to be upgraded; the section on existing houses says what the code does and does not reach. For the practical question of what to do with the vents on a house you already own, see should crawl space vents be open or closed.

Status panel

Edition in force: 2021 International Residential Code with Washington amendments (chapter 51-51 WAC)
In force since: March 15, 2024
Based on: The 2021 International Residential Code, adopted by reference in WAC 51-51-003 with Washington’s amendments. Chapters 11 and 25 through 43 are not adopted; energy is in chapter 51-11R WAC.
Energy code in force: 2021 Washington State Energy Code, Residential (chapter 51-11R WAC), since March 15, 2024
Pending edition: 2024 International Residential Code with Washington amendments
Status of the pending edition: Proposed. Not adopted.
Proposal: WSR 26-10-071, filed May 5, 2026
Council vote, October 30, 2026: Not confirmed. When this was checked, the council’s meetings page showed an agenda and a recording for the October 30, 2026 meeting and no minutes; its 2024 cycle page still gave the residential code’s adopted-rule date as “date tbd”; and the Register filing lists for WAC 51-51-008 and 51-51-0408 showed no permanent rule after the proposal. The recording was not watched.
Planned effective date: May 3, 2027, in the council’s schedule. No rule sets it yet.
Last checked: November 11, 2026
Next check: November 24, 2026

Proposed, not adopted. The State Building Code Council filed the proposal on May 5, 2026 (WSR 26-10-071). On October 16, 2026 it delayed final adoption and put the vote on its October 30 agenda. The outcome of the October 30 vote is not confirmed: no minutes of that meeting were posted when this was checked, and no permanent rule had been filed. Whatever the council voted, the 2024 edition is not adopted code until a permanent rule appears in the Washington State Register, and the council’s planned effective date for it is May 3, 2027.

Checked on November 11, 2026 against the Washington State Register filing lists for WAC 51-51-008 and 51-51-0408, which are the record of whether a rule has been adopted, and against the State Building Code Council’s 2024 cycle page, its meetings page and its announcements. The filing lists showed the proposal as the newest entry and no permanent rule after it. The sources are linked in full at the foot of this page, each with the date it was opened.

The five stages, and why the distinction matters

A code provision passes through stages that sound similar and are not. The most common error in writing about a new code edition is treating a proposal as a requirement; the second is treating a council vote as the rule. Every row on this page is tagged with exactly one of these.

Current law: Required today. The provision is in the Washington Administrative Code as in force, in the editions that took effect March 15, 2024, and this site read it there on the date shown.
Proposed: Printed in a proposal the State Building Code Council has filed, and not adopted by a permanent rule. It has no legal effect, it can be changed or dropped before adoption, and nothing may be built to it. Every pending item on this page is at this stage.
Adopted, final text not yet read: Part of an edition adopted by a filed permanent rule, where this site has not yet read the adopted wording of the section. Nothing is at this stage today.
Adopted, not yet effective: Adopted by a permanent rule filed with the Code Reviser, with an effective date still in the future. Real, dated, and not yet the rule you build to. Nothing is at this stage today.
Effective: A provision of the newer edition once it is adopted, effective, and checked by this site against the adopted text. Nothing is at this stage today.

The pending edition as a whole is at the second stage: proposed. The 14 rows in the three tables below are current law. The 2 rows under what is pending are proposals.

What Section R408 requires today

Section R408 is the under-floor space section of the residential code. Washington prints its own version of R408.1 to R408.3 in WAC 51-51-0408 and leaves R408.4 to R408.6 as the model code has them. The third column records what the pending proposal does to each rule, which for this section is close to nothing.

Washington crawl space requirements in Section R408 as in force, with what the pending 2024 proposal does to each, its status and the homeowner impact
Topic	Requirement today	In the 2024 proposal	Status	Homeowner impact
Ground cover R408.1	A ground cover of 6-mil black polyethylene, or an approved equal, must be laid over the ground in the crawl space, overlapped at least 6 inches at the joints and extended to the foundation wall. It may be left out where the crawl space has a concrete slab floor at least 2 inches thick. WAC 51-51-0408, R408.1	No change proposed. The proposal reprints R408.1 word for word.	Current law	This is the basic rule for a vented crawl space, and it is modest: a 6-mil sheet, lapped 6 inches, run to the wall. It does not ask for the sheet to be sealed, taped or turned up the wall. A bid that calls a thicker, sealed liner “required by code” for a vented crawl space is describing an upgrade, not this section.
Vent area R408.2	The under-floor space must have ventilation openings through the foundation or exterior walls, with a net area of at least 1 square foot for each 300 square feet of under-floor area. Openings are placed to give cross ventilation, and one side of the building may have none. WAC 51-51-0408, R408.1 first sentence and R408.2	No change proposed to the ratio or the placement rule.	Current law	Venting is the default in Washington: the section says the space “shall have” openings, and the unvented option is an exception with its own conditions. A vent blocked by soil, landscaping or an addition no longer counts toward the ratio.
Vent coverings R408.2 (coverings)	Vent openings must be covered for their full height and width with one of six listed materials, such as hardware cloth, cast-iron grating or corrosion-resistant wire mesh, and the least dimension of the covering’s openings may not exceed 1/4 inch. Operational louvers are permitted. WAC 51-51-0408, R408.2, items 1 to 6	No change proposed.	Current law	The 1/4-inch limit is the figure a vent screen is checked against. A torn or missing screen leaves an opening larger than the rule allows.
Reduced venting, and the radon vent it triggers R408.2 (exception)	Vent area may be reduced to 1/1,500 of the under-floor area where the ground is covered with an approved Class I vapor retarder and the openings still give cross ventilation. If the installed ventilation is less than 1/300, or operable louvers are installed, a radon vent must be installed from a point between the ground cover and the soil, built to the radon appendix. WAC 51-51-0408, R408.2, exception	Editorial only. The proposal reprints the exception and changes the appendix reference from AF to BE, following the 2024 model code’s relettering.	Current law	Closing most of the vents is allowed, but it is not free: it needs a Class I vapor retarder on the ground and it brings a radon vent with it. So do operable louvers, even at the full vent area.
Unvented (sealed) crawl space R408.3	Vents are not required where two numbered conditions are met. One: exposed earth is covered with a continuous Class I vapor retarder, its joints overlapped 6 inches and sealed or taped, its edges run at least 6 inches up the stem wall and attached and sealed to it, and a radon system meeting the radon appendix is installed. Two: continuously operated mechanical exhaust ventilation is provided at 1 cubic foot per minute for each 50 square feet of crawl space floor area, terminating to the exterior. There is an exception for under-floor plenums in existing structures. WAC 51-51-0408, R408.3, items 1 and 2	Editorial only. The proposal reprints both conditions and changes the appendix reference from AF to BE. Its own summary classes the change as alignment with the model code’s reorganization.	Current law	A sealed crawl space in Washington is a liner, a radon system and a continuously running exhaust fan, together. The rule prints the two conditions as a numbered list with no “or” between them, and the City of Seattle’s marked-up copy of the same text shows why: the model code offered a menu of ways to condition the space, and Washington struck the menu and kept exhaust. A dehumidifier is not in the section, and neither is conditioned supply air. Either can be added, but neither takes the place of the exhaust fan or the radon system as the rule is written.
Access, clearing and grade R408.4 to R408.6	Washington does not amend these sections, so the model code applies: an access opening at least 18 by 24 inches through the floor, or 16 by 24 inches through a perimeter wall; the under-floor grade cleared of vegetation and organic material, with forms and construction debris removed; and, where groundwater can rise to within 6 inches of the finished floor at the perimeter or surface water does not drain readily, an under-floor grade as high as the outside finished grade unless an approved drainage system is provided. 2021 Seattle Residential Code, Chapter 4, pages 4-47 and 4-48	Not addressed in Washington’s proposal, which amends none of them. Under a 2024 edition they would read as the 2024 model code prints them, which this site has not compared with the 2021 text.	Current law	The grade rule is the only place R408 touches water, and it says when drainage is needed, not how to build it. The access sizes are minimums for new work.
Model code vapor retarder section R408.8	Not adopted. The model code’s section on an under-floor vapor retarder does not apply in Washington; the ground cover and vapor retarder rules that do apply are in R408.1 to R408.3. WAC 51-51-0408, R408.8	No change proposed. The proposal again prints the section as not adopted.	Current law	A reader with a copy of the International Residential Code can find R408.8 and cite it by mistake. It is not a Washington requirement.

On the unvented option: the rule prints its two conditions as a numbered list, with no “and” or “or” between them. This page reads them as applying together because the City of Seattle’s marked-up copy of the same text shows the model code’s list of alternatives struck out and exhaust ventilation left as the only second item. If a design depends on reading them as alternatives, get the building department’s answer in writing first.

Radon: why a sealed crawl space in King County still gets a vent pipe

Washington’s code names seven counties as high radon potential, and King and Snohomish are not among them. EPA’s map puts both counties in Zone 3, its lowest of three categories. That keeps the radon rules away from an ordinary vented crawl space here. It does not keep them away from a sealed one, because the code attaches the radon appendix to the unvented method wherever it is used.

Washington radon requirements that attach to crawl spaces, as in force, with what the pending 2024 proposal does to each
Topic	Requirement today	In the 2024 proposal	Status	Homeowner impact
Where the radon rules apply R332.1 and Table AF101(1)	The radon appendix applies in the seven counties the code lists as high radon potential (Clark, Ferry, Okanogan, Pend Oreille, Skamania, Spokane and Stevens), and to every building built with an unvented crawl space under R408.3, in any county. King and Snohomish counties are not in the table. In a listed county an unvented crawl space is not permitted at all. WAC 51-51-0332, R332.1; WAC 51-51-60101, AF101.1 and Table AF101(1)	Editorial only. The proposal moves R332 to R335 and Appendix AF to Appendix BE, and reprints the same seven counties.	Current law	A house in King or Snohomish County is outside the high-radon table, so a vented crawl space built at the full vent area, without operable louvers, carries no radon requirement. Choosing the sealed option brings the radon appendix in anyway. Being outside the table is a statement about the county, not about one house: EPA says every home should be tested whatever its zone.
The radon system under a crawl space Appendix AF, AF103.5	For a crawl space the appendix describes a passive system: a continuous soil-gas retarder of polyethylene at least 6 mil thick, lapped at least 12 inches and extended to the foundation walls, and a 3- or 4-inch vent pipe from beneath the sheeting up through the building to at least 12 inches above the roof. It has an exception for buildings with an approved mechanical crawl space ventilation system. 2021 Seattle Residential Code, Appendix F, AF103.5, page AF-9	The proposal reletters the appendix BE. This site has not compared the 2024 model code’s wording of the section with the 2021 wording.	Current law	A sealed crawl space scope that follows the appendix includes a pipe that starts under the liner and ends above the roof, which is worth looking for in a bid. The appendix asks for 12-inch laps where R408.3 asks for 6-inch sealed laps; which applies to a sealed crawl space, and whether the appendix’s exception for mechanical ventilation covers the R408.3 exhaust fan, are questions for the building department.

A county’s zone is a prediction about an area, not a measurement of a house. EPA says its map should not be used to decide whether a home needs testing and that every home should be tested, whatever its zone.

The energy code: floor and duct insulation

Insulation is not in the residential code at all. It is in the 2021 Washington State Energy Code, Residential (chapter 51-11R WAC), in force since March 15, 2024. By its own terms that code is both the minimum and the maximum for residential construction in every city and county, so these figures are the same in Seattle, Bellevue, Everett and everywhere else in the state.

Washington residential energy code requirements that apply to crawl spaces, as in force, with what the pending 2024 proposal does to each
Topic	Requirement today	In the 2024 proposal	Status	Homeowner impact
Floor insulation over a crawl space WSEC-R Table R402.1.3	R-30 for floors, in new construction and additions. The table has one column for the whole state. Insulation squeezed into a cavity shallower than its label thickness counts at its compressed R-value. WAC 51-11R-40213, Table R402.1.3, Floor	The 2024 energy code proposal keeps R-30. It would also add two alternatives, listed under the pending changes below.	Current law	R-30 is the figure to hold a new floor insulation job against, and it would stay the figure under the proposal.
How floor insulation is installed WSEC-R R402.2.7	Three ways to comply. The usual one: cavity insulation kept in permanent contact with the underside of the subfloor, on supports no more than 24 inches apart, with foundation vents placed so the top of the vent is below the bottom of the insulation. WAC 51-11R-40220, R402.2.7	The 2024 energy code proposal rewrites the section; see the pending changes below.	Current law	On new work, cavity insulation has to be held against the subfloor or against sheathing below it; batts sagging between the two meet none of the three options.
Crawl space walls WSEC-R R402.2	The residential energy code gives no insulation value or method for crawl space walls. Its table has columns for ceilings, walls, floors, below-grade walls and slabs, and the insulation section mentions crawl spaces only for access hatches. This matches R408.3, from which Washington removed the model code’s perimeter-wall insulation option. WAC 51-11R-40213, table columns; WAC 51-11R-40220, R402.2.4 to R402.2.10	The proposal’s text mentions crawl space walls in its air-barrier and insulation-protection provisions. This site has not read enough of it to say whether it creates a wall-insulation path.	Current law	Insulating the crawl space walls in place of the floor is not a prescriptive option in the Washington text this site has read. Do not accept it as code-compliant on a permitted job without the building department’s confirmation.
Ducts in a crawl space WSEC-R R403.3.1	Supply and return ducts outside conditioned space, which includes a vented crawl space, are insulated to at least R-8 where the duct is 3 inches in diameter or larger and R-6 where it is smaller. WAC 51-11R-40320, R403.3.1	The proposal reorganizes the duct section. This site has not read where the insulation values land in it.	Current law	R-8 is the figure for new or replaced ductwork of ordinary size in a vented crawl space. The rule is for work done under the current code; it does not by itself require older duct insulation to be replaced.
Insulation work on an existing house WSEC-R R503.1.1	Altered parts of the building envelope meet the new-construction values, with exceptions where the building’s energy use is not increased: a floor cavity exposed during construction need only be filled with insulation, and nothing is required where the floor cavity is not exposed. WAC 51-11R-50300, R503.1.1, exceptions 2 and 3	This site has not read what the proposal does to the section.	Current law	The energy code does not make an owner insulate a floor that is not being worked on. Where old insulation is removed and the joist bays are open, the rule on a permitted alteration is to fill the cavity, not to reach a stated R-value.

In force since March 15, 2024. A 2024 edition is proposed (WSR 26-14-101, filed June 30, 2026) and not adopted. The council’s schedule puts its final adoption on November 20, 2026 and its implementation on May 3, 2027, and the council was still working through public comments on the insulation sections in November 2026.

How Seattle’s code relates

Seattle publishes its own residential code, the 2021 Seattle Residential Code, which is the 2021 International Residential Code with the state’s amendments and the city’s. For crawl spaces the city adds nothing this site could find: Seattle’s R408.1, R408.2 and R408.3 carry the marker the city uses for a Washington amendment and read the same as WAC 51-51-0408, apart from calling the radon appendix “Appendix F”.

What that rests on. A comparison of the two texts, not a statement by the city. Seattle’s PDF marks its additions by underline, which does not survive when the text is extracted, so a Seattle-only addition could have been missed.
Why Seattle’s copy is still worth opening. The city posts its chapters free, including the model code sections Washington does not amend, so it is the easiest place to read R408.4 to R408.6 and the radon appendix in full.
What does differ by city. Permits. Each city and county writes its own administrative chapter, which decides whether a job needs a building permit. This page does not cover that; the section hub says what has and has not been read.
Other cities. The state code applies in every county and city. This site has read Seattle’s local copy only, so it makes no statement about local amendments elsewhere in King or Snohomish County.

What is pending

Two proposals are open. For the residential code (WSR 26-10-071), the crawl space and radon sections are reprinted with one editorial change: the radon appendix is relettered from AF to BE and the radon section moves from R332 to R335. The proposal’s own summary classes those as alignment with the model code’s reorganization. For the energy code (WSR 26-14-101), 2 changes would touch a floor over a crawl space. The council’s schedule puts final adoption of the energy code on November 20, 2026.

Changes to crawl space floor insulation rules in the proposed 2024 Washington State Energy Code, with the rule in force, the proposed change, its stage and the homeowner impact
Topic	In force today	Proposed change	Status	Homeowner impact
Continuous-insulation alternatives for floors WSEC-R Table R402.1.3 (proposed)	Not available. The table in force gives one value for floors, R-30.	The proposed table prints the floor value as R-30, or R-19 in the cavity with R-7.5 of continuous insulation, or R-20 of continuous insulation. WSR 26-14-101, proposed Table R402.1.3, Floor	Proposed	If adopted as proposed, a floor could meet the code with less cavity insulation plus a continuous layer under the joists. Nothing may be built to it today, and the council was still revising the insulation sections in November 2026.
Floor insulation installation, rewritten WSEC-R R402.2.7 (proposed)	The section in force says floor framing cavity insulation shall comply with one of three options.	The proposal replaces that with requirements that all apply (insulation to the table value, air-sealed floor framing, supports no more than 24 inches apart, vents below the insulation), followed by a choice of three installation methods. WSR 26-14-101, proposed R402.2.7	Proposed	If adopted as proposed, air sealing of the floor framing would become an explicit requirement on new work. It has no effect on a job permitted under the code in force.

A proposal prints whole sections as they would read, including an implementation section with the date May 3, 2027 in it. That is proposal text. Nothing in either proposal is a requirement until a permanent rule adopts it, and the text can change before then.

Adoption timeline for the 2024 edition

The council’s schedule for the residential code, set against what its documents and the State Register show happened. A milestone is marked confirmed only where a published document records it. A date being in the past is not by itself evidence that the step took place.

April 1, 2025 Confirmed

Preproposal statement of inquiry for the 2024 International Residential Code (WSR 25-08-085).

The date is the one the council’s 2024 cycle page gives; the proposal cites the same filing.
May 5, 2026 Confirmed

The council files the proposed rules adopting the 2024 edition, WSR 26-10-071. They include the text of Section R408 and the radon sections as they would read.
June 10 and June 12, 2026 Confirmed

Public hearings on the proposal, in Olympia and Spokane Valley.

The council’s cycle page links the recordings and transcripts of both.
August 28, 2026 (intended) Not confirmed

The adoption date the proposal gave.

Not met. The council’s schedule, revised August 14, 2026, moved the meeting for final adoption to October 16.
October 16, 2026 Confirmed

Council meeting. Final adoption of the non-energy codes, the residential code among them, is delayed to October 30.

From the approved minutes, which also record that no public comments had been received on the residential code.
October 30, 2026 Not confirmed

Council meeting, with final adoption of the 2024 International Residential Code on the agenda as an action item.

The meeting was held; how the council voted on the residential code is not confirmed. No minutes were posted when this was checked. The one action from the meeting that the council has announced is an emergency energy-code rule for owners rebuilding after wildfire, which has nothing to do with crawl spaces.
November 18, 2026 (planned) Planned

Permanent rule filed with the Code Reviser. This filing, not the vote, is what adopts the edition.

The date in the council’s August schedule, set before the vote was delayed, so it may move.
May 3, 2027 (planned) Planned

The 2024 edition takes effect statewide.

The council’s planned implementation date, and the date the proposal would write into WAC 51-51-008. It becomes real only when a permanent rule sets it.

What the code does not address

Silence in a code section is not permission to guess, and it is not a requirement either. These crawl space topics are not settled by Section R408, so this page makes no claim that the code requires or forbids them:

Vapor barrier thickness beyond 6 mil. Washington’s code names a thickness once: the basic ground cover is 6-mil black polyethylene or an approved equal. Where it asks for more, in the reduced-vent exception and the unvented option, it asks for a Class I vapor retarder and gives no thickness. Nothing in Section R408 calls for 10, 12 or 20 mil.
Dehumidifiers. The section does not mention dehumidification, in the edition in force or in the proposal, and it is not one of the conditions for an unvented crawl space.
Drains and sump pumps. The section says when the under-floor grade must be raised unless an approved drainage system is provided. It does not say how a drain, a sump basin or a discharge line is built.
Mold. The section sets ventilation and ground cover requirements. It does not mention mold, its removal or who may remove it.
Existing crawl spaces. Nothing in the section says an existing crawl space must be brought up to it. When a repair or alteration has to meet the rules for new work is governed by Section R102.7.1 and applied by the building department.
Permits. Whether a given job needs a building permit is set by each city’s and county’s own administrative chapter, not by this section.

What this means for a house that already exists

The residential code governs new construction and work that is being done. It is not a maintenance standard, and no edition of it requires an owner to alter a crawl space that nobody is working on. A house built to an earlier code, or before any code, is not in violation because the code has since changed.

When work is done, the new work has to comply. Section R102.7.1 says additions, alterations and repairs conform to the requirements for a new structure without the existing structure having to comply, and that the work must not leave the house less compliant than it was.
Radon and alterations. The same section says alterations that do not involve building a foundation, crawl space, slab or basement are not required to meet the radon section and appendix. Section R408.3 separately lists a radon system as part of the unvented method. This page does not say how the two fit together when an existing crawl space is sealed. That is a question for the building department, and worth asking before signing an encapsulation contract.
Insulation. The energy code asks an alteration to meet the new-construction values, with exceptions: an exposed floor cavity need only be filled, and nothing is required where the cavity is not opened.
Whether a permit is needed is a separate question. It is answered by the city or county, not by Section R408, and a job that needs no permit still has to meet the code.

What to do with this

If you are comparing bids for a sealed crawl space, look for three things in the scope: the liner sealed at the joints and up the stem wall, a radon vent, and a continuously running exhaust fan sized to the floor area. A scope with a liner and a dehumidifier and nothing else does not match Section R408.3 as written. Ask the contractor which section the design is built to.
If you are being told the code has just changed, as of November 11, 2026 it has not. The edition in force dates from March 15, 2024, and the pending edition would not change the crawl space rules in substance. Ask which section and which document.
If your permit will be applied for near May 3, 2027, ask the building department which edition it will be reviewed under. That date is the council’s plan, not yet a rule.
If insulation is part of the scope, the figure for a new floor is R-30 and the figure for new ducts in a vented crawl space is R-8. Where old insulation is only being replaced in an existing floor, the rule is to fill the cavity.

How this page is maintained

Last checked: November 11, 2026. That means the status sources below were opened on that date and said what this page reports. Each source carries its own date, and one that was not re-read keeps its older one.
Next scheduled check: November 24, 2026. The council’s schedule puts the filing of the permanent rule on November 18, 2026 and its next council meeting two days later, so the check falls in the week after both. While an adoption is pending the checks stay weeks apart, not months.
What is checked, in order. First the State Register filing list for WAC 51-51-008, because a permanent rule there is the adoption and nothing else is. Then the council’s 2024 cycle page and its meeting minutes. If a rule has been filed, its text of the crawl space and radon sections is read before any row on this page moves.
Where the status lives: in one configuration file, not in prose across several pages. When the status changes, every page that states the edition in force changes with it in the same build.
Found an error, or is a date now wrong? Tell us, and name the section and the document. Corrections to a regulatory claim are made ahead of the next scheduled check, not at it, and are recorded in the public corrections ledger with the wording as it stood.
This site is not a code authority. Nothing here is a code interpretation, and your building department’s reading of the code governs your project.

Sources

Every regulatory claim on this page traces to one of these documents, and each row names the section it was read from so you can check it directly. The rules in force were read in the Washington Administrative Code, and the model code sections Washington does not amend were read in the City of Seattle’s copy. The two proposals are cited only for what is pending; they are not the code.

Washington State Building Code Council (via Washington State Legislature), WAC 51-51-003 — International Residential Code (Last amended by WSR 23-23-104, effective March 16, 2024) Checked November 10, 2026

Which edition Washington’s residential code is: the section adopts the 2021 edition of the International Residential Code by reference, with Washington’s additions, deletions and exceptions, and does not adopt chapters 11 and 25 through 43. Energy is regulated by chapter 51-11R WAC instead.
Washington State Building Code Council (via Washington State Legislature), WAC 51-51-008 — Implementation (Last amended by WSR 23-23-104, effective March 16, 2024) Checked November 10, 2026

The date the edition in force took effect: the International Residential Code adopted by chapter 51-51 WAC became effective in all counties and cities of the state on March 15, 2024. This is the section whose date changes when a new edition is adopted.
Washington State Legislature, WAC 51-51-0408 — Section R408, Under-floor space (Washington State Residential Code, amended by WSR 23-23-104) Checked November 10, 2026

What Washington requires under a house: ventilation at 1 square foot per 300 square feet of under-floor area; 6-mil black polyethylene ground cover with joints lapped 6 inches, extending to the foundation wall, which may be omitted where the crawl space has a concrete slab floor at least 2 inches thick; vent coverings with a least dimension of no more than 1/4 inch (corrosion-resistant wire mesh at 1/8 inch), and operational louvers permitted; ventilation reducible to 1/1,500 where a Class I vapor retarder covers the earth, with a radon vent built to Appendix AF required where ventilation is below 1/300 or operable louvers are installed. Vents may be omitted only where both numbered conditions of R408.3 are met: a continuous Class I vapor retarder lapped and sealed 6 inches, extending 6 inches up and sealed to the stem wall, with an Appendix AF radon system; and continuously operated mechanical exhaust at 1 cfm per 50 square feet. R408.8 is not adopted in Washington.
Washington State Building Code Council, WSR 26-10-071, proposed rules adopting the 2024 International Residential Code (chapter 51-51 WAC) (Filed May 5, 2026; intended adoption August 28, 2026) Checked October 7, 2026

That Washington’s proposal to move its residential code to the 2024 model code carries WAC 51-51-0408 forward with R408.3 unchanged apart from the radon appendix being relettered from AF to BE, so dehumidification is still not among the conditions it lists for an unvented crawl space.

Note: A proposed rule. When the section’s register filings were checked on the date above, no permanent rule had been filed after it.
Seattle Department of Construction and Inspections, 2021 Seattle Residential Code, Chapter 4, Foundations (Section R408, pages 4-47 and 4-48) Checked November 10, 2026

Two things. First, that Seattle’s R408.1, R408.2 and R408.3 carry the Washington-amendment marker and read the same as WAC 51-51-0408, apart from “Appendix F” where the state rule says “Appendix AF”; the copy shows the model code’s wording struck through, including its menu of alternatives for an unvented crawl space (dehumidification, conditioned supply air, perimeter-wall insulation), which is how the two numbered conditions of R408.3 are read as applying together. Second, the sections Washington does not amend, R408.4 to R408.6: an access opening not smaller than 18 by 24 inches through the floor or 16 by 24 inches through a perimeter wall; the under-floor grade cleared of vegetation and organic material, and forms and construction materials removed; and the under-floor grade as high as the outside finished grade where groundwater can rise to within 6 inches of the finished floor at the perimeter or surface water does not drain readily, unless an approved drainage system is provided.

Note: The PDF marks additions by underline, which is lost when its text is extracted. “No Seattle-only change to R408” is drawn from the match with the state rule, not from a statement by the city. R408.4 to R408.6 are model code text and are summarised here, not reproduced.
Washington State Building Code Council (via Washington State Legislature), WAC 51-51-0332 — Section R332, Protection against radon Checked November 10, 2026

Where the radon appendix applies. R332.1: to buildings constructed in the high radon potential (Zone 1) counties of Table AF101(1), and also to all buildings constructed using the unvented crawl space method of Section R408.3, in any county.
Washington State Building Code Council (via Washington State Legislature), WAC 51-51-60101 — Appendix F, Radon control methods (AF101 and Table AF101(1)) (WSR 07-01-090, effective July 1, 2007) Checked November 10, 2026

The high radon potential counties and what follows from being one. Table AF101(1) lists seven Washington counties: Clark, Ferry, Okanogan, Pend Oreille, Skamania, Spokane and Stevens. King and Snohomish are not in the table. AF101.1: unvented crawl spaces are not permitted in any high radon potential county, and in other areas the appendix applies to any structure built with an unvented crawl space under R408.3.
U.S. Environmental Protection Agency, EPA Map of Radon Zones — Washington Checked November 10, 2026

That King and Snohomish counties are in Zone 3, EPA’s lowest of three radon categories (predicted average indoor screening level below 2 pCi/L); Zone 2 is 2 to 4 pCi/L and Zone 1 is above 4. Washington’s Zone 1 counties are Clark, Skamania, Okanogan, Ferry, Stevens, Pend Oreille and Spokane. A low zone does not remove the Appendix AF radon vent that WAC 51-51-0408 requires where crawl space ventilation is reduced or the crawl space is built unvented.

Note: EPA states the map must not be used to decide whether an individual home needs testing, and that every home should be tested regardless of zone. A county-level prediction says nothing about one building.
Seattle Department of Construction and Inspections, 2021 Seattle Residential Code, Appendix F, Radon Control Methods (Section AF103.5, page AF-9) Checked November 10, 2026

What the radon appendix asks for under a crawl space. AF103.5 describes a passive sub-membrane depressurization system: a continuous soil-gas retarder of polyethylene at least 6 mil thick, lapped not less than 12 inches and extended to the foundation walls, with a 3- or 4-inch vent pipe run from beneath the sheeting up through the building to at least 12 inches above the roof. The section has an exception for buildings with an approved mechanical crawl space ventilation system or equivalent.

Note: Model code text read in the city’s copy and summarised, not reproduced. The appendix asks for 12-inch laps of the soil-gas retarder where R408.3 asks for 6-inch sealed laps of the vapor retarder; which governs on a sealed crawl space is a question for the building department.
Washington State Building Code Council (via Washington State Legislature), WAC 51-11R-10100 — Section R101, Scope and general requirements (2021 Washington State Energy Code, Residential) Checked November 10, 2026

The energy code edition in force and its reach. R101.1 adopts the 2021 edition of the Washington State Energy Code, effective in all counties and cities on March 15, 2024. R101.2 makes it both the maximum and the minimum energy code for residential construction in each town, city and county, so a city cannot set a different floor insulation value for houses.

Note: An emergency rule filed November 4, 2026 (WSR 26-23-002) touches R101.2 for owners rebuilding after the 2026 wildfires. It is not about crawl space work.
Washington State Building Code Council (via Washington State Legislature), WAC 51-11R-40213 — Table R402.1.3, Insulation minimum R-values and fenestration requirements by component (2021 Washington State Energy Code, Residential) Checked November 10, 2026

The floor insulation value for new construction: R-30, in a table with a single column for Climate Zone 5 and Marine 4, which covers the whole state. The table’s columns are fenestration, skylight, ceiling, wood frame wall, floor, below-grade wall and slab; there is no crawl space wall column. Footnote a: insulation installed in a cavity shallower than its label thickness counts at its compressed R-value.
Washington State Building Code Council, WSR 26-14-101, proposed rules adopting the 2024 Washington State Energy Code, Residential (chapter 51-11R WAC) (Filed June 30, 2026; intended adoption September 25, 2026) Checked November 10, 2026

What the pending residential energy code would do to floors over a crawl space. The proposed Table R402.1.3 keeps the floor value at R-30 and adds two alternatives that use continuous insulation, printed as “30 or 19+7.5ci or 20ci”. The proposed R402.2.7 replaces “comply with one of the following” with a list that all applies (insulation to the table value, air-sealed floor framing, supports no more than 24 inches on center, vents below the insulation) followed by a choice of three installation methods. The proposal would move the effective date to May 3, 2027.

Note: A proposed rule, still being revised: the council held work sessions on its R402 provisions in November 2026. Additions are marked by underline in the filing, which is lost when its text is extracted, so this entry reports only what could be read without the marks.
Washington State Building Code Council (via Washington State Legislature), WAC 51-11R-40220 — Section R402.2, Specific insulation requirements (2021 Washington State Energy Code, Residential) Checked November 10, 2026

How floor insulation over a crawl space has to be installed, and what the section leaves out. R402.2.7 gives three ways to comply; the first is cavity insulation kept in permanent contact with the underside of the subfloor decking, with supports no more than 24 inches on center and foundation vents placed so the top of the vent is below the lower surface of the insulation. R402.2.4 requires access hatches and doors from conditioned space to unconditioned space, crawl spaces included, to be insulated to the level of the surrounding surface and weatherstripped. The section has no subsection for crawl space walls: the word “crawl” appears in it only for access hatches.
Washington State Building Code Council (via Washington State Legislature), WAC 51-11R-40320 — Section R403.3, Ducts (2021 Washington State Energy Code, Residential) Checked November 10, 2026

Duct insulation outside conditioned space, which is where ducts in a vented crawl space are. R403.3.1: supply and return ducts insulated to not less than R-8 for ducts 3 inches in diameter and larger, and not less than R-6 for smaller ducts.
Washington State Building Code Council (via Washington State Legislature), WAC 51-11R-50300 — Section R503, Alterations (2021 Washington State Energy Code, Residential) Checked November 10, 2026

What the energy code asks of an existing house. R503.1.1: altered parts of the building envelope meet the new-construction requirements, with exceptions that apply where the building’s energy use is not increased. Exception 2: existing ceiling, wall or floor cavities exposed during construction need only be filled with insulation. Exception 3: no requirement where the existing roof, wall or floor cavity is not exposed.
Seattle Department of Construction and Inspections, Residential Code (2021 Seattle Residential Code) Checked November 10, 2026

That the City of Seattle has adopted the 2021 International Residential Code with amendments, published as the 2021 Seattle Residential Code, and where the city posts its chapters.

Note: The page read does not state the date the 2021 Seattle Residential Code took effect.
Washington State Building Code Council (via Washington State Legislature), WAC 51-51-0102 — Section R102, Applicability (Last amended by WSR 23-23-104, effective March 16, 2024) Checked November 10, 2026

How the residential code reaches an existing house. R102.7.1: additions, alterations and repairs conform to the requirements for a new structure without requiring the existing structure to comply with the code, unless otherwise stated, and must not make the existing structure less compliant than it was. Its second exception: additions or alterations that do not require the construction of foundations, crawl spaces, slabs or basements are not required to meet the radon protection requirements of Section R332.1 and Appendix AF. R102.5 names Appendix AF, Radon Control Methods, as included in the adoption.

Note: The section states the general rule. How it applies to a particular repair, and how its radon exception sits with the radon system named inside R408.3, is decided by the building department reviewing the work.
Washington State Legislature, Office of the Code Reviser, Register filings affecting WAC 51-51-008 Checked November 11, 2026

Whether a newer edition of the residential code has been adopted. The list shows every Washington State Register filing that touches the implementation section. When opened, the newest entry was WSR 26-10-071, a proposal filed May 5, 2026, and the newest permanent rule was WSR 23-23-104, filed November 15, 2023. No permanent rule adopting the 2024 edition was listed.

Note: The list can lag a filing by some days: on the same date it did not yet show an emergency energy-code rule the council said it filed on November 4, 2026.
Washington State Legislature, Office of the Code Reviser, Register filings affecting WAC 51-51-0408 Checked November 11, 2026

Whether the crawl space section itself has been amended since the edition in force. When opened, the newest entry was the proposal WSR 26-10-071 (filed May 5, 2026) and the newest permanent rule was WSR 23-23-104 (filed November 15, 2023).
Washington State Legislature, Office of the Code Reviser, 2026 WAC-to-Register Table, Title 51 WAC Checked November 10, 2026

A second view of the same record: for WAC 51-51-003, 51-51-008 and 51-51-0408 the 2026 table lists one action each, a proposed amendment under WSR 26-10-071, and no permanent action.
Washington State Building Code Council, 2024 Code Adoption Cycle Checked November 11, 2026

The council’s own record of where each 2024 code stands. For the 2024 International Residential Code (chapter 51-51 WAC) it lists the preproposal statement of inquiry (dated April 1, 2025), the proposed rule WSR 26-10-071, the public hearings of June 10 and June 12, 2026 with their transcripts, and an adopted-rule entry that still read “date tbd” when opened. The 2024 residential energy code’s adopted-rule entry read “date tbd” as well.
Washington State Building Code Council, Remaining 2024 Code Adoption Cycle Schedule (Revised August 14, 2026) Checked November 10, 2026

The council’s planned dates. For the building, residential and related codes: final draft approval September 25, 2026; council meeting for final adoption October 16, 2026; permanent-rule filing November 18, 2026; implementation Monday, May 3, 2027. For the energy codes: final draft approval October 30, 2026; final adoption November 20, 2026; permanent-rule filing December 2, 2026; implementation May 3, 2027.

Note: A plan, not a record. The October 16 date for the residential code was not met; see the council’s minutes for that meeting.
Washington State Building Code Council, Council meeting summary minutes, October 16, 2026 Checked November 10, 2026

That the council did not adopt the 2024 residential code on October 16, 2026: the approved minutes record a consensus to delay final adoption of the non-energy codes and to return with updated drafts of the adoption filings on October 30. For the residential code, staff reported that no public comments had been received.
Washington State Building Code Council, Council meeting agenda, October 30, 2026 (updated) Checked November 10, 2026

That final adoption of the 2024 International Residential Code (chapter 51-51 WAC) was listed as an action item for the council’s October 30, 2026 meeting.

Note: An agenda says what was scheduled, not what was decided.
Washington State Building Code Council, Meeting Schedules and Agendas Checked November 11, 2026

What the council has published about its October 30, 2026 meeting. When opened, the entry for that meeting carried an agenda and a recording and no minutes, so this site has no document recording how the council voted on the residential code. The page also lists the council meeting of November 20, 2026.

Note: The recording was not watched. Minutes are normally posted after the council approves them at a later meeting.
Washington State Building Code Council, State Building Code Council, home page announcements Checked November 11, 2026

The one action from the October 30, 2026 meeting that the council has announced: an emergency rule on Section R101.2 of the residential energy code for owners rebuilding after the 2026 wildfire season, filed November 4, 2026 (WSR 26-23-002) and effective on filing. Cited so that announcement is not mistaken for the adoption of the 2024 codes. When opened, the page carried no announcement about the 2024 residential code.

Sources last checked between October 7, 2026 and November 11, 2026; the date beside each one is when it was last opened. Where a figure comes from a document that is revised on a schedule, the edition or version is named above so you can confirm it against the current one yourself.

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