When does a crawl space need a dehumidifier?
Not when it feels damp, and not as a substitute for anything the code asks for. The deciding questions are whether the space is open to outdoor air, whether liquid water still gets in, and whether a sealed space still runs humid with its exhaust fan working. EPA’s moisture-control guidance for builders treats the decision as a measurement: monitor the relative humidity to decide whether dehumidification is needed. A hygrometer read over weeks is the evidence, and what counts as high humidity explains how to read it.
| Your crawl space | Dehumidifier? | Do this first |
|---|---|---|
| Standing water, or water after storms | Not yet | Stop the water with crawl space drainage or waterproofing. A dehumidifier takes vapor out of the air; it does not remove puddles, and the manufacturer cited here wants water intrusion fixed first. |
| Vented, bare soil | No | Cover the soil. Washington’s code expects the ground of a vented crawl space to be covered with 6-mil black polyethylene or an approved equal (R408.1); see vapor barrier installation. |
| Vented, soil covered, readings acceptable | No | Nothing beyond a look each season. |
| Vented, soil covered, humidity stays high | Not while it is vented | Decide whether to seal it: should crawl space vents be open or closed. |
| Sealed (encapsulated), or about to be | Optional, on top of the exhaust fan | Get the liner, the radon system and the continuous exhaust the code asks for. Add a dehumidifier only if the space still measures humid with those in place. The rest of this page covers that case. |
Can a dehumidifier work in a vented crawl space?
It can run, but it is working against the vents. Therma-Stor’s installation manual for its Santa Fe Advance crawl space units says the space must be free of water intrusion and of excessive outdoor air before the unit goes in, and that both problems should be fixed first. Open foundation vents are outdoor air by design, so a unit in a vented crawl space is drying a supply that never stops.
How much water that outdoor air carries depends on the season. EPA’s guidance for drying a crawl space during construction draws the line at an outdoor dew point of 55°F: below it, ventilation can hold the space under 65 percent relative humidity; above it, the job takes dehumidifiers. Warm summer air carries more water than cold winter air, which is why summer, not winter, is when outdoor air works hardest against a cool crawl space.
Winter brings the other load. Seattle-Tacoma International Airport’s normal year delivers 39.34 inches of precipitation, 75 percent of it from October through March (the climate and housing data has every station), and bare soil keeps handing that moisture back to the space above it. That is a ground-cover problem before it is an air problem.
What does Washington code require in a sealed crawl space?
Start with the default, because it is the opposite of what many national product pages assume. Washington’s residential code (WAC 51-51-0408) expects a crawl space to be vented: the under-floor space “shall have” openings, at 1 square foot for each 300 square feet of floor. 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.
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.
A crawl space may go without vents only under Section 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.
Both conditions apply together. 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.
So a quote that seals the vents and offers a dehumidifier as the conditioning is not describing one of the arrangements the section allows. Ask how it meets the code, and look for the radon vent pipe and the continuously running exhaust fan as line items. The radon system matters in King and Snohomish counties even though neither is on the code’s high-radon list. 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 full R408 text and the radon question are set out on the vents guide.
Washington’s proposal to move to the 2024 model code reprints R408.3 with the same two conditions, relettering the radon appendix and nothing more. It is a proposal, not adopted law; the Washington crawl space code page follows where it stands.
What size dehumidifier does a crawl space need?
Washington’s code is no help here, because it does not ask for a dehumidifier and so says nothing about its size. The 70 pints a day for every 1,000 square feet that turns up in bids and on contractor sites is the wording of the 2018 model International Residential Code, which Washington does not follow on this point. Size to what the manufacturer specifies for your space, which makes the useful skill reading a specification. Three things move the answer.
- Floor area and air-tightness together. Therma-Stor’s data sheet for its Santa Fe Advance90 rates one unit for up to 3,400 square feet of very tightly sealed crawl space, 2,600 moderately sealed and 2,200 with some leaks and imperfections. The same machine covers about 35 percent less floor in a leaky space, so the quality of the sealing belongs in the sizing conversation.
- The test behind the pint figure. Federal standards that took effect in June 2019 rate portable dehumidifiers at 65°F instead of the old 80°F, and whole-home units at 73°F and 60 percent humidity, so a machine tested the new way reports a smaller capacity. ENERGY STAR’s rough conversion puts a whole-home unit rated 80 pints a day through 2019 at about 70 under the new test, and a 70-pint portable at about 40 to 45. The Advance90 sheet shows the effect inside one document: 90 pints a day at 80°F and 60 percent, 71 at 70°F. Two bids naming different pint figures may describe the same capacity, so ask which test each one comes from.
- How cold the space gets. The same manufacturer says its Advance units run between 49°F and 95°F, work best from 56°F, and make little or no water at 55°F or below. Ask what the crawl space reaches in midwinter and whether the unit is rated for it.
ENERGY STAR’s own advice is that a whole-home unit be sized and installed correctly by a professional.
How much does crawl space dehumidifier installation cost?
This site’s Seattle-metro model puts a crawl space dehumidifier, supplied, installed and wired, at $1,350 to $2,700 before sales tax. That is a first installation including the electrical work to feed it; a like-for-like replacement into an existing circuit is the lower part of that band or below it. In Seattle the new circuit needs an electrical permit. It is a modeled estimate rather than a collected quote, and the cost methodology explains how the ranges are built. The unit is usually priced on top of the sealing work rather than inside it: encapsulation cost sets out the full system, and how long each part of an encapsulation lasts covers the replacement, filters and warranty terms that follow.
How much does a crawl space dehumidifier cost to run?
Running cost is the unit’s power draw, times the hours it runs, times the price of a kilowatt-hour. The hours are the unknown: the humidistat starts and stops the compressor, and how long it runs depends on how much moisture still reaches a sealed space, which nobody can know before the unit is in. So here is the arithmetic for one real unit at one published Seattle-area price, with the hours left as a variable.
Therma-Stor’s data sheet lists the Santa Fe Advance90 at 640 watts at its 80°F rating point. Seattle City Light’s Flat Rate for residential customers in Seattle charges 13.38 cents for every kilowatt-hour, whatever the amount used. Each hour the compressor runs therefore costs about 8.6 cents at that rate. Customers of other utilities, and City Light customers outside the city, pay different rates; the one that counts is on your own bill.
| Compressor hours a day | kWh a day | Per day | Per 30 days |
|---|---|---|---|
| 6 | 3.84 | $0.51 | $15.41 |
| 12 | 7.68 | $1.03 | $30.83 |
| 24 | 15.36 | $2.06 | $61.66 |
Read the table as arithmetic, not a forecast. The wattage is the maker’s figure at its 80°F, 60 percent rating point rather than a measurement in your crawl space, the energy charge leaves out City Light’s daily base charge (which you pay whether the unit runs or not), and the run hours are yours to find out. A plug-in energy meter between the unit and its outlet for a month gives the real number. In a sealed Washington crawl space the exhaust fan runs all the time as well, so a dehumidifier is a second running cost, not the only one. The same uncertainty is why the 15-year cost-to-own model leaves electricity unpriced.
Where does the water go, and where should the unit sit?
Every dehumidifier makes water, and the water has to leave the crawl space. Therma-Stor’s manual for its Advance units requires a trap in the drain line and a 3/4-inch PVC drain falling at least 1 inch in every 10 feet, offers a condensate pump where the water has to be lifted to reach a drain, and says to check the line for algae or sludge whenever the filter is changed. Ask the installer where the line will end, and whether it needs a pump. The same manual’s placement rules make a fair checklist for any brand:
- intake and exhaust at least 12 inches from any wall or obstruction;
- level, and raised on supports rather than set directly on structural framing without vibration absorbers;
- never in standing water, and hung from the joists with the maker’s kit wherever the space could flood, since a flooded unit is not covered by the warranty;
- within reach of a 115-volt outlet on a circuit of at least 15 amps, dedicated and GFCI-protected if it sits near water;
- with room on every side for service and filter changes.
Can you seal a crawl space without a dehumidifier?
Yes, and in Washington that is the arrangement the code describes. The conditioning R408.3 asks for is continuously operated mechanical exhaust at 1 cfm for every 50 square feet of crawl space floor, terminating outdoors, alongside the sealed liner and the radon system. For a 1,000-square-foot crawl space that is 20 cfm; for 1,500 square feet, 30. The model code’s conditioned-supply-air option is struck from Washington’s version, as is dehumidification, so neither replaces the exhaust fan.
A dehumidifier earns its place on top of that only where measurements show the space still runs humid. Any change to how air moves under the house matters for radon and for any gas appliance that takes its combustion air from the crawl space. The crawl space encapsulation page covers the radon question for King and Snohomish counties.
How do you keep a crawl space dehumidifier working?
Three habits cover most of it. Leave the humidistat where the maker recommends; Therma-Stor’s “Normal” band of 45 to 55 percent is the one it suggests for most uses. Check the drain line whenever the filter is changed. And keep a hygrometer you can read without crawling in, because a sealed crawl space whose exhaust fan or dehumidifier has stopped has lost the humidity control it was built around.
A free inspection is where the space is measured, checked for water, and the conditioning method and its sizing are put in writing, in a no-obligation quote. Check that the quote names the exhaust fan and radon system a sealed crawl space needs here and, if it adds a dehumidifier, the unit's capacity and where its condensate drains.
Sources
Code text is from Washington’s WAC 51-51-0408 as in force since March 15, 2024, read with the City of Seattle’s marked-up copy of the same section; the 70-pint figure is traced to the 2018 model code. Unit specifications are one manufacturer’s, used as a worked example and not a recommendation. The electricity price is Seattle City Light’s flat residential energy charge for customers in Seattle, read November 11, 2026.
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Washington State Legislature, WAC 51-51-0408 — Section R408, Under-floor space (Washington State Residential Code, amended by WSR 23-23-104) Checked
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.
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Seattle Department of Construction and Inspections, 2021 Seattle Residential Code, Chapter 4, Foundations (Section R408, pages 4-47 and 4-48) Checked
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.
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Washington State Building Code Council (via Washington State Legislature), WAC 51-51-0332 — Section R332, Protection against radon Checked
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.
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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
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.
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International Code Council, 2018 International Residential Code, Section R408 Under-Floor Space (2018 IRC, R408.3 item 2.4) Checked
That the 2018 edition of the model residential code listed, among the ways to condition an unvented crawl space, dehumidification sized to remove 70 pints of moisture a day for every 1,000 square feet of crawl space floor. Cited only to show where that figure comes from.
Note: A model code, not Washington law. Washington’s R408.3 (WAC 51-51-0408) does not list dehumidification among its conditions for an unvented crawl space.
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United States Environmental Protection Agency, Moisture Control Guidance for Building Design, Construction and Maintenance (EPA 402-F-13053, December 2013) Checked
That damp-proof or waterproof coatings on below-grade walls provide a capillary break that excludes the rainwater saturating the surrounding fill; that at the bottom of the below-grade wall a footing drain system carries rainwater and ground water away from the footing and the floor slab; the principle of draining the site first; and that damp basements or crawl spaces may add as much water vapor to the air in a day as all the other internal sources combined. Also its construction-phase guidance for a crawl space once the floor above encloses it: monitor the relative humidity to decide whether dehumidification is needed, and hold it below 65 percent with ventilation when outdoor dew points are below 55°F, or with dehumidifiers.
Note: Design and construction guidance for building professionals, not a code. The crawl space humidity guidance is written for the construction phase, before a house is occupied.
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NOAA National Centers for Environmental Information, U.S. Climate Normals (1991–2020) — Annual/Seasonal and Monthly (normals-annualseasonal-1991-2020, normals-monthly-1991-2020) Open dataset Checked
Every precipitation figure in the Seattle-metro climate and housing dataset: annual and monthly precipitation normals, the October–March total summed from the monthly normals, the average number of days per year at or above 0.10, 0.50 and 1.00 inches, and the completeness flag and years of record NCEI publishes alongside each value, for the eight stations within 15 miles of a served city with at least 20 years of record (Snoqualmie Falls, Landsburg, Monroe, Tacoma #1, Seattle-Tacoma International Airport, Seattle Sand Point, Renton Municipal Airport and Everett Snohomish County Airport). Pulled through the NCEI data access service on 2026-11-10.
Note: A normal is a 30-year statistic for one instrument at one point, not a value for the surrounding city. NCEI publishes a completeness flag with every value and this site republishes it, because a station with 20 years of record and one with 29 are not equally strong evidence. Seattle Boeing Field has 19 years behind its annual normal and is not published.
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ENERGY STAR (US EPA and US Department of Energy), Dehumidifier Testing and Capacity Checked
That federal standards based on the Integrated Energy Factor took effect in June 2019 with a new test procedure that rates portable dehumidifiers at 65°F instead of 80°F, so the same machine now reports a smaller capacity; that portable and whole-home units have separate minimum efficiencies; that a whole-home unit should be sized and installed correctly by a professional; and its rough correlation of old and new ratings, among them a whole-home unit rated 80 pints a day through 2019 at about 70 from 2020, and a 70-pint portable at about 40 to 45.
Note: The correlation table is ENERGY STAR’s approximation, and it says the change for any specific model will vary.
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U.S. Department of Energy, Uniform Test Method for Measuring the Energy Consumption of Dehumidifiers (10 CFR Part 430, Subpart B, Appendix X1, as published in the eCFR) Checked
The standard test conditions behind a current dehumidifier rating (Table 1 to paragraph 4.1.1): 65°F dry-bulb and 60 percent relative humidity for portable dehumidifiers, and 73°F and 60 percent for whole-home dehumidifiers.
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Therma-Stor LLC (Santa Fe), Santa Fe Advance90 product data sheet (Part number 4034180; specifications dated 7/23/18) Checked
One crawl-space-class unit’s published specification, used for the sizing and running-cost examples: 640 watts and 5.7 amps on a 115-volt supply at 80°F and 60 percent relative humidity; water removal of 90 pints a day at 80°F and 60 percent and 71 pints at 70°F and 60 percent; and crawl space coverage of up to 3,400 square feet very tightly sealed, 2,600 moderately sealed and 2,200 with some leaks and imperfections.
Note: Read from a distributor’s copy of the manufacturer’s data sheet, which is dated 2018 and marked subject to change without notice; the manufacturer’s own site did not serve it to a plain request. Ratings at 80°F predate the 2019 federal test conditions. One unit, used as an example and not as a recommendation.
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Therma-Stor LLC (Santa Fe), Santa Fe Advance90 / Advance120 Dehumidifier — Installation and Operation Instructions (TS-1152, Rev B, 08/20) Checked
Three things a dehumidifier costs money for after it is installed: the manufacturer’s recommendation that the air filters be replaced every three to six months; a warranty of two years on the product and six years on components; and the warranty’s exclusion of transportation and of labor costs incurred in repair or replacement of those components — which is why a six-year component warranty is not six years of free repairs. Also the installation conditions the dehumidifier guide relies on: the space must be free of water intrusion and of excessive outdoor air before the unit goes in; it runs between 49°F and 95°F, best from 56°F, and makes little or no water at 55°F or below; intake and exhaust stay 12 inches from any wall; it sits level and raised, never in standing water, and is hung from the joists where the space could flood, a flooded unit not being covered; it needs a 115-volt outlet on a circuit of at least 15 amps, dedicated and GFCI-protected near water; its drain needs a trap and a 3/4-inch line falling at least 1 inch in 10 feet, or a condensate pump; and its “Normal” setting, 45 to 55 percent relative humidity, is the one recommended for most uses, the driest setting being advised against in a space below 65°F because it means long stretches of ineffective running.
Note: One manufacturer and one product family. Cited because it is a crawl-space-class unit whose terms are published in full, not because its terms are universal. Confirm the filter interval and the warranty on the unit actually quoted to you; both vary by model and by brand.
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Seattle City Light, Residential Rates Checked
The electricity price in the dehumidifier running-cost example: City Light’s Flat Rate for residential customers in Seattle, an energy charge of $0.1338 per kWh with every kWh charged the same rate, plus a base service charge of $0.3945 a day. The page also lists a time-of-use option and different flat rates for customers in other jurisdictions, and says the 4% Rate Stabilization Account surcharge was removed on April 1, 2026.
Note: The page gives no effective date for the rates shown. Customers of other utilities, and City Light customers outside Seattle, pay different rates.
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Seattle Department of Construction and Inspections, Electrical Permit Checked
That in Seattle an electrical permit is needed whenever wiring is installed, altered, extended or connected to equipment, which covers a new receptacle or circuit for a sump pump or dehumidifier; that a professional applying needs a Washington electrical contractor’s license and a Seattle business license; and that owners may do their own electrical work on a property they own and occupy.
Sources last checked between and ; 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.