Before you go in: safety

You can do most of this inspection from the hatch with a good flashlight, and for plenty of homes that is the right call. Read this before deciding to crawl in.

Do not enter the crawl space if there is standing water anywhere near electrical wiring or junction boxes; the insulation is loose fill rather than batt in a home built before about 1990, which raises the possibility of vermiculite containing asbestos; there is heavy rodent activity, since droppings and nesting material carry real disease risk when disturbed; you see extensive mold growth; or you have asthma, allergies, or any respiratory condition. In any of those cases, inspect from the opening and have a professional handle the rest.

If you do go in, wear a respirator rather than a dust mask, long sleeves, gloves, and eye protection. Tell someone you are going under the house. Bring a real flashlight rather than a phone, and take photos as you go so you can compare against next season without a second trip.

September / October: pre-rain check

This is the important one. At Seattle-Tacoma International Airport, October through March brings 75 percent of an average year's 39.3 inches of rain, and the share is 69 to 76 percent at all 8 long-record stations around the metro. Anything wrong now becomes a problem you live with until spring.

Outside the house, no crawling required

  • Clear the gutters. Overflowing gutters dump water directly against the foundation.
  • Check where every downspout discharges. Seattle Public Utilities advises at least 5 feet from the foundation, and says 10 feet is better, without sending the water onto a neighbor's lot or over a steep slope.
  • Look at the grade. Soil should fall away from the foundation for the first several feet. Flower beds and settled soil often reverse it over time.
  • Clear debris from foundation vents so a vented crawl space can actually breathe. Washington's code sets the vent area for a vented crawl space, and a vent buried by soil or bark does not count toward it.
  • Check window wells and area drains for leaves and blockage.

At the crawl space opening

  • Smell first. A musty odor at the hatch means elevated humidity, and if you can smell it upstairs, air is moving into the house.
  • Look for standing water or damp soil, and for high-water marks on piers and stem walls that record past flooding.
  • Check the ground cover, if there is one, for tears, gaps, and sections that have slid out of position. Washington's code asks for 6-mil black polyethylene lapped 6 inches at the joints and run to the foundation wall.
  • Look up at the insulation. It should be tight to the subfloor. Sagging, dark, or fallen batt means moisture.
  • Scan for rodent evidence: droppings, shredded insulation, chewed material. If you find any, do not disturb it. Dealing with rodents in a crawl space starts with sealing the way in, not with cleaning up.
  • If you have a sump pump, test it. Pour a bucket of water into the basin and confirm it kicks on, discharges, and shuts off. Confirm the discharge line is not blocked or disconnected.

Mid-winter: wet season spot check

After the first sustained storm, ideally while it is still raining or within a day of it stopping. This is the only time of year you can see the problem actually happening.

  • Look for active water entry. Where is it coming in? A corner, along a wall, up through the soil? That answer determines the fix.
  • Confirm the sump is cycling if you have one, and that the discharge is not freezing or backing up.
  • Watch downspouts during rain to see where the water really goes rather than where you think it goes.
  • Note condensation on ductwork, pipes, or the underside of the subfloor.
  • Check for new sagging in insulation, which means it is absorbing moisture.

If you find active water, take photos and video. A contractor who can see where water entered in January will diagnose far more accurately than one walking a dry crawl space in July. This is useful evidence, so capture it while it exists.

April / May: post-rain assessment

The wet season is over and the crawl space is showing you the cumulative damage.

  • Has standing water drained? Water still present in May points to a high water table or failed drainage rather than a one-off storm.
  • Check joists and subfloor for dark staining, soft spots, or visible mold. Press suspicious wood with a screwdriver; it should be firm.
  • Assess insulation condition now that the season has done its worst. Wet batt does not recover its R-value.
  • Look at piers and stem walls for efflorescence, the white mineral crust that marks water passing through concrete.
  • Reassess the ground cover for winter damage.
  • Note whether the musty smell faded or persists into dry weather. Persisting means a continuing moisture source.

Summer: the repair window

If your spring assessment turned up problems, summer is when to fix them. Drier ground, easier access, and contractors are not yet booked solid by everyone who discovered water in November.

  • Schedule any drainage, insulation, barrier, or encapsulation work now. The 2026 Seattle cost guide covers what each of those scopes runs.
  • Handle exterior work while the ground is workable: regrading, downspout extensions, gutter repairs.
  • If insulation is being replaced, have the existing insulation measured and photographed before anything is removed. Puget Sound Energy's floor insulation rebate, for homes PSE heats, depends on there being 4 inches or less to start with and does not allow removing insulation to qualify. The 2026 rebate guide has the rest of the conditions.
  • Service or replace a sump pump ahead of the season rather than during it.

The printable version

Condensed to the fifteen items that catch nearly everything:

  1. Gutters clear
  2. Downspouts discharge at least 5 feet from the foundation (10 is better)
  3. Grade slopes away from the house
  4. Foundation vents unobstructed
  5. No musty smell at the hatch
  6. No musty smell in the living space
  7. No standing water on the crawl space soil
  8. No high-water marks on piers or walls
  9. Ground cover intact and in position
  10. Insulation dry and tight to the subfloor
  11. No sagging or fallen batt
  12. Joists and subfloor firm, unstained, mold free
  13. No rodent droppings or nesting
  14. Sump pump cycles and discharges correctly
  15. No condensation on ducts or pipes

Anything unchecked is either a maintenance task or a reason to get an inspection.

What warrants a call

Some findings can wait for next season. These should not:

  • Standing water that persists beyond a few days after rain, which is a drainage question rather than a cleanup one.
  • Soft or spongy joists, beams, or subfloor, which is structural and gets more expensive monthly.
  • Visible mold across a meaningful area rather than an isolated spot, which is past the point of homeowner cleanup and into remediation.
  • Musty smell reaching your living space, meaning crawl space air is entering the house.
  • Wet or fallen insulation, which has stopped working and is holding moisture against the framing. Replacing it only lasts if the moisture source goes with it.
  • A sump pump that failed the test. Replacement is a great deal cheaper before the next storm than after it.
  • Sagging floors upstairs or doors that have started sticking, which can indicate structural movement below.

None of these are emergencies on the day you find them. All of them get more expensive the longer they cycle through another wet season. If your checklist turned up several, a free in-home inspection will tell you which are urgent and which can wait, at no cost, with a written, no-obligation quote for the ones that need work. Some of the fixes are reasonable to take on yourself, and this job-by-job guide sorts out which.

Sources

The seasonal timing is built on NOAA's 1991–2020 precipitation normals for the eight long-record stations around the Seattle metro. The downspout distance is Seattle Public Utilities' advice. Humidity targets and the point at which cleanup stops being a homeowner job are EPA's. What a crawl space is required to have is Washington's residential code (WAC 51-51-0408), and the rebate condition is Puget Sound Energy's.

  1. 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.

  2. Seattle Public Utilities, Drainage System Explanatory summary, not the governing text Checked

    SPU’s homeowner advice to direct downspout flows at least 5 feet from the foundation (10 feet is better), not onto a neighbor’s property or over a steep hill, and to coordinate approved discharge points with SDCI.

  3. United States Environmental Protection Agency, A Brief Guide to Mold, Moisture and Your Home Checked

    Indoor humidity targets, EPA’s position on when sampling is and is not worth paying for, its position on biocides, the general health effects of damp indoor spaces, and the 24–48 hour drying window — which EPA states as a PREVENTION target ("dry water-damaged areas and items within 24-48 hours to prevent mold growth"), not as a time by which mold begins. Also that molds produce allergens, irritants and in some cases potentially toxic substances, can trigger asthma attacks in people allergic to mold, that it is impossible to get rid of all mold and spores indoors, that moldy surfaces should not be painted or caulked over, that a moldy area under about 10 square feet is in most cases a job a homeowner can handle, that larger growth sends the reader to EPA’s remediation guidance for schools and commercial buildings, and that damage from sewage or other contaminated water calls for a professional experienced with it.

  4. 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.

  5. USDA Forest Products Laboratory, Wood Handbook, Chapter 14: Biodeterioration of Wood (General Technical Report FPL-GTR-282 (2021 edition)) Checked

    The wood moisture content and temperature ranges at which decay fungi become active, which is what separates a surface mold problem from structural rot. Also, for the white-mold guide: mold shows as fuzzy or powdery surface growth in colors from light shades to black, and on softwoods can often be brushed off (p. 14-2); mold and stain fungi affect strength only slightly but make wood more absorbent, which can lead to decay (p. 14-2); fresh surface growth of decay fungi can look like fan-shaped patches, strands or root-like structures, usually white or brown (p. 14-3); white rot leaves wood bleached and spongy, brown rot darkens it and cracks it into cubes, and some brown-rot fungi form water-conducting strands that carry soil water into dry wood (p. 14-4); and that crawl-space condensation, whose primary source is soil moisture, can cause serious decay, while wood debris and concrete forms left under a building can seed it (p. 14-7). And, for the termite guide (pp. 14-11 to 14-15): anobiid powder-post beetles generally need wood at about 15% moisture or more, which inadequate ventilation can create and lower moisture through ventilation, insulation and vapor barriers can relieve; subterranean termites need a constant source of moisture, build mud shelter tubes that are a good sign of their activity, swarm usually in early spring or fall, and leave galleries along the grain under a shell of sound wood, so suspect wood is probed with a pick or knife; infestation hazard is greatest over a crawl space that is poorly drained and lacks a moisture barrier; in a crawl space, joists at least 18 inches and girders 12 inches from the earth, a polyethylene barrier over the soil running partly up the foundation wall, drainage away from the house with gutters, downspouts and grading, posts that bear on the ground made of concrete, and concrete forms, stakes, stumps and waste wood removed; control of termites already in a building means breaking the contact between the soil colony and the woodwork by blocking runways, treating the soil, repairing leaks such as plumbing leaks, or a combination, then inspecting often; carpenter ants use wood for shelter rather than food, prefer wood softened by decay and so can point to a moisture problem, and species such as Camponotus modoc can be particularly damaging.

  6. Puget Sound Energy, Weatherization rebates Checked

    The PSE amounts and conditions this site quotes: floor insulation at $2.50 per square foot up to $5,000, in a site-built home built 1989 or before with 0 to 4 inches of existing insulation, finished to R-30 or a full joist cavity, where existing insulation may not be removed to qualify and the thermal boundary may not be moved; floor air sealing at $0.20 per square foot up to $400; duct sealing and insulation up to $1,000 per home, duct sealing only up to $550; that the customer must use PSE electricity or PSE natural gas as the primary source of space heating and live in a single-family home of four units or fewer or a manufactured home; that the work must be done by a PSE Recommended Energy Professional or Trade Ally, who takes the rebate off the invoice; that self-installed work does not qualify; and that the rebate cannot exceed the price of the work.

    Note: PSE states that this is a tariffed service that can change or end without notice, and the page gives no program year. The page lists no rebate for a ground cover, vapor barrier, dehumidifier or drainage. It still says a federal tax credit may be available; the IRS says that credit ended for work after December 31, 2025.

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.

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