Crawl Space Insulation Calculator

In most climates, seal it. Building Science Corporation states plainly that crawl spaces should not be insulated between floors, and Advanced Energy's field study on twelve identical houses found vented crawl spaces above 80% relative humidity through most of spring and summer while sealed ones stayed below 65%. The exceptions matter though: in a dry climate the vented approach is reasonable and much cheaper, and where FEMA flood openings are required you must not seal, because that turns the crawl space into a basin.

Because of when the air comes in. In summer, outdoor air at a dew point of 70°F enters a crawl space whose surfaces sit nearer 60°F, and condenses on them. The ventilation that was supposed to dry the space is what wets it. This is why the building-science consensus shifted, and why older advice to "keep the vents open" has not aged well in humid regions.

No standing water or persistently damp soil, no active mold or rot, and no atmospherically-vented gas appliance that sealing would starve of combustion air. Covering wet ground with plastic can be worse than doing nothing, because it traps the moisture against the structure. If any of those apply, fix them first — the calculator flags each one.

It can. Encapsulation is not radon mitigation and occasionally makes readings worse. The EPA action level is 4 pCi/L, with action worth considering from 2. Test before and after. If you are sealing the ground anyway, have a passive sub-membrane depressurisation stub installed at the same time — it costs very little while the liner is open and can be made active later if a test comes back high.

No. Polyiso has the highest R-value per inch of the three common boards, but it wicks moisture out of concrete and loses R-value doing so, and should not be used below grade or against concrete or earth. Use XPS, which is explicitly rated for below-grade use, or EPS, which is cheaper and dries far faster — a fifteen-year buried comparison found EPS returning to under 1% moisture where XPS stayed near 16%.

You need one of four things, and a dehumidifier is the usual choice. IRC R408.3 requires a ground vapor retarder plus either mechanical exhaust, supplied conditioned air, a plenum arrangement, or dehumidification — a sealed crawl space with no humidity control is a worse outcome than a vented one. Practitioners default to a dedicated crawl space dehumidifier, sized around 70 pints a day per 1,000 sq ft, draining to daylight or a condensate pump.

Some of it. Laying a ground vapor barrier in a clean, dry crawl space with decent clearance is realistic, as is rigid foam at the rim joist or on the walls. Closed-cell spray foam is not — it needs two-component equipment, supplied-air respirators, and a 24-hour re-entry delay. Nor is anything involving mold, standing water, a combustion appliance, or clearance under 24 inches. A crawl space is a confined space: check for a second exit, test for carbon monoxide, wear a P100 respirator, and never work down there alone.

Yes, and it is one of the best-value parts of the job. The rim joist is a small area with disproportionate air leakage, and it is counted separately in the estimate because it usually gets a different material — rigid foam or closed-cell spray foam rather than whatever is going on the walls.