Should crawl space vents be open or closed? Check your zip, month by month

It depends on your climate and the month. Vents only dry a crawl space when the outdoor air is drier than the crawl space can hold at its cooler surface temperature. Across 459 NOAA stations, our model finds open vents likely to add moisture in at least one month at 342 of them (75%), almost always in late spring and summer, and in no month at the other 117. In humid summers, closed vents with humidity control usually perform better; in dry climates and cool, dry months, venting can help. In freezing weather vents are usually closed to protect pipes. Enter your zip on this page for your months.

Close them in two situations. In humid months — late spring and summer in most of the East and South — open vents tend to add moisture, so closed vents with humidity control usually do better; the tool on this page shows which months that applies to your zip. And in cold weather, close them once nights regularly drop to around 40°F, the temperature at which automatic foundation vents shut themselves, to keep freezing air off the pipes. A sealed or encapsulated crawl space stays closed all year.

Only when the outdoor air is dry enough to carry moisture out and it is not freezing — typically cool, dry spring and fall days, and much of the year in arid Western climates. Automatic foundation vents open fully at about 70°F. If a hygrometer shows crawl space humidity rising after you open the vents, the outside air is too wet to help and they should go back to closed. Never open the vents of an encapsulated crawl space.

In humid climates, usually closed. Summer outdoor air carries a lot of water vapour, and when it meets a crawl space's cooler soil, walls and ducts its relative humidity climbs — in our model, to an average of 93% at Raleigh–Durham in July, above the 80% line ASHRAE Standard 160 uses for mould risk. Field measurements in North Carolina point the same way: wall-vented crawl spaces averaged above 80% humidity on 39–86% of summer days, depending on the year. In dry Western climates summer air often stays dry enough that venting does no harm. The tool on this page shows which applies to your zip.

For most vented crawl spaces with plumbing, closed during freezing weather, to protect pipes and keep floors warmer — the moisture question matters less in winter, because cold air holds little water. This tool marks months when the average outdoor temperature is under 40°F. Our guide to closing crawl space vents in winter covers when to close them, the gas-appliance exception, and when to reopen.

Aim to keep it below 60%, the upper limit EPA gives for indoor humidity, and ideally in the 45–55% range that lets wood stay dry. Sustained readings near 80% or higher mean conditions where mould can grow on surfaces. A digital hygrometer left in the crawl space is the way to check; our humidity guide explains the numbers.

Sometimes. Vents were designed to let outside air carry moisture away, which works when outside air is dry — in cool, dry months and in arid climates. In humid months, vents bring moisture in. In our model the median station has 4 months a year when open vents are likely to add moisture, but Gulf Coast and Southeast stations typically have 4–5, and desert stations such as Denver, CO and Phoenix, AZ and Las Vegas, NV have none.

Because the air coming in is warmer and wetter than the crawl space. When warm outdoor air cools against the soil, foundation walls and air-conditioning ducts under the house, it cannot hold as much water vapour, so its relative humidity rises and, on the muggiest days, water condenses on the coolest surfaces. The vents are working as designed; the outside air just is not dry enough to dry anything.

Only as part of a sealed (closed or encapsulated) crawl space: a continuous, sealed ground vapour barrier, water sources dealt with, and a way to keep the air dry such as a dehumidifier or conditioned supply air, as the building code's unvented crawl space provisions require. Sealing vents over bare or wet soil without that can trap moisture. Check local code, and have any gas appliance's combustion air checked first.