What should the humidity in a crawl space be at different times of year?

Keep your crawl space below 60% relative humidity year round, and below 55% is better. That one number does most of the work. Mold and wood-rotting fungus need high humidity to grow, so once you hold the crawl under 60%, the framing stays sound and the musty smell upstairs goes away. The target does not change much with the season. What changes is how hard you have to work to hit it, and summer is the hard part in New Hampshire.

That is the short answer. Here is what is going on down there, season by season.

Why one number, not a different target each season

People expect a summer number and a winter number. The mold does not care about the calendar. It cares about moisture. The EPA points to keeping indoor humidity low to control mold, and once relative humidity climbs past about 60%, mold has what it needs to start eating wood, cardboard, and anything else organic in your crawl. Dust mites, the big indoor allergen, need humidity above 50% to live. So below 60% is the line, and below 55% gives you cushion. Aim there in January and aim there in July. What changes by season is how hard you have to work to hit it.

Summer is the dangerous season

This catches people off guard. A crawl gets wettest in summer.

The ground under your house stays cool all year, near 55F. In summer the outside air is warm and full of water. When that air gets into a cool crawl, it cools fast. Cooler air cannot hold as much water, so the relative humidity shoots up. On a muggy New Hampshire day you can push past 60% down there without a drop of rain. The framing sweats. That is condensation season, and it is why a vented crawl gets worse in summer. A sealed crawl with a dehumidifier holds under 55% while it is 80% and sticky outside.

Winter and the shoulder seasons

Winter air is dry. Cold air holds little water, so a meter in the crawl usually reads lower in the cold months. Good for mold. Bad for everything else. Those open vents pour cold air right under your floors, the floors above turn freezing, and your heat fights it all winter while NH frost heave works on the walls. So winter humidity can look fine on a meter while the crawl still costs you comfort and money.

Spring thaw and fall rain bring water in at the foundation. Most crawls sit on disturbed backfill, the looser soil the builder put back when the house went up. It drains slowly and holds water, so water pools at the wall after a thaw and works its way in. Then the meter creeps back up toward summer levels.

How you actually hold it under 60% all year

You cannot vent your way there in this climate. The fix is to seal the crawl off from the ground and the outside air, then dry what is left. That is crawl space encapsulation, and it is a different job from our Forever Dry System, which is a basement job. Crawl spaces get encapsulation.

Here is the system we put in:

• A 12-mil wall vapor barrier up the foundation walls.
• A 20-mil floor vapor barrier across the dirt.
• Dimpled drainage matting under the floor barrier so any water has a path.
• Spray foam to seal the seams.
• A dehumidifier to hold the space under 55% year round.
• A sump to handle water if the crawl takes any.

Encapsulation runs $3,000 to $25,000 in New Hampshire, and the price tracks the size of the crawl and how much prep it needs first. Headroom, standing water, and old debris all push it. We give you the number after we see it, and you get the written quote back within 24 hours.

If you put a cheap meter down there and it reads under 55% in every season, with a sound vapor barrier and no musty smell, you may not need this yet. We will tell you that to your face. The Department of Energy notes up to about a third of a home’s heat loss can come through an uninsulated foundation or crawl, so when there is real moisture, sealing it earns its keep through a NH winter. If you have a dirt or fieldstone crawl and a radon reading, ask us about a sub-membrane system while the crawl is open. You only open it up once.

Crawl encapsulation carries a 25-year warranty on our work, the wall vapor barrier 25 years, the dehumidifier 5 years, and the sump system 3 years. Trusted by 5,000+ homeowners in New England, BBB A+ accredited, licensed and insured.

Talk to us

Call us at 603-610-1770 or book your free inspection. We put a meter in your crawl, tell you what it needs and what it does not, and send a written quote within 24 hours.

More on the work: crawl space services, radon mitigation, supplemental support, and the 603 guarantee. If your problem is really a basement, start with basement waterproofing.

Related questions

What humidity reading means I have a problem? Anything that holds over 60% for long stretches. That is when mold and wood-rotting fungus have what they need. If your meter sits in the high 60s or 70s, especially through summer, the crawl needs sealing and a dehumidifier. Surfaces can read damper than the air, so do not wait for a perfect number.

Will a dehumidifier alone fix a humid crawl? Not by itself in an unsealed crawl. A dehumidifier in a vented dirt crawl is trying to dry the whole outdoors and the bare ground at once. It runs forever and loses. Seal the crawl first with the vapor barrier, then the dehumidifier can hold it under 55% on reasonable power.

Does crawl humidity affect the air upstairs? It does. Air moves up through a house, so damp crawl air rises into the rooms above and brings the musty smell and the moisture with it. Drying the crawl is part of drying the whole house.