How much wood moisture in your crawl space can you tolerate

Keep the wood in your crawl space under 20% moisture content. That is the line. Below 20%, wood-rotting fungus cannot get going, so your joists, girders, and sill plates stay sound. Once the wood holds 20% or more for long stretches, decay fungi have what they need and the framing starts to break down. The USDA Forest Products Laboratory points to that same threshold. Wood that stays under about 20% does not rot. Wood that stays wet does.

That is the short answer. Here is what is going on under your floor.

Why 20% is the number

Wood soaks up moisture from the air around it and gives it back, and it settles at a level that tracks the humidity in the crawl. Damp air means damp wood. Dry air means dry wood. There is a direct line between the two.

Decay fungus needs wet wood to live. Researchers put that floor right around 20% moisture content. Below it, the fungus stays dormant. Above it, especially up near 28% to 30% where wood fibers are fully saturated, the wood is wet enough to rot and mold spreads on top of it. So 20% is where the biology turns on.

You measure it with a moisture meter. They run about [verify], you push the pins into a joist, and you read the percentage right off it. A handful of readings around the crawl tell you fast whether you have a problem.

What drives the wood up over 20%

It is almost never a leak. It is the air.

The ground under your house stays cool all year, near 55F. In summer the outside air is warm and full of water. When that warm air gets into a cool crawl, it cools down, and cooler air cannot hold as much water, so the relative humidity shoots up and surfaces sweat. The wood drinks it in. A muggy New Hampshire week can push your joists past 20% without a drop of rain falling. That is why a vented crawl gets worse in summer, not better.

Water at the foundation makes it worse. 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 spring thaw and fall rain pool at the wall and feed moisture into the space. NH frost heave pushes on the walls all winter on top of it.

How you hold the wood under 20%

You cannot vent your way there in this climate. You dry the air, and the wood follows.

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 dry year round.
• A sump to handle water if the crawl takes any.

Hold the air dry and the wood drops back under 20% and stays there. 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.

Here is the part where we teach instead of sell. If you put a meter on your joists and they read under 20% 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 the wood is wet, sealing it pays you back through a NH winter on top of saving the framing. 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 on your framing, 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, sill replacement, and the 603 guarantee. If your problem is really a basement, start with basement waterproofing.

Related questions

What humidity reading keeps the wood under 20%? Hold the crawl air below 60% relative humidity, and below 55% is better. Wood tracks the air around it, so dry air keeps the joists dry. That is what the dehumidifier is for once the crawl is sealed. A meter that sits in the high 60s or 70s through summer means the wood is climbing too.

My joists already read over 20%. Is the wood ruined? Not always. Wood that got wet and dried out can still be sound, and decay fungus goes dormant once the wood drops back under 20%. The danger is wood that stays wet for months. We check the framing, tell you what is still solid, and if a sill or beam is too far gone we handle that too. See sill replacement.

Will a dehumidifier alone dry the wood? 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 the air dry and the wood comes down with it.