Skip to content

Crawl spaces

Crawl space humidity and moisture

By Robert Stulpt Last reviewed

Short answer

I like to keep crawl space humidity between 45% and 55%. Most crawl spaces are fine below 60%. Between 60% and 65%, I would keep an eye on it. Above 65% for extended periods is a problem, and at 70% or more I want to know why. For the wood, moisture levels of 10% to 15% are very good in many homes. At 20% or more, I want to know why.

On this page
  1. What the readings mean
  2. How to measure
  3. Where crawl space moisture comes from
  4. When to call someone

What the readings mean

Relative humidity in the crawl spaceWhat it means
45–55%Where I like it
Under 60%Normally fine
60–65%Keep an eye on it
Over 65% for long periodsA problem
70% or moreFind out why
Crawl space humidity and wood moisture readings Two scales. Relative humidity: 45 to 55 percent ideal, under 60 fine, 60 to 65 watch, above 65 a problem if it lasts, 70 or more find out why. Wood moisture content: 10 to 15 percent very good, 16 to 19 watch, 20 to 24 find the cause, 25 and above serious if it lasts. Robert Stulpt's working ranges. Relative humidity in the crawl space Ideal Fine Watch Problem if it lasts 45% 55% 60% 65% 70% 80% Moisture content of the wood framing Very good Watch Find the cause Serious if it lasts 10% 16% 20% 25% 30% 35% 70%+: find out why

Scroll sideways to see the whole drawing.

Robert Stulpt’s working ranges from his inspections in Virginia.
Moisture content of the woodWhat it means
10–15%Very good in many homes
16–19%Watch the trend and where it’s happening
20% or moreLook for the cause
25–30%, persistentA serious concern

Robert Stulpt’s working ranges from his inspections in Virginia.

I know a 20% moisture level isn’t going to make your joists rot the next day. But I don’t like to ignore it either. Researchers at the USDA Forest Products Laboratory put it this way: wood will decay above 30% moisture content and won’t decay below 20%. Everything in between is a gray area, which is why I start looking for the cause at 20%.

For comparison, the EPA recommends keeping humidity in the living space below 60%, ideally between 30% and 50%.

How to measure

I don’t go off one reading. I check the joists, the girder, the subfloor and the rim joist; near plumbing and HVAC equipment; around the perimeter and in the center of the crawl space; and in stained areas as well as non-stained ones. I use a hygrometer, a pin-type moisture meter and, if I need to see what happens over a few days, a data logger. If you’re a homeowner, a good remote hygrometer and a pin-type moisture meter will catch a lot.

  • Joists, girder and subfloor
  • Rim joist
  • Near plumbing and HVAC equipment
  • Around the perimeter and in the center
  • Stained areas and areas that look normal

Where crawl space moisture comes from

There are several reasons for high humidity in a crawl space: the soil, outside air, leaks and water getting in. During the summer, outside air is warm and humid. When it enters a crawl space that’s cooler than outside, it condenses on anything below its dew point. In winter, outdoor air holds less moisture, so the problems shift to cold surfaces and air leaking from the house, as well as leaks that have nothing to do with the season. To check a specific surface, use our dew point calculator.

What to do about it depends on the source: a vapor barrier for moisture from the soil, closing the vents and a dehumidifier for humid outside air, and finding the water when it’s getting in.

When to call someone

If wood is at 20% or higher and stays at that level, I start investigating. At 25% or higher multiple times, I want to see it corrected soon. If I see wood that is soft or getting thinner, decay at the bearing points of joists and beams or where they connect, joists or beams that are visibly bending, supports that have moved or a sagging floor above, I want a structural professional to look at it. Moisture meters show you where to look. They don’t show you how bad the wood is.

Sources