What is the Ideal Humidity Level for a Crawl Space?

It is a must that you keep a close eye on your crawl space so that you can prevent expensive structural damages.

So, what is the ideal humidity level for a crawl space? The ideal relative humidity should be kept at or below 50% and it has to be maintained all year round.

Controlling the accumulation of moisture is the number one secret to deter mold growth, pest infestation, and rot. It can be made through maintaining a safe relative humidity inside.

When you speak of relative humidity, this refers to the water content in the air in relation to the maximum amount of water that it can hold at a certain temperature.

Once warm air reaches your crawl space, it gets cooled and this will then raise the relative humidity. The result is higher relative humidity that can lead to energy loss, mold, rot, not to mention that it can also attract pests.

Mold requires a minimum of 70 percent relative humidity for it to grow and develop. But, this is still regarded as a loose number.

There are some mold types that can grow and thrive at a much lower relative humidity as based on a couple of factors, namely, the surface’s relative humidity, and the relative humidity of surrounding air.

These differences in temperature can make mold grow even at less than 70 percent.


Encapsulation is the best method to control the humidity level in a crawl space. Properly sealing it can help stop the water vapors from getting inside and to create an environment that is free from pests and moisture.

Encapsulation will involve the installation of a 20 mil vapor barrier system, sump pump installation, sealing crawl space doors and vents and adding a dehumidifier.

A properly done encapsulation can help quickly decrease moisture and humidity concerns through preventing water from getting in.

Remember that mildew and mold can have an impact on the overall structural stability and integrity of your home.

It is a must that you get the help of a professional that can perform an inspection of your crawl space and give you the best solutions for your problems.

Can Crawl Space Humidity Be Too Low?

Yes, lowering the relative humidity can actually get too low. For instance, if the wood’s moisture is less than 5 percent, the wood is going to dry out, and this is not good either.

This means that for you to have an ideal happy medium, you need to maintain the relative humidity at around 30 to 50 percent and it will keep the wood moisture at around 5 to 12 percent.

But, before you get too concerned about dehumidifying your crawl space, there are some realities involved here. The truth is that you cannot really over dry a crawl space. But, you can actually make it much dryer than it already was, and this can affect its floors.

It is natural for buildings to run at a certain moisture level. This level is used for the acclimation guidelines, whether it is an existing house or a new one. The floors need to be acclimated properly and the problems must be kept to the least number possible.

However, when the long term levels of moisture in the home change, the floors installed in the past can behave in a different way and this can then lead to several issues.

One of the major reasons to have a dehumidifier installed in the crawl space is to reduce moisture to control fungal decay and mold. If there is decay or mold, the floors, as well as the house itself, may still be fine yet this decay can lead to structural damage, with the mold causing health concerns.

The best way to stop decay and mold is to lower the levels of moisture. A dehumidifier will be able to help you do this as can closing vents and using vapor retarder membrane to cover the soil.

Overdrying somewhat needs a proper definition here. Can it be considered overdrying if you lower the moisture level in such a way that decay and mold will not grow? Maybe not. Many of you don’t want decay or mold in your house. Does the building or wood care if it is already too dry? Not really. Many things will end up shrinking once they dry yet they are still fine otherwise.

From the equipment standpoint, typical dehumidifiers are rated at 80 degrees F and 60 percent relative humidity. The efficiencies can drop once the air becomes cooler or dryer.

You could still reduce the relative humidity to 30 percent inside a sealed and warm crawl space with plenty of dehumidification yet this very same space could have already dropped lower than 30 percent relative humidity during winter before it was sealed. With no special equipment for dehumidification used,  it will not become dryer than around 40 percent relative humidity.

So, to answer the original question if crawl space humidity can get too low, the answer is yes, it is possible. But, of course, this is not something you want to do or happen.

Following a new moisture routine that is dryer than before can lead to some flooring issues. Stabilizing the moisture levels prior to installing the floors is crucial in spite of dehumidification.