# Closed vs. Open Crawlspace & Automatic Vents vs. Dehumidifer?

So, my dad (non-scientist) was debating with his cousin over whether he needed to spend $$on a crawlspace dehumidifier and get closed vents. It was both a heated/fierce debate and interesting. Mostly, I just listened to two old dudes arguing in partial amusement, but some intrigue as well. Curiosity got to me, so I thought I'd ask. There is debate over whether a crawlspace's vents should be open or closed. That much I know. Some say open and some say closed. However, they have a third option of automatic temperature vents (which my parent's house has) that open when it's hot and close when the weather is cold. Supposedly, this vents the crawlspace during hot and humid times and traps in heat when it's cold. So, you get the best of both worlds . My dad's inspector said he should replace his automatic temperature vents and go with closed crawlspace vents and buy a dehumidifier. The logic was that the automatic ones can sometimes let in warm air and humidity at times when it's not best to and over time that can lead to mold, wood rot, and moisture issues. Sealing the crawlspace with closed vents + installing a dehumidifier down there can best control humidity and rot problems he said. 5,000 please! My uncle, who has this exact set-up agrees with the inspector guy. However, my dad says no, b/c the previous inspector (they changed companies) said the automatic temperature vents give you the best of all worlds and already account for this. They open mostly when it's hot and humid and this allows good air flow that would make a build up of moisture issues not a problem (or, at least, a low probability issue). And in the cold, they do the reverse, which helps. He thinks the new inspector guy is just trying to make$$\$. Secondly, he thinks having a totally sealed crawlspace (even with dehumidifier) may strap in "stale"/old/bad air (whatever that is or means).

Any idea if these "arguments" from both sides have any merit? Is one method better than the other?

Related General Engineering News on Phys.org
hutchphd
Is the living space air-conditioned during the summer? Seems to me that would make a difference in the decision.

Well, dry air is indeed good for the wood. You can indeed do this with dehumidifier. It is kind of a sure kill solution.

But depending on the circumstances you might get just a bit worse result with less fuss. There are plenty of multi-century old wood still 'in service' around the world, with dehumidifiers not even invented when they were built in.

So it is not really a question what can be decided so easily.

jrmichler
Mentor
Here is a good discussion of crawlspace ventilation from the Building Science Corporation: https://www.buildingscience.com/doc...ce-insights/bsi-115-crawlspaces-either-or-out. Short version is that crawlspaces should be either fully ventilated or fully sealed.

My own house has 4 inches of foam insulation outside the foundation walls, under the footings, and under the crawlspace. The heating ducts run through the crawlspace, but do not discharge into the crawlspace. The only source of heat in the crawlspace is the heating duct surfaces, and that is enough to make the crawlspace slightly warmer than the house. We have nice warm floors in winter. Because our crawlspace is sealed from the outside, humid air from the outside does not get in. The crawlspace has about the same humidity as the house. We use a dehumidifier to keep the house at 50% RH in the summer. The result is a nice clean dry crawlspace with ZERO mold.

hutchphd
anorlunda
Staff Emeritus
Wouldn't the answer strongly depend on how wet the dirt is?

I once had to dig a 1 meter sump and put in a sump pump. After that, the air in the crawl space became much dryer.

Averagesupernova
jrmichler
Mentor
The bottom of my crawlspace is a few inches above the peak water table. The drain tile around the house has water running several months out of the year. There is a 6 mil polyethylene vapor barrier underneath the foam insulation. The foam is covered by 2" - 3" of sand for fire protection:

The white pipe on the left is the drain line for condensate from the air conditioner, boiler, and heat recovery ventilator. It drains into the drain tile instead of into the sanitary sewer because trickle flows are a primary cause of frozen sewer lines. We have a conventional septic system.

Is the living space air-conditioned during the summer? Seems to me that would make a difference in the decision.
yes

The bottom of my crawlspace is a few inches above the peak water table. The drain tile around the house has water running several months out of the year. There is a 6 mil polyethylene vapor barrier underneath the foam insulation. The foam is covered by 2" - 3" of sand for fire protection:
View attachment 271737
The white pipe on the left is the drain line for condensate from the air conditioner, boiler, and heat recovery ventilator. It drains into the drain tile instead of into the sanitary sewer because trickle flows are a primary cause of frozen sewer lines. We have a conventional septic system.
What's that big silver/metal thing? It looks like a refrigerator!!

What is the location, type of soil and type of construction of your father’s house?

Baluncore