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

In summary, there is a debate over whether to have open or closed vents in a crawlspace, with a third option of automatic temperature vents. One argument is to seal the crawlspace with closed vents and use a dehumidifier to control moisture, while the other argues that the automatic vents already provide the best solution. The decision may also depend on whether the living space is air-conditioned during the summer and the location, soil type, and construction of the house. Ultimately, the key is to define a secure bulkhead and determine the most effective and economical solution for each individual situation.
  • #1
kyphysics
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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! :smile:

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?
 
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  • #2
Is the living space air-conditioned during the summer? Seems to me that would make a difference in the decision.
 
  • #3
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.
 
  • #4
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.
 
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  • #5
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.
 
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  • #6
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:
Crawlspace.jpg

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.
 
  • #7
hutchphd said:
Is the living space air-conditioned during the summer? Seems to me that would make a difference in the decision.
yes
 
  • #8
jrmichler said:
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!
 
  • #9
What is the location, type of soil and type of construction of your father’s house?
 
  • #11
I believe it comes down to defining a secure bulkhead that separates the interior and the exterior of the house.

The question becomes; given an existing construction, where the bulkhead is missing or is imperfect, what actions would improve the situation, and over what period of time could those be economically justified ?

There are several possible solutions for each situation, including doing nothing.
 

Related to Closed vs. Open Crawlspace & Automatic Vents vs. Dehumidifer?

1. What is the difference between a closed and open crawlspace?

A closed crawlspace is completely sealed off from the outside environment, while an open crawlspace has vents or other openings that allow for air flow. Closed crawlspaces are typically more energy efficient and help to prevent moisture and pests from entering the space.

2. Which is better, automatic vents or a dehumidifier for a crawlspace?

This depends on the specific needs of your crawlspace. Automatic vents can help to regulate air flow and prevent moisture buildup, but they may not be enough in areas with high humidity. A dehumidifier can be more effective in controlling moisture levels, but it also requires regular maintenance and may use more energy.

3. Do automatic vents work in all climates?

No, automatic vents are not suitable for all climates. In areas with high levels of humidity, they may not be effective in preventing moisture buildup. Additionally, in colder climates, they may allow cold air to enter the crawlspace, making it more difficult to maintain a consistent temperature.

4. Can a dehumidifier replace the need for automatic vents?

In some cases, a dehumidifier may be able to effectively control moisture levels without the need for automatic vents. However, it is important to consider the specific needs of your crawlspace and consult with a professional to determine the best solution.

5. Are there any other options besides automatic vents and dehumidifiers for controlling moisture in a crawlspace?

Yes, there are other options such as encapsulation, which involves sealing off the entire crawlspace with a moisture barrier. This can be an effective solution for preventing moisture and pests from entering the space. However, it may also require additional measures for air flow and ventilation.

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