Crawl space question

joec

joe
User
I have a very dry crawlspace and that is very high as well. I did not need to put a liner in and the dehumidifier, so I took out the batt insulation and replaced it with closed cell foam insulation. Terminix came by to do an inspection and told me he saw evidence of rat infestation. My house was built in the 1970's and the vents are old and in bad shape. I was thinking of putting plywood over them and closing off the crawlspace. Are vents needed? When the encapsulate, they close off everything (I think). Any help appreciated.
 

Gofor

Mark
Corporate Member
According to our termite company, the new theory is that it is best to seal all vents. However, this can cause some problems, so they put an air inducer on the A/C plenum that allows a little bit of conditioned air into the crawl space, or if humidity is a problem, they run a dehumidifier in the crawl space. The air inducer can cause pressurization differences in the crawl space, so guess what? You may need a vent!!

We have had a humidity problem since the house was built in 2005. The contractor made some bad errors in sloping, and I actually had water running through the crawl space when it rained. Have fixed most of those problems, and I laid plastic sheeting over the soil, which made a big improvement. We just did some major landscaping changes to the flower beds in the front of the house this year. If that doesn't completely solve the problem, I plan on sealing mine. When I do, I plan to put in an air inducer (they cost about $100). As for sealing the vents, the inspector suggested I use rigid foam and seal it with expanding foam around the edges.

If it were me, I would seal the vents and check annually to see if there is a humidity problem. We don't have a rat problem, but did have signs of mice. Last year when I went under the house, found no signs of mice, but did pull out 7 shedded snake skins. This year, no signs of mice and no shedded skins, so I guess nature worked.

Not sure any of this will help you.
 
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Chris C

Chris
Senior User
When I replaced one of my units and the main trunk in 2020 the installer told me to close mine and keep them closed 365.
 

Oka

Casey
Corporate Member
Building Code states venting for under floor is 1 sq ft per 150 sqft of floor space. If you close it off then you have to provide elec fan venting
One thing you can do is use ss mesh for those vent screens. Animals cannot chew through that stuff
 
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Phil S

Phil Soper
Staff member
Corporate Member
My building inspector said to install a small fan in one of the vent openings and close all of the rest. The fan should blow out which will pull some of the conditioned air in the house above down into the crawl space. Air volume should be 50 cfm per 1000 sq ft. I have a small computer fan running in mine - seems to be working well
 

JimD

Jim
Senior User
I used to have a mice problem in my crawl space. They started getting into the house at night and chewing on things. I started putting poison in the crawl space and I have seen no more signs of them. But I check the poison monthly and have to replace a little block of it in one of the three stations usually. So at least a few are still hanging around.

I've also spent time and energy sealing up potential entry points but what seems to have made the most difference is the poison. I got some in little plastic bait stations so my dog cannot get to them - but I keep her out of the crawl space now too.
 

sandfarm

Joe
User
For about 2 years now, I have sealed my vents with 2" rigid foam board and silicone caulking and installed a dehumidifier. I have no ground cover and everything looks good with about 64 - 68 % humidity.
According to crawl space moisture tables, this is the correct humidity. Even the wood construction humidity is correct at about 12 -16 %.
I have been told that a ground cover would stop the continuous moisture from the dirt, but I don't want a cover so that the moisture condenses beneath the cover. Moisture content is correct without the cover.
The dehumidifier will have to work more this way.
 

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