Sealing up a crawlspace and running a dehumidifier vs std foundation vents

drkelly

drkelly

Dipstick who put two vehicles on jack stands
Joined
Mar 21, 2005
Location
Oak Ridge/Stokesdale, NC
We just had a new house built last fall. During construction I read a bit about sealing up a crawlspace and installing a dehumidifier instead of the old school vents in the foundation walls. There was some small discussion on this in another thread recently. I’d like to hear from anyone who has done this.

Thanks,
Danny
 
We just did it and I HIGHLY recommend it!! The humidity in our house dropped about 7% the first day and our house stays alot cooler. We didnt use a dehumidifier b/c our AC unit is in the crawlspace. it removes alot of the humidity on its own. The house smells fresher when you walk in and its also supposed to drop the utilities cost (will see in time). It was a bit expensive, but we had to have a mold remediation done too.
 
What exactly was done to your house. I think it might have been your thread where it was mentioned. Our HVAC unit is under the house too. I'm just beginning to learn about this, but thought you had to run a dehumidifier if you sealed up the crawlspace. The humidity level in the downstairs floor of our house has been fluctuating between 58-62%, which from what I have read is a just a little high for summer time. When I went under the house last w/e, I noticed water droplets were on the outside of the silver insulated duct work and on the bottom of the metal unit near/after the coil. I'm guessing that is normal just like a cold glass of water would have droplets form on the outside.
 
We have the same set up as RobMcBee and it's nice! Before we bought our house they, the previous owners, had the mold remediation done. We also have those old school vents on there, but they stay shut 24/7. The crawlspace is very comfortable year round.
 
I've installed several hundred crawlspace encapsulations in the last 8 years or so and there has never been a case where the home owner was dissapointed with the results. A lot of factors can determine what type of liner or moisture barrier is needed. Priority one: eliminate water intrusion. Moisture in a crawlspace leads to mold growth, poor functioning of HVAC system, termites, warped floors, pests, rank smells, etc. Keeping the humidity level below 55% IN THE CRAWLSPACE will greatly benefit the rest of the house. A fully sealed crawlspace meaning vents blocked (or eliminated) and a heavy vapor barrier curtained down the interior walls, wrapped around pillars and tape sealed to the floor liner creates the air and water tight seal necessary for proper humidity control with or without a dehu.
 
We have the same issue with ours to the point that the air distribution box (the thing that has the drain attached) is a solid sheet of dripping condensation. We have vents around the entire exterior (brick). When we added out addition on to the front of the house, they placed the heat activated vents that open when hot, but even still it's like a sauna. The vents in the main part of the house are the old fashioned kind and most are broken anyway (I had been meaning to replace with the heat activated kind anyway).

Would the dehumidifier work at all if we didn't seal up the vents? Would we notice any appreciable difference? Our crawlspace is dirt floor - which I'm not used to since I'm used to actually having a cement floor in a basement/crawlspace from growing up in Michigan.
 
A dehu would be pretty much useless in that application. Even with the crawlspace floor left dirt and the vents blocked the dehu would run constantly. Think of it like running the ac full blast with all the windows and doors open. You must contain off the area intended to control.
 
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drkelly said:
When I went under the house last w/e, I noticed water droplets were on the outside of the silver insulated duct work and on the bottom of the metal unit near/after the coil. I'm guessing that is normal just like a cold glass of water would have droplets form on the outside.
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If you have this, you need better insulation on your duct.
 
Crawl space vents are a terrible idea. In the summer time, you're letting air that is hot and humid enter the crawl space through the vents, where it drops in temperature below the dew point and all of the condensed moisture falls out. This is bad for the house, bad for you, good for bugs, good for mold, etc. Encapsulation is the way to go.

Dehumidifiers are basically big heaters. The problem is that the heat-rejection portion of the unit is on the interior of the house. Compare that to an air conditioner, where the heat rejection is on the exterior. If you can use the AC unit to supply a small amount of conditioned air to the crawl space, it will have the same dehumidification effect that you'd expect from a dehumidifier, but without the heat gain. This also helps with air transfer, as you're adding a small amount of positive pressure to the crawl space.

rockcity said:
If you have this, you need better insulation on your duct.
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No, his crawl space should be better conditioned.
 
XJsavage said:
I've installed several hundred crawlspace encapsulations in the last 8 years or so
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How thick of a liner are you guys installing? Is it just heavy-weight visqueen with the seams taped, or a heavier membrane product? Just curious.
 
The house I just bought had a slight water intrusion problem...I laid down a new vapor barrier, French drain and replaced all the old vents with automatic vents based on temp, fans based on a temp and a dehumidifier based on humidity level. It was right at $2300 to do it all myself (that did include some new insulation and a mold treatment) and about 2 days, but I swear the house is staying cooler and the highest interior reading I've seen is 56%, was previously constantly in the low 60's.
 
shawn said:
How thick of a liner are you guys installing? Is it just heavy-weight visqueen with the seams taped, or a heavier membrane product? Just curious.
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Over the years I've installed everything from 10 mil clear poly to 20 mil. The best stuff is a Basement Systems product known as Cleanspace. There's a 12 mil (white on one side, black on the other) that's fiberglass thread reinforced and nearly indestructible. We used this on walls and pillars. On the floor wed use either the 16 mil (blue/white) or 20 mil (white/white) on rockier terrain or heavy traffic crawlspace. Both are fiber reinforced as well. On every job there is a 4" white vaporbond tape used to bond the seams and the material is always used white side in. Different companies use different stuff but this is by far the best. Also carries a warranty of 20-35 years depending on thickness. Advantaclean uses a thick poly style which works OK too. Not sure the longevity of it but that like most things depends on how it was installed.
 
What about using closed-cell spray foam? Wouldn't that kill 2 birds w/ one stone, vapor retarder (barrier if thick enough) plus insulating the envelope, then plastic on the floor?
 
RatLabGuy said:
What about using closed-cell spray foam? Wouldn't that kill 2 birds w/ one stone, vapor retarder (barrier if thick enough) plus insulating the envelope, then plastic on the floor?
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Keep in mind that foam insulation can be used as a transportation route by termites. You can use rigid sheets on the exterior walls too, but whichever product you use, you want to make sure to leave a 4-6" high inspection void near the sill plate so that you can check for mud tunnels. The inspection void negates a lot of the benefit of the insulation, though.
 
RatLabGuy said:
What about using closed-cell spray foam? Wouldn't that kill 2 birds w/ one stone, vapor retarder (barrier if thick enough) plus insulating the envelope, then plastic on the floor?
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I've done that too and yes its great! We used closed cell foam in crawlspaces since its compact and packs a crazy R value per inch. 1"=R-20ish. We would use a thick moisture barrier on the floor, 16 mil or more, and lap it up the walls a few inches and spray the CC foam from floor up. It does get messy though and it is quite pricey.
 
shawn said:
Thanks. Term bar to the walls and pillars, I guess?
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Hammer drill with 1/4 masonary bit and 1" plastic rib pins similar to what holds in door panels. One pin per every 2-4' of wall and then a heavy caulk seam between the material and brick to create a seal. Pillars are usually tight enough to only need a caulk seal, unless they're funky T shaped pillars then we pin it in the corners to form material to the contour.
 
We had the full remediation and encapsulation done. Heres a list of what all was done.

  • removed all floor insulation and debris (we had alot of crap under there)
  • sprayed a sporicide on all joist, pipes, and surfaces. Wiped everything down with rags
  • removed old vapor barrier
  • raked dirt down to level out the floor
  • sealed the old vents
  • lined walls with 2" foam board
  • laid down liner and taped all the joints
Im sure much more went into it than what I listed. We opted out of the dehumidifier b/c of cost. Our sales rep said our ac unit would probably handle it on its on, but we could add it later if we want. @XJsavage can give you better details. but Ill go down here in a minute and take some pics.
 
our humidity inside the house never dropped below 60%. now it ranges from 45-55%...... 55% on the nasty humid days we've had lately. Our AC stays set at 78 and it feels like walking into a refrigerator coming in from outside.
 
RobMcBee said:
I put some pics up....

http://www.nc4x4.com/forum/index.php?threads/crappy-home-inspections-pic-heavy.150800/
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interesting... no insulation at all under the joists? I guess the idea is that the crawlspace is now part of the envelope and the new foam board on the walls covers the area?
How much of that space is above ground? 2" rigid foam = R10. not ideal for anything above ground.
Also might want to pack some foam board or spray along that rim joist.

I like the sealed door panel
 
^ that looks really good.
 
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