crawlspace sealing/insulating question


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Old 02-23-15, 07:35 PM
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crawlspace sealing/insulating question

Hello all, thanks for taking a look. I am going to attempt to seal and insulate the walls of my crawlspace. Hopefully my picture shows up. I am in Montana, so was planning on using 4" of foam board (iso or xps). Unfortunately I can only get 2" foam in my area (that I know of). So, I am considering two options. One is to cut a square of foam to fit over the OSB board, in between the 2x4s, and then run a long sheet across the 2x4s. I think this would help with thermal bridging, and I would not have to do so much cutting. However, I worry that this may trap moisture. The second option is to just cut 2 squares of equal size, to stick against the OSB. This would leave the 2x4s without any insulation, but I don't think there would be a moisture problem. Does either of these methods sound better than the other? Or maybe some third option I am not thinking about...thanks for your time.
 
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Old 02-24-15, 05:13 AM
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I would be concerned about trapping moisture too. That could be why it wasn't done from the start. Is there any ventilation, in that section?
 
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Old 02-24-15, 05:43 AM
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Above those 2x4's behind all of that pink stuff should be the rim joist, where the floor joist assembly rests on that knee wall. Your air sealing (#1 in importance) needs to start up there.

Now the air sealing should include some detail caulking of all exterior surfaces. You kind-of go crazy with the caulk or when warmer the can foam. Continue right down to where that kneewall rests on the foundation where there is often a lot of leakage.

As for insulation, I would fill the cavities with batt insulation, maybe some of the existing pink stuff or better some Roxul, you'll love it. Then cover as you suggested the inside surface with the 2" rigid, also taped and air sealed.

Not mentioned is the need to cover the dirt floor with a good black plastic vapor barrier and extend it up and sealed to the wall. Then cover the concrete with a layer of that 2" rigid, detailing it at the top to cover all concrete and mate with the rigid across the 2x4's.

Once the kneewall and the concrete are sealed and insulated with a vapor barrier on the ground, the pink stuff in the crawl ceiling become unnecessary as the crawl space can now share some of the heat and air circulation from the house. Warm floors and much reduced heating costs.

Bud
 
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Old 02-25-15, 11:23 AM
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I have used Roxul before but did not think to use it down here, but it makes sense. And my main reason for wanting to do this is to pull out the fiberglass, it is nasty and deteriorating in most areas. Thanks.
 
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Old 02-25-15, 12:00 PM
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Generally you insulate the ceiling of a crawlspace when it's not conditioned and the walls when it is conditioned space. Which is this going to be?
 
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Old 02-25-15, 03:02 PM
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This will be conditioned. Right now there are only 2 vents, I will be sealing those and laying down a moisture barrier, and insulating the walls.
 
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Old 03-11-15, 06:39 PM
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I will be conditioning my crawlspace once I replace and add piers to get the main beam jacked back up and level. I have read where some put the plastic vapor barrier up the wall first, then the 2" foam insulation over top of it. I have also read where some have applied the insulation to the block wall first and then covered it with the vapor barrier.

Which one is it?
 
 

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