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Old 11-23-2009, 11:52 PM   #1
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Insulating rim joists


I know the proper way is to use foam and tightly fit it then seal it off with caulk, but can I get by with fiberglass? Perhaps the rock based stuff that is mold proof? There's lot of spots where it would be too hard to start working with foam (electrical wires, plumbing etc) and I'm thinking it might provide better insulation as I can extend it maybe a foot or so along the joists. I've even thought of insulating the whole ceiling as it would probably help dampen sound a bit too.
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Old 11-24-2009, 05:51 AM   #2
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Fiberglass is OK, anything that keeps out cold and keeps in warm, provided it does not cause moisture problems. Urethane foam also works, but be careful about the expanding type, it can quickly get out of control.
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Old 11-24-2009, 06:19 AM   #3
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For you location, fiberglass will not work. For the previous poster's location, fiberglass may be okay. Insulating with fiberglass in such a cold climate will cause much condensation which will encourage mold growth and possibly rot your rim joist.
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Old 12-23-2009, 10:03 PM   #4
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I find my basement quite cold now that we're getting -20's outside, so I for sure need to do this as I'm probably losing lot of heat. I'm not really heating the basement to try and keep the heat upstairs as much as I can, for now.

I'm wondering, if I put rigid foam, is it safe to then put fiberglass over that for even extra insulation?

What I'm thinking of doing is adding one or two batts per joist space so I insulate maybe a couple feet of the floor along with the rim joists. Maybe I'd just fold a batt in two and insert it in, the joists are 2x10's so it would probably fit in nicely and I can staple a rope or something to help hold it.

Could I also just double or triple the foam for extra insulation? I could just add layers to fill the whole rim joists cavity.
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Old 12-23-2009, 10:23 PM   #5
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That is a good idea as some County's require a fire barrier over the exposed foam board. 1-1/2" thick of compressed or folded tightly glass batt, unfaced. Fiberglass is a poor air barrier, seal the foam at edges: http://www.rd.com/57548/article57548.html
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Old 12-24-2009, 07:26 AM   #6
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Rim joists areas are heat losers - and drafty; up here, I'd put the XPS against the rim joist, seal that for effective air/draft reduction and some insulation, then put unfaced fibreglass over that for more insulation. But fibreglass won't help the drafts. I'd drywall it too.

But both fibreglass and XPS will be as important, up here, as a good sealing job since the whole thing can be negated by incomplete sealing. Just don't think more insulation is the best if you short-change the sealing.
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Old 12-24-2009, 09:02 AM   #7
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I've always used fiberglass, its a quick fix
I have found some mold due to an air leak in one area
I did seal the leak....but the rim joist wood is very cold

After reading various posts on here the past year I will be removing the fiberglass & putting rigid foam board up & sealing
Then re-install the fiberglass as added insulation

I'm going to do the same when I frame out the basement walls
The basement was dipping down to mid 40's when we 1st moved in
So a quick fix was needed right away



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Old 12-24-2009, 09:29 AM   #8
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1 1/2" Thermax or 2" rigid foam is what you need per code, I think. Like already stated, in the cold climates the fiberglass doesn't work. It usually just molds, especially if it's finished.
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Old 12-24-2009, 11:59 AM   #9
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Yeah forgot about the sealing. I'll definataly do that too.

I was also thinking of putting a vapor barrier just to keep the fiberglass insulation intact so pieces don't fall on the ground or spiders don't make it their home, is this a good idea as well or is it important that it's not totally sealed in?

now the challenge if figuring out how I will put 4'x8' sheets of foam in my car. Might have to borrow a truck! LOL.

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Old 10-05-2010, 11:24 AM   #10
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Did you end up using a vapour barrier along with the rigid foam board and fiberglass insulation?

I used rigid foam board, then Roxul (instead of fiberglass), then a 6mil vapour barrier but I'm affraid that might of created a double vapour barrier??

Does the rigid foam board act as a vapour barrier?
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Old 10-05-2010, 06:38 PM   #11
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I never got the money to do this yet, but may do it this winter.

I'm also wondering myself if the foam acts as VB. I have a feeling it may.

I can't wait to do this, as I hope it will also cut down on the spider webs. I get lot of spiders that find their way in the basement and leave very long line webs all over. Lot of them are diagonal from joists to ground which tells me they are originating from high, and then drop themselves down. Most of the time they are near corners too.
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Old 10-05-2010, 07:05 PM   #12
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spiders... that's easy, get a frog!
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