# Insulating rim joists properly



## cleveman (Dec 17, 2011)

I'm not sure how much to use, but I would stay away from the 1/2 material.

I start out with the outside sheathing going down over the rim joist, and I use 1" xps for the sheathing. So it is already covered with 1" on the outside. Then I take all my door and window cutouts and rip them down to the floor joist width, and cut them piece by piece to the width of the cavity. You'll have some 14 1/2's, some 14 7/16ths, 14 5/8, etc. I use my fist or a hammer to squeeze them in there very tightly. I may have enough material to get 3 layers on the inside, so I'm up to r-20. Then I put r-13 fiberglass on top of that.

I think 2 layers of 1" material on the inside will work, then fiberglass on top of that. As I said, I smash it in there very tight and I have xps on the exterior as well, so I don't caulk around the edges. No big deal to caulk around the edges if you want.


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## mikegp (Jul 17, 2011)

I would cut them a little smaller than the opening and then use a canned spray foam around each piece. It will be more work, but will end up being like you used real spray foam. I would aim for 2 inches of XPS if possible. Either by stacking or just getting 2 inch off the bat.


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## jklingel (Dec 21, 2008)

I'd chuck the fiberglass while you have it out. It is pretty poor insulation to start with, and if it is not in an airtight cavity(putting it on the outside) it is even worse. If you want more info on this, greenbuildingadvisor.com has tons there. Your R value should exceed what you code calls for (codes are minimum) and air seal like mad, as mentioned above, or you will possibly have rot.


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## Beepster (Oct 19, 2009)

I am in the process of finishing of and plan to spray in. Either hire out if not too expensive or buy a froth pack (many youtube vids) and DIY. I am not that far yet.

The area directly above the cement block has "poured" in cement between the joists making it difficult to put in pieces of XPS. Spray it and do it right.

B


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## blk94f150 (Apr 29, 2010)

Froth-Paks are too expensive for the R-value and board feet you get out of them. I guess I'll use XPS with great stuff around the perimeter. Is 2 inch Polyshield XPS? It only has an R-value of 7.7 with 2 inches. The Owens Corning Foamular boards are R-5 per inch. Would 1 inch of Foamular along with Great stuff and R13 fiberglass do the trick to keep my rim joists from getting moist? I figure that no matter what I do, I'll stick that fiberglass stuff back up there as it can't hurt as long as it's not getting wet.


Also is there any reason to caulk the joints behind the foam board before putting it in?


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## cbaur88 (Jan 25, 2011)

blk94f150 said:


> Froth-Paks are too expensive for the R-value and board feet you get out of them. I guess I'll use XPS with great stuff around the perimeter. Is 2 inch Polyshield XPS? It only has an R-value of 7.7 with 2 inches. The Owens Corning Foamular boards are R-5 per inch. Would 1 inch of Foamular along with Great stuff and R13 fiberglass do the trick to keep my rim joists from getting moist? I figure that no matter what I do, I'll stick that fiberglass stuff back up there as it can't hurt as long as it's not getting wet.
> 
> 
> Also is there any reason to caulk the joints behind the foam board before putting it in?


For my crawlspace I caulked all the joints in my rim joists just to be extra careful and stop that air flow. I then placed 2" XPS board over it and sealed it with great stuff foam. I used Roxul vs. fiberglass but same difference for the most part. Good luck!


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## Gary in WA (Mar 11, 2009)

I agree! Caulk the wood joints to stop incoming air, foam the 4 edges of the foamboard to stop interior air from condensing on the wood rim: http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/

If under the IRC, the foamboard can be left exposed: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec016_par015.htm

Of course, always check with your local AHJ.
I was surprised to read that as little as a 1/32" gap can degrade your cavity insulation. Plus, water vapor molecules are much smaller than air molecules, we can't win...LOL.

Gary


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## jklingel (Dec 21, 2008)

GBR in WA said:


> I was surprised to read that as little as a 1/32" gap can degrade your cavity insulation.
> Gary


I think it was on the ORNL site where I read that a 1" sqr hole in sheet rock will bleed as much moisture as will diffuse through the rest of the sheet. Hence, air seal.


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## blk94f150 (Apr 29, 2010)

Can anyone comment on 1 inch vs 2 inch XPS? Will 1 inch be adequate to keep the condensation away? Obviously 2 will double the R-value, but I'm going to put the R13 glass insulation back in there as well anyway.


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## Windows on Wash (Aug 30, 2011)

jklingel said:


> I think it was on the ORNL site where I read that a 1" sqr hole in sheet rock will bleed as much moisture as will diffuse through the rest of the sheet. Hence, air seal.


More than that.


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## jklingel (Dec 21, 2008)

Windows: Chit. That is pretty dramatic. Where did you get that pic (I'd like to site it if I even use the photo). Thanks. john


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## Gary in WA (Mar 11, 2009)

blk94f150 said:


> Can anyone comment on 1 inch vs 2 inch XPS? Will 1 inch be adequate to keep the condensation away? Obviously 2 will double the R-value, but I'm going to put the R13 glass insulation back in there as well anyway.


---- depends on your location; http://www.buildingfoundation.umn.edu/FinalReportWWW/Chapter-5/5-optimum-main.htm Cruise the links given...

The pic. is from: http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11760

I was referring to air movement degrading cavity/rigid insulation.

Gary


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## jklingel (Dec 21, 2008)

GBR in WA said:


> The pic. is from: http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11760
> Gary


 GAry: Thanks for the link.


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