# do i i really need to insualte rim joist if i dont feel draft/air?



## Bud9051 (Nov 11, 2015)

The rim joist that sets on top of the foundation should always be sealed and insulated. Air leakage occurs mostly during cold weather, called stack effect. I do infrared inspections and prefferably during cold weather and there is always some leakage, On top of that a single rim 2x? has an insulation value below most windows and improving windows id very expensive. This is an inexpensive improvement with good savings.

Bud


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## jackallis (May 4, 2019)

Bud9051 said:


> The rim joist that sets on top of the foundation should always be sealed and insulated. Air leakage occurs mostly during cold weather, called stack effect. I do infrared inspections and prefferably during cold weather and there is always some leakage, On top of that a single rim 2x? has an insulation value below most windows and improving windows id very expensive. This is an inexpensive improvement with good savings.
> 
> Bud


thanks bud. 
what kind of foam board/size do you recommend I use? I am hearing all kind of recommendation.


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## Bud9051 (Nov 11, 2015)

Technically the pink or blue need to be covered with drywall which your walls will be. But local enforcement can vary from they don't care to covering every sq inch. Without knowing what your building department might be looking for we can only guess.

I grabbed *this* link (it has pictures) but there are many. If you resolve closing off the top of the wall cavity to meet fire codes I would probably just insert a piece of rigid and seal around the perimeter and all other seams as well. If it needs to be covered now would be a lot easier than later. 

Bud


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## ktownskier (Aug 22, 2018)

Cut the foam at a slight bevel. Put the foam in with the bevel facing inward. Fill the bevel in with low expansion foam to seal it in place. This will also help with air infiltration. 

If you need to, cover it with drywall, but put some space between the foam and drywall.


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## NotYerUncleBob2 (Dec 29, 2017)

Best foam cutting tool is a larger putty knife with the back sharpened so you cut pulling toward yourself. Works better than any blade or tool I've tried. 
The alternate to cutting a bevel is to just leave it a 1/4" small and foam the gap. If you cut it too small, or it doesn't want to sit in place while you foam, just lightly tack a 3" nail next to the foam board to hold it.


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## chandler48 (Jun 5, 2017)

We lived here for nearly 20 years with only a double band keeping out winter's fierceness. I knew better. A couple of years ago I cut and installed Roxul in the cavity. No vapor barrier needed, fireproof, water proof, vermin proof and mold proof, Plus R 15 factor. The difference is unimaginable. I would recommend it over foam since you have to cover foam.


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## Nealtw (Jun 22, 2017)

Cold floors next to the outside walls upstairs when the rim is not insulated, worse when the basement is finished and no direct heat to that area.


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## Dave Sal (Dec 20, 2012)

My crawlspace (tri-level home)used to get just above freezing during the winter, which always scared me because the copper water pipes run through there on the way to the kitchen and upstairs bathroom. I decided to insulate the rim joist using Owens Corning Foamular rigid foam as well as a bunch of cans of spray foam to help seal them in place. Now, the crawlspace stays an average of 63 degrees all year round without doing anything else. I have a remote temperature probe down there with the display here at my desk. During the winter, the hardwood floors above the crawlspace are no longer ice cold. It took me a few days to complete the job, and it was miserable work as I had to crawl along the perimeter but it was well worth the effort.


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## jackallis (May 4, 2019)

did you air seal the corner before puting the foam board in? how is that held on the joist? did you use spray foam?


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## Dave Sal (Dec 20, 2012)

Just cut the foam boards to fit the spaces and fill the outer perimeter with the spray foam. That will sort of lock it in place. Or, you can use some glue made for foam board and apply some on the back to stick it to the rim joist, then spray foam the edge as before. I used this in a few spots and it worked well. There was no need to air seal the corner because the foam boards would be sealed in place with the spray foam.


https://www.homedepot.com/p/Loctite-PL-300-10-fl-oz-Foamboard-Adhesive-1421941/202020476


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