# Rim Joist Insulation Questions



## adamz (May 13, 2018)

Hi,


I plan to insulate my basement rim joists. Today, I took a closer look at one of the inside basement walls and saw something that I didn't anticipate. I found an air cavity between my brick exterior wall and the basement concrete block foundation. This air cavity sits below the sill plate, between the sill plate and rim joist.

I attached two photos. One is the top view of the rim joist cavity. Using that picture, you can see there's an empty air cavity that's between the sill plate and rim joist. 

The gap is about 2.5 inches. Can I merely use 3 inches of XPS foam board (2 pieces sandwiched together), then seal around the foam board? This method will cover the top of the air cavity completely with the bottom of the foam board.

Or should I stuff the top of the air cavity, then spray foam seal the cavity, then put XPS foam board against the rim joist, then seal the perimeter of the XPS as usual?

Or something else?

BTW, the exterior wall behind the rim joist is just plywood sheathing and vinyl siding, no other wrap or insulation is outside. The brick is there to cover up the concrete foundation that sticks up above the ground. I'm in Zone 5, Pennsylvania.


Thanks.


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## Nealtw (Jun 22, 2017)

I would put bug screen over the gap, insulate with bat and fit a foam board above the sill plate. When we do a bump out on a house the bottom just gets vented soffit so the end result is much the same as you have.


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## 3onthetree (Dec 7, 2018)

Why not a good R30 batt like the fiberglass in your picture? The construction looks textbook to me.


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## adamz (May 13, 2018)

The reason I'd like to air seal. I have forced air heating and cooling.

My 1st floor outer walls near the baseboard are drafty/cold and floors are colder in some areas. I believe the rim joists might be causing it. 

Plus my unheated garage has bat insulation in the ceiling joists and drywall on the ceiling too. In the room above the garage, the outer wall baseboard area is the coldest. 

The R30 bat leaks air, that's why I think the rim joist needs air sealed. 

My basement is not heated, but from what I can gather, rim joist air sealing can improve those items.


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## adamz (May 13, 2018)

Nealtw,

You mentioned above: "*the bottom just gets vented soffit"*

What do you mean by that statement?

Thanks,
Adam


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## Nealtw (Jun 22, 2017)

adamz said:


> The reason I'd like to air seal. I have forced air heating and cooling.
> 
> 
> My 1st floor outer walls near the baseboard are drafty/cold and floors are colder in some areas. I believe the rim joists might be causing it.
> ...


 The bottom of the wall and the floor are being attached by cold in two directions, no matter what you d, you do have a lot of bridging in an area like that. 


In the basement area you might put a 3" foam board over the gap and seal that with spay foam and then one against the rim.
In the garage maybe one against the rim and one against the floor. Or maybe two against the floor.


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