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DKP
Advice Wanted about Insulating Bump-out
DKP

We live in Topeka, KS. Our house is an 85-year-old airplane bungalow with no insulation in its original plaster walls. I would like to insulate a section of the dining room that is a bump-out, about 1.5', and about 8' long.
So, the floor joists simply extend out over the foundation, leaving about 1' above the exterior ground.
As you could imagine, that whole area gets freezing cold in winter, as the "boxes" created by the rim and floor joists are open, bare, and not insulated in the basement. It also doesn't help that there is an old single-pane window in the basement right below the bumpout.

I've seen on another forum a way to insulate this from the inside:

--rigid foam on the bottom of the interior of the bumpout; rigid foam pieces on rim joists (on far vertical "wall" of the "boxes", as I call them)
--closed-cell spray foam insulation on the upper portion of the bumpout, the part that is the dining room floor sub-floor. The idea is to provide a vapor barrier, and it should be against where the warm air is?
--And a piece of rigid foam on the inside of the basement, lining up with the normal rim joists, to seal off the bumpout "boxes" from the basement.

It would be like this, from the Terry Love forums, but instead of fiber I'm thinking of using the rigid foam, to seal air and keep critters out:

|
|...interior dining room
|
|
|__________subfloor_________
|^^^the spray foam only goes here^^^
|
|
|<< band joist
|
|{fill space between foam & bottom sheathing with fiber** ME: rigid foam on bottom and against band joist
|
|______bottom sheathing_____

...outdoor air...

_____ground__________

I'm wondering is this is the way to go, if the placement of the vapor barrier is correct, considering the region we live in (NE Kansas). And if the spray foam is even necessary; could something else less expensive take its place?

Any advice would be appreciated! We're still having warm weather, so I think there's still time to do this.

MichaelInOr
Re: Advice Wanted about Insulating Bump-out
MichaelInOr

I am a big fan of spray foam because of its sealing properties. I had a 3' x 3' x 1' uninsulated area that acted as a pipe to channel cold air into my house. The quotes from the spray in foam contractors were astronomical. I ended up using a half dozen cans of "Great stuff" high expansion foam and several dozen 1 liter plastic soft drink bottles washed with the caps firmly screwed on embeded in the foam to fill the volume. It worked excellent and completely eliminated the drafts for under $50.

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