# Insulation under bay window



## nolan (Feb 21, 2009)

I have a bay window in my kitchen, facing south, that gets very cold (on the floor) during the winter months. Outside, under the bay, is open. I want to insulate outside under the bay but fiberglass insulation is too thick, as I only have 1 1/2" between the bay 'ceiling' to the bottom of the siding. Would construction foam (R-value of 5) be enough to help insulate? I'm considering stacking patio bricks under the bay to conduct heat from the sun and also to help keep the wind (from the west) out. Any other suggestions?


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## Just Bill (Dec 21, 2008)

Use rigid EPS foamboard(1 1/2-2"-pink or blue at big box), glue it to the underside with Liquid nails or similar. after carefully trimming it for a perfect fit.


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## buletbob (May 9, 2008)

whats fastened to the bottom of the floor joist now? is it plywood? If it was me I would investigate and see if the floor cavity is properly insulated under the bay. then proceed as mentioned above. BOB :thumbsup:


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## DangerMouse (Jul 17, 2008)

maybe i'm not reading it right, but would spray foam be a better option?

DM


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## buletbob (May 9, 2008)

I would think so, as long as there isn't any insulation in there already,to keep it from expanding into the gaps.
with most walk bays without foundations, you should be able to see the floor joist cavity from the crawl or basement, if its not blocked by blocking. this way you could address the situation. BOB


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## Clutchcargo (Mar 31, 2007)

Just Bill said:


> Use rigid EPS foamboard(1 1/2-2"-pink or blue at big box), glue it to the underside with Liquid nails or similar. after carefully trimming it for a perfect fit.


Exactly what I did. I also caulked around the perimeter of the foamboard and spray foamed any remaining gaps. It's still a bit colder but not bad. That's the nature of having a projection from the wall.
My window was drafty too so I removed the two corners (the 22.5°) on the outside and add 2" foam, cut to 22.5° to resemble a wedge, and sealed with caulk. The bay window was much warmer this year.


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## nolan (Feb 21, 2009)

Thank you all for your responses. I'll probably have to go with the foamboard. You can't see under the bay from the basement. The bay jets out from the kitchen and there is open space underneath it outside. The bottom of the bay is covered with plywood, and without insulation. So unfortunately the spray foam would not work here. I guess the foamboard is better than nothing. :wink:


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## buletbob (May 9, 2008)

is there any blocking that is closing the bays between the flor joists, if so remove them and install the foam board and then install the insulation in the floor cavity from the basement. that will help allot. then reinstall the blocking. BOB


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## Tom Struble (Dec 29, 2008)

maybe im not reading this right now, but if you cant see in between the joist and there is plywood there how do you know if its insulated or not?


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