# Insulating Cantilever Floor in a bump out bay window



## mae-ling (Dec 9, 2011)

OH MAN,
Due to it being improperly built and sealed and poly for insulation I think spray foam may be your best option.
Also should be solid (Non-perforated) soffit.


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## PeteSf (Aug 23, 2015)

Spray foam may be the best option I agree but its a reach when working from the basement into the cavity as the rim joist is 43" away from the foundation wall due to the finished wall. I could spray the rim joist from the outside after removing the soffit and then replace the soffit and spray from the inside spraying the soffit and sealing out any daylight as you can see from the pictures. But the tricky part will be spraying the soffit where it lays in the j channel against the exterior wall to seal out any air that may enter via the j channel.

Or I could do the following and strickly work with foam board and batt/roxul insulation
1. Remove all exterior soffit.
From within each bay cavity
2. Cut a square piece of foam board with notched corners and seal into place on the rim joist. (Vertical position)
3. Cut a rectangular piece of foam board and seal into place on the subfloor. (Horizontal position)
4. Where multiple I - Beams create an uninsulated 2" cavity, seal the exposed cavity from the outside with foam board. (Vertical position)
5. Cut a square piece of foam board with notched corners and place it on the foundation wall plate sealing off the cavity. (Vertical position)
6. Insert batt or roxul insulation in foam board lined cavity (rim, subfloor and foundation wall plate) fill until no more can fit.
7. Cut a rectangular piece of foam board and slide it between the bottom of the I beams. I would do this since I don't have any finishing room between the soffit and the bottom of I beams. This would be sealed between I beam the bottom most position laying horizontally flush with the bottom of the I beam.
8. Seal bottom of I beams connected to foam board with special tape.
Replace soffit

For the duct work cavity I will not install the subfloor piece of foam board. As any warm air should heat the floor space. I'm wondering if I should seal off as best I can the entrance of the duct above the foundation wall plate into the cantilever bay

All of which will be sealed with great stuff or a caulk made to seal foam board.
My thinking is to line the rim joist and subfloor with foam board working from the outside after removing the soffit. Then where there is 2 or more I beams connected I would seal off those cavity ends with 1" foam board.


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## Gary in WA (Mar 11, 2009)

Welcome to the forum!

Pull soffit, caulk/canned foam the rim joists with 2" foil-faced XPS. Roxul the cavities, add ff XPS to whole bay underside (on all cant. joists, per *Fig.7*;http://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces) air seal, add soffit material.

Gary
PS FB on joist bottoms warms the cavity against cold/condensation just like a wall.


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## PeteSf (Aug 23, 2015)

Hi Gary.

I did the following
Removed soffit
Cut cobbled 1.5 foil face rigid board to the rim joists and subfloor and top off sill plate blocking access to the basement.

I was going to install roxul in the foil faces cavity and then insert 1.5 foil faced rigid board to the bottom of the cantilever bay caulking all joints. Then reinstall the soffit. 

As I dont have a lot of room between the soffit and the bottom of the joists I may insert 1/8 laun board sealing the complete cantilever bay but it depends if I have the room.

Question with 1.5 foil faced glued to the subfloor and the roxul and the 1.5 foil faced glued to the bottom of the joist cause moister issues? I live in north east PA and the bay window that the cantilever exists on is stucco installed on rainscreen.


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## Gary in WA (Mar 11, 2009)

As per link, foam board the underside (on joist bottoms only), not against the subfloor... 

Gary


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## donniedawg (Sep 7, 2015)

*May give you some ideas to use.*

Could give you some of the 10 new ideas we each need to invent every day. As for the pipes, I would wrap them with the denser pipe foam available I could get my hands on. Not the cheap crap but the best R-value possible. So dense you cannot see the pours in it. Get silver duct tape wrap, about 3"X 100' that's used to repair A/C duct work. Wrap the foam on the pipes with the tape. As for insulation, others had very good ideas and use rigid foam, I think red carries the highest R-value, R-10 per 1 inch. Easiest way and fastest to cut this would be a jig saw with a blade that looks like a razor knife, no teeth on it, just a very sharp edge. Cuts the board like butter.I see electrical wires, making me feel if you need to ever expand the wiring or have a pipe leak or break, you will need quick access again. Don't get me wrong I LOVE spray foam, for many uses, but it can create more problems in certain areas than it's intended use. Have you ever removed it to get to a problem area? I've ripped exterior doors and windows out because my stupid brother, who used the expansion type(3X) on doors and a window, I installed the day before. The pressure broke the window and I made him jack hammer the doors back off. To attach the rigid foam you can use a quick setting construction adhesive and very little at that. Also if there are any little nooks or cracks to seal get some cans of the NON-Expanding foam to insulate these areas. Good Luck, you'll need it.


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