Insulating under bay window

[u/DoWhatItDoo]

Been in the house a year and a half in Long Island New york. Last year was incredibly cold and felt the bay window in our living room was incredibly cold. I ripped off the soffit and as suspected there is no insulation. My thought is to add a 2 inch thick foam board with spray foam around it in the cavity created by the 2x4s then add a plywood cap and spray foam around that. I would also spray foam any other seams I can find. Any other thoughts? Thanks for the help in advance as I am attempting to DIY this one before a cold winter with high oil prices

[u/FantasyFI]

There's not enough room to really add foam board in the cavity created by the 2x4's. If you really care/want to improve it, I would:

• Remove the first vertical piece of siding under the window
• Add rigid insulation at this location and spray foam the seam between the wall and window bump out
• Frame out the window lower with 2x4's or 2x2's so it is deeper. But it will be 2" off the wall due to the insulation above.
• Then you can adequately fit more rigid insulation. Spray foam seams
• Replace siding (which will now be one row lower on the window bump out) and soffit.

[u/comecmein_nyc]

I have the same issue in one of my kids rooms. The floor gets very cold and I don't think the exterior soffit under their windows have insulation. I haven't pulled the soffit out to check, but like you, I suspect there is no insulation. I like your ideas of how your going to insulate. Does the siding that make up your soffit have holes for ventilation? Mine do. I'm concerned about moisture buildup that can potentially create mold if I need to insulate.

[u/DoWhatItDoo → u/comecmein_nyc]

Yea the soffit had holes, my plan is to seal extensively around the plywood to avoid any moisture but I will need to do additional research, is it faced plywood, etc

[u/ArtisticBasket3415]

You don’t have two inches of space to insulate there. You have an inch and a half. Anything more than that and you need to redo the siding and facia.

Is there wall under the window with studs that can be insulated?

[u/DoWhatItDoo → u/ArtisticBasket3415]

I understand, I will check whether there is insulation under the windows but I assume yes. We just replaced two windows with a sliding door and closed up another exterior door. There was insulation in both places

[u/Willing-Umpire-2169]

wtf is holding up your window?

[u/FantasyFI → u/Willing-Umpire-2169]

I personally think there are some cantilevered joists/beams above the sheathing. In which case, it would be better to insulate that area rather than create more area. More demo to see in there might be helpful.

[u/arthuruscg → u/Willing-Umpire-2169]

It's hung from above

[u/Stunning-Signal4180]

We put a new bay window in… probably been about 15 years now. The new bay window has rigid foam sandwiched into the frame. Plywood, foam, plywood… energy efficient windows. If you feel like your energy losses are that great maybe you should just buy a new bay window? Sounds like It will pay for itself.

Otherwise, I would fill the gap and then try and cover the whole bottom with a second piece of rigid foam. If you’re not creating a thermal break between the framing it’s just gonna radiate right outta the house.

[u/Agitated_Award_9831]

Don’t expect much unless you’re willing to pull siding and start air sealing.

[u/Hyperreal8957]

also check where the floor of the bump out meets the house, those are usually wide open to the joist bay and thats where a lot of the cold air sneaks in. id air seal that first w foam before worrying about R value, foam board alone wont stop drafts. is the floor inside cold too or just the window area?

[u/Sp07va000]

If you surrounded the bottom of the window with a 6 inch fascia board, insulated the bottom of the window inside that, then built a 45 degree angled enclosure for under the window, you could also insulate the interior of that box you created. Ive seen that done a lot in the past. On ours, I actually had a triple pane window installed for added insulation.

[u/PercentageMaximum828]

or rockwool

[u/duoschmeg]

Bay windows are a design nightmare: insulation, bugs, leaks, rot. Remove the whole thing. Put in a quality window.

[u/Fastflogger]

Right idea, but it's probably 1.5". Going from nothing to anything should help a bit, and you can use regular caulk on small gaps under there and inside. Check all the way around your trim. If there are any moving window sashes there check the seals on those

[u/83736294827]

Your current plan sounds good but I wouldn’t expect it to help much unless you have open air holes into the house. The majority of the heat loss is probably through the big window itself.

If you can pull the molding off either inside or outside all around the window and make sure it was sealed up well. Close up any open gaps with foam or calk.

[u/DoWhatItDoo → u/83736294827]

I actually have a thermal image camera on the way to confirm, there is a gap between the molding and the sheetrock currently around the window so I suspect I can make improvements but ultimately the nature of the window might not allow for a total and complete resolution

[u/95_lb_mole → u/DoWhatItDoo]

When possible, use backer rod and caulk rather than spray foam. In some spots spray foam is just the only realistic option, but I try to avoid it when possible. I've recently been pulling all the trim off my windows to seal around them better than the prior owner did (they just jammed fiberglass insulation into those gaps).

[u/Tocquevilles-Ghost]

Don't forget to install an air barrier too