Bay Window Insulation in Basement

[2000max]

Hi, new home owner here (long island, NY). I have a bay window in my house and noticed that once the temperature dropped that the floor is very cold compared to the rest of the living space. What do you guys think is the best way to insulate the bottom of the bay window?

I read some threads on here I checked out how the basement cavity below the bay window floor was insulated currently - what I found is in the pictures posted. Also on the exterior I"m pretty sure that there is only a plywood sheathing at the bottom of the bay window which is not air sealed. I'm pretty sure the insulation in the picture is sitting right on the plywood sheathing with nothing else in between. Hope you guys can help. Thanks!

From the other threads I read, I got a little lost at understanding climate zones and the need for vapor barriers. Any and all help is appreciated.

 

[2000max]

After some research my initial plan is to air seal the bottom sheathing of the bay window with a high quality caulk and fill the cavities with R30 rock wool. I may also add some vertical pieces of rigid foam to the rim joists at the far end of the cavity at the same time.

Once the weather gets nicer I may pull down the plywood sheathing at the bottom of the window and add a layer of rigid foam. Do you guys suggest using EPS for that and if so what thickness?

 

[Sponsor]

 

[Dana]

The far end of the cavity and bottom of sheathing doesn't need the rigid foam, but the plank sheathed top could use some. There also needs to be an air barrier between the cavity and the interior wall. With plank sheathing there is the potential for a lot of air leakage into the cavity, which would accumulate in the bottom sheathing. Installing foam board on the bottom sheathing would trap the moisture in the rock wool. What's needed is basically this:he fiberglass batts on the interior side aren 't needed if it's con:

The fiberglass batts on the left side of the picture aren 't needed if it's conditioned space, but the air-dam (the vertical foam board in this drawing) is essential no matter what.

The primary function of the foam here is a low vapor permeance air barrier- it doesn't have to be very thick as long as it has a facer, and it doesn't need to be double layered. Half-inch foil face polyiso is what I'd use since it's dead easy to cut cleanly with a steel wallboard taping knife sharpened on the edges an a straight edge. Foil or vinyl faced EPS would work too, but leaves an EPS crumb mess. Polyurethane caulk works better than can-foam for achieving the long term air seal.

The batts need to fill the entire void. R30 rock wool batts are only 7.25" of loft, designed for 2x8 framing (I assume that's what you have?), but compresses easily to the 6.75" cavity with half-inch foam up top.

Not that it really matters in this case, but Long Island is US climate zone 4A. In zone 4A you were to install rigid foam on the exterior side (the bottom sheathing) it would be advisable to make the foam least 15% of the total R for reliable dew point control at the foam/rock wool boundary. But if the foam is on the warm-in-winter side of the assembly it doesn't matter- the assembly dries to the exterior. In zone 4A you could get away with just dense packing the cavity full of 3lb per cubic foot cellulose and skip the air barriers, but for a project this small the foam-board air barriers and rock wool is a better choice.

 

[2000max]

Dana, thank you for the reply. I think I understand the approach here. I'm pretty sure the framing is 2x8. If I use foil faced iso as the vertical air dam do I have to put a thermal barrier of some sort between it and the basement (which is conditioned). I didn't know if there were fire safety considerations to be made.

This may be a dumb question (but I'm learning) but for the vertical air barrier would I want to face the foil side of the foam towards the interior /basement or does it not matter than much? And when applying it to the top planked sheath the foil should be face up towards the first floor?

 

[Dana]

Dana, thank you for the reply. I think I understand the approach here. I'm pretty sure the framing is 2x8. If I use foil faced iso as the vertical air dam do I have to put a thermal barrier of some sort between it and the basement (which is conditioned). I didn't know if there were fire safety considerations to be made.

This may be a dumb question (but I'm learning) but for the vertical air barrier would I want to face the foil side of the foam towards the interior /basement or does it not matter than much? And when applying it to the top planked sheath the foil should be face up towards the first floor?

If it's open to the basement use half inch or thicker plywood as the air barrier rather than foam board. You could use fire rated Dow Thermax polyiso, which would not need a thermal barrier against ignition. If it's not a finished basement most inspectors would allow the use of any foil-faced foam without thermal barriers as long as it didn't extend much below the joists.

Foil faced polyiso has foil facers on both sides. If using single foil-faced EPS foam (not really recommended here) it'll be slightly more fire-safe with the foil facing the basement.

Is the basement insulated?

 

[2000max]

Thank you. The basement is not insulated but does have some paneling up on the walls covering the foundation, and carpet on the floor.

 

[Dana]

Thank you. The basement is not insulated but does have some paneling up on the walls covering the foundation, and carpet on the floor.

Current IRC code minimum for basement walls in US climate zone 4A (L.I.) is R10 continuous insulation, or R13 if between studs. If there is very much above-grade exposure of the foundation an uninsulated basement is going to account for well over 10%, sometimes more than 20% of the annual heating fuel use.

With credit for the additional air films created by the paneling you're looking at about R2.5 "whole wall" for the basement, which means for every degree of temperature difference from indoors to outdoors, each square foot is losing 1/R2.5= a U-factor of 0.4 BTU/hr. So when it's +15F outside (the 99% outside design temp for Long Island) even at a cool 60F indoors that's a 45 F difference. If you have 2' of above grade exposure and 150' of perimeter it's 300 square feet x 45F x 0.4 = 5400 BTU/hr. At 70F indoors it would be 6600 BTU/hr (which is over 20% of the heat load of my house at +15F.)

With R10 continuous insulation the U-factor drops to about 0.09, and a loss of less than 1500 BTU/hr at 70F indoors, 15F out.

And that's just the above-grade portion. The losses below grad don't fluctuate daily but are a slow steady seasonal loss that adds up too.

If you ever decide to gut the walls and insulate, don't just stuff them full of R13s and cover it back up or you'll have a mold farm on your hands. It's safer if you can slip even a half-inch of foil-faced foam between the studwall and foundation, which will block ground moisture migration into the studwall, but will still have an average temp at the foam/fiber boundary to not accumulate condensation or frost over the winter. There are a number of details to attend to which I won't go into until/unless you're really considering it. It's been covered on this forum multiple times, if you want to dig up some examples using the search function.

 

[2000max]

Thanks for the follow up. I agree the next large step would be to insulate the entire basement. I will do some research here and get a understanding of the mechanics. I will probably do it in parallel with a remodel of the basement.