# Insulation under bay window



## EdLank (Apr 23, 2010)

I have a similar problem. My first winter in this 18 year old house revealed bad drafts in the basement from the cantilevered joists for a "bay window". I could reach over the basement sill plate and tap on exterior "siding" below the joists. There was no insulation in the cantilevered portion, so I assumed I would need to remove the bottom covering, install insulation, and apply plywood someday. At the time, I installed blocking boards to prevent basement air infiltration. Now it is time to do the outside work. From the outside, I tapped on the bottom horizontal weather covering and it sounds like it has wood behind the "siding." I am guessing the plywood may have a gap at one joist space, which may be the one I checked from the basement. I will have to do some digging first to start the work, since the wall protrusion is about 3" off the ground. I can imagine this is a site of rodent entry and air infiltration.

I intend to remove the bottom "siding" and fill the joist space with multiple foam boards and seal with expanding foam. What thickness and grade of plywood should be attached to the joists before putting on the "siding?" Currently I think all of this is covered with vinyl siding, but I cannot see until I dig. How do I weather seal the vertical siding to the bottom horizontal material once I have taken the current "siding" off? My new plywood may (depending if there is already some there) change the position of the final bottom weather covering (whatever I finally use). I have a roll of aluminum sheeting left by previous owner, but not 2' wide. Should I use that to cover the plywood rather than vinyl siding?


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## eisert (Oct 20, 2009)

Ed- you're making this more difficult than it needs to be. If you can access the cavity from the basement, just have it sprayed full of foam.


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## EdLank (Apr 23, 2010)

Well, I installed blocks that are caulked at the basement wall between all the joists, and those are currently preventing access to the cantilevered space. Furthermore, I can see rodents (at least chipmunks) are enjoying the protection the overhang provides by the periodically rearranged mulch at the perimeter of the overhang. I suspect mice have found a way into the house because if a little vinyl is all that separates them from a life of warmth and gluttony, the vinyl will be a barrier no longer.

Also, seeing the links to the condensation analysis, I am concerned about creating joist rot. Vapor has two sources here...the living space, and the outside, so this gets more complicated. The overhang is only a few inches off the ground, which will be a nice moisture source nearly around the clock...not just during a rain like with siding.


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## eisert (Oct 20, 2009)

Proper spray foam insulation will provide its own vapro barrier. I would remove the blocks you said you installes in the joist cavities, then fill with foam.


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## HomeSealed (Jan 3, 2008)

Mulch underneath that overhang is a horrible idea and as long as it is there it will breed issues with moisture, bugs, and rodents.


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## Scuba_Dave (Jan 16, 2009)

I'd remove the mulch & block the area permanently off after insulating
With brick or something else


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## EdLank (Apr 23, 2010)

I thank you for your suggestions. I can remove the blocking inside, but if I find one space really has no plywood on the underside of the joists, I am still likely to remove the bottom siding to try to create more of a rodent barrier. Putting foam into the area without a solid floor will very likely push the bottom "siding" down in that space permanently. I guess I could shim it up tightly from outside until the foam hardens, but this seems too uncontrolled and sloppy.

I am not sure it I can get my head high enough in the basement joist spaces to see out into the cantilevered space. I am not comfortable blindly putting foam out there.

We have not put any mulch in the area, but the previous owner had, but near the overhang, not under it. Regardless, the overhang is not far above the ground, so in the summer I can imagine it stays rather steamy, with a constant source of humidity. Chipmunks are probably responsible for building the mini berm near the perimeter of the overhang, using mulch. I had thought of blocking the area with bricks, but figure whenever we restrict access, we may do a better job of restricting predators than rodents, which can get into almost any space, and then they would have uncontested entry/egress into the house!

I thought I might dig out a few inches to lower the ground near this overhang, which might also make it less attractive to animals, but I will have to do it in a large area to avoid having a grade toward the house.


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## EdLank (Apr 23, 2010)

I have removed the draft blocks I had installed in the basement at the sill plate. There is nothing but aluminum siding below the joists. The siding is 12" wide by 24" long pieces, running perpendicular to the foundation. There is some faced insulation out there in the joist spaces, but that did nothing to prevent air infiltration. I have dug out enough dirt to do some work below there. How do I remove the aluminum siding on the bottom without destroying it? It is in channels as would be used on vertical external walls. I may need to lower the channels once the plywood is in place, or may instead apply a single sheet of aluminum on the bottom of the plywood.


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