I recently replaced the windows in my house, which was built in the 1970s, and I'm getting ready to tackle the old siding. My plan is to add foam insulation before putting on the new siding. I've read that it should go in this order: plywood (which I believe is underneath), then house wrap, followed by foam insulation, strapping, and finally the siding. However, after chatting with a Home Depot employee, I learned that the thickness of the foam I can use is limited by the new window trim, which is only 1" thick. This means that if I use 1" foam plus 3/4" strapping, it'll be too thick and I might have issues. My 'crazy' idea was to just put strapping over the house wrap and cut the foam to fit in between. I see issues with this, such as thermal bridging and a 1/4" gap. So my questions are: Is this foam insulation plan a bad idea? Is there a better way to insulate beneath the siding? Should I even try to add more insulation?
3 Answers
Here's a link that might help you understand how to address your window issue better: https://www.finehomebuilding.com/2019/07/09/windows-in-thick-walls
I doubt there’s any plywood under your house, especially one built in the 70s. You’d be lucky if it even has some in strategic corners for bracing against lateral forces. Also, extensions on the exterior jambs can be done if installed correctly!
It's not a dumb idea, but the way you're planning to attach the strapping through the foam will undermine the insulation's effectiveness. With just a 1" trim, you could either opt for thinner foam or bypass it entirely and focus on improving the insulation in the attic or the walls instead. That might provide better performance overall.
The store said the thinnest they have is 1". If air can come in, the foam won’t offer much benefit, right?
What if I just used 1" insulation and skipped the strapping? I’ve heard that for vinyl siding, you can nail right through the insulation.
Actually, I know there is plywood because we had to repair part of the interior wall. I could see it when I replaced some insulation. But I'm confused about what you mean by 'jambs'.