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Community and Q&A

New windows — siding and insulating

Matt Cibula | Posted in Energy Efficiency and Durability on

We are needing to install new windows (which we’ve purchased) with nailing flanges. We currently have steel siding and realized that there wouldn’t be a great way to install the windows without taking siding off. The siding has come of fairly easily, but we’ve discovered two additional layers of siding under our steel siding, which has made it virtually impossible to properly install the new windows.

We wondered if we could remove the 3 layers of siding (which would cause our soffits to be off) and add some insulation while building out the old sheathing by 1.5 or 2 inches. We live in rural Iowa and winters are windy where we live.

My question is how can I best insulate this with foam board (if that’s even a good option)? Or would it be better to forget the rigid insulation all together?


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  1. GBA Editor
    Martin Holladay | | #1

    Frankly, it sounds like you are in over your head. It sounds as if you bought flanged windows without realizing that flangeless windows would have been a better choice.

    There are lots of ways to mess up the installation of windows and rigid foam. If you don't get the water management and flashing details right, you can rot your walls very quickly. So if all this is new to you, you might want to work with an experienced contractor.

    Here is an article to get you started: How to Install Rigid Foam Sheathing.

  2. Matt Cibula | | #2

    I could not have installed replacement (flangeless) windows properly because of the current windows as they were replacements as well. The siding needs replaced, but we anticipated (or hoped) we could get a few more years out of it. Given our new-found information we realize this may not be possible. I've installed many windows prior as window installation is nothing new. I am now deciding how best to go about siding and possibly insulating our home since the siding will be taken off.

  3. GBA Editor
    Martin Holladay | | #3

    Well, if you are removing one layer of siding, it makes sense to remove all of the siding, right down to the sheathing. Then you can either install the windows without any exterior foam, if that's what your prefer, or install exterior foam. It's your choice.

  4. Matt Cibula | | #4

    My main concern is moisture. After reading about rigid insulation it sounds like it can hold moisture. I'm not sure if adding an R4 is worth it or not. I would either install the insulation and then furring strips for cement board siding, or I could install the furring strips and add insulation board in between the strips and seal up with house wrap. I'm just not sure which would be best or if there's a different way.

  5. GBA Editor
    Martin Holladay | | #5

    Iowa has two climate zones -- climate zone 5 in the south and climate zone 6 in the north.

    The map is here:

    In your climate zone, R-4 rigid foam won't work, because it isn't thick enough. If you live in climate zone 6, you'll need at least R-7.5 foam to cover 2x4 walls, or R-11.25 foam to cover 2x6 walls. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    When rigid foam is installed on the exterior side of wall sheathing, you can't put furring strips between the sheathing and the foam. The furring strips are installed between the foam and the siding.

  6. Dan Kolbert | | #6

    You wouldn't want to insulate between the furring strips - you'd lose the value of having a rain screen.

  7. Matt Cibula | | #7

    Does house wrap (tyvek) go on before the foam board, after, or not at all? Do you need an other underlayment such as building paper to be layed over the board before siding?

  8. Nick Welch | | #8

    Matt, the article Martin linked earlier answers all that and more. Or at least, it gives you a starting point to figure out how to make those decisions. How to Install Rigid Foam Sheathing

    There is so much valuable info on this site regarding the project you're looking at; just search the blog posts and Q&A and read for hours... and re-read... and read again. That's what I do, anyway.

  9. GBA Editor
    Martin Holladay | | #9

    Nick is right -- the place to start your research is How to Install Rigid Foam Sheathing.

    That article includes many links, including a link to an article that focuses on your latest questions: Where Does the Housewrap Go?

  10. Matt Cibula | | #10

    I've read the articles and checked the hardie specs, but can't seem to find where it says how thick the furring strips need to be. If I install 1.5" of foam board can I attach 1x4s as furring strips before hardie panels? Or will I need to and 2x lumber?

  11. GBA Editor
    Martin Holladay | | #11

    Check out this article: Fastening Furring Strips to a Foam-Sheathed Wall.

    The article notes, "James Hardie Corp. accepts 7/16 in. of penetration for fiber-cement lap siding."

  12. Matt Cibula | | #12

    Since my walls are true 2x4 with plaster and lath and I can't be sure how much, if any insulation is in the wall cavity, does that impact the thickness of foam board on the exterior? I am in the northern part of Zone 5, about 30 miles or so away from zone 6. I read the updated minimum thickness, but do I go with the 2x4 numbers and add R-5, which is just 1" board or do I add 1.5" fomular panels and get an R-value of 7.5? 1.5" of Polyiso-the foil faced type has an R value of 9.8. Since this is a retro fit, it seems like some of the rules that I'm finding don't specifically apply to me.

  13. GBA Editor
    Martin Holladay | | #13

    The difference between the amount of insulation in a wall that is 3.5 inches thick and one that is 4 inches thick isn't enough to worry about.

    The table you are talking about can be found here: Calculating the Minimum Thickness of Rigid Foam Sheathing. According to that table, the minimum R-value of the exterior foam for your wall is R-5.

    If it turns out that your walls actually have less than 4 inches of insulation in them (or no insulation in them), the R-5 foam won't cause any problems.

    Of course, as with any insulating job, feel free to add more than R-5 if you want.

    R-5 is the minimum, not the maximum.

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