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Community and Q&A

Best way to Insulate around windows and doors from exterior

Pete Fabio | Posted in GBA Pro Help on

Hello, We are planning to replace our siding and want to properly insulate and flash around windows and doors while they are exposed,or what to look for when we remove the old siding The home was built in 1981 . The new siding will have new foamboard over the biltrite. Thank You

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  1. Aaron Vander Meulen | | #1

    How thick is the insulation? Is it sheets or fanboard?

  2. Pete Fabio | | #2

    XFP38 By the way the windows have had new thermal glass,frames are in great shape will be wraped,just looking for anything further to do with the windows.

  3. GBA Editor
    Martin Holladay | | #3

    I don't know what "biltrite" is.

    It sounds like you have had your existing windows reglazed. You have removed your existing siding, and your siding installer will be installing foam board of unspecified thickness under your new siding.

    1. What is your climate zone or location?

    2. What type of foam is being installed? How thick is it?

    3. You asked, "What's the best way to Insulate around windows and doors from exterior?" The answer is, in most cases, there is no insulation added around windows and doors -- just air sealing work and flashing to shed wind-driven rain. Hopefully, your contractor knows how to perform air sealing and flashing work; if you are worried about this issues, and if you are worried about the competence of your installer, you should start surfing this site and studying those issues.

    Some Passivhaus builders have been experimenting with a technique called "overinsulating the exterior of the window frames." This is really only possible with European tilt/turn windows, and it is rarely done in the U.S., so I wouldn't worry about it.

  4. Aaron Vander Meulen | | #4

    XFP38 is 3/8 green guard fan board insulation, correct? You really can't "properly" flash and insulate a window from the exterior. "Proper" flashing includes a pan flashing that can only be installed if the window is removed. "Proper" insulation/air sealing is done from the interior. remove trim and use gaskets/spray foam/Sika tape and then reinstall trim. Is 3/8 foam thin enough to prevent condensation on the back side of the foam? I think you may be better off skipping the foam or using thicker foam based on the IECC tables for you climate.

  5. Pete Fabio | | #5

    Well the climate is wi and this seems to be the method of choice for seamless steel siding contractors,using fanboard and sealing all the seams.Just want to do it right for the last time and get a second opinion. Thanks

  6. Albert Rooks | | #6

    Hi Pete,

    Perhaps your aware of the issue but... Built in 1981... Be sure to be aware and verify that there is no vapor barrier on the inside of the wall. Obviously trapping moisture would be an issue to avoid when adding foam on the exterior. Sorry, I could not place climate "wi".

    I agree with both Martin and Aaron above that adding insulation around doors and windows is not the best target in this case. Air sealing the window and door frames is a great target. Go for it!

  7. Pete Fabio | | #7

    Here is what the 2x4 wall has from the inside out sheetrock,plastic vapor barrier ,batt insulation,3/4 4x8 sheets of styrofoam,biltrite,siding.northern climate. Thanks

  8. GBA Editor
    Martin Holladay | | #8

    I hope "biltrite" isn't a rubber sheet -- this is what I found when I Googled "biltrite":

  9. Pete Fabio | | #9

    Hello Martin, This is what builders use instead of osb or plywood most likely cost driven heres what I have : if this makes more sense. Thanks Pete

  10. Pete Fabio | | #10

    This does bring up another question if I have plastic vapor barrier on the inside wall under the sheetrock would I tape all the seams on the XFP38 would this trap the moisture in the walls?

  11. TJ Elder | | #11

    Pete, you have two problems here. First, it's a bad idea to install XPS on the outside when there's poly on the inside. This wall would not allow drying in either direction. Second, even without the interior poly your XPS fan-fold is too thin. The minimum thickness depends on climate, and increases as you go north. See this article:

  12. Pete Fabio | | #12

    Thomas, So many different answers on this,the siding contractors should know,the do need to use it under steel siding. Greenguard claims it can be used in place of house wrap and it has been approved. It will breathe out but not in.To clarify XFP38 is the product our contractor is going to use.Well I need to really clarify this before we begin. Thanks Pete

  13. Aaron Vander Meulen | | #13

    According to their website, the XPS fan fold has a Perm rating of 1.3. The layer of 3/4 (I assume) XPS is more troubling. I'm kind of curious why they "have to" use it under the siding. I've seen fanfold used over old wood siding before vinyl on older houses, but can't imagine way its needed in this application. When you say "seamless metal siding" is it corrugated, like the Fabral product, or is it similar to the products used on the roof? While its not something I do every day, we had no problems installing Fabral over OSBa couple of years ago. Or are they using it instead of a house wrap?

  14. Pete Fabio | | #14

    Aaron, It is 3/8 and they are using it for house wrap and I believe to help with the straightening of the siding.They also have it approved by the inspector. It is US Seamless brand. It does look good installed, its what you cannot see that I want to get right.So back to the question will this trap moisture in the walls? Thanks Pete

  15. GBA Editor
    Martin Holladay | | #15

    You were told that the fan-fold foam "will breathe out but not in."

    No such product exists -- that is, a product with one-way permeance. The permeance works in both directions.

  16. Pete Fabio | | #16

    Here is what I received from Greenguard on their product using the fan fold as a house wrap: Taping the seams. Approved as a WRB. Back to the big question do you guys think that doing this will trap moisure in the walls and it would have to pass through the seamless steel siding,fanfold,bildrite sheathing,3/4" styrofoam sheathing, to get into the fiberglass batts. Thanks Pete

  17. Robert Hronek | | #17


    Are they stripping the siding and sheathing. Now is the time to insulate and air seal. With the sheathing stripped pull out the insulation and the poly and have the walls dense packed. Now you can add rigid on the outside. You will need from R7 -10.

    This blog goes over the exterior rigid R values.

  18. Pete Fabio | | #18

    I GIVE UP!

  19. GBA Editor
    Martin Holladay | | #19

    Don't give up.

    Thin fan-fold foam insulation used in a re-siding job like yours probably won't trap moisture in your house, but there are a lot of variables. These variables make GBA readers reluctant to provide a blanket thumbs-up or thumbs-down for your plan.

    Re-siding is one thing -- pretty simple, really -- but it would be a shame if you didn't take this opportunity to address several thermal weaknesses in your wall system. To do that, you need a knowledgeable contractor -- and that means someone different from the one you have, who sounds like a siding guy who recommends the use of fan-fold foam.

  20. Pete Fabio | | #20

    Hello Martin, I agree this is the opportunity to do this right,and I just discovered last nite that I have been mistaken about the sheathing under my old masonite siding,on the heated portions of my home it is only 3/4 sheets of styrofoam and unheated attached garage has bildrite. I should never have assumed that looking in my garage or attic was the same all over.As you can tell I'am no expert but should there be something more substantial next to the studs than just blue styrofoam? I just want to specify the way I want it done.Thanks so much

  21. Aaron Vander Meulen | | #21

    Yes and no. Typically, OSB or plywood is used at corners to provide bracing. This is generally covered with a thinner layer of foam. However, if your foam is only 3/4", this is unlikely. They may also have let in 1x4 bracing, or metal T bracing at the corners, which seems more likely. After removing the old siding, it would be an excellent time to tape and seal the seams of the foam, as well as all the holes from the old siding. You would also benefit from removing the interior trim and air sealing with foam/gaskets/tape. Its up to you weather or not the fan fold (R1) is worth it, personally I'd use house wrap.

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