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Community and Q&A

Rigid foam insulation over Bildrite during remodel/reside?

ttvedt | Posted in Energy Efficiency and Durability on

I have a recurring question / situation that comes up in my east central MN location (6b-7a border). There are many 1970’s homes undergoing remodeling and looking to add insulation for improved comfort and energy effciency. Miost common construction is drywall, kraft faced R13, 2×4 walls, 3/4″ Bildrite (asphalt fiberboard sheathing).

Can one add an inch or two of unfaced rigid foam to the exterior with out creating a vapor problem? How much foam is needed to be effective, without making the home too tight? Proposed is foam over current sheathing, then 1×3 strapping and cement board siding. Is this a good idea and would it be wise to put a ‘raindrop’ type house wrap over the asphalt sheathing – before the foam or after. New windows are generally part of the plan and obviously the flashing details are important. I am materials dealer, often asked for my advice or opinion by DIY homeowners who are trying to sort through all the information out there. Any advice would be appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    In Climate Zone 7, the minimum R-value for rigid foam installed on the exterior side of wall sheathing is R-10 for 2x4 walls or R-15 for 2x6 walls.

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    You may also be interested in reading How to Install Rigid Foam Sheathing.

  2. LucyF | | #2

    First you can always send interested people to this website. There is no other website where you can get such practical and GOOD advice on upgrading the energy efficiency of your home. There may be some disagreements, but the people here (Martin and staff as well as regulars) know their stuff and will help you make good decisions.

    Second, when homeowners are going through all the trouble of tearing off the siding to improve the insulation, they need to do enough to make a difference. You are in a cold climate, while every little bit helps, 2 inches of foam probably isn't that much more expensive but would greatly improve the home's comfort.

    Dana Dorsett will probably answer you about the vapor permeability of the different kinds of foam, faced or unfaced, etc. and the risk of creating a vapor problem. My thinking is that if you add enough foam, it is not a concern and it helps your home maintain its' temperature so much better. I had 2" of foil faced polyiso foam on my house in Upstate SC - zone 3.

    I hope that helps.

  3. ttvedt | | #3

    Thanks. Yes, there does seem to be a lot of good information on many topics throughout this website. I'll certainly refer people to it in the future. I agree with you on "doing enough to make a difference". I just needed a little more to go on when trying to help folks make good decisions.

  4. PAUL KUENN | | #4

    Two inches is NOT enough!

    Even in Green Bay, WI area (zone 5-6) we're pulling off siding over 2" R10 that was installed over the same walls as yours 8-10 years ago and you can see significant oxidation/rust on nails and screws that are imbedded in the "warm" 2x4 walls. This proves it's still too cold and moisture is condensing at the tips in the wall.

    So many were done with half or one inch insulation under siding and some nails are almost rusted through or any coating is gone. What a waste!

    I had the same on my house so I removed the siding (again) and added another 4 inches. Wow, what a difference in internal warmth. You can get recycled polyiso, XPS and/or EPS almost anywhere these days. Just check out Craig's list. Usually a 3rd of the price and works fine. Make sure they send you photos for quality control. A house we remodeled that had 5" insulation over the sheathing showed no signs of moisture where we pulled one wall apart. His house was done 7 years ago with 3" EPS and another 2" layer on top, furring strips then vinyl siding.

    If you want it to last, then put in the little extra time and money to secure the future.


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