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Community and Q&A

Polyiso on the inside?

Vlad O | Posted in Green Building Techniques on

A question from a DIY prospective…. I can see a point of adding rigid foam insulation on the outside. But for a 2-story house, and a do-it-yourselfer, it is a bit of a scary project – you have to complete a lot of work in a short time, and get a lot of details just right.

Adding insulation on the inside, on the other hand, allows one to work at a more flexible pace. Even if it takes a year to do it room-by-room, you don’t expose your house to the elements. You don’t get a continuous thermal envelope, of course. The general idea is to remove drywall, fix whatever is wrong with fiberglass batts, add 2″ of polyiso, furring strips, and then drywall. Sort of like insulating a basement. Does it make any sense, or is it just inviting trouble?

I am in northern part of Zone 4. Current insulation is 4″ fiberglass batts with paper to the inside.

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  1. GBA Editor
    Martin Holladay | | #1

    Plenty of people have successfully followed your suggested approach, including me. It's not as good as exterior foam, but it will certainly improve the thermal performance of your walls.

    For your approach to work, you need to be sure that your wall assembly can dry to the exterior. I would never try this on a house with exterior stucco, since stucco doesn't dry well to the exterior. If your house has vinyl siding, fiber-cement siding, or wood siding, you should be OK -- especially if the siding has a rainscreen gap.

    The best approach includes opening up the ceiling in each room so that you can insulate the rim joists. Make sure that your rim joist insulation is continuous with your wall insulation, without interruption. It's also important to expose the areas where partitions intersect with exterior walls, so that (ideally) you can remove the last partition stud and slide rigid foam behind the partition intersection.

    Remember, you'll have to pull all of your electrical boxes inward, and you'll need to build extension jambs for your windows and doors.

  2. user-1115477 | | #2


    I am in also in the northern part of zone 4A and a DIYer. I have taken an old 2x4 stick frame house and added 2 layers of 2" polyiso all around (among other things), and the work included two roof extensions to accomodate the layers. I did all the work myself; it's hard, but the results are good and it feels great. My house is tight (not sure how tight, right now), but I went from a 3-ton, ducted heat pump to two ductless minisplits of about half the capacity. Note that the floorplan has to be right to employ ductless minisplits effectively.

    I once had the same thought as you. I know now that if I would have taken the inside insulation approach, I would be very disappointed right now. I had a mindset that I wanted to take an existing house and make it as energy efficient as I possibly could. If you feel that way or ever think you will feel that way, then don't take the inside insulation approach.

    My first impulse is to say to you, "if I can do it, you can do it." But it depends on how good you want the energy efficiency to be and how involved and determined of a DIYer you are.

    Some things to consider: The inside approach will also take quite a bit of time and can be scary. There will be huge amounts of dust and smaller rooms as a result. Any serious attempt would involve moving kitchen counters, bath fixtures, extending electrical outlets.....whew, not easy.....and all for lesser results in the end. You mention that you could work at a more flexible pace without exposing your house to the elements. I worked at a very slow pace (all by myself) at a pretty flexible pace, and found that exposure to the elements was not a problem You don't have to remove existing siding all at once, and you can use housewrap to keep things sheltered good enough. The only problem I had with weather duing the entire time I was working (about 1.5 years) was a few rain incidents in the middle of extending roofs about one foot--tarps are cheap insurance. Another huge consideration: When I did the external insulation, my house needed both new siding and windows. If you don't need those things, then the outside approach is certainly harder to swallow. I framed out window extension bucks (outie windows) to accomodate the insulation layers. My house is also two stories--I realized early that I had to buy a pole jack scaffold (yes, I moved that around by myself, also). All the work would have been a nightmare without that scaffold. Afterward, I was able to sell it for almost as much as I paid. This might sound extreme to you, but I feel it was all worth it. I'm not sure about you, but I would not have been happy with the inside approach.

    I really think that if you are using words like "thermal envelope," then you may just be serious enough about energy efficiency to avoid a major, inside insulation approach.

  3. Vlad O | | #3

    Thanks you for your advice, Martin and Sonny. I am glad to hear that the inside approach is not crazy, and that outside approach is doable for a DIY-er.

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