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Community and Q&A

Adding rigid foam to living space side of wall

steveworks | Posted in Energy Efficiency and Durability on


I have a project in the Boston area where I would like to add and inch or so of rigid insulation to add some R value and knock down the thermal transfer and air-seal. The problem I’m saddled with is the wall that I need to work on is the gable end of the house and I’d have to extend the roof and move the electric meters and weather head if I thicken up the exterior wall by adding the foam and rain screen, and also add the new siding.

Some of the wall has the original fiberglass in it, and some we’re going to put open cell into ( repairing leaky wall and I will take down wall board and replace old fiberglass) . I am wondering if I put the rigid on the inside of the condition space ( preferably going over the wallboard where the fiberglass still is,) will I end up with condensation on the exterior side of the rigid or inside the wall cavity. I defiantly want the follow the “do no harm” rule.

Thanks much


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  1. GBA Editor
    Martin Holladay | | #1

    Unless there is something unusual about your house, installing interior rigid foam will do no harm. As with any insulation retrofit work, pay close attention to air sealing.

    There are several reasons why exterior rigid foam is preferred to interior rigid foam. Among the reasons: exterior foam does a better job of addressing thermal bridging at rim joists and partition intersections. But if you are creative and thorough, you can come up with ways to address these areas on the interior.

  2. steveworks | | #2

    Thanks Martin,

    i know putting the insulation on the inside is not an ideal way to go, but it's a plan B at any rate. I've already insulated the rim joists from the basement so that's been a help as far as air and thermal bridging down low. I won't get the framing studs into the package but it will be far better than what's there now. Also the return on investment as far as the client is concerned ( not reworking the roof shingles at the rakes and not having to redo the feed from the meter to the breaker box) will basically keep budget for the insulation. I'll do a 'smoke stick" test and keep an eye out for leaks.

    Thanks again

  3. user-901114 | | #3

    Are you planning to do this work to only one wall? If so, I would wonder how this could effect the other leaky walls. Put more pressure on them perhaps? Which floor is the wall on? How many floors in total? Removing window trim? Extending outlets?

  4. steveworks | | #4

    We're planning on doing two walls. the gable wall is basically two floors, ( the house is a cape with a shed dormer all along the back) The roof on the front of the house meets the front wall, no soffit at all. I could extend the roof a course and push the gutters out and the do a drainage plain and foam from the outside. I'd have to put a little porch roof over the front door as theer's no room above the head trim. I will be pulling of the window trim on the gable wall and air sealing that and probably get some Tyvek or felt paper on the exterior. This all started as an water leak from the attached garage onto the driveway side widow. probably even put some Ice & Water as extended flashing from the garage roof to the window as well. Defiantly have to move the electric boxes as well. Big side benefit, getting rid of the popcorn plaster ceiling!

  5. user-901114 | | #5

    Sounds like my cape in Waltham.. But I have no garage. What kind of shape is the roof and siding in? Does the dormer wall go all the way flush to the gable wall?

  6. steveworks | | #6


    Yup the dormer wall goes right to the gable. Roof is basically new, the siding is FJ cedar and needs a coat of paint ( I actually have the house on cycle where I only paint 2 walls and then do 2 wall the next year,waiting about 6 or so between).

    I'm going back and forth about the front of the house as far as residing. If it wasn't for having to but a roof over the front door ( more work and more roofing) I might just reside the front and put rigid on the outside. The flip side to that is ripping down the plaster inside, filling it with open cell, and then putting foam up and then new board. I guess it depend on down time between jobs.

    Pretty sure I'm going to do the insulation on the gable side from inside. I'm already opening up the wall to dig out the old (once wet) fiberglass. Also not having to remount the meter box, PV meter, cut out box and the Verizon box, plus residing and re-flashing will leave me time to clean out the truck :)

    My guess is that when I have to paint the back ( which is the full two stories) I may go and do it all from the exterior.

  7. user-901114 | | #7

    I guess a downside of putting some rigid on the inside (gable) and some on the outside (back dormer) is that the corners will still be thermal bridges. Unless there's a trick by wrapping the corners from the outside..

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