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Community and Q&A

Thoughts on adding rigid insulation over existing siding.

KAPLAN ARCHITECTS | Posted in Green Building Techniques on

We have a historic house that is undergoing a deep energy retrofit with rigid insulation being applied to the outside and then strapping and new siding. The contractor has ripped off a few layers of asphalt siding and is now down to the original siding. He would like to keep it on and simply put the new rigid insulation over the siding. It seems this will create some nice air pockets which be good insulators, and be a cavity for drying if needed. Does anyone have experience doing this and any input on pros and cons versus taking that original siding off and going down to sheathing?

Thank you,


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  1. user-723121 | | #1

    I would go further and get down to the original exterior sheathing, most likely pine boards. Some refastening of these boards may be in order, another reason to strip off the remaining siding.

  2. albertrooks | | #2


    Air pockets might be counter productive. You don't want air movement in that layer. Especially if it can exchange with another layer and bring in additional moisture that may condense.

    Curious of your climate zone and the type and thickness of foam that will be added... Would you mind sharing that info?

  3. user-788447 | | #3

    In the Twin Cities MN it was relatively common to stucco over existing lap siding. Different I know but this practice did not seem to lead to predictable failures.

    I just ripped the siding of my old home and was glad I did because it gave me an opportunity to replace old sheathing that had ant infestation and areas where air leakage was apparent. Also I was able to detail and insulate around my windows much better than had been done before.

    Probably not significant but if you do insulate over the existing siding you have a separation between the insulation layer and the air barrier layer (if there even is one). I've always wondered if this compromised the R-value of the wall assembly.


    Leaving siding would strictly be a cost saving measure. We're in Zone 6, adding 2" of polyiso. Existing 2x4 walls would be filled with new cellulose. So that's R-12.6 walls plus R-13.2 foam - a good ratio but not if cool air gets in the gap. I've only seen people spray foam over existing siding. My fear is that unless we blocked the airflow at the edges, that this could be a real condensation issue.


  5. wjrobinson | | #5

    At least cut holes and look inside walls. I just opened up 3 walls in a home that had small signs of mold showing externally. Inside the walls there was enough damage and mold to point toward rebuilding the entire lower four feet of all wallsof the home.

    So take a peak through the layers of your wall at the very least. Ridges foam is tricky. Two homes with it, out of two that I started unrelated work on, had complete absolutely destroyed rotted and turned to soil, hidden framing damage. Only the painted surface materials were ok. Behind that paint was not wood, only composted wood.

  6. LucyF | | #6

    I was wondering the same thing only I would like to install polyiso insulation over old, but intact asbestos siding. Is that possible?

    Lucy, upstate SC

  7. user-1023211 | | #7

    Hi all,

    This is a question I've had also. I'd like to hear more information on this issue.

    I'd like to install at > 2" rigid foam (Iowa), but stay away from polyiso since I've seen it eaten away from insects burrowing in it installed on other homes around here (have photos). Apparently they don't use boric acid in polyiso like the polystyrene manufacturers use.

    Anyway, my reason for wanting to install exterior insulation over the siding is because we have young kids and old lead paint on the cedar horizontal lap siding. Which we primed with Lead Barrier Compound and repainted a few years ago so the lead is 'contained' as best you can. I'd rather not make a toxic mess to get to the wood sheathing underneath.

    The underneath sheathing looked dry and sturdy in all the places I drilled for blowing in cellulose a few yrs ago. We have ~2' overhangs so it rarely gets wet on the siding.

    Any help would be appreciated,


  8. GBA Editor
    Martin Holladay | | #8

    My first reaction to this question: as a builder, I would always strive to strip the siding off and expose the sheathing, so that the condition of the sheathing could be ascertained. This is a rare opportunity to inspect walls for moisture damage.

    However, from a building science perspective, there is no reason you can't install rigid foam on the exterior side of the siding, as long as:

    1. You are fairly sure that the sheathing is dry and sound.

    2. You have come up with a way to air-seal the perimeter of the rigid foam to limit air flow through the gaps between the siding and the foam. (This is a challenge. It requires attention to detail.)

    The only reason to consider this approach, in my mind, is if the siding contains lead or asbestos. If the budget can handle the extra costs, though, removing the siding -- if necessary, with help from a lead-abatement contractor or an asbestos-abatement contractor -- is the best way to go.

  9. michaelbluejay | | #9

    I'm thinking that the top of the foamboard could be air-sealed with canned spray foam, and the bottom of foamboard could be sealed the same way. Since you advise air-sealing the entire permeter, I guess it's not important for the bottom to be open for potential drainage?

  10. GBA Editor
    Martin Holladay | | #10

    I'm not sure why you have decided to provide a comment on a Q&A thread from 2011.

    But to answer your question: When installing a continuous layer of rigid foam on the exterior side of a wall, it's more important to air seal the horizontal seam at the bottom of the wall than it is to leave the seam open for drainage. If the foam is the right thickness, and is installed with attention to airtightness, there won't be any moisture that needs to drain.

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