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Community and Q&A

Can an energy retrofit trap moisture?

user-651028 | Posted in Energy Efficiency and Durability on

I know we’ve looked at this many times, this question addresses a specific point for clarification. Energy retrofits can be expensive. If we are going to make a difference in climate change we need to make retrofits more attractive to home-owners by making them more affordable.

I have a project in NH designing an energy retrofit for a typical colonial home in zone 5, 7,000 HDD. It is 10 years old with a newer 3rd floor expansion. It has 2×6 stud walls w/ R-19 F.G. batts, craft faced in the original, poly under the drywall in the expansion. The sheathing is probably OSB. The owner and I want to get rid of the vinyl. We’re designing an energy retrofit with some sort of foam insulation over the existing vinyl siding (thick enough to avoid moisture accumulation) – either Larsen trusses with spray foam or rigid sheets with vertical strapping. We don’t want to remove the drywall and incur the added expense of all new interior finishes.

My question has to do with moisture. Does EPS, XPS or open cell spray foam have a high enough perm rating to not trap moisture in the wall? My experience tells me not to do it. But the manufacturers tell me these foams are vapor semi-permeable materials.

Dave Ely, AIA
Windy Hill Associates

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  1. user-869687 | | #1


    Generally the response from the experts is not to worry about the impermeability of foam, provided the thickness is enough to prevent moisture accumulation from the inside. Moisture will dry toward the interior. For this to work you must not have poly under the drywall, so maybe there's some way you could remove that.

    It is possible to install out-insulation that is vapor permeable, with something like a Larsen truss and a vapor open insulation material, including rockwool or cellulose. That could work for you, but it's not as simple to install as rigid foam boards. I would not consider any rigid foam insulation as vapor permeable even though EPS is somewhat permeable. Also you really need to remove the vinyl siding before installing any new exterior layers, if you were suggesting that it might get sandwiched in there.

  2. user-651028 | | #2

    Thanks Thomas. Vinyl siding is difficult to dispose of responsibly. Is there a reason you suggest removing it?

  3. user-869687 | | #3


    If you try to squash vinyl siding under rigid foam, there could be air circulation below the foam, undermining thermal value. It's also super flimsy and compressible, making a poor substrate. That would be doubly true if you did a vapor open assembly with something like a Larsen truss, which would need solid structural attachment to the wall. In that case the vinyl could also be a problem for moisture. As installed, vinyl siding allows moisture to flow through via air infiltration, but it could become a vapor barrier in the middle of an airtight wall.

  4. HDGFHkB5cP | | #4

    We have done repair work on comparably aged buildings in the New Boston/Amherst/Mont Vernon area and have found in nearly all cases extensive rot in the OSB, and in some cases framing, due to the lack of attention to flashing/wrapping details that seem to have been SOP during the building boom in this area. Aside from the disposal issue, and the relatively minor labor costs, stripping the siding is prudent and will reveal any existing issues before proceeding with your retrofit.

  5. bdrfab | | #5

    I certainly won't argue its more difficult than say, paper, however can be recycled. I was able to find a page on my states DEQ website that listed all permits issued to recycle vinyl and find someone who takes it. And i certainly agree with Jim's reasons for stripping it first.

  6. davidmeiland | | #6

    If you talk to vinyl siding suppliers, they will almost certainly direct you to recyclers who will take the scrap.

  7. user-947651 | | #7

    And don't forgoet the one man's junk is another's treasure. I once stripped vinyl off a house and sold the entire heap for $100. I am sure I could have done better but I needed it out of there fast. Try Craigslist for advertising.
    If you strip the siding and run a cut along the center of the stud bays about a foot or so of access you could remove the existing insulation and poly. Place the sheathing back and dense pack your stud bays and then sheathe your fexterior foam over the existing sheathing..

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