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Questions about insulation and siding

roseau | Posted in Energy Efficiency and Durability on

I have an old house that was built in 1945 , it has a lath and plaster interior with wood shavings used as insulation in the existing 2×4 stud walls. i would like to remove the exterior stucco and replace all the windows and doors and clad the exterior with new engineered wood siding. Can i add 1 inch of Permeable foam insulation to the outside , strap out the wall to provide an air space between the foam and siding ? or will this cause problems long term ? the product i was considering is a permeable type foam that adds insulation while preventing air infiltration from the outside , the product is sold by IKO , its called Ener Air . i live in Winnipeg Manitoba which has the same climate as Grand forks ND.

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  1. GBA Editor
    Martin Holladay | | #1

    You live in Climate Zone 7, which is quite cold. You can add exterior rigid foam if you want, as long as the rigid foam has a minimum R-value of R-10. That would mean 2 inches of XPS or polyiso, or 2.5 inches of EPS.

    If you prefer to install vapor-permeable insulation, you can use a continuous layer of mineral wool. If you use mineral wool, you can choose any thickness you want. There is no minimum R-value when you use mineral wool.

    Here are links to several articles with more information on these issues:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Install Rigid Foam Sheathing

    Wrapping an Older House with Rock Wool Insulation

  2. JeremyArch | | #2

    I hope its okay if i jump in to this thread.

    Martin, i have 2x6 r-19 in my walls in climate zone 6 with an interior polyethylene barrier. If i put 1-1.5" of roxul comfort board on when residing, I wouldnt have to worry about the dew point causing moisture issues on the plywood sheathing?

  3. Expert Member
    Dana Dorsett | | #3

    Jeremy: With the polyethylene sheeting you already don't have to worry about dew point issues, since you've blocked the diffusion path from the humid interior to the cold sheathing. Adding vapor permeable insulation such as rock wool doesn't make it any worse, it only makes it better.

    A mere 1.5" of rock wool isn't quite enough to be able to skip the polyethylene (or another lower-permeance vapor retarder) on the interior, but raising average temp at the sheathing will lower it's moisture content as long as it has a drying path to the exterior, which it does.

  4. roseau | | #4

    so i read the previous blogs and i understand the issues with moisture etc but what about the permeable foam board that i mentioned the has a perm rating of 60 and Tyvek has a perm rating of 57 so in my mind it should adequately vent moisture to the outside ?? this a relatively new product unlike the foil backed stuff which is NON permeable.And i should still strap it out past the foam ?

  5. Expert Member
    Dana Dorsett | | #5

    Yes, high-perm foam exists, and should work fine, but it's a specialty item.

    And yes, you still need at least some amount of ventilation space between even high-perm foam and the siding in order to have a good drying path to the exterior.

  6. JeremyArch | | #6

    I was under the impression there was a minimum r value suggested when adding exterior insulation. Good to know i can add some during my future re-side. I would prefer to add more but my roof overhang at the gable ends is only a couple inches

  7. Expert Member
    Dana Dorsett | | #7

    That minimum exterior R is only necessary if the exterior insulation has a low vapor permeance, and there is no interior polyethylene.

    If you have the interior polyethylene keeping to the minimum is still a good idea with exterior insulation that has low vapor permeance (like most rigid foam), but rock wool doesn't have that problem, but most rigid foam. With low-permeance exterior insulation it creates something of a moisture trap, but as long as the temperature of the sheathing is warm enough, it (usually) stays dry enough.

    If there is no interior vapor retarders (defined as < ~1 US perm in Canadian building codes) the minimum exterior R is mandatory even if it's high permeance insulation. This is because the plywood or OSB sheathing itself has too a low vapor permeance, and will accumulate moisture coming from the home's interior if it's too cold, even when there is a drying path toward the exterior.

  8. JeremyArch | | #8

    Thanks Dana, i guess in my situation it would make more sense to use something like the polyiso ener-air product that Roseau had mentioned. It would provide a little more r-value for my limited space.

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