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Can I leave the fiberglass insulation in place and add 1/2-inch Rmax polyiso to the interior?

user-5523794 | Posted in Energy Efficiency and Durability on

I am starting a renovation in a home with T-111 exterior siding, 2×4 walls, 3 1/2″ fiberglass batts for insulation. Can I add 1/2″ foil faced Rmax on the interior wall and then sheetrock? Is there a better way to increase the insulation/air sealing then what I ‘m describing?

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  1. Expert Member
    Dana Dorsett | | #1

    What is your climate zone/location?

  2. user-5523794 | | #2

    Zone 5

  3. iLikeDirt | | #3

    Do you already have all the drywall off or are you planning to do this for another reason?

  4. user-5523794 | | #4

    All the drywall is off

  5. Expert Member
    Dana Dorsett | | #5

    With T1-11 the sheathing is the siding, and when you have sheathing that gets direct wetting from rain it's good if there is at least SOME drying capacity toward the interior.

    In zone 5 the IRC code min is R13 + R5 c.i.. Half inch polyiso is only good for R3. From a thermal performance point of view it's better to install 3/4" or 1", bringing it up to something like code.

    From a moisture management point of view it's better to put the R5 on the exterior under new siding. If you install foil faced polyiso on the interior it's good to install vinyl siding (which is inherently back-ventilate) or some other siding with back ventilation over the T1-11. That way the amount & frequency of direct wetting of the T1-11 drops by more than an order of magnitude.

    If you're not going to add siding and are still going to add foam-R on the interior use UNFACED EPS (up to an inch).

    It's fine to put the rigid foam over kraft faced goods or unfaced.

    While you have the wall open, caulk the framing to the T1-11 in every stud bay with the appropriate sealants, and can-foam all electrical penetrations of studs & stud plates. If it's in the budget, R15 rock wool or R15 fiberglass are superior products to R13s, as is damp sprayed cellulose. R15 + R4.2 c.i. (1" of unfaced type-II EPS) would perform about as well as R13 + R5 c.i..

    At 1" the EPS layer would run 2.5-3 perms, which is somewhat tighter than latex paint, but not tight enough to cause a problem.

  6. user-5523794 | | #6

    I was going to add a cedar tongue and groove siding at a later date over top of the T1-11 with some type of fabric to allow for water to drain. This house also has a flat roof (hot tar apparently) with only R22 fiberglass batts, no overhangs and circle vents in the fascia. I was considering removing the sheetrock there also. the rafters are only 2x10s. What would you recommend in this situation?

  7. GBA Editor
    Martin Holladay | | #7

    Dana gave you good advice on the walls. Here is a link to an article that will give you guidance on your roof: Insulating Low-Slope Residential Roofs.

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