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Interior insulation for a part-stucco house

Ariff | Posted in Energy Efficiency and Durability on

We’re planning to gut and refinish our family room. The room has 3 exterior sides. Two of the exterior sides have traditional stucco and the other has vertical wood siding on the outside. The room also has a cathedral ceiling.

As part of the renovation I want to increase the insulation of the room. I understand that exterior insulation would be the best way but the stucco is in good shape and we are not planning to change the siding currently. So I’m looking for advice on which interior insulation to use.

I’ve been considering using spray foam (walls and ceiling) to seal and insulate the room. However, I’m concerned about the drying potential on the stucco walls. Do I need to consider different insulation types for the different walls? Or should I just use dense-packed cellulose on all walls? What about the ceiling?

Thanks in advance.

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  1. Ariff | | #1

    Forgot to add that we're near Boston.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, you didn't mention whether the existing walls have any insulation in the stud bays. (Many existing homes do not.)

    If the stud bays are now empty, dense-packed cellulose is definitely the way to go, because it is the least disruptive to interior finishes.

    If you decide to install spray foam, you'll probably have to open up all the walls from the inside. That's disruptive, but it's no big deal if this is a gut-rehab job and you were planning to do that anyway.

    Your question about whether the installation of spray foam in an existing house with stucco siding will reduce the drying potential of the sheathing is an intriguing one. Stucco is a very risky siding, especially in a cold climate like Boston. It doesn't take much of a change to tip a stucco wall into failure, because stucco walls usually dry very slowly to the exterior. Because of that, I'd be inclined to recommend dense-packed cellulose instead of spray foam.

    You also asked, "What about the ceiling?" If it's a flat ceiling with a vented unconditioned attic above, it's hard to beat cellulose insulation. Pile it on thick.

    If it's a cathedral ceiling, you have lots of options. I often advise people to install a thick layer of interior polyisocyanurate insulation, held in place with furring strips, and then to blow dense-packed cellulose insulation into the rafter bays through the foam.

  3. Ariff | | #3

    Thanks Martin.

    The existing walls have fiberglass insulation. The house is about 45 years old. Since we'll be removing the existing plaster boards, should I attach a layer of polyiso across the studs and then put cellulose behind the boards?

    The ceiling is a vented cathedral ceiling. Do I need to do anything special to ensure the baffles do not get damaged/blocked when installing the dense packed cellulose? Also how thick should the polyiso be? 1 or 2 inches?

    Thanks again.

  4. GBA Editor
    Martin Holladay | | #4

    It's entirely up to you to determine the R-value of the insulation you install in your walls and ceiling. In climate zone 5 (Boston), the 2009 IRC requires a minimum of R-20 in walls and R-38 in ceilings. I think these minimum code requirements should be your minimum goals as well. More is better.

    If your house has 2x6 studs, the stud bays are deep enough to accommodate R-20 of dense-packed cellulose. If your house has 2x4 studs, you can't achieve R-20 unless you use closed-cell spray foam in your walls, or unless you supplement the dense-packed cellulose with a continuous layer of R-7 (minimum) rigid foam.

    You can use a similar calculation to determine whether you need rigid foam for your ceiling. Dense-packed cellulose has an R-value of about R-3.7 per inch.

    Whether or not you can use the existing ventilation chutes depends on how rigid they are. The white styrofoam Proper-Vents are basically worthless, because they can't resist the pressure of dense-packed cellulose. If you are replacing old styrofoam vent channels, I suggest you substitute AccuVents.

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