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Community and Q&A

Best practice/advice- Three insulation assemblies

ckolloff | Posted in Energy Efficiency and Durability on

I need guidance/advice on insulating three separate assemblies as I finish the space on the 2nd floor of our 1952 Cape Cod.

The space: Total 2nd floor square footage equals 960- all conditioned space
No dormers on the front, single shed dormer spans nearly the entire backside of the house.
Williamsburg VA- Climate Zone 4A- more hot & humid than you might imagine.

Here is what we are starting with:

1. Exterior wall on either end (facing North & South) is constructed as follows- Brick, 1″ airspace, #15 felt, 1/2″ insulating (asbestos?) sheathing, 2×4 studs

2. Exterior wall on the shed dormer across the back (faces West) is constructed as follows- Aluminum siding, horizontal 1/4″ wood plank siding, #15 felt, 1/2″ insulating (asbestos?) sheathing, 2×4 studs

3. Roof is constructed as follows- Asphalt shingles, felt, 1×6 roof decking, 2×6 rafters, 11/12 pitch. I am not sold on unvented/cathedralized ceilings due to moisture issues and therefore do intend to vent the roof assembly with soffit to ridge venting.

The primary limitation is budget. Spray foam is not a possibility. Also, While rigid foam board above the roof deck would be desirable, it not realistic for us.

How do you recommend I treat each of these assemblies?


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  1. GBA Editor
    Martin Holladay | | #1

    That's a pretty general question.

    In your Climate Zone, the 2012 International Residential Code requires ceilings or roofs to be insulated to at least R-49. I suggest you aim for that level or better.

    For information on ways to insulate a sloped roof assembly, see this article: How to Build an Insulated Cathedral Ceiling.

    For information on insulating walls, I suggest that you start by reading these two introductory articles in the GBA Encyclopedia:

    Insulation Choices

    Insulating Roofs, Walls, and Floors

    Those two articles have lots of links to other articles with more in-depth information.

  2. Expert Member
    Dana Dorsett | | #2

    Have that sheathing board tested for asbestos if it's showing ANY signs of wear at all.

    Assuming the sheathing checks out as OK, caulk it to the framing inside each stud bay. Any lateral seams in the sheathing can be taped with housewrap tape then painted/troweled over with fiber-reinforced duct mastic to give it a chance of staying sealed over the long term.

    Then, carefully install R15 rock wool or R15 fiberglass, tucking it in at the corners & edges, then tugging it back out just proud of the stud edges so that the next layer will be a compression fit. Split the batts to accomodate wiring or plumbing runs, and trim carefully around electrical boxes (which need to be air sealed before installing the insulation.) A 10" bread knife makes a pretty good batt knife, if you don't have the purpose-made tool handy.

    You can then use 1" foil-faced polyiso tacked in place with a few cap nails and tape the seams with a quality aluminum tape, and caulk any exposed edges to the framing. Install the interior wallboard by through screwing it to the studs. You'll have some air-sealing details to attend to around electric boxes, connections, and and you'll likely have to re-mount or replace them to accommodate the extra inch due to the polyiso. (There are several methods for air-sealing electrical boxes.) It's also worth using can-foam to seal any stud penetrations of wiring or plumbing prior to insulating.

    With R15 cavity fill and R6 continuous sheathing on the interior you'll beat the R13 + 5c.i. prescribed by the IRC 2012 by a small bit.

    On the cathedralized roof assemblies, if they can be fully vented soffit to ridge (no blocking or obstructions. you can use housewrap to make a continuous air barrier by wrapping the rafters and stapling the housewrap at 3.5" out from the interior to accommodate R15 batts there as well. But rather than foil-faced polyiso, use 2" of unfaced EPS foam (not XPS) as the interior side thermal break. That will end up at about R23 center-cavity and is WAY sub-code, but it leaves you some options for later, and is nearly twice the performance of R13 batts thermally bridged by rafters. Hopefully lowering the ceilings by 2" won't make it a serious head-banger(?).

    By using unfaced EPS it leaves the interior side semi vapor-open at about 1.3-2 perms, which means at some later point you could fill the 2" vent space with blown cellulose or fiberglass for another R7.5-8 (bringing it up to R30-ish) and install R13-R20 rigid foam above the roof deck without creating a moisture trap. To go unvented in that stack up you need at LEAST R13 above the roof deck (3" roofing iso would be fine, performing at about R15 in mid-winter your climate with that stack up, R18 during the shoulder seasons). That wouldn't quite take you to the R49 code min on an R-value basis, but since you would have more than R20 of thermal break over the rafters (R8 inside, R13 outside), it might meet code on a U-factor basis by ducking under U0.026.

  3. ckolloff | | #3

    I am reading the recommended articles and associated links. Thanks.

    How can I test for asbestos? And what are my options if found?

    I do not anticipate placing rigid foam above the roof anytime soon and would like to increase the R value of the roof assembly and walls. In addition to the suggestions you offered above, What are your thoughts on :
    Furring the rafters down for additional roxul and also adding an additional inch of XPS?
    Framing a second interior wall using 2x3s to provide additional insulation space?

    Are there better options?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "How can I test for asbestos?"

    A. You should contact an asbestos abatement contractor if you want to test your sheathing's asbestos content.

    Q. "What are your thoughts on furring the rafters down for additional Roxul and also adding an additional inch of XPS?"

    A. You can insulate between the rafters with Roxul mineral wool, but only if you include a ventilation gap between the top of your insulation and the underside of the roof sheathing. Assuming that this ventilation gap exists, you can proceed as you have described.

    Q. "What are your thoughts on framing a second interior wall using 2x3s to provide additional insulation space?"

    A. You can do that if you want. If you decide to do that, I would install the 2x3s horizontally, not vertically, if I were you. This creates a so-called Mooney wall. This method interrupts thermal bridging through the studs.

    Q. "Are there better options?"

    A. The suggestions that you have come up with will work, but it's hard to say whether your methods are better or worse than alternative methods. Whatever methods you choose, pay close attention to airtightness.

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