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Community and Q&A

Seattle Insulation Retrofit

Jeremy Sharp | Posted in General Questions on

Location: Seattle, WA, Marine Zone 4C
Type of Construction: Wood frame single family residence
Year Built: 1947
Size: 690 sq ft main floor, 380 sq ft basement

I purchased a home several years ago that had previously been a rental house for 30+ years and owned by the same family since it was built. It has seen very little in the way of alterations and updates. Which is good and bad. No need to undo poor workmanship but just about everything needs upgrading. Perhaps a blank slate?!

This post is regarding insulation. There is a small amount of loose rock wool in the attic, none in the floor or walls. I am seeking feedback on my idea to create a conditioned attic, insulated walls and unvented crawlspace.

Insulation Goals
Roof: R-58 (6″ polyiso above framing, R19 batt inside framing)
Walls: R-21.75 (1.5″ polyiso on sheathing, R12 blown in cellulose)
Crawlspace/Basement: R 16.25 (2.5″ polyiso)

I am concerned about putting additional weight on the roof framing (2×4). I will consult a structural engineer, but wanted to float my ideas here first to make sure I am on the right track. Anyone have experience with 2×4 rafters?

I am occupying the home so projects will be competed in phases as budget and time allow. I want to get the details fleshed out before I begin. I have attached a few files showing the existing floor plans, sections and a wall detail with the proposed insulation.

I have learned a tremendous amount from this community and look forward to any insights and recommendations you all might have. Thanks!

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  1. Steve Knapp CZ 3A Georgia | | #1


    I think you are on the right track by consulting with an engineer. He or she can actually visit your home and inspect the roof assembly. Perhaps you can sister a beefier rafter to the existing 2x4s or maybe it will make more sense to rebuild the roof entirely.

    On the rockwool, take care that it is not contaminated with asbestos. See this article for more information:

  2. User avatar
    Dana Dorsett | | #2

    Note: On a U-factor basis 6" of R6.5/inch polyiso above the roof deck already meets IRC 2015 code minimum, even without the R19 fluff. With the stiffening effect of nailer deck through-screwed to the original roof deck there may be no need to increase the depth of the 2x4s, but it may need collar ties. If the 2x4s are douglas fir (likely, given the location & vintage) and 16" on center rather than 24", it may not need collar ties.
    Consult with the engineer.

    R19s are the crummiest straw-man excuse for insulation to come down the pike since R11s. If you're installing insulation under the roof deck, high density R15s between the 2x4 rafters would be a better choice. HD15 fiberglass or R15 rock wool is sufficiently high density to not lose performance to convection currents, even without an interior side air barrier. If you bump the depths to 5.5", go with R23 rock wool or R21 fiberglass, not R19s or low density R22s.

    Using reclaimed roofing foam from commercial building demolition & re-roofing is substantially cheaper than virgin stock polyiso (a 65-75% discount!) , but should be derated to R5.7/inch for application, stackup, and climate. With 6" you'd then be at R34 (closer to code-min performance than you might think on a U-factor basis( and with R15s in the rafters you'd be at R49 center-cavity, but would have higher performace than code, due to the fact that the R34 is not thermally bridged by rafters.

    There is a foam reclaimer in your neighborhood (Kent) with reasonably priced goods in good to excellent shape:

    ^^^ $22/sheet for 3" polyiso is "in the range". In my neighborhood it's typically anywhere from $15-25/sheet. It can be used to insulate the basement too.

  3. Jeremy Sharp | | #3

    Steve, thanks for the words of caution. Residential rock wool is generally not known to contain asbestos-I will have it tested before beginning work. Thanks!

  4. Jeremy Sharp | | #4

    Dana, Pro tip on the reclaimed insulation in Kent-will definitely be in contact with them.

    What do you think about forgoing the insulation under the roof deck and adding another 3" layer above the deck? 9" at R5.7 = R51. Cost wise is a wash (or cheaper) and would be much easier to install on the outside as some of the interior areas are very tight to get into. Thanks!

  5. User avatar
    Dana Dorsett | | #5

    The awkward length & expense of the 10.5-11" pancake head timber screws for mounting 9" of foam with 3/4" nailer deck on a pitched roof gets a bit out of hand, and the aesthetics of a 10" fascia board can look a bit odd. At 6" of foam you're only looking at 7.5-8" screws, which is do-able.

    A mere 6.5" of R5.7/inch foam guarantees you'll hit code min on a U-factor basis, no matter what you have for a framing fraction on the rafters.

  6. Jeremy Sharp | | #6

    Dana, I have attached a revised drawing. Just want to make sure I understand you correctly. 6.5" polyiso above the deck-no insulation under the deck. Thanks for you help and quick responses.

  7. User avatar
    Dana Dorsett | | #7

    It's better (= more air tight) if you use the self adhered membrane over the plywood you're adding to the skip sheathing. It's fine to use #30 felt under the shingles above.

    The skip sheathing can sometimes be the achilles heel for air leakage at the rake of the roof, even with well sealed plywood over it. If there's enough room to work with to apply a shot of 2-3" of open cell foam on the underside to seal it all up it may be "worth it", adding a bit of R. As long as you're not putting fiber insulation in the rafter bays that decision can wait until you've proven how leaky it is or isn't with a blower door. You can also just chase the leak points with a FrothPak kit or similar, rather than foaming the whole shebang.

    BTW: Most dry blown cellulose runs R3.7/inch these days, some dense pack runs R3.9/inch. So a milled 2x4 cavity would be more like R13-R13.7 rather than R12, not that it makes much difference in the "whole wall R".

    Code max U-factor is U0.026, which is (1/U=) R38.5 "whole assembly". With 6.5" of R5.7/inch polyiso (typical per inch rating applied for commercial roofs in stackups where polyiso is the only insulation) you have R37 of polyiso, to which you get to add ~R0.5 for each layer of half inch plywood, which brings it up to R38. You get another ~R0.8 for the combined interior & exterior air films, and another R0.5 or so for the shingle layup. The skip sheathing adds something too. You'll be > R40 (= < U0.025) with comfortable margin on the U0.026 max. A competent P.E. would be able to document the U-factor calculations in certified-document detail, if the inspector really cares. If it comes up short it may be cheaper and easier to just add "contractor roll" R13 batts between the rafters to get there rather than going for deeper foam.

    Your foundation detail shows a step-out /shelf before it drops to slab level. It's still worth insulating over that shelf and down to slab level (or an inch orr so above the slabe) with R15. Just be sure the cut edges of the polyiso don't rest directly on concrete- some sort of capillary break is necessary. (EPDM flashing tape over the cut edges would surely do it.)

  8. Jeremy Sharp | | #8

    Thank you Dana. This is fantastic information. I've got an inquiry in with my structural engineer and we'll go from there.

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