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Cape Cod house: Insulation

bjvanst | Posted in General Questions on

We are in the process of renovating the upper floor of our 1.5 story Cape Cod style home.  We are having closed cell spray foam applied directly to our roof deck to fill the rafter bays (3.5″ depth).

I know that adding insulation over the roof deck is the ideal way to further improve the R-value of our roof but…

I will be cross-strapping the rafters with prior to hanging new dry wall and was wondering if there was an opportunity to add additional insulation under the rafters.

I was eyeing something like the RockWool ComfortBoard, continuous under the rafters in the upper attic and behind the kneewalls, and separated by the strapping in the sloped ceiling.

Is this something that can be done?  Or should I drop the idea?

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  1. GBA Editor
    Martin Holladay | | #1

    Combining closed-cell spray foam insulation with some type of batt insulation is done all the time. This is called the "flash-and-batt" approach.

    For more information, see "Flash-and-Batt Insulation."

  2. Expert Member
    Dana Dorsett | | #2

    Where are you located? Local climate matters.

    The fiber has to be in direct contact with the foam- an air gap is a thermal bypass. With 2x4 rafters the foam is really probably only about 3.25" deep on average, to avoid having to trim the foam- a full 3.5" just isn't very likely.

    At 3.25" you're looking at anywhere from ~R20 to ~R24 depending on the exact product. That's enough for dew point control for up to R30-R36 (depending on actual value of the foam) of fiber insulation in US climate zone 5, but only R20-R24 of additional fiber in zone 6, but only R12-R16 in zone 7.

    How deep is your strapping? What's the deepest that can be tolerated?

    Bonfiglioli strips on the rafter edges may be preferable to cross-strapping, which would allow the use of batts in a compression fit, with no chance of 1/4" deep bypass channels anywhere between the foam & fiber:

    1. bjvanst | | #3

      Thanks for the reply, Dana.

      I'm located just outside of Toronto. I believe that we are climate zone 5b/6a.

      The spray foam contractor uses Walltite Eco and in our conversation during quoting, it did sound as though they planned to scarf the rafter bays after installation. I'll have to double-check with them.

      The depth of the strapping was going to depend on whether I add any form of insulation under the rafters. If I was to use Roxul/Rockwool's ComfortBoard 80, which is 1.5" thick, I'd be looking to strap out with 2x3s. If not, I'd be using 1x3s.

      The decision to strap perpendicular to the rafters to simplify the drywall install, keeping deadwood out of the rafter bays as much as possible. It also provides me with some wood to secure the internal walls along the sloped portions.

      The linked PDF was interesting. If I fur out the rafters, deadwood would be kept out of the rafter bays anyways as it would have to be brought forward to the edge of the furring. I'll have to give that approach some further thought.

      With that said, and the information provided above, are there any other issues to keep in mind with my original idea beyond the bypass?

  3. Expert Member
    Dana Dorsett | | #4

    >are there any other issues to keep in mind with my original idea beyond the bypass?

    The relatively low performance of the roof for the climate is one aspect. It's not nearly enough total R to avoid ice-damming issues, etc..

    If going with 1.5" thick Bonfiglioli strips (1x strapping & 3/4" polyiso) it roughly doubles the performance of the framing fraction, which helps. Taking rock wool batts designed for 2x4 stud bays and splitting them into ~1.75" thick batts for a complete cavity fill is probably easier to do and higher-R than compressing 2" Comfortboard ( 1.5" Comfortboard may leave voids.)

    According to the datatsheet ( ) 3.5" Walltite Eco runs about R20.5, compressed split batts adds another R6.5 so you'd be at about R27 at center cavity. That's about half what the IRC code minimum. But the framing fraction will be about R10, about the same as a 9.25" deep 2x10 rafter, so from a "whole assembly R" point of view it's comparable to R30 rock wool between 2x8 rafters.

    That's not terrible for the zone 5/6 boundary, even if it's below code min. The thermal break over the rafters will make a difference in ice damming potential, even though ideally it would be higher performance than that.

  4. bjvanst | | #5

    > The relatively low performance of the roof for the climate is one aspect. It's not nearly enough total R to avoid ice-damming issues, etc..

    Well, that's discouraging!

    If one was to go the Bonfiglioli route, would you suggest running the foam layer continuously from where the rafter meets the top plate to the peak? Under the knee wall top plate as well? And what of the rafter ties/ceiling joists?

    I will admit that I'm starting to feel like I'm in over my head but I do appreciate your time and input. You've identified a lot for me to learn about.

  5. GBA Editor
    Martin Holladay | | #6

    According to most climate zone maps, Toronto is in Zone 6. Here is a link: "Climate Zone Map Including Canada."

    In your climate zone -- cold in winter -- you need to aim for R-49, which is the code minimum in Zone 6 in the U.S.

    Achieving R-49 in the roof of a Cape Cod house is challenging but possible. For a list of all the different ways you can achieve that goal, see "How to Build an Insulated Cathedral Ceiling."

  6. Expert Member
    Dana Dorsett | | #7

    According to the climate statistics for the past 25 years Toronto is zone 5. It used to be zone 6. Only 6 out of the last 25 years have have nosed over the 4000HHD(C) base 18C mark (the definition of where zone 6 begins) , and only one out of the past 20 broke that mark. The 25 year average is about 3800, definitely zone 5.

    It's the same R49 roof for zone 5 in the IRC though.

    With 3"/R20.5 WallTite you can add up to R30 of interior side fiber insulation without running into wintertime moisture accumulation problems with no need for an interior side vapor barrier. At the cold edge of zone 6 it would be prudent to stop at R20, but I suspect you're not really going that deep.

    Running the roof deck foam and Bonfiglioli strips all the way to the top plate of the exterior walls is the right way to do it. Air sealing and insulating kneewalls and joist bays crossing under the kneewalls properly as a retrofit is something of a fools-errand- very time consuming. (I've starred as the main "fool" in that movie multiple times. :-) ). With the roof deck insulated the R-value and air tightness of the kneewall and joists below it become irrelevant.

    Making the Bonfiglioli strips deeper, or 2x4 or 2x6 framing lateral to the rafters deep enough to add R15-R23 batt insulation on the part of the roof behind the kneewall wouldn't take up any living-space headroom, and would be "worth it" on a performance basis. In the cathedralized ceiling part it's a trade off between efficiency and headroom, but at whatever depth, installing fiber insulation to fill the gap from the foam to the gyprock is well worth it. With 2" polyiso foamboard and 1x furring it would be the right depth to accommodate 3" sound & fire protection rock wool batts (~R12-R13), and the framing fraction would be higher-R than 2x12 rafters due to the polyiso. Even 1.5" polyiso would be a huge improvement. Compressing the 3" thick fire/sound batts into the space would run R10-ish, still a very real improvement over just the R20 WallTite.

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