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Attic insulation in older home without raised heel trusses

TRIADProperties | Posted in General Questions on

Renovating a home in Zone 4.  Rafters are 2×4.  Built in 1972.  Rafters rest directly on sill plate top of 4″ block wall with brick veneer.  Low slope roof, 5/12 (22 degrees).  Tallest point is 5′ inside attic.  There is no raised heel.  Interior gut job.  Owner wants to keep existing rafters, replace plywood roof, underlay, shingles.  Looking to put insulation at roof line.  5″ nailable polysio insulation on top of 2″ polysio (total of 7″ insulation – R38), then 5/8″ zip systems OSB on rafters; blown-in cellulose (3.5″) interior between rafters with netting.  This to achieve R49.
Thoughts on this detail?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >" Looking to put insulation at roof line. 5″ nailable polysio insulation on top of 2″ polysio (total of 7″ insulation – R38), then 5/8″ zip systems OSB on rafters; blown-in cellulose (3.5″) interior between rafters with netting. This to achieve R49."

    With continuous R38 polyiso up top (not thermally bridged by rafters) it already meets code on a U-factor basis without the cellulose. The code-max ceiling U-factor for zones 4 and higher is U0.026, which is (1/U=) R38.5 "whole assembly".

    The nailer deck adds another R0.5 or so, the shingles & underlayment maybe another R0.3, the ceiling gypsum another R0.5. The air film on the under side of the roof deck another Ro.5, the air film under the ceiling gypsum another R0.6. It's dead-easy to clear that the U0.026 hurdle without rafter-cavity insulation with margin when there's 7" of polyiso up top.

    If you ARE going to go with rafter bay insulation, kraft faced R13 batts or even unfaced R15 rock wool or fiberglass batts may arguably be "worth it", but the installation cost of cellulose in netting would be unduly expensive, with little marginal benefit due to the thin depth of the thermally bridging rafters.

  2. TRIADProperties | | #2

    Thank you so much for detailed response. Just to clarify, to meet code and pass inspection (IBC 2012) in my location), I just need the assembly above the rafters and not the rafter bay insulation? Appreciative of the tip for rafter bay insulation :-). Big savings.

    1. Expert Member
      Dana Dorsett | | #3

      >"Just to clarify, to meet code and pass inspection (IBC 2012) in my location), I just need the assembly above the rafters and not the rafter bay insulation?"

      Assuming it's residential, under the IRC 2012 the R49 or U0.026 prescriptives are the same as in IRC 2015 or IRC 2018. See TABLE N1101.1.1 and TABLE N1102.1.3 respectively (a bit more than half way down the page):

      https://codes.iccsafe.org/content/IRC2012/chapter-11-re-energy-efficiency?site_type=public

      It's even less stringent for commercial buildings.

      1. TRIADProperties | | #5

        Thanks for references Dana.....yes it is residential. Appreciate you guidance. See no need from code perspective for rafter insulation.

      2. TRIADProperties | | #6

        One more question.....with baffles and vent at eave. Since I have no insulation at floor, do I only need to take the baffle up the inside ceiling of attic a few feet and stop? House is basically a rectangle. Eaves run on the long sides of house. Also, I plan on having rigid roof ridge (12") with filter at the ridge and since there is no beam at ridge, I will be stopping the insulation above roof deck and decking 7/8" from peak either side. Will that work?

  3. walta100 | | #4

    I would avoid the insulated roof unless someone made the poor decision to put the ductwork and or HVAC equipment in the attic.

    When you move the insulation from the ceiling to the roof and gables and keep the R value the same the house has 20% more surface to lose heat and will use 20% more heat.

    The foam insulation on the roof costs more per square foot.

    If you have the plywood off you could spray foam the ceiling where you do not have the space for cellulose.

    From a price point of view new trusses may cost less than 7 inches of new foam and a nail bed.

    Walta

  4. TRIADProperties | | #7

    Hi Walter....thanks for taking the time to response and recommendation. We really don't have a choice but to have the system for the upper level of house in attic. The ceiling height in basement is 7'-8" to underside of floor joist above. Ceiling height to underside of rafters on main level is 8'-2". With ducts in attic space problematic with temperature in summer. Will run numbers to compare cost of new rafters to insulated roof. Thanks.

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