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Conditioning an existing attic in a 1900s Queen Anne wooden home

truluck | Posted in Plans Review on

conditioning an existing attic in a 1900s queen Anne wooden home.
Location: coastal South Carolina
two story wooden home built 1901
attic dominations: 22’x46’x(10’at peak of roof)
planed usable newly conditioned attic space 10’x46’
2 gable end vents & ridge vent.
no gable end sheathing or sheathing on first or second floors.
roof is a standing steam installed over asphalt shingles.
rafters are true 2×4 and roof decking is true 1×6 tongue and groove.
HVAC for upstairs is located in the attic with duct work through ceiling of second floor space.
reinforce floor in attic with 2’x12’s joist because of 14′ span to middle load bearing wall in 22’x46′ size attic.
spray foam with open cell 4” between rafters and 4” silver glow insulation on gable ends.
Close 2 gable end vents with glass.
Sheetrock walls gable ends and carpet entire attic floor.
The access to the attic is a new staircase that has no door and will leave attic space open to second floor.
We plan to replace exiting gas furnace with 3 ton split unit heat pump that will be located behind knee wall in attic. The return will remain in second floor stairwell and vents will remain for 2nd floor and be ducted between 2’ x12’ floor joist in attic with additional vents for attic space.

Not sure what to do with ridge vent? Should we spray foam over and just sheet rock at coiler ties, leaving 4” space at ridge and vent?
Should I use open cell or close cell spray foam and do you see any other possible problems with my plan to condition attic space?

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  1. GBA Editor
    Martin Holladay | | #1

    Lots of questions. You are located in Climate Zone 3. The 2012 International Residential Code requires that a roof assembly in your climate zone have a minimum R-value of R-38. Your plan to install 4 inches of open-cell spray foam results in an R-value of only R-14, which is about 1/3 of the insulation you need. So that's not a very good plan.

    Here is a link to an article that explains your options for insulating your roof: How to Build an Insulated Cathedral Ceiling.

    Your choice to use SilverGlo (a brand of foil-faced EPS insulation) to insulate your gable walls is unusual. Since your walls have no sheathing, you might want to read this article: Insulating Walls in an Old House With No Sheathing.

  2. Expert Member
    Dana Dorsett | | #2

    With 10' of head oom at the peak there's sufficient head room to build on a gusseted truss to the existing rafters deep enough to allow 10" of open cell foam under the roof deck. If you use 0.7lb foam that's enough to hit R40. It would have to be applied in two lifts of 5" to be fire-safe during the curing of the first lift.

    If you rip a sheet of half-inch OSB 10" wide and cut it into 10" lengths you can then nail & glue the 10 x 10 gussets to the side of a 2x4 (or finger jointed 2x3), every 4', then nail and glue it to the side of the existing rafters you'd then be able to hang the ceiling gypsum on the new truss chords. To get it in in the truss would often have to be cut into two pieces.

    In a zone 3A climate you'd want to cover the foam with half-inch gypsum and paint it with half-perm "vapor barrier" latex primer to mitigate daily moisture cycling into/out-of the foam and roof deck during the summers in an air conditioned attic.

    Build out the kneewalls AFTER the gypsum layer is installed. You'll probably have an easier time of it installing the air handlers & ducts before installing the kneewalls too. Just be sure to locate and size the service access to that space such that it's at least possible to replace/repair the air handler.

    You definitely DON'T want to spray open cell foam direcly on the back side of siding that has no sheathing. Ideally you'd have at least 1/4" of air gap between the siding and the next layer, and apply the spray foam to that layer. Using 1/4" fan-fold XPS siding underlayment for both spacer strips and the spray-apply-to layer is a nearly ideal product for the application, since it functions as the weather resistant barrier, and is sufficiently rigid to handle the expanding foam.

    Adding 4" of EPS to the foam-filled gable end walls is overkill in this climate. With 0.7lb foam even 3.25" plus an inch of polyiso would more than meet code minimums, and with 1.5-2" of polyisocyanurate it would be pretty much at or past the financially rational limit of where rooftop solar to offset heat pump use becomes a better investment. If you want to use EPS instead of polyiso, anything more than 2.5" is probably beyond the limit. From a $/R point of view polyiso and EPS are usually very comparable, but polyiso eats up less interior space.

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