Greetings All,

I've read many of the great articles concerning insulating this style of home (usually in the context of retrofit) but being a visual guy, I drew out what I "think" I understand about the subject, and wondered what everyone's thoughts were on the subject.

First of all, the design under consideration here is the typical 1.5 story "Cape Cod" style, which in my area (NH) almost always has a shed dormer on the back.  They almost always have two "doghouse" dormers on the front, too - but adding those seems like it would make it even *harder* to insulate properly.  This design always has minimal or no overhangs as well, to reduce wind grab in the event of Nor'easters, blizzards, hurricanes (rare), etc.

My thoughts:

- Instead of starting with a gable roof and adding a shed dormer, just make the shed dormer on one side full length.  (Turns the design into a "saltbox" I believe)

- Frame with 2x6 on 24" centers walls, single top plates.  Fill with spray foam, then rigid foam panels, then horizontal nailers for drywall.  Vapor barrier is either provided by the rigid foam board, or would go on top of it.  (Right?)

- Frame rafters and ceiling joists/floor joist/collar ties with 2x12, on rafters add in a nailing strip and another layer of (thinner) sheathing 3" from the exterior surface, glued/caulked/screwed in place.

- Break with tradition and add in 1' of overhang, divide soffit space so that an air channel is preserved, but that spray foam can be placed in the eaves.

...at this point, I thin a drawing helps to explain - blue shows cold air space to vented attic, green shows where the foam would go:



I think some blocking near the transitions and some more foam might help too.

The question:  Is this reasonable?  Crazy?  Looking at it entirely the wrong way?

Thanks for your input,

-John-Eddie

Oh - meant to add that since the infamous "knee walls" aren't load bearing, I'd think they could be addedd after the walls were insulated as described, moving them into the conditioned space.

Losing the doghouse dormers is the right idea- valleys are hard to air-seal, accumulate snow, and become heat sources for starting ice damming issues.

Rather than a vented attic, go with exterior roofing foam (rigid foam) with a vented nailer deck. There are commercial nailbase iso or EPS panels available from several vendors (Hunter Panels, Atlas, etc), but a DIY version using 2x furring atop the foam through-screwed to the structural rafters 24" o.c. with pancake head timber screws on which an OSB nailer on which to mount the roofing. The vent space keeps the OSB nailer deck dry, just as a vented attic would keep a structural deck dry.

In NH you'd want at least half the center-cavity R value to be exterior foam in order to keep the strucural roof deck warm enough to not incur wintertime moisture accumulation without using interior side vapor retarders (standard latex paint is enough.) With 2x8 rafters w/blown cellulose or open cell spray foam you'd hit ~R25, so you'd need about 4.5-5" of polyiso on the exterior, which would put it slightly above the IURC2012  R49 code for NH (climate zones 5 & 6).  Going this route is significantly higher performance than code-min using other methods, since there is R25 foam thermally breaking the R7-R11 (depending on depth) rafters.

Even virgin-stock rigid polyisocyanurate is cheaper than 2lb foam per unit-R when installed on simple-geometry flat surfaces (for minimal fitting & installation labor) but there are several vendors of reclaimed roofing iso in New England trading in goods culled from commercial re-roofing  & demolition that reduce the $R cost to roughly that of high density batt insulation. (Insulation Depot in Framingham MA is one, but you can find others advertising on craigslist & elsewhere.)

Even at 24" o.c. framing, closed cell spray foam in the wall cavities would be a waste, and would barely make R17 for the "whole-wall" average performance, even with 5" of R6/inch foam.  For the same wall thickness you could use 2x4s  16" o.c. with open cell foam or cellulose fill, and 2" of iso, and you'd be at about R22-R23 whole-wall for less money.  The structural sheathing would then be plenty warm enough to avoid wintertime moisture accumulation.  If you make the foam continuous with the roofing foam you will minimize thermal bridging.

Air seal all of your roof & wall sheathing to the framing with acoustic sealant or construction adhesive as you go- it'll be as tight or tighter than a spray-foam solution.  If using spray foam for cavity fill, the high expansion rate of open cell foam yields a slightly tighter wall than with closed cell.  (see:  http://www.greenbuildingadvisor.com...ance-walls )

Thanks - that is a much more comprehensive response than I had hoped for!

I'll check out the materials you've mentioned and let the concepts sink in.

Thanks again!

-John-Eddie

You may find this bit o' bloggery (and the numerous embedded links to further details) useful to think about too.