Wall assembly question
Good morning,
I’m planning a small ‘pretty good’ house in southern New England, technically and codewise a zone 5, but being on an island offshore, more practically a marine zone 4.
My initial wall design is as follows: cedar shingles, 15# felt paper, 1/2″ CDX, 2″ polyiso, taped zip sheathing, 2×6 frame with dense packed cellulose, blue board/plaster, airtight, latex paint. Seems like a good economical easy-to-build wall, maybe approaching an R-28 or so. An experienced passive house builder has suggested upgrading to a 2×8 wall, with or without staggered studs, for an extra R-7. Questions:
In the big picture, is the extra r-value worth the increased cost and complexity of the build, and the loss of interior dimension? And, would the foam/fluff ratio then be wrong?
Thanks for any insights. If this goes well, I’ll proceed to the real fun- roof design!
Best,
John
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Replies
John,
I disagree with the passive house builder. By far the best and most cost-effective way to improve the thermal performance of your wall is to make the rigid foam layer thicker -- that is, to go from 2 inches of rigid foam to 3 inches or 4 inches of rigid foam.
The question of whether additional R-value in your wall is worthwhile has too many personal factors for people on this forum to give you much insight. It depends on how much you value energy savings, climate impact, interior space, etc.
If you are reasonably proficient with Excel, I suggest making an energy model of your house. Make things like wall R-value, ceiling R-value, window U-value, blower door test result, etc. be inputs as opposed to hard coding them in formulas throughout the spreadsheet. Then, you have a good tool for answering the question "If I am going to improve something, where should I spend my money." It will also be a good way to check whether your manual J is reasonable.
At 16" o.c. stud spacing (framing fraction ~25%) your current stack up yields a whole-wall R a hair north of ~R26.5, if derating the polyiso to R5.5/inch for climate averages, or R25.5 if derating it to R5/inch (excessive, for a zone 4-ish climate).
Adding another half inch of R5.5/inch polyiso would bump you it to over R29, adding an inch it would move to over R32.
Going to 2x8 studs and leaving the polyiso at 2" would bump it to just shy of R31 while adding 1.75" to the wall thickness.
The advantage to thicker foam is warmer average temps at the sheathing, for higher dew point margin, and less moisture cycling of the cellulose. With R26+ cavity fill and R10-R11 on the exterior it meets the ~27% exterior R-ratios found in the IRC chapter 7 prescriptives for zone 5, but with no margin. With R13.5+ on the exterior of R20 cavity fill you have a lot of margin.
Of course you could hire a qualified energy nerd or a P.E. to run a WUFI simulation on the 2x8/cellulose + 2" polyiso stackup to prove that it works since you're on the warm edge of zone 5, but the money spent on the analysis is probably better spent just improving the ratio with a half inch to inch thicker foam. The napkin math models such as R-ratios don't tell you what's really going on or with any precision the way a competently done WUFI sim might, but since the dumb math model gets you within a few percent of the "right" answer on a stackup this simple, does it really matter?
This is a great site and resource! Thank you so much for your advice and insight, exactly what I needed. It seems pretty obvious now, both from an energy and a construction standpoint, that I should add foam and keep the simpler framing and interior space. I'll do some more investigation of foam availability and pricing- if I'm de-rating the polyiso for cold temperature, I should look at eps instead for its other benefits.
My next issues will be about roof design, after I consider how to distill it into coherent and answerable questions.
Thanks again,
John
I'm going to jump the gun on your roof design. Distilled, the one word you should be looking for is "vented". I read articles and Q&A's here every day about how to fix problems with non-vented roofs. It's possible to design and build durable and cost-effective vented roofs, but it does not appear common.
Long answer: see Martin's article "10 Rules for Roof Design" that has distilled answers to the questions that you are likely to ask.
P.S. Make your overhangs bigger.
Designing and building a PGH should be a great experience. Have fun and good luck.
Yes, Andrew, that's a pre-emptive strike! I appreciate that, and am grappling with the issues. If I owned a lot in the hinterlands, and my only requirement was a PGH or better, I'd design a simple gable roof with only two eaves, two planes, and a ridge. But there are other factors in play that I can't ignore....
Thanks,
John