Should I adjust my wall plan to increase air seal/insulation performance and durability of my wall design?
My project is a single family home in the Seattle area (http://www.flickr.com/photos/48876863@N08/) Even though Seattle is rainy during the winter months, it has a dry season during the summer http://www.beautifulseattle.com/clisumm.htm. My goal is to decrease my heating requirement with proper ail sealing/insulation while maintaining indoor air quality with a HRV. I am heating with a closed system, concrete radiant. Outside: cedar shingles top, cedar planks on bottom. Taped Typar as weather barrier. ½ “ cdx sheathing. 24 ga Galvalume roof over Tri-Flex underlayment. Inside: standard framing, 2×6 walls, scissor trusses. closed cell flash on rim and band joist after rough-in, Blown-in-bib (fiberglass) exterior walls, sound proof batts interior walls, R38 scissor trusses, airtight drywall using EPDM gaskets and air sealing electrical switches and receptacles. I will limit can lights / holes to my attic where possible. This plan limits disruption to the construction process. Any suggestions to increase performance? Since my timeline to keep the house is long, should I flash and batt all exterior walls?
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Replies
Frank,
Your question is very open-ended. It's your house; invest your money where you want.
If you want to lower your energy bills, you can always improve your insulation. Many New England builders are specifying R-40 walls, R-60 ceilings, and triple-glazed windows.
Extreme insulation measures have a long payback, especially in a mild climate like yours. But many homeowners like the security of low energy bills for life. If you'd rather spend your money on something else, though, the decision is yours.
Martin
thanks for your response. Then from a bldg science perspective, my plan is sound? ... and then adding additional air sealing/insulation measures, like a a closed cell flash at the right depth, is something I should consider for even lower bills? My concern for the flash and batt coupled with air tight drywall is in making the exterior walls too tight (when moisture intrudes)...is this a valid concern considering I will be using a vapor retarder latex primer and HRV?
Frank,
I have heard of a few failures blamed on flash-and-batt installations. The failures were in very cold climates, and were due to insufficient thickness of the closed-cell foam. According to Joe Lstiburek, you should be fine in your climate zone with one inch of foam.
Your plan to allow the wall assembly to dry to the interior is a good one. Don't install interior polyethylene or vinyl wallpaper.
Frank
First off, well done in doing your homework, and it looks like you're doing fine. I'll throw in my two cents for improving the assembly only because your question asked exactly that. If you were to improve it, what might you do? Flash and batt has seemed to me to be useful in retrofits and tight spaces, or applications where a vapor barrier or air seal needs to be added to an assembly. In new construction, it would seem that you could achieve the same or greater performance with less cost. For example, if you strap the interior wall with 2x lumber (usually 2x4s ripped in half) then blow in dense packed cellulose. You would have the same R-value (R-27) as the flash and batt you mentioned above with the added benefit of a thermal break and a lower material cost. HOWEVER, you'd have to be prepared to loose an 1-1/2" all the way around your house as well as deal with the added depth for window and door trim and sealing details.
Likewise you could add exterior rigid insullation board to your assembly. There are numerous articles on this website (including details if you're a member) about doing that, but again you'd have to be prepared to handle the new details at the windows and doors.
Christopher
I have considered exterior rigid insulation but didn't go any further for the very reasons you've mentioned. I appreciate your response.