Retrofit exterior foam wall assembly: Sanity check
First a quick thank you to all that contribute and answer here. I have been reading GBA for some time and you all have helped and inspired me greatly.
I am located in Central Oregon, Zone 5. My existing house is about 30 years old, uninsulated slab on grade, 2×6 framing, OSB sheathing, perforated house wrap, fiberglass insulation, horrible air sealing, vinyl dual pane windows and a decked unfinished second floor/attic (so limited insulation depth and air sealing access.)
The building footprint is about 2200 SF with 1150 SF of living space, attached garage space and an unfinished full second story attic.
I plan to install Tyvek Drainwrap (Mind the Gap, Eh!), 3″ R-Tech Faced Type I EPS on the exterior, new windows and doors with innie details and a 3/4″ thick rain screen to attach the metal standing seam cladding along with LP or wood lap siding.
On the interior, I originally planned to flash and batt primarily for air sealing. Now I would like to install dense pack cellulose even though I have been unable to rent equipment or locate a contractor to do this in my area. Fiberglass, blown and batts seem to rule here. I may have to purchase equipment to do this which isn’t ideal but I feel comfortable performing the work.
Does this sound like a reasonable wall assembly, are there more economical or effective ways to achieve a similar higher-performance envelope?
Given the situation, is cellulose worth the extra effort and cost over fiberglass? I favor the “green” value, gapless blown installation, fire and pest resistant properties, and near air barrier performance. However, batts would be cheap and easy to self perform.
Exterior foam, foil face in or out? I feel like the solar gain in winter is worth more than keeping it out in the summer. With this much insulation is it even going to matter?
The second story space is fully decked with 3/4″ T&G plywood except for about three feet that is open along most of the eaves. The decking makes air sealing the ceiling fixtures and wall top plates challenging as well as limiting insulation depth to the 2×12 floor joist depth. The open insulation along the eaves has accumulated much dirt and attracted mice over the years. I plan to replace the decking in these areas to protect the insulation and reduce air flow through the ceiling assembly. The eave vents are still open to the attic space. Any issues or opportunities I am missing here?
Is properly detailed Tyvek Drainwrap an adequate air barrier or should I “belt and suspenders” with taped foam or sheathing seams, airtight drywall, and/or spray foam flash the interior cavities? I considered a self-adhered membrane wrap, which would be much easier to install working solo, but went the drainwrap route for a cost-effective WRB and drying gap combo (Mind the Gap, Eh!) between the exterior foam and sheathing. I like things that serve multiple functions.
Are there other options or strategy to consider for slab insulation? The slab is uninsulated but with termites, existing damage in the house, I am very hesitant to install perimeter foundation insulation and create a hidden pathway into the house. Interior ceilings are already too low for my preferences at 8′ nominal so I wouldn’t want to change that much.
Thanks in advance!
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