Exterior rigid insulation over existing home roof and walls
Hello, I have a question pertaining to exterior eps rigid foam insulation. I own a fixer upper home in the 3300 ft elevation range of the mojave desert in Ca. (not certain on climate zone, about a dozen days a year in the 32F range and about 2 dozen days in the 100F range). The house was built in the mid 1960s with zero insulation. My intent is to insulate the entire exterior of the home with 4’x8’x1″ styrofoam sheets. I have free access to these and keep them out of the landfill by using them.
The walls: 2×4 stud, tar paper, chicken wire lath, stucco (no sheathing). The roof: 2×10 16 on center, 1/2 plywood, tar paper, low slope asphalt roll roofing 1st layer. Both walls and roofing good/very good condition.
My question is, can I just go right over the existing outer layers with eps?
This is my plan:
The walls: 4in stagger seamed sealed foam, 9ft tyvek home wrap, 1/4in thick by 1in wide reclaimed flooring strips,thereby 1/4in outspaced lath, attached with 6in screw through the existing stucco to 2×4 stud, then new stucco exterior. No drainage plane, no rain gap. The current wall doesn’t have one. Desert = not much rain plus uber rapid evaporation. The window sills sloped and flashed to drain correctly, which along with the doors will now be “innies” which I love.
The roof: follows the same pattern as the walls except: more eps 7in? 11in? (depends on reclaimed eps supply I have), tyvek, 1/4in thick light weight version cement fiberboard(more rot/fireproof than sheathing),2nd tyvek sheet, all screwed down to the underlying joists. Now this next part is very non traditional, and may not be possible, but since everything above the existing plywood roof is water/rot resistant I would like to try it. Multiple thick coatings of elastomeric roof coating directly painted on top of the final screwed down tyvek sheet, then before the final elastomeric coating dries, adding a coarse silica sand to increase the uv protection of the elastomeric.
My main question here is can I exterior insulate over the existing exterior. The wacky idea of tyvek and elastomeric as a diy seamless roof is a separate question. Thank you
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