Spray foam installation on underside of roof sheathing–existing house
We own a circa 1920s renovated farmhouse in eastern Nebraska. Over the years we have been making improvements to the home’s performance. This summer we finally knocked a hole in the second-story ceiling to get a look at the attic space for the first time. The second story was always bitterly cold in winter and extremely hot in summer.
What we discovered was haphazardly thrown-in batt insulation on top of 1″ thick 1920s insulation (black on one face). In a large portion of the roof, only the 1″ thick insulation was present. The second floor is not a real second story, so the walls slope due to the gabled roof. Where the walls are sloped, only 3″ of space exists between the sloped ceiling/wall and the roof deck, and even this is broken by the rafters and protruding roofing nails.
The spaces available are so small (especially that 3″ space) that the “standard” techniques of blown fiberglass fill and ventilate are really not an option to achieve the R-values that we desire. We were shooting for R-38 to 40 with the blown-in fiberglass, but would be willing to accept a lower “R” with foam with the understanding that reducing infiltration boosts the actual performance as compared to loose insulation.
So now we are thinking of tearing off the ceilings to allow clear access to the underside of the roof decking, eliminating any ideas of ventilating the attic space, and having a contractor install several inches of open cell foam to the underside of the roof.
1) Should we install a vapor retarder against the decking before the insulating foam?
2) What about the protruding nails? Can we snip these shorter?
3) Whats a reasonable number of inches to install on the underside of a roof in Nebraska, both from the standpoint of any limitations of the foam and reasonable R-value when using spray foam.
I thank you in advance for responses–cold weather is coming soon, and I think giving that 1920’s stuff an R-value of 1 would be generous!
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