R-Value recommendation unvented low slope zone 6 roof
For Zone 6, maine, an unvented low slope 1:12 roof using flash and batt of closed cell (51%) and dense packed cellulose 49%, with the code changing to R60 soon, what is recommended on this site as a goal to shoot for for an Rvalue? I know there is a diminishing return at some point and the problem with my roof is that I have to keep adding spray foam which is costly in order to keep the 51% ratio….but want to make sure its not a week point in structure and I want to make sure the engineers use trusses big enough for what I need but not too big that its wasted space. I have already explored other roofing options and flash and batt is what works here. The walls at double studded 12″ DPC R42, on a slab of R20, Logic windows but there are a lot of larger windows on the north side (lake) which is an obvious week point but cant change that.
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R49 is pretty reasonable in terms of overall energy loss, but R60 is obviously a bit better in that regard. If your goal is to keep costs down but still be reasonably efficient, I'd go for the R49 code requirement. If your goal is higher energy efficiency, which I'm guessing is the case here due to your R42 wall design, then try for R60.
If you're thinking "will I ever see a noticeable difference in energy consumption between R49 and R60 over the life of my home?", then I'd say probably not. In terms of overall difference in energy losses over your entire home, I'd expect that difference between the roof being R49 and the roof being R60 will likely be a few percent at most.
Bill