Rigid foam board insulation
You won`t get R4 per 1/2" out of any board I know of, R7.2" new and
stabilised at R6+" is the best Polyisosyanurate gets. I covered my
house in 2" polyiso for R 14.4. Can you replace siding on a soft surface
and not over nail it making it uneven? I covered mine in Osb.
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