"real" R-value is unimportant here, effective is what matters but most engineers can't figure out how to do a heat loss with only masonry. They grossly oversize the boiler. My double brick house has a lower cost to heat than the average 2x6 stud wall construction with fibreglas insulation so that makes it an "effective" R20 or so. The real insulative value of a 2x6 stud wall with air barrier and vapour barrier is about R13, if you are lucky.
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