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I work on old houses as a carpenter-designer-builder and GC. More and more I see on other people's jobs and on mine as well, roofers putting Ice & Water Shield (Bituthane etc.) on the whole roof deck before applying slate or wood shingles. It is handy when a roof is complicated, has a low pitch, will be uncovered for a long time in winter and so forth, but is it a good idea? Is it not a vapor barrier on the wrong side of the insulation? Particularly if there is blown-in cellulose insulation, installed above existing plaster and therefore no interior vapor barrier, it seems to me that there is a possiblity of condensation on the backside of the roof deck. If it were possible to install soffit vents and ridge vents that might address my concern, but on many historic houses that is not possible, or on hip roofs or other awkward shapes. My roofer points out the 30# felt, which we would otherwise use, is also a vapor barrier. However I assume that punching it full of nail holes makes it permeable after roofing installation.
I am going to be installing a new wood roof on a house in Cambridge, Mass. that is a national historic and architectural landmark and am very concerned that I do nothing to jeopardize its future. Of course, installing a roof that does not leak or fail prematurely is part of this concern, as well as vapor worries. Any opinions, studies etc. that could help me with this?