Ice & Water Shield on whole roof
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?
Henry
Replies
I installed the water guard on my entire roof and I was very concerned about the points that you mentioned. In particular, the vapour barrier topic. I solved the problem by ensuring that I had LOTS of ventilation. I have one square foot of ventilation for every ninety square feet of ceiling. I don't have soffit vents or a ridge cap vent.
Taunton Press has a "For Pros by Pros" book on shingling. The author discusses ventilation and comments that too little ventilation can cause problems but too much ventilation never seems to have an adverse effect. I went with the later.
Your problem may be figuring out how to install vents on a historic building that pass architectural review...
I had my vents built to fit between the roof joists out of stainless steel.
Douglas T in Saskatoon, Saskatchewan, Canada
Hi; Given your location, if you went through a winter with no condensation problems then I assume what you did has worked. My understanding is that unless you have a high and low intake and outlet, respectively, that simple holes through the roof deck move very little air. It also sounds as if you have an attic space under the roof. The situation in my job is that the top floor is fully finished and there is no possibility of air flow under the roof deck.