Am I creating a double vapor barrier roofing disaster?
Hello everyone, I've been researching about green building and building science and trying to learn the lingo for a couple of months now. I'm working on a renovation to a ground floor addition on the north side of our house.I live in a cold climate in south eastern ontario according to the hygrothermal map at building science corp: (http://www.buildingscience.com/doctypes/enclosures-that-work/etw-buildin...). The IECC climate zone map is for the USA, but I believe I'd fall into Climate zone 5 and not 6 (http://apps1.eere.energy.gov/buildings/publications/pdfs/building_americ...).
The project goal is to reroof and reinsulate a ground floor addition on the back (north side) of our two story house. We started the project because the roof has had problems leaking since day one (20+ years ago) due to an improperly installed skylight and lousy roofers (why bother with flashing against the house? just butt it up and tar the sucker!). The roof is an insulated unvented shed roof which slopes down from the second story to the north side and on the interior it's a cathedral ceiling (half of a cathedral? term unknown). The addition was initially insulated with pink fiber glass batts in both the ceiling and walls, vapor barriered and finished with drywall.
The existing steel roof, the large skylight, and the shingles underneath the steel roof are going to be removed so that rotten sheathing can be replaced, the skylight area sheathed over and a new steel roof installed. My plan for the inside was to insulate with closed cell spf (BASF WAllTite Eco, tech specs: http://walltite.basf.ca/FoamMasters/English/DownloadCenter/DLC%20PDFs/06...) to a depth of 6" if we can afford it. My understanding is that spraying ccSPF to this depth means that it will act as a class II vapor retarder (between 0.1 and 1.0 perms).
My main concern is with the installation of the synthetic roofing membrane under the steel roof in combination with the ccSPF. The steel roof materials are applied in this order: 5/8" sheathing > synthetic roofing membrane > 1x4 strapping (a ventilated space correct? don't want ice dams!) > 29 or 30 gauge steel roofing
The synthetic membrane proposed is a product called Resistor (Tech specs: http://www.resisto.ca/openfile.aspx?Id=299) which has a Perm rating of 0.5 Perms. This in effect is also a class II vapor retarder. My understanding is that I would effectively be placing a class II vapor retarder on both sides of my 5/8" sheathing which is BAD. From what I've read from Lsitburek and Straube and comments on here again and again is that the assembly needs to dry in at least one direction. The sheathing in this scenario can technically dry in both directions but in a very limited way. Now I've also read (from those same fine folks at building science corp) that unvented roof assemblies with ccSPF on the underside of the roof deck, paired with a perfectly installed vapor semi-impermeable roofing membrane work well - no problemo! The crux of the problem is the workmanship needs to be PERFECT, and the membrane mustn't fail and the roof sheathing needs to essentially never get wet since it has a very limited ability to "breathe". For me this perfect storm of excellence seems unlikely.
Also, I cannot insulate on the roof deck because of clearance issues with the windows on the second floor of the house. If I raise the roof deck with insulation, the roof is going to hit the window sills or worse, the glass. My insulation needs to be in between the rafters.
My gut instinct is that I need to install a vapor semi-permeable class III vapor retarder as my roofing membrane (between 1.0 and 10 perms) OR a vapor permeable Class IV vapor retarder (this option seems very strange to me). Am I nuts? Do I need to hang my head in shame because I've missed something so blindingly obvious? Should I bulldoze the addition into the adjacent field and return to my regularly scheduled life? I apologize the explanation is so lengthy, I just hope I've covered all the relevant information.
Posted Thu, 01/16/2014 - 14:26
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