Most of the people developing vented cathedral ceilings use the interior drywall as their ceiling air barrier. The regular way to generate the transition among the exterior wall air barrier and the inside ceiling air barrier is to put in a broad piece of plywood in addition to the wall's top plate.
You're right that this type of roof assembly is not going to dry to the interior. But it is not purported to. I would tape the polyiso -- the tape will not trigger any complications.
Should really I deal with the flat roof as a cathedral ceiling and select spray foam (or board), fiberglass, and no moisture barrier?
During the absence of the airtight ceiling, roof ventilation can do much more damage than very good, since air motion in rafter bays can persuade indoor air to leak as a result of ceiling cracks.
Code does not look to apply because it's grandfathered in. It looks like I ought to redo the hall ceiling.
Insulated cathedral ceilings are a comparatively the latest phenomenon. The trend for insulated cathedral ceilings (and great rooms) really took off within the seventies and eighties, when examples started appearing like mushrooms after a warm rain.
Code interpretations are subject for the dedication of your neighborhood constructing inspector. In the long run, it won't make any difference what I believe -- what issues is what your local code official thinks. So check with him or her.
Having said that, if your neighborhood making inspector approves of your respective approach, and When you've got loads of your time in your fingers, and you also are able to executing a meticulous job of air sealing, it is possible to go ahead and check out the Minimize-and-cobble strategy.
Simply because a roof assembly won't Keep to the guidelines proven by the making code or suggestions of setting up scientists doesn't mean the roof has
Your plan to setup 8 inches of rigid foam is a superb 1, however you could possibly put in thinner foam when there is a compelling reason to take action. Needless to say, 8 inches of rigid foam has an increased R-benefit, and may execute much better, than the usual thinner layer.
I've a partial cathedral ceiling while in the finished attic of my home in zone 4 (TN). Just about every slope of the roof is divided about in thirds by three different scenarios: knee partitions in the outer thirds, "cathedral" ceiling in the middle thirds, and also a vented "attic" in the center 3rd which has a ridge vent - a typical scenario with concluded attics, a minimum of all over right here. I have a big issue in the summertime with overheating. The insulation is not Doing work well as it was improperly insulated with just R-19 fiberglass batts in between the rafters with out a air flow baffle or enough soffit venting and only r-19 blown into the "attic House". Furthermore, my upstairs HVAC ducts are run during the really hot "attic" House under the ridge. Based on what i haved realized from numerous articles or blog posts listed here, my prepared Answer should be to cathedralize all the roof, eave to ridge: install continous air-limited web-site-constructed two" vent channels employing foil-confronted polyiso board (as prompt in ), setting up continuous soffit vents to dramatically Enhance the airflow from soffit to ridge, view website making an air-tight ceiling down below the rafters, and, most importantly, deliver the ductwork In the conditioned envelope.
The code constraints on the use of air-permeable insulation between rafters were being designed to avoid the roof sheathing from rotting. When fiberglass batts are installed in unvented rafter bays, the batts allow for moist indoor air to get to the cold roof sheathing.
As your home flexes with alterations of temperature It can be probable, even likely that several of the foamed-in air sealing will leak. It might be well worth carrying out 2" of open cell foam on the inside aspect of which might be a far more adaptable and trustworthy lasting air seal.
2nd, can you please affirm this: "If you want to use only one type of insulation in unvented rafter bays, you're limited to spray polyurethane foam."