Insulating Rooms in Attic
We are building a home in SE Kansas (Zone 4A) with a bonus room over the garage and a little bit of bonus space of the adjacent part of the attic over the house. About 1/2 the house attic has no bonus space. I’ve been reading a lot of articles and comments on how to insulated cathedral ceilings, all about attic venting, etc and am still faced with a few questions. I would consider putting foam on top of all my roof sheathing but about 1/2 my roof is not over any bonus space and part of it is a large overhang in a vaulted living room that hangs over an outdoor deck which would be a waste of foam on top but would be needed since they share the same roofline. In the half of the house with no bonus room, my plan was to flash n cellulose fill the ceiling which would be separated by an insulated wall / air barrier in attic from the other half of the house which contains the bonus rooms. Because of numerous hips and details, I don’t think venting is a good option unless mushroom vents will work ok and there is open attic at the top portion of the attic.
Therefore, here are my questions and I refer to page: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
1) Can I do unvented rafter bays by flashing with 1″ or more of closed cell foam, then packed cellulose (or other type), then 2″ of rigid foam under the rafters to stop thermal bridging? This would be variation of assembly #5 in the link above. Is this permissible or would this cause any moisture issues. I’ve looked and saw something like this reference in this article, but wanted to confirm.
2) Alternatively, could I use open cell foam to fill the bays and still rigid foam over these with 2″ foam, and if so, what type? We are out of city so codes are not enforced but I believe code calls for R49 ceiling so depending on my rafter bay depth, we may have to do something different or settle for less in this area.
3) In bonus truss, Is it permissible to go unvented from soffit area with the above mentioned insulation method, then insulate the top cord (ceiling) of the bonus room the same way? (see attached drawing) In this case, would I need a ridge vent at the peak over this area? The roof is hip style so I’m guessing I’ll probably need to add in some mushroom vents in the open attic above the conditioned room in the attic.
4) The decision is not yet made to use roof trusses or cut rafters / I -joist on site. If using rafters, it seems fairly easy to apply 2″ rigid foam across the bottom of the rafters and then fill with blown cellulose. However, if using trusses, how would one accomplish something similar as the you can’t just fix rigid insulation along the bottom of the joist bay as easily because of the truss design. The foam can’t lay flat w/o cutting it and piecing wherever a wood strut is tying the top/bottom cord of truss. I guess I’m not sure of a good way to insulate between trusses and getting it sealed up fairly well. Any suggestions here would be appreciated.
5) Could use rigid foam on top of sheathing like Assembly #3 to get an r-18 to 21 on top of the roof, but I have 2 issues: Due to having to keep rooflines same elevations, I’d be insulating a few large overhangs that go out over a porch needlessly as it is the same roof line that takes it over the insulated space inside. Secondly, again, I need to add more insulation inside how do you properly finish out insulating the inside / bottom side of deck if using bonus roof trusses? All I can think of is stuffing batt insulation in there unless somehow we could get celluose to stick or be held in place.
I’m just trying to see if I can build a non-vented roof in this attic space or if there would be a better way to do this. I’m open to a vented assembly but only about 70% of my rafter bay’s have soffits below and the ridge vent would be even less space with the hips. In addition, venting 1.5″ would reduce my overall thickness of insulation so I really feel non-vented wins out here. I attached a sample detail sketch up – pardon my drawing capabilities.
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