roof and 3 leaf construction

Ok, I am trying to figure out the roof design for a new build. I had previously posted on my original thread that I thought a flat roof would be good because it did not need ventilation, but have been told by roofers that this assumption is incorrect. So, if I have internal walls and ceiling and external walls and ceiling, wouldn't an A-frame roof be a thrid leaf in the ceiling structure? Can you build and airtight box with an inner wall and ceiling free from the exterior and just build an A-frame on top of that so that the roof is vented so as not to rot out? Thanks, Aaron

Aaron, I have been fighting this battle for lot of years now - and there really isn't any way (with standard wood frame construction) to get out of this dilema. So I decided follow these paramaters: For new construction design a decent 2 leaf isolator into my roof -and then use a seperate "within the room" construction for my rooms - with mass only on the bottom of the inner frame. For rooms within existing buildings - I alloow the roof to breathe however it was designed to in the first place - build my room within a room inside that envelope - and apply plywood with drywall over the top of the inner room's ceiling frame - so my 3rd leaf is there instead of at the exterior frame. Although I get better isolation in the first case - this is probably more due to the the fact that the outer shell as a whole is designed with isolation in mind - and not just the roof. But - I also get excellent isolation in the 2nd case - The bottom line is this - don't sweat things you can't change - just make the best of them - and in either situation - pay very close attention to details - it's adhearance to getting the smallest details right that make or break your project from an iso point of view....... not the 3rd leaf in a ceiling that needs to breathe in order to exist for the next 30 years........ Rod

Rod, Thank you so much for your reply. In addition, thank you so much for writing such an amazing book. Puts so many things in such a great perspective.

For new construction design a decent 2 leaf isolator into my roof -and then use a seperate "within the room" construction for my rooms - with mass only on the bottom of the inner frame.

Ok, so the inner frame has the layer of gyp board attached to the studs with insulation. Then the ceiling or roof joists above that are free of sheathing. Then the roof rafters above that has the layers of gyp board attached to them on the inside with insulation and baffles to allow airflow and then the exterior sheathing. Is that my correct reading? I am feeling confused. What I had been thinking was from inside out: layers of gyp. attached to studs on walls and ceiling with rockwool insulaton, 1 inch airgap, rockwool insulation between outer studs, with layers of osb on the outside walls and ceiling, (siding on exterior walls of course), then on top of the outer ceiling (I guess this could be considered an "attic floor") have an a-frame roof built on top of plates on the outer perimeter. Please tell me the pitfalls of this plan. Also, If I am building on a monolithic slab at what point is flanking going to take over and make the point moot? Thanks in advance! Aaron

Something to think about: If you have rigid insulation directly attached to the underside of your roof decking you DO NOT need to ventilate your roof. If you have fiberglass or some other form of insulation then you MUST vent. The only potential drawback with not insulating a roof (when done properly) is that the life of the shingles can be shortened. This is basically moot if you use high quality shingles. On the other hand a vented roof with poor ventilation can ruin your decking. I had a roof with proper soffit vents and a proper ridgeline vent and it still failed because the cheap cobra ridge vent did not vent quick enough for the small space in between the sheetrock and the decking (cathedral ceiling). I installed a better ridge vent (lomanco) and the issue was solved. I was getting moisture on the back side of my decking and moisture was growing mold in the spring and fall when the temperature difference between outside and inside was not as drastic as it is in the winter and fall. Another roof in my house ( a post and beam house I built) has 2x cedar decking as the exposed ceiling inside, with rigid polyiso insulation sandwiched between the decking with NO air gap. Moisture can not get in there, thus the decking has stayed dry and will last. Habitat post and beam out of Mass. has been using this roof formula for years and they have seen shingles and decking last 30 plus years. Venting is key if you have an air space under your decking, not needed if you don't. I don't know if that changes anything for you. Also, if you ridge vent, use a rigid plastic vent like lomanco. Cobra vent eventually fills with dust and starts to fail, plus only vents enough for attic spaces where there is a lot of air (ironic that small air spaces need faster venting). It is widely used because it is cheap, easy, and the problems with it don't start until well after the contractor is out of the picture.