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I will be building a gambrel roof garage with a heated studio on the second floor. I plan to insulate with fiberglass. These are some of my questions. Should I use larger lumber for rafters to fit more insulation? Do I need to use the styrofoam channels (if that’s what they’re called) for ventilation? If so does it run the entire length of each bay? I plan on running ridge and soffit vents. I am open to other options for insulation or venting. One more question–How the heck does the moisture inside the wall space get into the channels to be vented?
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Where do you live and what sized rafters are you planning on? Lake Podunk, Minnesota / 2 x 6's - yes you need more depth for insulation. Monterey, California / 2 x 10's - that's plenty.
Styrofoam backing panels are not the only option. One appraoch is to just staple up one size smaller of FG batts. That will leave a gap under the sheathing. The concern is that when the FG sags over time, you lose insulation in the top of the bay and block ventilation in the bottom. In my own house, I used 2" blueboard as a backer because it added R-10 and I needed a surface to spray the urethane onto. But then we have extreme winters.
Yes the ventilation gap runs the entire length of the bay. Whats more, you must drill or notch any blocking so the air can continue up to the ridge vent. This gets tricky in the valleys of gables.
Regarding moisture: I try to keep the moisture out of the walls and roof in the first place. My air gap under the sheathing is to keep the roofing cold to avoid ice dams. But if you have moisture in the bays, it moves by diffusion from areas of high humidity to areas of low humidity. With a winter temperature gradient, that is enhanced because, unfortunately, the water vapor will condense and/or freeze within the insulation. It then melts in the spring making it look like the roof is leaking. Or do you mean how does the moisture get through that apparently impermeable styrofoam baffle? Good question. A bit through gaps, a bit through the studs. Better to keep as much moisture out of the wall as possible. -David
*Dear DavidThank you for the information. So many more questions arise. I worry about vapor barrier to keep moisture out of the wall cavity because I have seen so many problems develop because the vapor barrier wasn't quite perfect. I also don't like the environment in a house that is so airtight. I think I prefer to let the moisture migrate and then vent it away. I may be wrong in this assumption. I live in Western MA.,so insulation is important. If I think of the lower rafters as my wall, then 2x6's are not a bad wall. 2x8's with some sort of air space would probably be better, but I still can't see where styrofoam channels are going to help. I still don't understand how the moisture in the wall cavity gets into those channels. As for the upper rafters, they could be as small as 2x4's or a truss and it would be easy to put any size insulation hanging from the ceiling joists with a space above, vented out the ridge vent. Has anyone ever used wire to hold the insulation away from the plywood? Thanks again for your response. Mark
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I will be building a gambrel roof garage with a heated studio on the second floor. I plan to insulate with fiberglass. These are some of my questions. Should I use larger lumber for rafters to fit more insulation? Do I need to use the styrofoam channels (if that's what they're called) for ventilation? If so does it run the entire length of each bay? I plan on running ridge and soffit vents. I am open to other options for insulation or venting. One more question--How the heck does the moisture inside the wall space get into the channels to be vented?