Best Insulation Scheme for a Finish Attic?

[Daniel Rubin]

Hello, I am re-finishing an attic in an older home in Ohio. I have removed all of the existing walls and insulation. I am down to rafters and I am now ready to start the project. I would like to insulate the attic the best way possible as prior to the remodeling it used to get pretty hot up there in the summer. I have 6" trusses with about 5 1/2" available space between the roof sheathing and the living space for insulation. The house has 2 gable vents but does not have soffit vents, but I am going to put them in. I was going to put those styro rafter vents starting at the soffit vents through the section of the roof that is the angled ceiling of the finished space, but they are a whooping 1 1/4" thick which really reduces my space for insulation. While wondering in the home improvement store I came across reflectix radiant insulation. My idea now is put the styro rafter vents at the soffit vents to keep the blow in insulation on the floor of the attic from blocking the air flow. I was then going to staple relflectix between rafters starting at the bottom of the section that is the angled ceiling, up over the top flat area and then down the other section of truss that is the angled cieling on the other side. I plan on doing this leaving a gap of 3/4" between the reflectix and the roof sheathing in the sections that is the angled cieling. I am then going to shim out the rafters with 2x2s and add 5

1/2" thick high density unfaced R-21 insulation between the reflectix and the inside edge of the living space. I plan to finish it all off with a poly vapor barrier. This should give me about R-30 for the attic space. Does this sound like a good setup? Is it ok to put fiberglass under radiant insulation such as reflectix in roof trusses? Am I going to have any moisture problems? I would think that reflectix is a 100% vapor barrier so it sounds like I'll have a double vapor barrier with this setup? Is that a problem? I am thinking this is ok because those styro rafter vents would also act as a vapor barrier, no?

[Joe Bobst]

Just wondering, if this is installed with the reflecting surface up, will it cook the shingles? Maybe there's enough airflow from soffit to ridge vent just under the decking to keep things cool on the roof top?

Joe

[Ken Carlson]

One suggestion would be to add 2x4s underneath the existing 2x6 rafters to make an actual depth of 9 inches. Definitely use the spacer baffles on the underside of the roof deck to provide continuous ventilation from the eaves to the ridge. This will give you 7-3/4 inches of space, so you'll be close to R-30 if you use R-30 fiberglass batts intended for 2x10 rafters.

If you don't already have a ridge vent, install one, or use some of those roof mounted vent caps.

One thing I'm thinking about doing in our semi-finished attic, but haven't researched yet, is to put in unfaced fiberglass as described above, and then add a layer of rigid foam underneath that. There would be two benefits to the rigid foam, it would add R-value, plus it if it was continuous, it would be a thermal break between the drywall and the rafters. Then the drywall would be installed on the rigid foam. But like I said, I haven't researched this yet to see if it really makes sense or if I'm missing something.

Of course with all of this, you will lose headroom under the sloped portion of the ceiling.

Ken