Insulation, Radiant and Vapor Barriers - how to insulate 2x6 rafters to r-30

hi all,

i have a cape-style home, where the front half of the bedrooms on the second floor have a sloped ceiling. per local building code, the ceiling needs to be insulated to r-30; however, the sloped ceiling (which is actually just the roof rafters) are only 2x6, which will only hold r-21.

i have a couple of ideas as to how to get to r-30, and would appreciate other's input as to which way may be the best (cost-wise, and effort-wise). also, if anyone has another idea, i'd love to hear it. if it makes a difference, i plan on ultimately covering the ceiling with 1/2" drywall, and i do need to frame a couple of interior non-LB walls up against the ceiling.

so my ideas are

1. install the r-21 between the rafters, then screw or staple as many layers of rigid board insulation to achive r-30.

2. for the length of each rafter, attach the 1.5" side of a new 2x6 to the 1.5" side of the existing 2x6 rafter, essentially creating (almost) a 2x12, and then fill rafter bay with r-30.

3. nail or screw a new 2x12 next to each existing 2x6 rafter, then fill rafter bay with r-30.

so, any help on this would be much appreciated! thanks in advance

I think I have the same type of house. Is this an upstais beroom where the ceiligs are sloped? If so, wouldnt bring the rafters down another 6 inches make the room even smaller? My upstairs rooms are way to low (the sloped part) for me. Id die with another 5 or 6 inches gone. sounds like you have the back dormered, though, I dont. so maybeif its just 1/2 the room it wouldnt be so bad.

If thats the case, Id look into other type os insulation maybe you can get more r value with something other than fiberglass?

Also, double check your code where you are. Near me weget pretty cold winters and i can get away with R25 in a cieling out here. If you can, look at certainteed and Johnsmanville websites, they make a compressed R30 and R25 (R30c and R25c) Each fits in a bay about 2.5 inches less than the noncompressed. So, the R30c fits in a 2x10. If you can do an R25c, maybe you onlyhave to drop your rafters 2 inches or so.

What I have done on an extension built with a cathedral ceiling but with the same profile as my cape rooflinec(as suggested by the architect and town inspector) is use regular R30 in my 2x10 rafters. But, I used the foam baffles to ensure an airspace. The R30 got compressed doingthis, but the architect and inspector didnt think anything of it.

So I did it that way.

Just some thoughts.

Hi. I just went through a similiar project with my home, similiar upstairs. I stapled vents to decking, installed high-density r-21 (which a Corning rep said would be r-19 when compressed to 5") and then nailed w/plastic cap nails 1" polyiso rigid foam for another r6.5. I used 3.5" drywall screws to anchor drywall through the foam. Its been 6 months and so far so good. Naturally the rising gas prices bite, but I also wanted to benchmark the additional insulation better. I managed a r25.5 for the slopes, then added more above the collar ties for r48. This is how I managed it. Good luck.

That's very informative. I only wish there was a video for me to follow along. I'm hoping to finish an attic with insulation that's better than the 50 year old radiation barrier that's currently there.

But first I have to remove the 4 layers of asphalt shingles and cedar shakes before I start sticking new insulation into these spaces.

J