I have an interesting situation where I need to insulate where there is a roof within a roof (due to odd remodeling decisions). Over our 12' x 12' mudroom, there is hardly insulation but there is a newer carport "attic" that is disconnected from the attic for the main part of the house. Within this large carport roof, there is the 12' x 12' original mudroom roof. Currently the only hole into the small mudroom roof is a hole for its gable vent.
What would you do? I think option 3 makes the most sense.
1. Put insulation between the internal mudroom roof and external carport roof. There is enough room to fit R-38 batts between the roofs and that would be the quickest thing to do. However the mudroom may not be kept as warm as heat will escape through the ceiling before being trapped by the inner roof. I also don't know if there would be a problem with fiberglass insulation over asphalt shingles and under roof sheathing. Plus I would need to insulate the inner wall (where the gable vent is) and that could cause issues with the wood shingles there.
2. Blow insulation into the inner roof. This would only require a few holes to be cut into the inner roof to blow the insulation in. However the edges where the roof is not tall would still not be adequately insulated.
3. Cut off the inner roof to lay down batts the normal way. Extra work that I wasn't planning on doing, but it's only a 12' x 12' room, and how hard can it be? This would allow for an evenly distributed >R-38 insulation. I'd like to hit the R-50 to R-60 range anyway. Seems like the right thing to do. Any potential problems with this approach to look out for before I start hacking away at the original roof?
Insulating a ceiling/roof within a roof
Re: Insulating a ceiling/roof within a roof
Is there a way for you to take a picture or draw a couple of 'technical' drawings so we can get an idea of what everything looks like? My spatial imagination is struggling to provide me with helpful imagery 
Re: Insulating a ceiling/roof within a roof
Yes that would help right? Here is what it looks like, where Original Roof is entirely contained within Carport Roof. The small square is the gable vent, "venting" under the carport roof, not venting to the outside.
Option 1 would be the lazy way of throwing insulation on top of the original roof, under the carport roof. Option 2 would be to blow in insulation with minimal disturbance to original roof. I'm showing where insulation would go for Option 3 if the original roof was not in the way.
Option 1 would be the lazy way of throwing insulation on top of the original roof, under the carport roof. Option 2 would be to blow in insulation with minimal disturbance to original roof. I'm showing where insulation would go for Option 3 if the original roof was not in the way.
Re: Insulating a ceiling/roof within a roof
Good point. A triangle is stronger than a rectangle, from what I remember of my science classes.
I'm putting in a large order of fiberglass batts for the rest of the house, so I'm hoping to stick to batts instead of having to deal with the blown in stuff too. Now I'm thinking I would like to remove the original roof shingles and sheathing but keep the rafters intact. If that's too much of a pain I think I can cut open the wall (where the original gable vent is) enough so that I could get in and install without messing with the roof. There's not much space but I could crawl in.
I'm putting in a large order of fiberglass batts for the rest of the house, so I'm hoping to stick to batts instead of having to deal with the blown in stuff too. Now I'm thinking I would like to remove the original roof shingles and sheathing but keep the rafters intact. If that's too much of a pain I think I can cut open the wall (where the original gable vent is) enough so that I could get in and install without messing with the roof. There's not much space but I could crawl in.