We are building the 20 X 30 house with the 1 1/2 stories and have used the site built trusses using 2 X 6 lumber as per the plans. We would like to floor the tiny attic space. Can we use R-19 insulation in the rafter bays and above the the ceiling in lieu of R-38 above the ceiling?
Is this making sense? We plan to use the R-30 fiberglass bats in the rafter bays until we reach the the cross ties. The continue up the rafter bay with the R-19 and then fill the cross tie spaces with more R-19. The R-19 is 6 1/4" thick so there would be no air channel in the attic space but it won't be covered with drywall so we are hoping that won't be an issue.
If this won't work, is there any type of insulation available that would give us the required R-38 in the 5 1/2 inch space that's available?
Thanks,
Sasha
You should preserve the vent space for air flowing over the sloped portion of the rafters and up into the vented attic at the peak. If you insulate this flat attic floor using Extruded Polystyrene foam panels such as Dow Blueboard or FormulaR (R5/in) then you will have only a small compromise in insulation for the use of this storage space.
Be sure to insulate and weatherstrip the access hatch.
Another way to insulate this space is using R-19 between the trusses as you suggest, then laying 2x4s across the attic trusses and putting R-13 in this space before laying down the attic floor. This takes a few more inches out of the attic storage height however.
Spray foam (closed cell) would come close to R39 and would allow you to eliminate the roof ventilation requirement since it avoids a cold surface at the roof sheathing and the condensation problems. Unfortunately, spray foam is more expensive and is not usually considered a DIY process.