How to insulate flat part of attic ceiling?

Question

I have a large (860 sq. ft.) attic that is uninsulated. I'm planning a project now to turn this into a TV room, so I'll need to insulate, run electrical, put up drywall, etc.

An insulation company will be insulation the roof slope behind the kneewalls, and I'll do the rest of the insulation, which will be the ceiling slope from the kneewall up to the collar ties, and the flat part of the ceiling. See attached sketch.

Code says the ceiling need to be R38. I have a plan for the slopes, which are 2x10s: fiberglass batts with 1" rigid board over it. (This won't get me up to R38 but I'm hoping it's close enough for the inspector.) But I'm not sure what I should do for the flat part of the ceiling--the collar ties are only 2x6s, so the R30 insulation won't fit within that space (it's thicker than 6"). But on the other hand, there will be plenty of space above that in the uninsulated top of the attic (which will be vented with gable vents). So can I just use those batts anyway, and have the insulation project above the collar ties? And can I lay additional batts on top to increase the R value? I don't want to come down any further into the attic living space (e.g., by adding rigid-board insulation under the drywall) because I want to maximize headroon.

Answer 1

You can certainly put the R30 batts in between the 2x6 collar ties, but it won't result in an R30 ceiling installation, due the thermal bridging of the collar ties and the resultant gap above them between the insulation batts.

I think abetter plan might be to stuff the 6" thick insulation in between collar ties as normal, then lay another layer of 6" thick insulation batts at right angles to and on top of the first course, thereby completely insulating the collar ties and eliminating the thermal bridge.

Also, don't forget to install rafter vents on the sloped portion of the wall so that air from the soffit can flow up to your roof vents and prevent moisture damage inside the attic.