Ventilation is the main concern.
You can use R-13 which fits in a 2x4 wall (made for walls and can be used
elsewhere)although it is not as insulating as R-30
You can use R-30 insulation (Fiberglass batts) but have to pay attention to
venting the space left in the attic area. some put gable end vents(be sure
they are large enough for the space to ventilate. k 12" x14" or 14"
x......14, 16 18, 24 etc. all kinds of sizes.
Near the plate and the roof getting smaller area some cut or compress the
insulation to allow air flow.........
Some use roof mounted vents if gable end vents do not work. Also allowing
air space between the roof sheathing and the insulation is important. Some
put screened "bird holes" in the blocking at the plate to allow air flow in.
Again, some use rigid insulation but in an existing roof, and trying to cut
it tight is tough.
And, if a person is really doing a major job, solid insulation can be placed
on the roof sheething with 2x4 on edge and plywood attached to that.. Then
roofing over and not needing ventilation. (it has to be a tight space and no
air space....) 3.5" solid insulation to match the 2x4 sleepers.....
So many ways......not sure of your situation.
My biggest question is how you intend to "cathedral out" trusses.
You cannot cut or change trusses without an engineered solution.
There are such a thing as scissored trusses that can have a
limited cathedral ceiling, but this would very difficult after the
fact in an occupied space.
Most current thinking would maintain an air space above the
insulation and below the roof sheathing. It also common to
insulate completely with no venting. A hard decision with experts
on both sides.
Keep the whole world singing . . . .
I was under the impression that these were scissored trusses but maybe not?
That too is a consideration as you stated.
So now there is also evidence that an enclosed space with no ventilation is
I have fallen/stepped through roofs with improper ventilation and roof
More than 10......Have had to repair/replace/re-insulate many.
Here in the UK it would depend on what's above the trusses.
If you have a vapour permeable membrane then you can fill the trusses with
insulation and put another layer underneath. eg 4" between and 4" under to
give 8" of insulation. Then on the underside add a vapour barrier (plastic
sheet) and plasterboard (drywall).
If you have a vapour impermeable membrane then you need a 50mm ventilated
void below it. So only 2" between the trusses and 6" below them to give your
8" of insulation. Then as above. The void would be ventilated to the outside
at eaves and ridge.
..but either way you can't just go cutting out the trusses. The 2x4 on it's
own won't be strong enough. You need a proper engineered solution or the
whole lot is likely to fall down pushing the walls out in the process.
If the roof is in poor condition one option is to remove the lot and build a
new roof with insulation above rafter level. That way you can leave the
rafters exposed on the inside if that's wanted.
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