Cathedral ceiling retrofit
I have a section of my house with a cathedral ceiling.
SW Michigan - Zone 5A
~50 lb snow load area
One side of the cathedral runs up to the house.
The current ceiling is (from inside the outside):
Roof is 17' wide and 20' run, gutter to ridge.
Timber Rafters - ~6' on center, about 4'x10'
T&G boards on top of the rafters.
About R-20 (guessing it is 6-8" thick) insulation.
Roof deck and black asphalt shingles.
I have 2 problems.
Bad ice dams
Noticeable temperature change to the next room. (hotter in summer and colder in winter).
The basic outline of my plan is to take off the existing shingle layer, then add 6" of foam board and then add a cold roof. This should take the total R value up to R50 or so. (I'll need to inspect the existing roof deck and fix any issues there.)
The question is how to I site build the insulation and take care of the structural component of the roof?
Two basic options:
Air barrier made with peal and stick ice guard (no can lights, make sure there are no air leaks).
Dimensional lumber "ring" around the edges.
Lay down foam sheets with staggered and taped seams 6" layers thick. (foamed gaps.)
Lay down roof paper.
Lay down 2x4s to make up the cold roof air gaps - screwed through the whole thing to the OSB.
OSB nailed to the 2x4s.
Ice and water, felt and shingles
(taking care to do ridge and eave vents and dress gaps in the foam sheet with spray foam.)
The alternative I've seen does basically the same thing, but with each layer of foam sheet layed at 90* with 2x4s between them and staggered so the thermal bridges are minimized. Seams are taped and foamed to control air leaks.
Thinking through this, it seams that the difference is that using 2x4s on a bias trades more wood for fewer long (8" or so) screws.
When you build up the roof with 6" of foam board, how to you manage sheer loads?
Is "B" overkill?
What is the fastener pattern for the 2x4 air gap and OSB roof deck?
Posted Sep 2, 2014 2:47 PM ET
Edited Sep 3, 2014 4:52 AM ET
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