Cathedral Ceiling/ Roof Insulation upgrade
We have a 23 yr. old log house we built in the Adirondacks of NY (zone 6). Lots of snow and cold winters. We are replacing our asphalt shingle roof with standing seam metal so we thought we should also upgrade our roof insulation.
Currently we have cathedral ceilings throughout with 2×6 tongue and groove, felt paper, 2×10 rafters 24″ on center, 12″ fiberglass batts with prop a vent, felt paper, plywood and shingles.
It is a large (4,700 sq. ft.) complex roof with 2 valleys, shed dormer,2 uninsulated porches, 4 skylights and large overhangs. We are looking to work with ou current roof layout because of costs.
I understand thermal bridging (our whole house is one) but to put foam board under the rafters or on top of the sheathing and add another layer of sheathing seems labor intensive.
We are thinking about closed cell spray foam for the air and vapor barrier it provides in a small space. One insulation installer mentioned flash and batt. 2″ closed cell on the warm side against the T&G and using our faced fiberglass batts to fill the rest of the rafters.
Could we used the faced batts this way?
Would this also work with a combo of closed cell on the warm side and open cell foam to the sheathing?
Would this need to be vented?
Any advice would be appreciated. Thanks
Kathy Hall
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Replies
Kathy,
Lots of questions. If you plan to open up your roof from the top and pull out the existing insulation, you have many options. Closed-cell spray foam isn't cheap, but once you have the spray foam truck there, you might as well pay for a significant amount of insulation instead of just a flash job.
But if you want to save money, you can install spray foam against the T&G boards with fiberglass batts above. Some of the spray foam is likely to ooze out between the boards and show up on the inside of your house.
My own recommendation is to install as much rigid foam on top of the existing roof sheathing as you can afford. That will result in a better job. If you go this route, but sure to plug the air intakes and air outlets for the existing roof ventilation.
Martin's approach seems logical, but if you add foam on top of the roof deck you will need to add to all of the rake and fascia boards on the house, so the look will change and the work-flow will tend towards doing it all at once. What you might consider is using rigid insulation board between the existing rafters, either removing the fiberglass entirely or installing part of it back into the cavities along with the rigid. I'm a contractor, so I tend to think about the workflow, and what I like about this is that I could strip part of your roof at a time... let's say a 12-foot wide section from eave to ridge.... retrofit the insulation, and then install that much metal. At no time would I have much of your house exposed to the weather. Metal installs vertically so it really lends itself to this type of approach. If you were to open the entire 4700 feet for a spray foam truck, you would need massive manpower on site to replace the deck, re-felt and re-roof, etc. That's a huge risk, you couldn't get me to do that no matter what sort of recession we might be having.
My tendency would be to put the rigid right against the T&G ceiling, rather than putting the fiberglass in first. I wonder what others think would be best...?
We were definately doing this in stages. I thought that the felt paper put on years ago would keep the spray foam from getting into the tongue and groove. My main concern in reading about flash and batt is the correct ratio for northern climates and that the foam was on the cold side against the roof sheathing, not the warm side. Since we are working from the outside, this is not possible, and I thought the air and vapr barrier of the closed cell would be a benfit.
I was also worried whether the facing on the batts would cause problems,
Venting and condesation were other concerns. With investing in metal roof we want to try and get it right.
Kathy,
1. If you have felt paper behind your T&G ceiling boards, then spray foam shouldn't ooze out.
2. If you go ahead with your backwards flash-and-batt plan, your roof sheathing will be cold and somewhat vulnerable to moisture accumulation and rot. The spray foam will create a decent air barrier, however, lessening your risk. Nevertheless, I wouldn't attempt such a cold-sheathing strategy without roof ventilation. That means you'll need to figure out a way to install an air barrier on top of your fiberglass batts, followed by a ventilation channel. The ventilation channel can be below the roof sheathing or above the roof sheathing -- your choice.
3. Kraft facing on your batts is not a concern.