How should I insulate the slope of my catehdral roof?
Background:
• I am building a cottage with a steep 14/12 pitch roof which is structured with a 3 part truss system with a central beam and a truss on that sits on each side of the beam (see attached picture). It is like a cathedral ceiling in that from inside the sides slope at the same pitch as the roof, although at the center of the ceiling it is flat. That flat part of the ceiling is 22′ from the main floor below. The flat section of the ceiling is about 12′ wide. The cottage size is 30 * 34.
• The cottage is in the Ottawa, Ontario – Canada area
• I am trying to create an efficient insulated structure (walls are 2*6 24” on centre (insulation = batt or blown in (TBD) ) with 1.5 inch of ridged insulation on exterior)
• The roof has no hips, valleys, dormers, or skylights. It has a straight shot from the soffit to the ridge. It is already shingled
• I have already installed two large attic vents per the cottage plan
• I was planning to insulate the upper flat section of the ceiling as a traditional vented attic space with blown in fiberglass or cellulose to achieve R50 or so
• The vertical height of the sloped section of the truss is about 9 feet.
• My questions are centered on the best way to insulate the sloped component of the roof.
• You may not be able to see it in the photo, but the sloped component of the truss is pretty deep (over 24”) so there is lots of room for insulation. The interior slope is also 14/12 (it isn’t a scissor truss)
• I was planning to use a vented approach over the slope as well as I think I have plenty of space
• I haven’t found a company that advertises dense pack cellulose in the area and understand that it is hard to DIY dense pack cellulose with the typical rental machines so I am inclined to think dense pack is not an option
Planned Approach for Sloped Component:
• I was hoping to create a baffle under the roof decking (at least 3 inches deep) and then fill the truss bay between the underside of baffle and a 6 mil vapour barrier under the truss (vapour barrier is reinforced by 1*3 straps on its underside). Again this is for the sloped section.
• I could use batt insulation to fill the truss bay but I am concerned about thermal bridging so I’d prefer blown in or dense pack (although again I don’t think I have access to dense packed cellulose)
• I am open to adhering ridged insulation under the trusses with strapping, but with the significant space I have I think I should be able to overcome thermal bridging and achieve high r values with cheaper blown insulation
Questions:
• Do you have any concerns with my approach in general?
• Per Martin H’s article on Cathedral Ceilings it says I could create the baffles with thin osb or plywood, but I don’t understand why moisture wouldn’t rot the underside of that baffle if I filled the space below with insulation. Doesn’t having a wood baffle with insulation filling the space below it essentially create an unvented space under the baffle where moisture would gather on the underside of the wood baffle? Don’t get me wrong, I would love to do this approach as it would be simple, but I don’t understand why it is ok.
• I was confused by the following paragraph from the article. Does the baffle need to be sealed? I wouldn’t have thought so. I think I am missing something. What does he mean by sealing the panel? I understand you need to have a constant uncompromised vapour barrier on the bottom face of the truss. I was going to use 6 mil vapour barrier there and airtight dry way.
‘When installing any type of vent baffle, it’s important to pay attention to air-sealing, especially if you plan to install fibrous insulation in the rafter bays. Seal the edges of each panel with caulk, and tape the seams between panels with a high-quality tape.’ – Martin H
• If I don’t need to seal the baffle, could I make the channel with insulation netting?
• Assuming I can go with the proposed channel approach, can I fill that channel from the flat attic section of the attic using a blown in loose fill fiberglass insulation? I understand cellulose compacts over time but not sure about fiberglass.
Thanks in Advance for your help. I have been thinking about this issue for a long time and haven’t found a clear answer yet.
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Replies
Mike,
First of all, I have a message for other GBA readers (not you): you should never frame a roof like the one in this photo, or even order roof trusses from a truss company, before you have developed a plan for insulating your roof. It makes no sense to build a roof before you think about insulation. After all, certain types of trusses preclude some types of insulation. Once you have bought your trusses, you have narrowed your options for insulation.
I will also repeat my oft-repeated mantra: the best place for insulation on a cathedral ceiling is above the roof sheathing. However, it's clear that you don't want to take that approach.
Now, to address your questions.
Q. "I am inclined to think dense pack is not an option."
A. The space in the cathedral sections of your trusses is over 24 inches -- an awkward depth. The only way to dense-pack this space would be to fill the entire space (other than the ventilation channel) with insulation. That's a lot of insulation. It's very difficult to dense-pack a deep volume like this, so I agree with you. Of course, if you had discussed the situation with your truss company before you ordered the roof trusses, you might have chosen a depth that was easier to insulate.
Q. "Do you have any concerns with my approach in general?"
A. Yes.
Q. "I don’t understand why moisture wouldn’t rot the underside of that baffle if I filled the space below with insulation."
A. The main vehicle for moisture transport from the interior of a home into the attic or the roof assembly is air leakage. If you pay attention to sealing the air leaks in your ceiling, you won't have significant moisture migration, and your ventilation baffles will stay dry. Remember, too, that these ventilation baffles are bordered on one side by a ventilated air space; that ventilation space helps keep the baffles dry.
Q. "Does the baffle need to be sealed? I wouldn’t have thought so. I think I am missing something. What does he mean by sealing the panel?"
A. If you use rectangles of OSB or plywood as ventilation baffles, the perimeter of each piece of OSB or plywood needs to be sealed with caulk, canned spray foam, or high-quality tape. You want an air barrier above the insulation (unless, of course, you are insulating with spray foam, in which case the spray foam creates the air barrier). That air barrier will improve the thermal performance of any air-permeable insulation like fiberglass or cellulose.
Q. "Could I make the channel with insulation netting?"
A. No. If you are insulating with an air-permeable insulation, you want your ventilation baffles to be as airtight as possible.
Q. "Can I fill that channel from the flat attic section of the attic using a blown in loose-fill fiberglass insulation?"
A. Yes. Another possible approach is to insulate the cathedral section of your roof assembly with spray polyurethane foam.
Thanks for the quick answer. I totally agree with your advice on alignment of design of the insulation with the roof system. I thought I had a good plan but realized it wasn't as good as could be achieved after further research. The roof is shingled or I would seriously consider your good advice on insulation above the sheathing.
I will assess the cost of spray polyurethane foam. Assuming I don't switch to the foam approach, I do have couple of follow-up questions based on your response in regards to a baffle approach:
*if I create baffles as you describe with thin plywood and air seal them, as I read your response, the risk of leaks is reduced efficiency of the insulation below. An air leak at the baffle does not increase risk of rot, etc. Am I interpreting that correctly?
*Will loose fill fiberglass compress over time if used to fill the slopped bays
*You replied in the affirmative to concerns with the approach. Recognizing that insulation of sheathing is your preferred approach, and that spray foam is another alternative, do you have concerns if I use the baffle approach using thin plywood - air sealed as you describe – with at least 3 inches of airflow under the sheathing, with 6mil vapour barrier under trusses, and loose fill fiberglass filling the truss bay? If yes, please elaborate.
Thanks again for your advice on this
Mike,
Q. "If I create baffles as you describe with thin plywood and air seal them, as I read your response, the risk of leaks is reduced efficiency of the insulation below. An air leak at the baffle does not increase risk of rot, etc. Am I interpreting that correctly?"
A. Yes, that is correct. But if you are going to the trouble of installing site-built ventilation baffles, you might as well do a good job. The Energy Star Homes program requires the installation of a six-sided air barrier (bottom, top, front, back, left, and right) for installation of fiberglass or cellulose insulation, with few exceptions.
Q. "Will loose fill fiberglass compress over time if used to fill the slopped bays?"
A. No. The worry with blown-in fiberglass is that it will be installed at too low a density -- a practice (called "fluffing") employed by some dishonest insulation contractors. High density is good. However, blown-in fiberglass does not settle.
Q. "You replied in the affirmative to concerns with the approach. Recognizing that insulation of sheathing is your preferred approach, and that spray foam is another alternative, do you have concerns if I use the baffle approach using thin plywood - air sealed as you describe – with at least 3 inches of airflow under the sheathing, with 6 mil vapour barrier under trusses, and loose-fill fiberglass filling the truss bay?"
A. My only concern with that approach would be the 6 mil polyethylene. I know that 6 mil poly is still used frequently in Canada, and is required by some building inspectors. But if there is any chance that your building will ever be air conditioned, using 6 mil poly is a bad idea. If you building inspector insists on a vapor retarder that comes in a roll, use a smart retarder like MemBrain. Otherwise, just use vapor retarder paint.
More information here:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!