How should I insulate the slope of my catehdral roof?
• 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
• 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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