Above roof insulation
AndyMMP
| Posted in General Questions on
GBA Community,
We have a part time cabin in zone 6A. We are completely off grid, solar and generator for power and wood stove for heat with propane furnace for back up. Cabin is unoccupied from January- April. Cathedral ceiling with 1.5″X5.25″ tongue & groove ceiling. 4/12 pitch with felt paper, asphalt shingle and steel rib over the top. We need new roof and would like to insulate over top of plank sheeting. Vented or unvented? Ice & water shield, 2- 2″ layers of polyico, plywood or strapping ?
Any advise on this would be greatly appreciated.
Andy Muzik
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Replies
Do you have any existing insulation in the assembly already? If you do, there is a minimum amount of "above the sheathing" insulation you need to make the assembly safe. If you don't have any existing insulation, then any insulation on top is better than what you're starting with. 4" of polyiso is only about R26, which isn't much -- especially in such a northern climate zone. I would try for AT LEAST 6", which would be about R39 or so. That's not really to code, but it's a lot better than nothing (assuming that's what you're starting with). Code would need nearly 8" of polyiso.
You don't need to vent an assembly that is all, or mostly, insulation above the sheathing. Putting a vent channel above the polyiso won't hurt though.
Bill
Thanks Bill for taking the time to respond to my question.
To answer your question, no there is no insulation in the current assembly.
Andy
Andy,
There are quite a few different ways to achieve some amount of insulation on the exterior of the roof. Since you're not there during the coldest part of the year, I wouldn't think you would want to go full out and overly spend, when you can do a pretty good job otherwise.
You have it easier than some, by having 1.4" T&G that you can screw into and not worrying about hitting a rafter dead on. Before I get into it, let me ask - do the walls currently have insulation? Are there a lot of windows? Is it a log cabin? etc. Some of those details might help us all with other suggestions.
Kyle, Thank you for your response. The cabin is 24' X 48'. Post and beam ceiling and typical 2"X4" wall 16" on center. Interior wall is tongue and groove 3/4"X 5 1/4" pine. paper face fiberglass insulation with T111 on the outside. Plenty of windows, 10 in total all double glaze sliders. the west end of the 24' end is all glass from floor to roof and is single glaze. Our plan is to reroof and side entire structure and replace windows on west end. I would like to add foam panels on roof and walls to improve insulation and add some type of house wrap as well. We are looking at steel rib panel for roof and siding.
Something similar to attached less the insulation in cavity ceiling.
Gotcha. Thanks for the details!
The reason I was asking, it because it would have made little sense to insulate the roof, if the walls were still uninsulated. It looks like that is basically taken care of with the fiberglass insulation.
You'll probably benefit the most cost wise by choosing a single insulation type, and taking advantage of any bulk discount you can get. If you want somewhere around R40 on the roof, as Bill suggested above, you'll need 6" of polyiso. That does well, as polyiso is easily found in 1.5" and 2" thicknesses. I'd go with a single layer on the exterior of the walls, since you've already got cavity insulation, and three or four layers on the top.
I'd put the house wrap directly over the T111, and let the insulation rest outside of it. For the roof, you can run 2x4 battens over the top layer of insulation and attach the metal directly to that. Same for the walls.