retrofit insulation in cathedral/shed roof–any other ideas or suggestions?
for the experts left around here, question. After some queries at GreenBuildingAdvisor, I’ve settled on an insulation method for my 16′ wide shed/cathedral roof (4:12 pitch, 2×12 rafters on 16″ centers). I’m going to retrofit insulation from above when I re-roof (finishing up the metal roofing project on the rest of the house). Currently there is poorly-placed fiberglass that let’s a lot of thermal energy transfer (it’s a sad sight to see this ceiling on a 95 degree day through a thermal camera).
For a variety of reasons, I am going to maintain the vent channel on the underside of the sheathing that runs from the soffit to the peak. So I am not doing spray foam (way to complicated to remove sheathing and apply given the size of the roof and working with a spray foam vendor over multiple days). I am NOT going to do cut and cobble, or try to install baffles with dense pack under it, for all the reasons I have been given about why those are not ideal options.
What I am hoping to do is to place Roxul, the 9.5″ that achieves R-38. I am hoping to remove alternating pieces of sheathing, and be able to work in the r-38 from above and below. So working from the top down, remove a row of sheathing, leave a row, and then remove the next row. That way I can place the roxul into the rafter cavity from above and below the sheathing, and then replace the sheathing one or two pieces at a time. It might be more tedious, but I don’t want to get in a position of removing all of the sheathing on a 16×40′ room at once and try to keep that wateright.
worth noting: given other areas of the house, I don’t think there is much, if any, deterioration of the roof decking. It’s 5/8″ plywood (from 1977) and in the other roof planes, looked like new under the tar paper. So it’s not necessarily the case that I already have to replace the decking.
I’ve thought this out a lot in my head, but am I nuts? Is there a more obviously good way to proceed. Like this shot in the dark: does anyone know of insulators that can inject into cathedral bays from the peak? There is a 12″ wide band of cedar that covers the ends of the rafters at the peak and there is easy access to the bays. I could even get the fiberglass bats out with a pole and a hook and a vacuum. Can anyone spray foam and keep a vent channel over a 16′ span?).
And in case you ask: The underside/interior has been lovingly cleaned up, painted, beautiful beams installed, etc., and over my dead body will anyone insulate from below.
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That sounds like a nightmare of labor! Not trying to crap on your plans, but it doesn't really sound like you're leaving much room for success, should anything deviate from your plan.
If you're paying people for labor, you'd probably come out the same if you removed all the decking in one go, then did the insulation, and covered it back up. Even if you're doing it by yourself it's easier to batch these processes than it is to save on what, 20 extra sheets of sheathing?
yes, that's crossed my mind few times. I have a labor crew that can help me/do what I ask. On the other hand, it is easier to work off the decking than open rafters. It may be that I replace all the decking, but work it in rows.
I have slid ROXUL between floor joists when insulating / sound proofing rooms. I could push it a couple of feet or so with no problem, after that it was grabbing the joists. Using some cardboard strips along the joists and drywall sides allowed me to push it in as far a needed. I could wiggle out the carboard after. Electrical wires were a show stopper. Had to open up to get around those.
I would start with a single bottom row sheet and see how difficult it is to slide in the insulation. Compare the effort with that required to pull a sheet. I suspect pulling a sheet is easier.
If you are sliding in the insulation, slide in a baffle to make sure the insulation does not bunch up and close off the ventilation gap.
thanks. excellent suggestions. My thought with the "every other row of decking" was that 2' was manageable, but not anymore than that, especially working on a slope. 9.5" Roxul is also going to be harder than the 3.5 and 5.5" I've worked with. Part of my aim here is to just preserve decking if it is not in need of replacement. I hate to waste good material. But it will come off if it's easier to replace. I'll likely start from the top, since there is a lower shed roof that I can stand on and work at stomach height to start this out.