Air-Sealing and Insulating Shed Roof of Addition
Hi folks,
I have a question regarding use of rockwool in small shed roof style addition. I have not used mineral wool insulation before.
Here’s the scenario:
Small cabin in upstate NY, zone 5/6. Just framed up an 8’x10′ addition, with shed roof attached to side of existing building. Walls are 2×4, roof is 2×6 rafters 16″ o.c. The roof and the end wall are convex arched curves. The rafters are parallel, but due to the arc the sheathing will be bent over them. So, rigid polyiso on the inside or outside of rafters is not an option. Cut and cobble polyiso sealed with great stuff in between rafters is an option, but would be time consuming. I’m not worried about thermal bridging of rafters in this case. It’s a small building, heated with a woodstove and only for short periods of time. I’m more concerned with good air sealing.
I’m thinking of using R-23 mineral wool, which is 5.5″ thick. The rafters are also 5.5″ so, in order to allow for airflow, I could add 1/2″ plywood strips to underside of each rafter. The rockwool would then sit at the bottom of the rafter bays, with 1/2″ gap between the top of the rockwool and the underside of the roof deck. Roof pitch is 2/12. The roof deck is 1/2″ plywood. I would add blocking between rafters at the top plate, but leave it 1/2″ low to allow air from soffit to flow into rafter bays. The soffit would be vented, and the upper end of the roof where it attaches to the building would be vented. I’d add two or three narrow strips of 1/2″ plywood the the underside of the roof sheathing in each rafter bay to maintain gap for airflow from soffit to ridge.
I’m trying to figure out how to air seal this room… would it be recommended to add an air barrier on the inside of the rafters? The ceiling will be wood, the walls will likely be drywall. I should mention there will be a shower in the room with an exhaust fan.
Any thoughts on air sealing and moisture management in this situation would be appreciated. It’s not a super tight budget, but not looking to have the spray foam truck come for such a small project. I thought of a spray foam kit, but prefer to do it cheaper/greener. I’ll likely use rockwool in the walls as well, and am also trying to figure out how to air seal, which is more complicated due to the curve. The walls will also be sheathed with plywood, felt paper or housewrap, and sided with vertical corrugated metal.
Thanks!
Ben
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Replies
Ben,
You can air-seal behind your wood ceiling with poly, a variable perm membrane, or drywall.
Although the size and occasional use of your addition makes it a bit more forgiving, a 1/2" airspace is too shallow, especially with a low slope roof. I would aim for at least 1 1/2", if you aren't using baffles.
Thanks, Malcolm.
So to clarify, a vapor barrier like poly is acceptable with rockwool because the roof cavity will still be able to breath to the exterior? And when you say baffles, you mean a proper vent type product? I was hoping to replicate that with the plywood strips up the rafter bay, but I understand what you're saying about 1/2" not being enough.
If cut and cobble polyiso sealed with great stuff isn't too labor intensive, do you think that would be a better option? I'd do two layers for a total of 4", and still vent the rafter bays with 1.5" gap but the foil would act as air/vapor barrier. Thanks!
Ben,
I think your plan is fine with minor tweaks.
- Rather than 1/2" plywood, fur down the rafters with 2"x2"s. With an 1 1/2" vent space you can forgo the baffles, except at the exterior wall.
- Use poly as an interior air-barrier. The only downside is in climates where you might be air-conditioning, and it doesn't sound likely for your cabin.
Ok, that makes sense. No air conditioning here, open windows and fans if needed in the summer. I'm always nervous about using poly as I've seen it cause moisture problems by being used incorrectly in multiple cases. But, it makes sense that it would work in this case.
One more question on the topic... If I were to use rockwool in the walls as well, R15 in the 2x4 stud bays, with 1/2" plywood sheathing, housewrap/felt paper and metal siding on the exterior, would you recommend any type of vapor barrier on the interior?
Thanks for all your input!
Ben,
Luckily vented roofs are about the best place for poly - as long as you aren't cooling the space.
A lot depends on whether the wall can dry to the outside. If you use a perforated flashing at the base of the corrugated metal siding, and better still mount it on horizontal strapping to create3 a deeper air-space, you are fine using poly on the walls too.