Baffles, Venting, and Insulating Over Top of Exterior Wall in Attic
Sorry I am unfamiliar with terminology here.
I live in Southern Ontario, and this will be an unfinished attic with insulation batts on the attic floor.
From inside to out, The roof deck has truss, then horizontally oriented spaced 1 x 3 boards, then solid plywood, then shingles. So the 1×3 boards sort of act like furring for the plywood?
Our roof has a large overhang outside, but no soffit. Inside, At the angle where the exterior wall joins the plane of the roof deck, there is a vertical oriented board about 3 inches tall (where we had thought to add soffit venting), then under that, a 3inch board oriented horizontally over top the exterior wall. This horizontal board is flush with the floor of the rest of the attic.
I’ve read how important it is to insulate over the walls, but also how important it is to not allow the insulation to touch the roof deck. Also read that it’s important to leave the vented area clear for air flow, yet provide a wind wash block.
I can’t do all of that in this small space. If I vent where we intended, the top of the exterior walls cannot be insulated because of the space needed for wind block and baffles.
The other option for venting is: the space at which the vertical board and the roof deck meet has a gap 1/8- 1/4” in several places. We were thinking of using a saw and making sure this small gap exists consistently along the top of that vertical board.
I bought Pink Styrofoam Attic Baffles (Raft-R-Mate). I can butt those to the top 1/3 of the vertical board, just inferior to the gap and this will provide the vent/baffling and wind wash.
But will it be worth it all? Such a small gap… will it provide enough venting? We do have a large mechanical roof vent and ridge vents.
If you a agree with the “gap” alternative, would I then spray foam the baffle into place? So I can get a seal and some insulation over the wall? It is very difficult to maneuver to get any seal in place in order to then place only 2-2.5 inches of batt insulation between baffle and wall plate.
Your guidance is very much appreciated!
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The roof you have used to have cedar shakes thus the skip sheathing. This was replaced at some time by shingles. Good thing is that you have big overhangs so there is a fair bit of distance between your top plate and the roof deck. You can get a lot of insulation in there.
Staple the baffles to your rafter and above your top plate as shown here:
You don't need any insulation in the overhang area. It looks like you have some pretty big gaps there that can work as eave air intakes. I would disable any mechanical roof vents, the negative pressure they sometimes create makes the problem they supposed to fix even worse by pulling in house air through ceiling air leaks.
The important detail is to air seal your top plate. This can be done with one of the small two part spray foam kits. In colder weather this can be challenging but as long as you warm the tanks in hot water before you use it, it should work. You only need about an inch or so over the top plate and ceiling joint. You can also spray over all your interior partition top plates and any ceiling light fixture device boxes. Instead of spray foam you can also use duct seal mastic. This is essentially caulking sold in a pail that you can brush on. Slower application but works just as well.
Once you have your ceiling air sealed, fill the attic with insulation. The vent baffles will keep the insulation away from your roof deck and provide enough space for air flow.
Maybe the photo doesn’t depict accurately, but the overhangs can’t be accessed from inside the attic. The overhangs are on the exterior.
With spray foam or mastic, I am sealing the entire top plate? From the exterior joint between the top plate and the vertical piece of wood, to the interior edge of the top plate and just over the interior drywall (keeping in mind the superior edge of the top plate sits flush with the interior drywall).
So I would do the air sealing of the top plate before installing the baffles? Or can I install the baffles and then air seal just the interior edge of the top plate?
You want to seal between the top plate and the ceiling. You also want to seal any wire holes.
After you air seal, staple the baffle to the outside edge of the top plate and run it up vertically to the roof deck than along the slope up. Since you don't have trusses, there will be a gap between baffles. Probably the simplest is to cut a piece of 2x4 to height tack it in place with a couple of screws and staple the baffle to that. You can also tape the gap, just doesn't feel as solid as wood.
You should not need to get into your overhangs, about the only thing you want there is some vent holes, generally older houses have enough caps and cracks that you don't need to install dedicated vents.
Can’t I just staple to the roof deck or the skip sheathing? Instead of using 2x4s? The bottom 18-24inches will be sealed with the spray foam...up to the level of the batts...so does it matter if there are gaps in the baffles above that? The Raft R Mate baffles are 22-23” wide. It looks like I’ll have to trim the width to fit in most of the roof joist bays.
There is not need to seal up the baffles with spray foam. They are just there to contain the insulation, they don't need to be air tight at all. If you can do that with staples and tape, there is no need for extra support such as the 2x4s I mentioned. The only thing that needs to be sealed up with SPF is your ceiling since it is the main air barrier.
There is nothing wrong with stapling to the skip sheathing. Just make sure the vent channel stays clear when you blow in the attic insulation.
That piece of vertical oriented wood sits right on top of the top plate.