Baffled by baffles and air sealing
I’ve overloaded myself on roof venting research and am now confused. Some write that you should air seal the baffle edges. That does not make sense to me, and I cannot find an explanation for doing it. I mean, if the goal is to get airflow via the baffles (and avoid insulation blocking them), why would it matter where the air leaves the baffles?
I’m in Zone 6A (Minnesota). 12′ x 16′, internally air sealed structure with Membrain, unconditioned attic, 12″ energy heel trusses, 24″ OC, R-49 batts between the trusses, venting in the soffit, ridge, and possibly gables, 2″ space between the baffles and the underside of the roof deck.
I’ll likely site-build the baffles using furring strips on the truss faces and span them with 1/2″ plywood.
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Vent baffles are usually used out at the eaves, and they serve primarily two purposes:
1- Prevent attic insulation (especially loose fill blown insulation) from getting out into the soffit areas and plugging up the vents, and
2- Prevent wind washing of the insulation over the top plates of the exterior walls
Neither of those two things requires that the baffles be perfectly air sealed. All you need is a reasonably well fitting baffle that can keep the insulatin in place and block the majority of FORCED AIR (i.e. wind) from getting up through the vent and blowing along the exposed edge of insulation in the top plate areas. There is no need to air seal things with caulk or foam here, all you need is to avoid gaps large enough for insulation to get through at install time, and a solid enough baffle structure to keep significant amounts of wind from getting into the edge of the insulated area. That's it.
I typically build baffles with either 1/2" polyiso (lightweight, relatively inexpensive, and easy to work with) or 1/4" waferboard (basically very thin OSB). Either material works. I like to use 1x2 furring strips to "frame out" the baffles, which I do by tacking the furring strips against the inside edges of the rafters up tight to the underside of the roof sheathing. Quick, cheap, and easy to install. There is no need for material as heavy as 1/2" OSB here -- this isn't a structural application after all. You don't really need a material that has R value, either, I just use 1/2" polyiso because it's convenient to use here. Sometimes with a roof that has minimal space over the top plate, I'll use some spray foam or cut'n'cobbled polyiso to get some additional R value right over the top plate, but usually there is enough space for blown cellulose to insulate that area sufficiently well that I don't do anything additional.
Bill
Thanks Bill.
How would you attach the polysio or waferboard to the furring strips?
Waferboard is easier to attach: I just use more finish nails. I use a 15 gauge finish nailer to tack the furring strips up, then I use the same finish nailer to tack the wafer board to the furring strips. I intentionally drive the nails through the wafer board at an angle diagonally through the wafer board, through the furring strip, and into the rafter, since I don't want to rely entirely on the small head of the finish nail to hold the waferboard. Driving the nail in diagonally helps to get a bit more holding power on that relatively thin piece of waferboard. This method works well, and goes fast.
With polyiso, nailing doesn't work very well due to the squishy nature of the material. I use a very wide (usually 4") cloth duct tape for this. I wrap the duct tape around the outside edge of the furring strip and under the polyiso, then tack up the furring strips at the accessible ends. I'm usually doing it this way when I can only work from the soffit side, so I often only attach the furring strips with 3-4 nails at an exposed tail sticking out past the polyiso. The polyiso/1x2 assembly is lightweight enough that this works OK. I finish off by putting a vertical piece in taped to the edge of the polyiso I just put in with the furring strips, then I tack or glue the lower edge of that vertical piece to the exterior side of the top plate. Usually you can use the tape like a hinge when doing this, so tape first, then swing the piece down into a bead of sealant on the top plate to secure it. Fast and easy. This also gets you a bit of R value on the exterior side of the top plate, but you have to have a soffit assembly that allows that to be concealed.
I will very ocassionally use some canned foam if I have some large and irregular gaps to deal with around the edges of that vertical baffle piece. The goal isn't really to air seal in this case, it's just to plug up holes that are big enough for blown insulation to "leak" through when installed in future construction stages. Canned foam is quick and easy for this. I use sealant on the top plate for the same reason: not to air seal, but because it's easy to apply and does a good job securing the polyiso panel in place. I usually just use polyurethane sealant for this because I usually have that on hand, but pretty much anything that can stick reliably to both the wood framing and the polyiso will work fine here.
Bill
Excellent!
Thanks again Bill.
Tiziano,
Depending on how crazy you want to get in cutting (and how cramped your work area is), you can pressure-fit polyiso fairly easily. Furring strips are still helpful to keep your distance off the sheathing, but you can eyeball it and do pretty well. If you're worried about them getting knocked out of place, a little adhesive or caulk on the edges should lock it in well enough.
Yep. That's going to be tricky since the trusses are 5/12 and only 12' long so my room to move around in the attic is restricted.
I was thinking I would attach the furring strips to the trusses before raising them, and then maybe delay sheathing the roof for a few hours to get the waferboard or polysio in.
If you do it that way, then I'd tack up the furring strips, put some sealant/adhesive on the face of the polyiso out near the edges, then pull it up from above to set the sealant/adhesive on the polyiso against the lower edges of the furring strips. The polyiso should be light enough for the sealant/adhesive to hold it in place while the stuff sets up and cures. This would save the somewhat fussy taping step I mentioned earlier. I don't normally have access through the roof to work, since I'm usually doing retrofits on existing structures. With the roof open, you have more options.
I'd avoid friction fits with your scenario though, since the rafters are going to move around a bit while the roof is completed, and that might cause some of the baffle panels to drop out.
Bill
That should still go pretty fast. Thanks.
The easiest way to do this, especially in a very cramped space, is to use polyiso boards only with canned spray foam as the adhesive.
Decide how deep you want the baffles and the thickness of your polyiso.
I used 1/2" polyiso, and wanted 2" baffles. So I measured the width of each rafter bay (my rafters were site built, so varied +/-1") Lets say the rafter bay is 15" face to face of rafter. With 1/2" polyiso, I cut a piece that was 18" wide. Then I scored the polyiso 2" on the non-foil side, being careful to leave the foil intack. Once scored, bend the 2" leg over. You now have a piece of polyiso with a 14" wide solid piece, and a 1/2" wide, 2" tall leg at each side, making the total width 15" (15 + 1/2 + 1/2). Take this piece an fricton fit is where you want, then spray foam it in place to cover the open scored 90 degree corner and adhere is to the sides of the rafter
There is no upside down nailing in an ackward position, and the polyiso goes in fairly quickly. If you can't reach all the way out to the edge, it's no big deal. The polyiso is rigid enough - as long as you get 2/3 of each side foamed it will stay. As we all know, the spray foam sticks to anything.
Thanks. That's clever.
Tiziano,
The design and installation of baffles matters a lot on cathedral ceilings where they are continuous from eaves to peak. For baffles that are just there to allow air to get above the level of the insulation in vented attics, why not buy staple up ones from a lumberyard?
Now that you mention it, I bet I missed that cathedral ceiling factor in regards to air sealing the baffles. That was likely what confused me.
As for the staple ones... I am considering those.
I'm further along with the design and have run into an issue. The truss blocking up slope will block the vent channels. I seem to need that blocking so as to have nailing surfaces for the roof sheathing.
That makes me wonder if I can skip the soffit vents altogether and simply use a full-length ridge vent along with gable vents? The roof is only 5/12 and 12' x 16'.
I'm using boxed eaves, air sealing the ceiling with Contega HF in the joints and MemBrain across the span. I'm using batt insulation in the attic (which also makes me wonder if baffles are needed anyway - does batt insulation suffer from wind washing?).
Tiziano,
Fiberglass batt insulation is very susceptible to wind-washing. Mineral wool less so.
Practically you can probably get away with a combination of ridge and gable vents, although it doesn't meet code. Can you post a drawing of the truss blocking? Your description of it being necessary as a nailing surface is unusual.
I wrongly assumed that wind-washing was a physical thing where blown insulation would get pushed by the wind.
Drawing attached. I hid quite a few elements to make the issue cleared.
Tiziano,
Isn't that blocking on the outriggers above the level of your batts in the attic.
For that first one, just barely. So if I run the baffles along that furring strip I'd have to keep the baffles quite short.
Plus, I've got blocking across all the trusses, in rows all the way up separated 2 ft oc for sheathing nailing. I'm thinking it's overkill and all I need is blocking at the ridge and the tops of the bottom chords.
Tiziano,
The only blocking you need is that called out on the truss drawings.
They only show the common and end trusses - no connections or how it all fits together.
Pages 2 and 3 of the attached.
Tiziano,
There is nothing to fit together. You stand the trusses, frame the lookouts, put on a rough fascia, and sheath the roof. No blocking, and the only bracing required is one on the bottom chord if you weren't putting in a ceiling.
Thanks Malcom.
I tend to over-build things, especially when I'm ignorant of the process.
I'm reading up on framing a trussed roof now.
There isn't much "framing" to framing a truss roof. Stand them up like Malcolm said, then put some temporary supports while you put in the ridge board. The roof sheathing acts to tie everything together as much more as is needed in most cases. You don't need blocking aside from any overhangs like Malcolm mentioned.
BTW, "windwashing" CAN sometimes move loose fill insulation around, but usually it refers to wind-driven air currents that circulate THROUGH the insulation out over the top plates and a bit past that area. Those air currents essentially cancel out a big chunk of the R value of the insulation where the windwashing is occurring. Denser insulation is less affected (i.e. mineral wool is less affected than fiberglass, cellulose is less affected than fiberglass, etc.). Denser insulation is more restrictive to airflow, but remember that any batt-type insulating material is NOT an air barrier, so they will all be somewhat affected to varying degrees.
Bill
Thanks Bill.
Decades ago I saw a neighbor build a garage. He put blocking every 2' all the way up to the ridge. That must have stuck with me.