House wrap as rafter baffles?
shedworkshop
| Posted in General Questions on
I’m thinking about using Owen’s Corning PinkWrap as site-built baffles for my vented cathedral ceiling. It states that it has a perm rating of 15. Would that be a good option?
I have some Lowe’s housewrap that claims a perm rating of 12 that I’ve left sitting outside and water definitely pools in it. If I leave it crumpled up, I often see water just sitting in it days after a rain. So I’m also curious how effective the stated permeance is at allowing water to migrate out.
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I think this would be very difficult to use to form baffles. Normally you want to use a material with at least some amount of rigidity to it. Permeance is not particularly important, either, since the baffles are "leaky" in almost all cases, allowing a way for any moisture underneath them to migrate up and out. I also don't see that a membrane would be able to keep the vent channel open as you press fit insulation underneath.
Personally, I usually use 1/4" waferboard or 1/2" polyiso for site-built baffles, usually "framed" with 1x2 furring strips. You want materials that are cheap and easy to work with here, since you generally have to build a number of them and you don't want to have to use fussy materials for something like this.
Bill
Bill, thank you for the response! You and Michael have convinced me to just pick up some insulation boards and use them with furring strips. Any thoughts between using polystyrene vs expanded polystyrene?
https://www.lowes.com/pd/Kingspan-Insulation-Common-0-25-in-x-4-ft-x-50-ft-Actual-0-25-in-x-4-ft-x-50-ft-1-R-1-Faced-Polystyrene-Foam-Board-Insulation/999972970
https://www.lowes.com/pd/Insulfoam-Common-0-5-in-x-4-ft-x-8-ft-Actual-0-5-in-x-4-ft-x-8-ft-1-R-1-9-Faced-Polystyrene-Garage-Door-Foam-Board-Insulation/3033278
EPS, the expanded stuff, is usually harder to work with because it's more "crumbly". The 1/4" XPS stuff should work fine, just have SHARP blades in your knife when you cut the stuff.
Note that polyiso is a greener material than XPS, and so is the 1/4" waferboard I mentioned earlier. I actually really like using waferboard for baffles because it's so easy to work with and it's also cheap (~$11 or so for 4x8 sheet). You don't really NEED an insulating material for baffles, it's just commonly used because it's on hand or readily available.
Bill
...and if you are no longer concerned about the baffles being vapor permeable, see post #8 below on a way to avoid furring strips...
If you enjoy working with fabrics, which most construction-oriented people do not, housewrap would work just fine as insulation baffles. I would install 1x2 battens on each side of the rafter bay, cut the membrane several inches wider than the bay, then carefully stitch it up both sides using heavy-duty staples (the kind that you need a gun to install) or cap nails. Then trim the membrane flush on both sides. It would not hold up to dense-packed cellulose or wood fiber but batts would be fine.
I have used a similar technique several times when insulating rim joists on foam-free homes. It's finicky work and I would lean heavily toward a more rigid material, as Bill suggests.
Thanks Michael! It sounds like 1x2 battens and some sort of insulation board is the way to go. I posted above: any difference between going with 0.25" polystyrene vs 0.5" expanded polystyrene?
Shedworkshop.
I’ve used house-wrap as baffles over batts. Found it quick and easy to install. The lack of rigidity is probably a lot more important in areas where the standard vent space is only 1” deep, unlike here where the minimum is 2 1/2”.
I think the permeance is more important than Bill does. Vapour-retarders act as a percentage of their area, so whether the baffles have gaps makes no appreciable difference to the amount of moisture they let through - and a high perm material like house-wrap will allow a lot more moisture to be removed by the vent channel above than rigid materials do. That's particularly true if you use foam as baffles.
The water you see pooling on house-wrap is it performing exactly as they supposed to. They stop bulk water from moving through them while allowing water vapour to do so.
My vent space is going to be 1.75". In my head house wrap sounds easier to install over furring strips. Just stretch it, staple it, and apply tape over it to air seal. I'd get the benefits of greater permeance as well.
Edit: I guess my vent space would be 1.5" if I use 1x2 furring.
There is nothing wrong with cardboard baffles. Cheap, green and effective. No need to air seal baffles, you want to air seal the ceiling bellow, that is what must be tight.
I needed baffles and had two issues:
1. The bays were not exacty a consistent width = they were off by up to 1" and then there were irregular sized bays, so ordering baffles from the big box, even the special 1.5" ones, would not work.
2. I did not want to use cardboard due to all the articles about it disintegrating.
I used 1/2" foil faced polyiso.
Measure each bay, then cut a width of foil face polyiso that is that width + 2x the height of free space you want over your baffle - 1"
Then score your baffles on either side for the height you want - if you want a 1.5" free air space, score the piece of polyiso, on the OPPOSITE of the foil side, 1.5" from each edge.
Bend the scored polyiso up 90 degrees. If you did this right, the baffles will sit in the rafter bay loosely via friction.
Then use spray foam to affix the baffle to the rafter on each side, running a small bead of spray foam down the underside where the polyiso is cut where you scored it.
These will last longer than cardboard, there is less work to do up in the rafter bay because you cut & score the pieces on the ground and the work to do in the rafter bay is easy as you are just setting in the baffle and spraying foam. No nailers, blocking etc involved.
User...022,
But no diffusion through the baffles, so you rely on any interior moisture moving through the framing members before being vented.
Malcom - that is correct in the case of a vaulted ceiling where you are installing continuous baffles from soffit vent to ridge vent, and you are correct - the OP specified a vented cathederal ceiling and I missed that. OP should NOT use the method I described for a cathederal ceiling. I used it for a conventional vented attic...
Depending upon OP's climate zone, OP may want to consider Joe Lstiburek's vapor diffusion vent, but I agree that a continuous vent under the decking is preferable..
Deleted
Ok, in that case I would propose a variation of some of the above.
Get 2x2 and nail them up to the side of the rafters. Get a small roll of house wrap and cut it on a miter saw about 1/4" less than the distance between the rafters (ie 14.25" for 16 OC).
You can now take these small jelly rolls, staple and unroll as you go along the 2x2. If you want to air seal, you can put a bead of acoustic caulk on the 2x2 before the house wrap gets stapled on.
I ended up going with 1x2's nailed to the rafters. Pic of the in-progress work attached. The miter saw cut idea is genius, but some of my rafter spaces are different sizes (I know, I goofed). I'll probably skip the air sealing since it doesn't sound necessary. Anyway, seems to be going well so far, but critiques welcome!
There is no need to air seal vent baffles, your air barrier should be whatever is going under the rafters -- usually some kind of panel product (drywall). It's much easier to air seal the ceiling, and better too, because it helps keep moisture from getting up into the roof assembly.
A lot of the tricks you'll get from commercial building people on here will have to do with ways to save time on the job, since we are ALL trying to be efficient on jobsites. Akos idea to cut the rolls down with a saw is one such idea, and will save you tons of time installing the stuff into the rafter bays. I like to use a finish nailer to tack the furring strips up for a similar reason: it goes fast, the finish nailer fits into the rafter bays easily, and the 15 gauge nails my finish nailer shoots are plenty strong enough for furring like this.
Careful planning and lots of thinking ahead of time can save you tons and tons of time further down the road. The forums here will get you lots of ideas other people have come up with over time as well.
Bill