Quantifiable effects of wind washing in vented unbaffled cathedral ceilings over fibrous insulations.
ClimateZone8
| Posted in Expert Exchange Q&A on
I am trying to find information that quantifies the effects of wind washing of air permeable insulations in cathedral ceilings. I’ve done some pretty extensive searching online and can’t seem to find anything other than it’s good practice to baffle the vent space in a cathedral ceiling if the rafter bays below that vent space are insulated with air permeable fibrous insulations.
I’m particularly interested in any studies or other documentation that provides information as to the loss in r-value in an unbaffled vent space insulated with fiberglass batts…….but any studies or other tangible information on thermal performance effects of wind washing/venting over fibrous insulations in cathedral ceilings would be useful.
Point being, out in the wild here in AK contractors and builders in general are less likely to engage in a practice if the benefits or detriments aren’t quantifiable in some way.
Any insight would be greatly appreciated.
Ilya Benesch
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Replies
Ilya,
Not directly dealing with wind-washing in cathedral ceilings, but the analogous situation of batt insulation in vented wall cavities should give you some idea of the reductions. See tables 8 & 9 in this link.
https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
My own take is that if you forgo baffles over the whole length, it might still make sense to baffle the first three feet.
Here in Canada it's still a common practice not to use baffles, but our code mandated airspace is deeper than that found in the IRC, which means there is less chance of the batts blocking air-flow.
Hi Malcom. Here in AK it's so variable. Some places on Alaska's coast we've had to design homes that could resist 160mph winds (and weather) and other places the wind is generally pretty mild. And then there's the comparatively extreme sustained cold in Zone 8 which I expect also factors into elevated convective heat losses in vent spaces. Fiberglass batts are pretty ubiquitous here - particularly off the road system as the expense of flying in blowing equipment is often hard to justify. The batts appear to be the most vulnerable product due to density/fiber structure and I was hoping to find some information that focused on batts. That said, the document you linked is a very helpful. Thank you.
Ilya,
I design and build on Vancouver Island where the temperate climate means the risk of the roof assembly failure outweighs the losses to reductions in the insulation, so I like to err on the side of good drying potential.
I also like assemblies that take into account how their performance may deteriorate over time so I'd rather take an occasional reduction in the loss of the insulating value of the roof to keep the assembly safer.
Solid baffles worry me. With good air-sealing they shouldn't but again that might change. One assembly I think represents the best of both worlds is this: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Malcolm,
That is the way we have been doing them up here the last 10-15 years as well. Typically 2x4 on the flat and Tyvek House Wrap or Commercial Wrap if its one of our jobs. With existing framing and roof sheathing already on, which is a situation that comes up periodically, then the overventing approach doesn't work and it's back to baffles. The plastic vent baffles that are saturating the industry right now don't sit well with me. I expect properly installed and not squished flat, they do fine keeping the roof deck cold (assuming adequate insulation and air tightness in the thermal and pressure boundaries below) but the impermeability of the baffle material undoubtedly reduces the drying potential in the roof assembly. So that's what got me on this question to begin with. I'd rather use 1/4" or 3/8" CDX plywood for baffles for greater permeability. Or if there is a comparable durable, strong, permeable and CHEAPER material as an alternative to plywood baffles, I'm looking for other options. I'd love to have some targeted information that gauges the thermal performance improvement in baffling the entire length of every rafter bay in a cathedral ceiling that was insulated with fiberglass batts.
Batt insulation manufacturers don't distinguish how batts are used in terms of R-value as far as I know. An R-19 batt laying on an attic ceiling is R-19 just like it would be in a wall, but it isn't. Fiberglass batts must be enclosed on all sides to perform to the rated R-value. When I build cathedral ceilings the insulation is enclosed on all sides from top to bottom without exception.
Doug