Air-Sealing Cathedral Ceiling Baffles
I am building a house in the mountains of Colorado in Climate Zone 7. The house will have ~1600 sq ft total of cathedral ceiling. Roof will be stick framed with 2x12s on 16-inch centers. We were going to do parallel chord trusses until we saw the price. Lol. Rafters will have depth increased using ½ inch osb/plywood gussets and 2x3s to get to ~r53 whole roof r-value using fiberglass batt insulation. The gaps between the 2x12s and rafters will be filled using ripped sheets of EPSs and nailed to the gussets to provide a thermal break (yes that does involve work that I am willing to do). Interior air barrier will be Intello plus. Lots of attention will go into the interior air barrier using methods described here. A ridge vent will be present.
Exactly how important is fully air sealing cathedral ceiling ventilation baffles assuming there is a good interior air barrier such as Intello plus and other interior air sealing details are sound?
GBA and a few references in Fine Homebuilding (often by the same GBA contributors) are pretty much the only places on the internet you will see actual suggestions to fully air seal cathedral ceiling baffles from soffit to peak whether manufactured or site built. Doing this level of detail seems unnecessary and is not a common practice. I cannot seem to find anything definitive anywhere that comes close to quantifying the degradation in fiberglass batt performance in a cathedral ceiling. All that is ever stated here in articles and comments and elsewhere is that you need to seal the baffles to reduce wind-washing and degradation in performance. There is almost nothing else out there whether blogs, forums, youtube, manufacturer suggestions or general google searches that reference fully sealing cathedral baffles. I have seen plenty of references where people put NOTHING at all for an actual ventilation baffle and the builders just stuff an r30, r38 or r49 batt into the rafter and simply leave a gap below the sheathing for ventilation. There are also manufacturers who state that their baffles in a cathedral application need an inch or two gap in-between each baffle up to the peak. An example of this is Owens Corning:
Even the often-referenced Energy Star Thermal Bypass checklist that states baffles are needed doesn’t show them air-sealed. In fact, you can still see daylight in their picture on page 16. That actively seems to go against the air barrier on all six sides mantra mentioned by them and often repeated here. I do realize that there is no reference to cathedral ceilings in the checklist, but I imagine the principle is the same at the soffit even if the insulation is just on the floor of the attic. My own house I am in now just has R-49 fiberglass blown on the floor of the attic. What is the level of degradation my insulation experiences? The attic ceiling is very well sealed, but the insulation is just open to the air above.
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/TBC_Guide_062507.pdf
One company has a piece of polyboard that helps reduce wind washing, but there is no actual “seal” via caulk, tape, or foam.
Through my research it appears that one of the best products among a slew of crummy ones for this application is the Accuvent Cathedral Ceiling Baffle and the extension:
https://jrproductsinc.com/product/accuvent-cathedral-ceiling-baffle-16-o-c-50-count-bundle/
https://jrproductsinc.com/product/accuvent-cathedral-extension-24-o-c-50-count-bundle/
I feel like a reasonable compromise would be to use these two products, install them properly and then air seal only at the soffit with canned foam along the seams there and call it good without air sealing the rest. The rest of the roof is described above.
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As I read it, air sealing the baffle (or even omitting gaps) violates code (because it creates a separate "enclosed rafter space" without ventilation). Owens Corning seems to understand this.
R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed
where ceilings are applied directly to the underside of roof rafters shall have cross
ventilation for each separate space by ventilating openings ...
I believe that wind washing with baffles+gaps is so negligible that it should be ignored. But you could use a layer of mineral wool and then fiberglass.
That is an interesting take Jon R. I don't THINK that is the case. If the insulation can dry to the interior with a type II vapor retarder and the area above the baffle can dry into the 1.5 inch gap, wouldn't that address that?
I do agree and also believe that the wind washing impact would be negligible in comparison to the extra cost and time to fully seal the baffles that it would not come close to worth it.
There is a common misconception along the lines of "part of my wall/roof can dry outward and part can dry inward - so it will perform well".
During Winter, when vapor drive is outward, "inward drying" does not prevent moisture accumulation on the cold baffles.
The best counter argument would be that baffles are so vapor permeable that there is no need (other than code) for ventilation of the formed rafter space.
I do see your point that while the proposed system can't stop the moisture, there is an avenue for it to get out.
I think the reason for the air sealing is the same reason you air seal the exterior of a wall and also the inside of the wall with drywall.
I think it's to reduce the possibility of air that has a large delta from coming in contact with a surface that it can condense on. If cold air comes in from the eaves and makes its way to the backside of your warm ceiling drywall then there will be condensation. It might be better to apply blown in cellulose instead of batts to help reduce the chance of air getting through the insulation if you don't want to air seal the baffles. Let me know your thoughts.
I do agree that the reasons for air sealing the baffles is the same as for a wall. That is surely going to happen even if I attempt to seal the baffles. However, being able to transport moisture out the baffle vent or to the inside to dry "should" take care of things as long as attention is paid to the details. Regarding blown cellulose, I am not ruling it out, but finding someone to do it in our ultra-high demand area and the associated cost vs DIY has me leaning towards the batts.
JWolfe1,
The effects of wind-washing on the insulation which is protected by the baffle but just not air-sealed, appears to be minimal. https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
Thank you! That is good information. The fiberglass reduction isn’t too bad in this study.
I'm with Malcolm on this one: the baffle acts to block the vast majority of airflow from getting to the insulation, so any remaining small leaks are unlikely to have any noticeable effect. I typically just use thin (last ones were 1/4" chipboard) wood materials to build baffles, and I tack them up with a finish nailer using furring strips tacked to the inside edges of the rafters. I don't bother sealing the connections, and I suspect thin wood products like this are reasonably vapor open anyway.
I do sometimes foam in small vertical pieces out at the eaves to connect the top plate to the baffle, but that's the only part, and I use the foam because I can't fit my finish nailer out into the eaves from the interior.
Remember that baffles are there to help direct airflow, not to actually block air movement the way drywall air barriers are intended to do. Baffles are NOT air barriers -- they don't serve the same function as "air barriers" as we understand them in walls and ceilings.
Bill
I had not considered the 1/4 inch chip board. The sum of what I have gathered it seems fully air sealing all the baffles won’t be time well spent.