Are insulation baffles necessary in this scenario?
Could really use some advice on insulating the following:
– I am having a 215 sq.ft addition built (will be a small kitchen and bath)
– addition will have a cathedral ceiling with 2×12 rafters (1 1/2″ x 11 1/4″)
– planning to insulate myself
– I am in N/E Kansas (zone 4 I believe?)
– I am required by code to have a minimum of R-30 ceiling insulation
– I’m leaning toward using R-30 mineral wool batts which are 7.25″ thick
– I understand that air flow above insulation is necessary.
– I intend to use soffit vents for each rafter bay (or continuous soffit vent)
– I intend to have a ridge vent installed with roofing
My Questions are:
1. Are baffles necessary in this scenario? (there will be 4″ of air space above rockwool batts)
2. If so….Is it necessary to run baffles along entire rafter bays…up to ridge vent?… or only at soffit end of rafter bays?…or not even necessary at soffit ends?
This may be a “no brainer” for many, but not for me! Any advice would be hugely appreciated!
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Replies
Leslie,
It's not a no-brainer, because there are two answers. In roofs like yours where the baffles aren't necessary to maintain the ventilation gap, their purpose is to stop air-washing - that is air moving through the batts and decreasing the efficiency of the insulation. Outside the world of high-performance building, you won't see baffles used in that situation. So I'd say the answer is: yes they are a good idea, and no they aren't necessary. In either case you do need baffles, or some other way of keeping the vent channels open at the exterior walls where the batt may block access to the soffit area.
You can infer from below yourself, but with the limited airflow through those vents, I can't see it being worth installing baffles just for the little R value loss in your case.
https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
Jon,
The study seems to be measuring the effects in essentially closed cavities with some pressure differentials and air-flow, but not where the wind is directly washing the surface of the batts the way it would in a vented cathedral roof. I haven't waded through the whole thing, but I'm not sure it is addressing what we are talking about here.
"It is assumed that openings are relatively small and that eddies within the wind are not carried into the cavity and the momentum of the wind is not impacting the material directly. Hypothetically, for large openings (such as slots larger than 50 mm [2 in.]) these phenomena could have significant scour- ing effects on fibrous insulations, increasing heat loss."
Hence "infer" - but please do post some better data. No, this isn't a case of direct wind exposure (velocity will be much lower).
With typical entrance and exits vents creating the resistance that limits flow rate, a large 4" duct/cavity will *reduce* velocity/scouring as compared to a smaller duct/cavity moving the same CFM.
Leslie: note that air sealing details would need to prevent air from flowing above and below the baffles.
Jon,
I have no other data, and know very little about the effects of wind washing. I'm interested in finding out whether it is as big an issue as is often said here on GBA, but that study doesn't seem to help me much, as I'm not as convinced as you it applies to intentionally vented roofs.
The issue is particularly relevant to me as this summer I am replacing my roof and will have access to all my rafter bays. Right now, as is the practice here, the batts aren't separated from the 3" vent channel. I have a few options. One is to leave things alone, another is to cover the top of the batts with rigid mineral wool boards, and a third is to site-build baffles. Which would you choose?
I agree that there should be a more definitive answer. I adjusted the numbers from the paper to fit Leslie's case, but don't feel it's accurate enough to say more than "< R1".
Fiberglass is much more susceptible to wind washing than mineral wool or cellulose. Also keep in mind perm ratio (I'd never use baffles made from foil faced foam).
Jon,
Given the very low numbers for mineral wool boards, I think I might use them. They would slightly depress the batts, but still leave 2" of vent-space.
It's appreciably more annoying to live in an older house that has some problems when y0u built it yourself.
Thanks so much for the input from both!
The way I'm now seeing it is that baffles at exterior walls "may or may not" be necessary, but are probably a worthwhile investment....especially if vent channels appear to need something to keep them open at the exterior walls. I'm thinking "when in doubt...and the additional cost is minimal compared to potentially losing insulation value or potentially having to fix moisture problems in the future....best to just go for it. Thanks a million for the input!
Something such as this? (doesn't mention anything about using with rockwool, but I can't think of any reason why it couldn't be) ....
https://www.sprayfoamsys.com/product/accuvent-acbp11754-14-5-x-41-16-o-c-flame-retardant-pvc-attic-vent-50-pack/?gclid=EAIaIQobChMIrJHX_sDU5gIVFP7jBx32OwweEAQYAyABEgJF8PD_BwE
...or these cheaper cardboard baffles o.k.?...
https://www.amazon.com/Cardboard-Attic-Baffles-Bundles-50/dp/B07WJV2F4F/ref=asc_df_B07WJV2F4F/?tag=hyprod-20&linkCode=df0&hvadid=385339520387&hvpos=1o10&hvnetw=g&hvrand=6294455673362736571&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9023847&hvtargid=pla-824482024507&psc=1&tag=&ref=&adgrpid=78829231576&hvpone=&hvptwo=&hvadid=385339520387&hvpos=1o10&hvnetw=g&hvrand=6294455673362736571&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9023847&hvtargid=pla-824482024507
Or even these...which say they're specifically for spray foam insulation but again, I'm not seeing any reason they wouldn't be sufficient for rockwool batts as well. (Am I missing something?)
https://www.sprayfoamsys.com/product/accublock-14-5-x-24-5-24-o-c-flame-retardant-air-block-vent-50-pack-acbp18600/?gclid=EAIaIQobChMIzbuagsPU5gIVqvfjBx050QDxEAQYAyABEgI4r_D_BwE
This is an interesting topic to me currently.
So you guys are saying I could friction fit a comfortboard and use it as a baffle? And wind washing would be negligible? This seems like a good option, although that amount of comforbard will be expensive. What about just regular rock wool batts with a tight fit? Or are baffles required by code?
Joe,
What most codes requires is a clear vent-space between the insulation and the sheathing above of a certain depth. There is nothing in ours that says you need baffles to maintain it.
I agree this is an interesting topic and I wish more posters would weigh in on it. If the penalty for not providing an air-tight seal above batt insulation is negligible, that represents quite a bit of work that doesn't need to get done.
When you redo yours, do all three options and put temperature sensors in the middle. Record the relative shifts in temperature with outdoor wind velocity.
My house would make a good test case. It has a variety of roof types and orientations.
I would have guessed that your roof would be too complex to rafter vent, especially without shingle vents.
It's got a flat ceiling at 8 ft, so about half the volume is rafters that all terminate in a small attic.
This is the one time I'm glad Martin has retired He's have a fit if he saw it.
>"– addition will have a cathedral ceiling with 2×12 rafters (1 1/2″ x 11 1/4″)
– planning to insulate myself
– I am in N/E Kansas (zone 4 I believe?)
– I am required by code to have a minimum of R-30 ceiling insulation
– I’m leaning toward using R-30 mineral wool batts which are 7.25″ thick"
Local codes may only require R30, but higher is still financially rational.
The current IRC 2018 code-minimum for zone 4 is R49.
There is sufficient space in a 2x12 cavity to stack R23 + R15 rock wool batts (9" nominal loft) for a total R38, and still have more than twice the IRC's code minimum 1" clearance between roof deck & insulation. I personally wouldn't install any LESS than R38, in a location on the cold edge of zone 4A, even if local codes are still stuck at IRC 2003 levels. YMMV.
IRC minimums for climate zone 4 ceilings bumped up from R30 to R38 in 2006, then up to R49 in 2012. At R38 you'd still be less than 10 years behind the times. Taking it to R38 rock wool would be more than a 20% reduction in heat transfer compared installing R30 rock wool in the same framing.
I have 24" depth cathedral ceiling to play with, so amount of insulation is not the concern.
It's the baffles and wind-washing I'm curious about.
Sorry to take the thread from OP but seems pretty related.
A consistently large duct (say 12") with code sized entry/exit vents is more like an attic, which certainly makes wind washing a non-issue, even with fiberglass.
Jon R,
I was planning to leave a 3-4" gap. I figure that will give me around R-60 up there.
Would be interesting to see an analysis of the cost of some options. Say mineral wool (of any kind) on the upper layer, adding a little more fiberglass (R2?), baffles and nothing (slightly increasing operational cost).
I don't know about anyone else, but I have a really hard time even locating mineral wool in my area. If I can even find it, it seems cost prohibitive in larger quantities.
Joe,
Isn't your roof quite shallow pitched? With 2 ft deep trusses why not use cellulose?
Good question. I'd like to but I live in a pretty rural area with limited contractor options. There are very few contractors willing to do cellulose around here.
I was also a little concerned about bulging with the weight of the cellulose up there. Although i was told the inspectors allow us to install sheetrock first, which would help with that.
I just got a quote back from a guy that does cellulose. Just to do the walls will cost me the same as doing the entire house(walls, ceiling, floor) with a combination of fiberglass BIBS and batts. Hard to justify.
Joe,
Is your 24" deep cavity provided by using I-joists as roof rafters? Because the web of I-joists is so narrow, will you have to modify your spacing to get a good friction fit using batts? For example, if I-joists are spaced 24" o.c., what is the actual distance between webs?
Also, when placing batts into I-joists bays, must the batts be in contact with the interior drywall? If so, would mineral wool batts have to be notched to fit tightly around each bottom flange?
I have trusses, so the web space is not an issue. However, all the spaces created by the geometry of the truss will leave gaps if I use batts. Blown-in would be more effective.