Continuity of Ventilation Baffles in Rafter Bays
Hi everybody. Thank you in advance for any insight, your expertise is much appreciated. My wife and I moved into our current house about a year ago and this past winter the house was drafty and uncomfortable. We also noticed the snow melts off our roof significantly faster than the neighbors, leading me to evaluate our attic insulation. The attic has about 9-10 inches of loose fill fiberglass giving ~r-25 and was not air sealed. I spent a few days air sealing all penetrations and top plates in the attic and noticed we have soffit baffles in every other rafter bay. Would it be worth while to add soffit baffles in every rafter bay before adding cellulose insulation to r-60. There is a continuous soffit vent below.
Thank you
Tony
Michigan Zone 5
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Replies
Tony,
Check if your soffit intake vent area meets the requirements of the code (half to two thirds of 1/300 of the attic area). If the your existing intake is enough, adding more ventilation will not do much. Ice damming is mostly from interior air leaks, hard to vent your way out of it.
Sounds like you are doing all the right things. The existing 9" of insulation is pretty good, with the air sealing I would check that the ice damming is gone before adding in more insulation in case there are missed air leaks. Trying to air seal after with 2 feet of insulation is a pain.
Note that code requires 1/150 or 1/300 depending. And when it comes to ice dams, 2x more vent area is almost twice as good - building to code minimum is rarely a good idea.
Hi Tony,
You can look up Section R806 Roof Ventilation on line using the IRC 2015, for example, for free. You cannot download it, but is read only.
That said, the minimum vent area is 1/150 and 1/300 is an exception with certain caveats. It looks like your insulation requirement is R49 in your zone. With regard to vents in each bay, I think that would be overkill, unless you don't meet the minimum ventilation requirement with what you have currently. Baffles only need to be higher than the attic ceiling insulation to allow free venting.
Tony,
If some rafter bays lack ventilation baffles, that lack can definitely make ice dams worse. If you have the patience to install the missing baffles, you should. Once you've done that (assuming you've done a good job of air sealing as well), consider adding more insulation, especially near the eaves.
I went up and took some measurements and counted vents. I have the following:
1. 7 air vents near the top of the roof. Assuming 50 square inches each = 350 in 2 = 2.43 ft2
2. 20 raft-r-mate soffit baffles. The product data sheet has 22.3 in2 listed as the free are, but i'm assuming it would be 11.15 since they are split in half to fit the 16 in oc rafter bays = 223 in2 = 1.55 ft2.
3. The total area of the attic is ~1000 square feet.
It seems like my intake is low. I plan on using provent baffles to add intake, each will add 15 in2 - will need to add at least 9 to achieve 50:50 exhaust to intake.
Does this all sound correct?
Also, I've been struggling with adding some polyiso to attic kneewalls created where a cathedral ceiling intersects the attic. I want to use polyiso on the attic side to cut down on thermal bridging, I verified with a local inspector that this is ok as the attic space would be considered inaccessible. I have found the actual walls are ok to insulate in this manner, but is the slope over the ceiling ok to cover with poyiso? The area in question is similar to the first picture. The next two pictures show the polyiso I started installing. I have no problem uninstalling the polyiso from the over the ceiling if it poses a problem, I have plenty of other uses.
Tony,
If possible it's good to have more intakes than outlets. It helps slightly pressurize the attic, which stops air from leaking through the ceiling.
I understand the concept but this seems to be a subject with minimal published data. Are we talking $1/year or $100/year with a well sealed attic floor?
This study found that a balanced soffit/ridge ratio was most efficient. And that simply adding more ventilation was more effective than variations in ratio.
https://pdfs.semanticscholar.org/11e8/3cf11adeb2483674cc07650a611876032435.pdf
Could you tell me where you received this info regarding pressurizing? Thanks
Robert,
From Martin's article All about Attic Venting :
"Lstiburek advises builders to include more soffit ventilation than ridge ventilation. “Building codes suggest balancing the intake and exhaust ventilation,” Lstiburek wrote. “The code, however, is wrong, and I’m working hard to get it changed. More ventilation at the eaves than at the ridge will slightly pressurize the attic. A depressurized attic can suck conditioned air out of the living space, and losing that conditioned air wastes money. For best results, provide between 50% and 75% of the ventilation space at the eaves; a 60/40 split is a good sweet spot.”
The quote from Lstiburek isn't fully accurate. Balanced would result in the air at the attic floor slightly depressurized. More intake reduces that effect, and brings the attic at the floor level closer to atmospheric pressure at that elevation. But that difference is small compared to the stack effect pressure inside the house that is trying to drive the leaks into the attic.
Charlie,
Yes - probably "try and minimize depressurization" might be a better way to state it.
I doubt the difference between equal or slightly more vent space at the eaves is consequential, but the opposite ratio certainly doesn't help, in much the same way mechanical extraction fans don't.
1. Did you insulate the top plate at the soffit?
2. I do not know where you’re from, but insulation against the roof deck is a no no, especially in the east…
3. I’ve read, and found that insulating all the way out to the gutter is the best way to keep From creating ice dams!
4. Depending on the pitch of your roof, better to insulate the roof deck to cut down on thermal bridging then internally adding a bunch of oil cans with foam ?
On this general issue of airflow and venting: I've got a knee wall situation and I'd like to insulate under the roof slope vs the backside of the knee wall. But I only have vent holes in the blocking on every other rafter bay. I suppose I could drill more vent holes otherwise I don't see how I'd get good airflow in those bays. Remind me never again to build with knee walls. What a pain.
I’m so intrigued by this thread. I had to step back and realize that whatever I ended up with is miles ahead of where my 100 year old house’s attic started before this rabbit hole.