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Accu Vent Baffle Installation Detail Questions

I’m installing Accu-Vent baffles in a new construction home and have a few questions concerning installation details. Background: Climate Zone 4A, 8” raised heel trusses, 4.5:12 pitch roof with flat ceilings (although a small portion of the house has a 2:12 vaulted ceiling), R-60 cellulose on the ceiling. I am foaming the edges of the Accu-Vent to the trusses to minimize any air leakage up to the roof sheathing.
1. At the vaulted ceiling area, I need to use 2 Accu-Vent baffles to get the baffles above the top of the insulation. Should the lower edge of the top/upper baffle be installed above the top edge of the bottom/lower baffle, or reverse, or does it matter? In another thread, one person suggested “shingling” the baffles in case there is ever a roof leak so that the water will run down the side.
2. I am assuming that the small penetrations/tears for where staples fasten to the sheathing are not an issue, since there will be air flow at the underside of the sheathing. Correct?
3. If I provide caulk to try to get air-tight drywall, it seems like going on the bottom of two top plates would create a better air seal, since the Accu-Vent baffle has the lip on the bottom.
4. What is the best way to get the joint between the two baffles air-tight…caulk, spray foam, something else?
5. Any other tips, advice, or lessons learned?

Thanks to all,
David

Asked by David Baca
Posted Wed, 01/30/2013 - 12:44

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6 Answers

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1.
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David,
Q. "At the vaulted ceiling area, I need to use 2 Accu-Vent baffles to get the baffles above the top of the insulation. Should the lower edge of the top/upper baffle be installed above the top edge of the bottom/lower baffle, or reverse, or does it matter?"

A. I think it makes sense to shingle the baffles so that any leaks or condensation are directed toward the soffit.

Q. "I am assuming that the small penetrations/tears for where staples fasten to the sheathing are not an issue, since there will be air flow at the underside of the sheathing. Correct?"

A. Right. Don't sweat the small stuff.

Q. "If I provide caulk to try to get air-tight drywall, it seems like going on the bottom of two top plates would create a better air seal, since the Accu-Vent baffle has the lip on the bottom."

A. I'm not sure I understand this question. Just make sure that you install the baffles in such a way that there is a deep layer of insulation above the top plates. I'm a little concerned, by the way, that you chose roof trusses with an 8-inch heel -- that's not enough room for the R-60 insulation you hope to install.

Q. "What is the best way to get the joint between the two baffles air-tight…caulk, spray foam, something else?"

A. I suggest caulk.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 01/30/2013 - 13:00

2.
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1. We usually water-lap them. Accu-vent makes both a cathedral style baffle and extension baffle which are designed to interlock. Less ventilation area though.
2. Don't sweat it. My preference is to avoid the lower staples.
3. IMO, this is flaw in the design. We usually cut away the bottom lip of the Accuvent entirely (the part that says "Staple Here"). Depending on the vertical height at the wall plate you may want to cut away more. We'll usually try to bridge from the sheathing to the outside edge of the wall plate with the Accuvent, which sometimes requires cutting away a part of the Accuvent designed to sit horizontally on the wall plate. Basically, try to seal the Accuvent to the exterior edge of the wall plate.
4. Single -part foam. We mainly retrofit in really tight spaces, so we'll use a 42" single part foam gun. If everything is open you should be fine with a short foam gun. Have fun!

Answered by Jesse Smith
Posted Wed, 01/30/2013 - 14:10

3.
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Thanks for the replies, guys.

I will have R-60 on the bulk of the ceiling. For a small portion of the house where I have this vaulted ceiling, I will have less than R-60 (4.5:12 roof pitch, 2:12 ceiling pitch, 10" clear at the inside of the exterior wall). I'll have R-23 at the inside face of the wall and R-30 at 12" from the wall, R-37, at 2' from the wall, etc. The only other thing I could do at this point (and I mean tonight, because drywall comes tomorrow), is to fur the ceiling down 2". I've had to make some compromises since I changed from ocSPF to cellulose, but I'm doing the best that I can with where I at. Do you think the ceiling needs to be furred down?

Answered by David Baca
Posted Wed, 01/30/2013 - 17:14

4.
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David,
Let's see -- you'll have R-23 at the inside face of the wall. That means you'll have -- what? -- maybe R-17 at the outside face of the wall. Not great.

That probably violates Joe Lstiburek's rule of thumb: "A reasonable rule of thumb is: Thou shalt never, according to Joe’s Rule of Thumb, have less R-value on the top of your top plate than in the wall."

For more information on Lstiburek's recommendations, see Lstiburek’s Rules for Venting Roofs .

Answered by Martin Holladay, GBA Advisor
Posted Wed, 01/30/2013 - 17:50

5.
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Agreed, not great, but the only alternative is to fur down the entire ceiling 2". I would change the detail if I did it over again. If it were your house, would you fur the ceiling down?

I will have R=20 at the outside face of the wall (6" x 3.3=19.8), which is pretty close to my R=21 walls. If the density is increased (and I believe it will be more dense than "loose fill," my R value should go up.

Answered by David Baca
Posted Wed, 01/30/2013 - 18:01

6.
Helpful? 0

David,
Not great, but close enough. Probably OK.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 01/30/2013 - 18:11

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