| Author |
Message |
J. Peter Jordan
Senior Member
Username: jpjordan
Post Number: 818
Registered: 05-2004
| | Posted on Sunday, April 19, 2015 - 08:49 pm: |    |
Plastic "spacer" installed under the roof deck in wood construction to provide a vent space between the underside of the plywood deck and the insulation. Durovent is a product I found; my client (the Architect) found a product called SmartBaffles. My client believes that when properly sealed, they will also serve as an air barrier. Does anyone have any experience with this or have any other products we should consider?
This is a 3-story, wood framed, residential facility (similar to a fraternity or sorority) with an asphalt shingle installed over underlayment and plywood roof deck. |
Robert E. Woodburn, AIA, CSI, CCS, CCCA
Senior Member
Username: bob_woodburn
Post Number: 160
Registered: 11-2010
| | Posted on Monday, April 20, 2015 - 10:28 am: | |
1. How would a vent baffle be "properly sealed"? Sealant or tape on each baffle, each side of each rafter?
2. What are the chances of a typical residential insulation installer (or whoever installs this) achieving a virtually flawless, leak-free condition (since that's what an air barrier requires)? |
Colin Gilboy
Senior Member
Username: colin
Post Number: 399
Registered: 09-2005
| | Posted on Monday, April 20, 2015 - 10:46 am: | |
I believe that most attic spaces are designed to be vented and should not be sealed. Even vaulted ceilings have a air space between the insulation and the plywood. That is what soffit vents and ridge vents are for.
Colin Gilboy
Publisher, 4specs.com
435.200.5775 - Utah
800.369.8008 |
J. Peter Jordan
Senior Member
Username: jpjordan
Post Number: 819
Registered: 05-2004
| | Posted on Monday, April 20, 2015 - 10:57 am: | |
The problem with venting the attic is continuity of the air barrier. There is really no way that you are going to put an air barrier in the attic roof assembly.
The idea is that the insulation contractor (who will probably install this stuff) will seal the edges of this product against the rafters (I didn't say this was a good idea).
I have pushed for a composite roof deck product (plywood factory laminated to insulation with spacers for venting), but everyone thinks this is too expensive. |
Robert E. Woodburn, AIA, CSI, CCS, CCCA
Senior Member
Username: bob_woodburn
Post Number: 161
Registered: 11-2010
| | Posted on Monday, April 20, 2015 - 11:06 am: | |
As you may know, Joe Lstiburek has long advocated eliminating the vented attic, in favor of a sealed conditioned attic space, which would allow an air barrier somewhere in the roof assembly. It may well be that it is too late for that decision, or unlikely to be accepted because it is so unconventional, but if one wants an air barrier at the roof in wood frame construction, it may be the only practical way to do that. |
Ronald L. Geren, FCSI, AIA, CCS, CCCA, SCIP
Senior Member
Username: specman
Post Number: 1297
Registered: 03-2003
| | Posted on Monday, April 20, 2015 - 11:38 am: | |
Don't forget that gypsum board, when properly taped and finished, is an acceptable air barrier (not a vapor retarder). Penetrations for light fixtures and vents will need to be properly sealed. Seems less expensive than trying to seal a rafter vent space.
There are two things that need to be ventilated per the IBC: attics and enclosed rafter spaces.
Attics: Defined as the space between the ceiling beams of the top story and the rafters. Insulation is above the ceiling at the ceiling beams (usually the bottom chord of trusses). The free flow of air through the attic is permitted/required. The gypsum board at the ceiling provides the air barrier.
Enclosed Rafter Spaces: The space between the bottom edge of a rafter that forms the ceiling and the top edge of the rafter for the roof deck. The IBC requires a 1-inch gap between the insulation and the roof deck connecting eave and ridge vents. Gypsum board applied directly to the bottom edge of the rafters forms the air barrier.
The IBC requires vapor retarders in Climate Zones 6, 7, and 8, so those zones will need to have a vapor retarder worked into the roof assembly.
Just a side note for what's coming in the 2015 IBC: Nonventilated attics and rafter spaces are permitted (conforming to the same requirements that have been in the IRC). There are several conditions that must be met, but spray-applied polyurethane foam insulation is one method.
Ron Geren, FCSI, AIA, CCS, CCCA, SCIP
www.specsandcodes.com |
Robert E. Woodburn, AIA, CSI, CCS, CCCA
Senior Member
Username: bob_woodburn
Post Number: 162
Registered: 11-2010
| | Posted on Monday, April 20, 2015 - 11:48 am: | |
The IBC (in certain zones) requires a vapor retarder in the roof assembly, but attics must be vented?! Does that make sense? What am I not understanding? |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 893
Registered: 12-2006
| | Posted on Monday, April 20, 2015 - 06:17 pm: | |
Ron, doesn't conditioning attic space qualify as ventilating?
The way I understood Dr. Joe's thought (and I don't mean to speak for him, this is just my understanding of what he said during a presentation a few years back) was that the underlayment product, such as the one that Peter is referring to, gets sealed from the underside (taped joints?). The ventilated space (gap) between the insulation and the plywood sheathing on top is ventilated by means of ridge vents and soffit vents. They are used to cool the shingles so the shingles don't melt from the heat that would otherwise build up. These vents do not ventilate the interior space, that is the work of the HVAC system.
From what I recall, unless you are placing HVAC equipment or ductwork in the attic, simple registers should allow air to flow from the spaces below the attic though that may make balancing the HVAC system tricky. Ducted air is obviously an option but it would presumably increase costs. |
Ronald L. Geren, FCSI, AIA, CCS, CCCA, SCIP
Senior Member
Username: specman
Post Number: 1298
Registered: 03-2003
| | Posted on Monday, April 20, 2015 - 07:06 pm: | |
Ken, if it is part of the conditioned envelope, then yes, the attic does not need to be ventilated.
However, you will still need insulation between the rafters, so the 1-inch gap requirement between the insulation and roof deck would apply.
Ron Geren, FCSI, AIA, CCS, CCCA, SCIP
www.specsandcodes.com |
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