Non-vented Attic
Faulted1
| Posted in Energy Efficiency and Durability on
Can someone describe or link to a good example of a non-vented attic assembly which minimizes or eliminates spray foam insulation. Suitable to a retrofit project — Min R-38 – Zone 4
Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
John,
Here are two choices:
1. Rigid foam above the roof sheathing -- I would use at least R-13 (2 inches of polyiso), but more is better -- and as much dense-packed cellulose under the roof sheathing as you need to achieve your R-value goals. It's easier to install dense-packed cellulose if your roof is framed with conventional rafters, not trusses.
2. Install a ventilation channel under the sheathing using Accuvent panels or site-built channels with rigid material like thin plywood, and then install as much dense-packed cellulose as you need to acheive your R-value goal.
John N.,
sorry that I hijacked your sprayfoam thread.
Since you are now talking about retrofit and non vented attic...
Don't throw sprayfoam out of the toolkit.
It may be the better option compared to the alternatives.
We have used dense-packed cellulose in the rafters and then rigid foam on the interior side with success on a couple of projects. The downside is reduced drying potential to the interior, so be sure to use a non-faced product.
Chris,
Another downside: your approach violates the code.
John,
In case you missed it, here's a link to an article on the topic: Creating a Conditioned Attic
John B,
No apology necessary It is interesting reading. Retrofit is not the most accurate description but it is a shorter story than project gone off the rails.
Martin,
Thanks for reminding me of that article. It is very good and to the point.
Martin,
I guess I need a little clarification on the semantics: Wouldn't an attic with "a ventilation channel under the sheathing" be a vented attic?
I'm guessing that John N. is trying to avoid soffit and/or ridge vents for some reason.
A similar approach that wouldn't require the soffit vents would be to fir out 3/4" for a second layer of roof sheathing. Semantically, at least, then the attic would be unvented.
Kevin,
Here in the U.S., unlike in France, we don't have a government-funded agency devoted to vocabulary and definitions. But I think it's possible to have an unvented conditioned attic with insulation along a sloped roof assembly that includes a ventilation channel between the top of the insulation and the bottom of the roof sheathing.
In the situation I just described, the roof assembly is vented, but the attic is unvented.
Martin,
It hasn't been a code issue in any of the projects that we used that install. Could you elaborate?
Thanks
Chris,
You can read more about code issues for insulated sloped roof assemblies here: Creating a Conditioned Attic.
Here's the scoop: The 2006 International Residential Code (IRC) sets out roof ventilation requirements in Section R806. However, in Section R806.4, the IRC permits conditioned attic assemblies without any roof ventilation; according to the code, if ventilation is omitted, only “air-impermeable” insulation can be used in contact with the roof sheathing.
Although the 2006 IRC neglected to include a definition of “air-impermeable insulation,” that omission was rectified in the 2009 IRC, which defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTM E 2178 or E 283.” In other words, an air-impermeable insulation must meet the same airtightness standard as an air barrier material. Although spray foam insulation and rigid foam insulation can meet this standard, dense-packed cellulose cannot.
If you choose to install a fibrous insulation like fiberglass or cellulose between your rafters, you must include a ventilation channel between the top of your insulation and the underside of the roof sheathing.
Section R806.4 of the 2009 IRC allows builders to insulate unvented rafter bays with a combination of air-impermeable insulation (for example, closed-cell spray polyurethane foam) and air-permeable insulation (for example, cellulose), as long as the thickness of the air-impermeable insulation that is “applied in direct contact with the underside of the structural roof sheathing” meets the minimum R-values required for condensation control shown in Table R806.4. These values are:
* Climate Zones 1-3 — R-5
* Climate Zone 4C — R-10
* Climate Zones 4A and 4B — R-15
* Climate Zone 5 — R-20
* Climate Zone 6 — R-25
* Climate Zone 7 — R-30
* Climate Zone 8 — R-35
Thanks Martin,
In our projects, the code officials have treated open cell and cellulose the same as closed cell foam for unvented roof assemblies. National Fiber also warranties (for what that is worth) unvented roofs with dense-packed cellulose as long as it is in at least a full depth 2x10 rafter bay.
Thanks again for citing the location/ text of the code.
Chris,
That's really interesting, I think I'd try that assembly in a dry climate. I wouldn't put anything impermeable on top of the sheathing, just tarpaper and shingles, or tarpaper and metal roofing on 1/2" furring strips.
Getting the permit may be difficult, though.
John
I have been dense packing cathedral ceilings with no ventilation for two decades with dense pack cellulose and have not heard of a single problem. However to meet code in this case I think that installing rigid insulation under the roof sheathing before dense packing would meet meet the code for an air impermeable insulation under the roof. I would like to have other input on this before you move forward
A recent article in JLC shows a blown-in cellulose unvented cathedral ceiling. The author also states having done "hot roofs" like this for 30 years without problems:
http://www.ihtmv.org/PDF/HPHomes.pdf