Subscribe or Renew Membership Subscribe Renew

Unvented Attic Space Right for Me in AZ

adamflyer's picture

I am wrapping up the plans for a house I am planning on building in the next few months. I'm located just outside of Prescott, AZ (Climate Zone 4B). I am doing a 1.5 story with 16" TGIs 24" o.c. for the vaulted ceiling/roof so enclosed rafter assembly. I am planning on going with a metal roof incase that matters. I would like the rafters to be insulated their full length (exterior wall to ridge board vs stopping at a knee wall. My plan is to use fiberglass batts. I don't want to use spray foam (cost prohibative) or ridgid foam on the outside of the sheathing (just don't like the idea and cost prohibative). I'm open to other options if there are any. I know the conventional way to do this is to leave an air gap between the top of the fiberglass batt and the sheathing, vent at the soffit, and vent through a ridge vent.

I know a lot of people are going with unvented attic spaces these days. I asked my building inspector if this works well for may area/climate and he is pretty much sold on spray foam but did not have any advice for other ways of doing it. He just said he would be good with what the code says (IRC 2012).

Does anyone have any experience going with an unvented enclosed rafter assmbly with fiberglass batts? I have read all kinds of congecture on the subject but yet to find anything that seems objective.

Per ICC 2012 R806.5

2. No interior Class I vapor retarders are installed on the
ceiling side (attic floor) of the unvented attic assembly
or on the ceiling side of the unvented enclosed rafter
assembly.
 
So just regular drywall should be fine right? Is drywall classified as a vapor retarder? I'm planning on using 5/8".
 
Per ICC 2012 R806.5
Air-impermeable insulation only. Insulation
shall be applied in direct contact with the under
-side of the structural roof sheathing.
 
This is what I plan on doing with fiberglass batts. Looks like Owens Corning sells R49 fiberglass batts that are 14" thick. Does anyone see a problem with this or a better option?
 
Thanks,
Adam
 

Take a look (post #215509, reply #1 of 2)

Take a look at the photo below, which is exactly like what you intend to build. I will admit, our decision to use batt insulation was cost driven. Spray foam meets the code and guards against moisture issues that MAY arise down the road. Your location is so dry that I can scarce imagine that moisture will be an issue. You could achieve a similar result packing layers of rigid foam boards into the rafter bays. 

The next way to meet the code and ward against moisture issues would be to use a 16" TJI. Create a vent pocket in the top 2" of each rafter bay, and be sure to allow for venting at the bottom and top of each rafter bay. At you can see in the photo, mine is a habitable attic, with rafters bearing directly on exterior walls. No o inport air can enter every bay due to a front porch roof (which is why spray insulation is commonly used).

Finally, you can plug up the whole rafter bay with batt insulation. Doing so exposes the risk that moisture may gather in the rafter bays and deteriorate the insulation. My code inspector was mostly concerned with meeting minimum IRC2012 standards, so he was OK with batting the entire rafter bay.

Since my TJIs are 14" deep and spaced 24" OC, I  used 5/8 drywall without vapor retarder, the aim being to not trap moist air in rafter bays. BTW, I typically use shellack primer (B.I.N.) in two coats as vapor barrier. This allows me to glue and screw drywall.

Keep in mind that the kind of TJI you use depends on both spacing and span, and the location of  knee walls. Typically a kneewall is not considered load-bearing, but its presence does determine TJI type. I'm sure you can do the math. If not, table may prove helpful, or you can contact the manufacturer directly. I received helpful advice from the in-house structural engineer.

Mel Fros froscarpentry.com

PreviewAttachmentSize
several_rafters.jpg
several_rafters.jpg35.63 KB

IMHO (post #215509, reply #2 of 2)

Were I in your situation, I would re-think the spray foam. I have found it to be very effective.

In the long run, the extra cost ought to be covered by your reduced heating and cooling costs. I don't know what you are seeing for prices, but the product appears to now be far cheaper than 5 - 10 years ago. I had 4" of foam installed in 3 large attics for a cost of about $2k.

Good luck.