Conditioned attic roof assembly
Many ideas have come and gone, nearly all have gone due to cost effectiveness in CZ-4A Chattanooga. And some due to area wide inexperience.
I have stuck with advanced framing, sealed crawlspace and Zip system sheathing to tighten up the structure. I also think creating a conditioned attic is the best method to air seal my roofline. Just to be sure that I’m thinking correctly I will summarize my options;
Spray foam under the sheathing – too expensive
Vent channel under the sheathing, air tight, fiberglass can then go under that airtight barrier.
Airtight sheathing, fiberglass, airtight barrier (IntelloPlus). Diffusion vent for moisture release.
I think this is it. I’m still trying to decide between the last two. I’m not sure which would be cheaper and easier to ensure airtightness.
Thoughts?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Brian,
Do you have a simple roof that allows venting of every rafter bay? (And how deep are your rafters?)
No hips, but there are several gables. I believe we could vent every bay but I think I see what you're getting at, if I can't vent that bay I have to spray foam.
2x8 rafters, I will probably need to gusset down some for extra depth.
Brian,
First of all, I think you should read this article, which explains all of the available options:
http://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
It's a building code violation to install fiberglass batts between rafters, unless (a) there is a ventilation gap between the top of the insulation and the underside of the roof sheathing, along with soffit vents and a ridge vent for every rafter bay, or (b) there in an adequately thick layer of rigid foam above the roof sheathing, or (c) you follow the rules for a flash-and-batt installation.
For more information on the hazards of ignoring this advice, see this article:
http://www.greenbuildingadvisor.com/article/smart-vapor-retarders-for-walls-and-roofs
In general a conditioned attic is a really bad idea but is better than an unconditioned attic full of ductwork and HVAC equipment but only slightly.
Once HVAC has taken up residency in attic it is hard to get rid of. If this is a new build do not do anything as stupid as installing HVAC in your attic.
I think the money you would spend getting a R38 roof is better invested in R60 on the attic floor with a mini split HVAC system keeping that junk out of the attic or moving the HVAC equipment into a well sealed crawlspace.
Walta
Martin, thank you for saving me from a doomed plan. I had thought I read up on nearly all the related articles here on GBA but missed those and thought I found a loophole.
So just to be perfectly clear, if I use ridgid foam to create vent channels, air seal. Then fiberglass and air seal again (here would be a good place for IntelloPlus, right?).
Walter, I hear you and I know it's not ideal. As I said, I think it's my best method of air sealing. I could be wrong, take a look at the link below and give me your thoughts.
https://www.architecturaldesigns.com/house-plans/country-home-plan-with-marvelous-porches-4122wm
We will be finishing the bonus room as well as adding a mirror image of the living room upstairs, we plan on a shed dormer in back from the ridge to grab that space.
Brian,
Q. "Can I use rigid foam to create vent channels, air seal, then fiberglass and air seal again? (Here would be a good place for IntelloPlus, right?)"
A. Yes, you can do as you describe, as long as you don't have any rafter bays that die into valleys or hips. (Ideally, you have a straight shot from your soffit vents to your ridge vent, in every rafter bay, without blockages or interruptions.)
Before you decide to use Intello Plus as your interior air barrier, make sure that (a) the manufacturer of Intello allows the membrane to be exposed on the interior, and (b) your local code official agrees. The issue is fire safety -- I don't know if Intello Plus can be exposed or not.
It might be easier to just use drywall as your interior air barrier.