Retrofitting an existing attic to a conditioned attic (without spray foam)
Zone: 3A W/H
Roof Type: 4/12 Gable Roof with Tar paper & Asphalt shingles
I have an existing house I am considering converting to a conditioned attic and have found it fairly difficult to find a feasible insulation strategy that doesn’t involve spray foam.
The problems I have run into …
1.) The house has roof vents and gable end vents, but no soffit vents, so I am trying to determine whether I need to cut in soffit vents and a ridge vent if I do the retrofit or if going unvented is even a possibility.
2.) What insulation assembly to use that would be relatively cost-effective for the relative R-value and feasible to do a retrofit install.
I considered doing something like poly iso boards but that isn’t exactly the most cost-effective approach.
Currently, there is ~ 20in of relatively new loose-fill insulation in the attic I had considered removing it, installing a smart membrane like Intello Plus and then blowing the insulation back into the rafter bays.
Something similar to this…
https://foursevenfive.com/blog/unvented-foam-free-roof-retrofit-with-dolphin-insulation/
My concern was depending on what I needed to do with regards to the venting, I didn’t want to potentially pack anything against the roof deck that wasn’t capable of drying properly in the event of any type of leak.
Any suggestions on a sensible way to approach such a project?
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Replies
Astono1,
If your ducts are in the attic, it would be better to seal the space and bring them inside the conditioned structure. You could use reclaimed rigid foam on the outside of the sheathing, but it's best to use this approach when you also need a new roof. You might want to consider a one or two inch layer of closed cell foam in combination with air permeable insulation. (See this article for more info: https://www.greenbuildingadvisor.com/article/flash-and-batt-insulation.)
I don't know what the exact ratio of foam to batt is for my zone, but my concern was that would need something like 50% of the R-value from the foam (which gets fairly costly) just to prevent condensation. I just don't know if that is something that is specific to the flash and batt method or if it is a problem regardless of the insulation assembly used.
As the article points out, the 2012 IRC requires R-5 in Zones 1-3. Martin also notes that in center-of-cavity terms this means about 2 inches of closed cell.
Perhaps open cell spray foam would alleviate any concerns about spray foam.