Attic flash-and-batt with photos
Dana turned me onto the idea of flash and batt for our completely uninsulated walk-up attic in our 1910 duplex, and after many months of comparison shopping of foam installers, a re-visit after not-great application, two pallets delivered of rock wool, and many many hours of trimming batts (very few bays were a standard size for either 16 on center or 23 on center batts) and trimming polyiso to extend rafters…
we are almost done! Just a little drywall left to do!
Our humidity numbers in the attic are good, and our temperature is generally within 4 degrees of first floor temperature… sometimes warmer when it’s particularly sunny (when southern exposure upstairs bedroom also is running 4 degrrees warmer), sometimes cooler when radiators are warming on the first and second floor and it’s earlier in the day.
(By the time we got full coverage, our mild March was underway… but even so, some of those nights were in the 30s, and the attic temperature was going no lower than 61, which is what the thermostat is set to overnight)
Thanks so much Zephyr and Walta — without the polyiso furring strips, this would not have worked at all. It only cost about $100 more for the rigid foam, though the plastic washers and extra long drywall screws were about $100, too.
For those thinking of doing this, our 1,000-foot-board attic decking/gable walls cost $3,400 to have 2 inches of closed cell foam sprayed in (one inch on walls) — bids varied a LOT, the cheapest offer was 2,500, one was $6,000, I think?
the rock wool was about $1,200 (we’ll end up returning four bags of five batts unused)
I learned to use a power drill for this project, as my spouse is afraid of heights, so I was the one working on pieces of plywood resting across beams to install furring strips at peak of roof over the stairwell. 🙂
Will it be okay to leave the drywall unpainted? Do we need to mud & tape for safety?
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Replies
You need to mud and tape if this drywall is to serve as a fire barrier. You don’t need to do a paint-ready job of it though (you could skip the sanding part, for example). If I was doing this, I’d do a quick sanding and prime the drywall and leave it like that. If you don’t apply primer, the drywall will yellow after a while. I prefer the new white look, and primer is enough to keep things looking new.
Glad to hear your project is going well! You’ll be really feeling good about the work when you start saving money during the next heating season :-)
Bill
I could not open your photo. Is it set to be public?
Thanks for taking the time to tell us how your project ended.
Taping and mudding is important to stop the spread of a fire, also it serves a second air barrier. Spray foam seals many gaps but it will miss some.
Paint will be mostly cosmetic as your spray foam is your vapor barrier.
Walta
Ah, I had not slid the album to share with anyone with link. So I fixed that, now you should be able to see.
We will tape and mud. We have the joint compound on hand -- any brand/specification tips on tape?
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I like the fiberglass mesh tape. Much easier to apply but it take more cotes to finish than paper.
Not that it matters in an attic it would be better to avoid having joints at the corners of windows and doors, you are less likely to get a crack.
Walta
+1 for fiberglass mesh tape, but not everywhere. Use the mesh tape on all flat joints. Stick the tape on, then a coat of mud over the top. You’ll probably need at least two coats. Build it up so that it fills in the tapered edges of the panels so that you get a flat finished surface.
Use paper tape for inside corners. Put a thin bed coat of mud in the corner (a corner knife works best for this). I moisten the paper tape in a bucket of water before I start. Take the moistened paper tape out, wipe it between your fingers so that it’s not dripping wet, fold it in half and press it into the bed coat of mud. I then use a regular drywall knife to flatten the tape out, and usually to put a finish coat of mud over the top too. It takes some practice, but it’s not difficult.
I’ve never payed much attention to the brands of the tape. I think I’ve been using USG brand the most lately, probably because it’s what was in stock when I bought it. I like the easy sand hot mud (the “mix it with water” kind), and usually use the 45 minute version which is a good fit for me. 90 minute is better if you go slow or have a large job.
Bill
Paper tape with USG All Purpose (green label and lid). Additional coats use Plus 3.