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Insulating & airsealing an 85 year old attic

PYTN | Posted in General Questions on
I just bought a 1930s Tudor, in East Texas and had central air installed. Now I need to insulate the attic. I just got the R23 Rockwool delivered, but had a few questions:
What prep work should I do? Air sealing, etc? And if so, how do you air seal an attic?
Vapor/air barrier? What type/kind is best?
What else do I need to do to get the best insulated space possible?
Important notes about my house/attic. 
Vast majority of my ceiling is tongue and groove shiplap and I’m leaving it that way. So an air/dust/vapor barrier to keep dust from falling from the attic would be great
I’ve got no wall insulation and can’t put it in. However, I have nice, thick brick walls.
And I’m planning to enclose the attic at some point. I’ve got around 750 usable square feet, so at some point this insulation will be under floor decking.

So, how should I airseal it? And what other prep should I do before putting the Rockwool in? 

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi PYTN,

    We have an excellent article on air sealing an attic that covers most of the areas you'll need to seal and how to do it. You can find it here:

    However, if you plan to leave your wood ceiling in place, you essentially have one huge air leak and you'll need to find a way to deal with that. I'm a fan of drywall ceilings as an air barrier for a house's lid and even recommend drywall behind wood paneling. It's an affordable material and as long as you don't punch a whole bunch of holes through it, it's easy to detail as an air barrier.

    If you are not up for the task of taking down the wood, installing new drywall, and reinstalling the wood, you could instead install closed-cell spray foam to the attic side of the wood ceiling. This would give you an air barrier and get you off to a good thermal start. You can add as much fibrous insulation as you'd like on top of the foam. The two big downsides here are that you are using foam when you have other more eco-friendly options and that it will make remodeling in the future much more difficult.

    While it is not relevant to the topic of this thread, I thought I'd point out that tongue-and-groove and shiplap boards are two different things. The distinction would be relevant if you discussing the ceiling with a trim carpenter, for example.

    1. Expert Member
      Dana Dorsett | | #2

      You don't need or want a true vapor barrier in the assembly, even humid east TX, even if running air conditioning, but you DO want it to be air tight.

      >"If you are not up for the task of taking down the wood, installing new drywall, and reinstalling the wood, you could also install closed-cell spray foam to the attic side of the wood ceiling. "

      If going that route don't apply spray foam directly to the wood- it'll glue it all together making it a real PITA to repair/replace, and putting expanding between the t & g joints could even create seasonal warpage issues. If sealing it all with foam, put down some sort of slip/release surface first, such as kraft paper or housewrap, folded & side-stapled to the joists so that the foam doesn't touch any of the wood.

      And don't use closed cell foam, use open cell. A 3" shot of half-pound density open cell foam costs about as much as 1" of HFC blown closed cell foam, and makes a much better air seal than a 1" flash of closed cell. It would deliver about twice the R-value, using only 3/4 as much polymer as a 1" shot of closed cell too, and uses water rather than climate damaging HFCs as the blowing agent.

      In my area 750 square feet of 1" closed cell foam or 3" of open cell foam runs about a grand (it' too small a job for lower pricing than that.) YMMV. For that kind of money it might be better to just go ahead and apply it to insulating at the roof deck instead. The roof deck usually has less than 1.5x as much surface area as the attic floor. In your climate going with an unvented roof, a flash-inch of closed cell foam on the roof deck is sufficient protection from interior side moisture drives to go to raise it to code min or higher adding fiber insulation, and wouldn't be so vapor tight as to preclude adding rigid insulation above the roof deck at some later date when re-roofing.

      1. PYTN | | #5

        Thanks Dana. Unfortunately, it was price prohibitive to do foam. Quotes I got were higher than my budget, so I'm going with Rockwool in between the joists.

        How should I airseal the attic with the shiplap ceilings? Would caulk work?

        I'll omit the vapor barrier. Wood is everywhere in this house, don't want any moisture issues.

        1. Expert Member
          Dana Dorsett | | #7

          It is sometimes possible to roll out housewrap over the tops of the joists, then start tucking & stapling them to the sides of the joists near the ship-lap ceiling. This doesn't work so well if there are 1001 nail points sticking out at you, or there is a lot of electrical/plumbing/ducts to work around. Where sheets get joined, tape them with housewrap tape, and tape over the staples as well. It won't be perfectly air tight, but it'll cut the leakage by more than 90% compared to what's leaking through the ship lap.

          Caulk will probably not work out so well. The hygroscopic contraction/expansion of the wood will open up leaks pretty quickly unless using something really flexible long term, such as acoustic sealant (which is pretty stinky nasty stuff to work with). It might work using something like polyurethane caulk, which is has very strong adhesion (Gorilla Glue is pretty much the same chemistry) and at least some long term flexibility.

    2. PYTN | | #4

      Taking out the shiplap is definitely not an option. It's in there tight. Took me nearly a day to take down some around the shower to get ready for tile.

      I wanted to do spray foam on the attic floor, but none of the spray foam companies here would do it. They all wanted to directly spray the roofline and it was pricy.

      On the shiplap, if foam or drywall aren't an option, any suggestions on air sealing? Would caulk sealing the backside of the shiplap in the attic side do it?

      1. Expert Member
        BILL WICHERS | | #6

        That would be a LOT of caulking work. It might be easier to get some thin sheets of rigid foam cut to fit between the attic floor joists, then just seal the edges of the foam. That would avoid the need to seal all the gaps between boards.

        In my own house, I have one room of about 12 feet square that has a tongue and groove ceiling that is very poorly air sealed (just kraft faced batts above it). In the winters, I can see snowmelt on that section of the roof so I know it's pretty leaky. Since I'm planning to rework that ceiling completely in another few years, I'm doing a quick and dirty experiment at air sealing: I'm painting the ceiling and only caulking the gaps between the ends of boards. A coat or two of BIN primer and a few coats of latex ceiling paint and we'll see how well it works. I'm thinking enough paint will wick into the smaller caps to seal stuff, at least seal it better than the "nothing at all" that it is now.

        Note that painting isn't really an accepted way to air seal ceilings like this -- I'm just doing an experiment since I plan to replace the ceiling in the near future anyway. At most I'll get two winters with the painted ceiling before it's completely redone with drywall (the hard part is vaulting the ceiling, which requires some structural work).


  2. GBA Editor
    Deleted | | #3


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