Attic insulation guidance

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
Old stone house (1700's vintage) . Basically a thick fieldstone and mud mortar shell, with a wood frame built inside for floors/ceilings. Space between second story ceiling and attic floor is sparsely insulated with blown in fiberglass, no air barrier.

Plan is to fill the small, uneven space between stone walls and wood frame with spray foam, and replace fiberglass with cellulose (both blown in). And, importantly, add a plastic sheet air barrier.

Finally my question:

Should the plastic sheet air barrier go on top of the cellulose and joists, or under the cellulose. On top would be easier, under would require lots of extra work, going over the plaster ceiling and wrapping each joist on the sides and top.

I may have just answered my own question, but I'd like some expert opinions on whether the air barrier should go above or below the cellulose insulation in the joist bays.
 

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
Here's an article that should be helpful.
Thanks CTyankee. I love old books and articles so much more than stuff from today. Much more soul. Even when reading on a small digital screen.

According to the article, the vapor barrier goes at the bottom of the joist bays.

Preferably, I suppose a vapor barrier would go up, fastened to the bottom of the joists, before installing drywall or plaster. As that's not an option, here, I guess I need to figure on a tougher job, rolling over the top of the joists and tucking into the bays with the cellulose on top of the plastic.

A thin 1/2" layer of spray foam over the top of the plaster would make this easier. Sure hate the cost of the foam, though.
 

BrianVA

New Member
Oct 28, 2020
19
Central VA
Any thought to insulating under the roof rafters with rigid foam board, rather than over the 2nd floor ceiling? If you have HVAC duct work up there it might make sense. It basically turns your attic into conditioned space. I'm guessing you don't, based on the historic nature of the house. But just thought I would throw an idea out there.
 

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
Any thought to insulating under the roof rafters with rigid foam board, rather than over the 2nd floor ceiling? If you have HVAC duct work up there it might make sense. It basically turns your attic into conditioned space. I'm guessing you don't, based on the historic nature of the house. But just thought I would throw an idea out there.
Thanks for the idea, I like it and had considered it. The attic walls, dormers and windows are way too leaky and uninsulated, and rafters are undersized, and low to the attic floor. So it would be a way bigger job, and probably less effective once completed.
 

BrianVA

New Member
Oct 28, 2020
19
Central VA
Thanks for the idea, I like it and had considered it. The attic walls, dormers and windows are way too leaky and uninsulated, and rafters are undersized, and low to the attic floor. So it would be a way bigger job, and probably less effective once completed.
That makes sense. Old houses are really cool, but they can be quite a challenge!
 

Highbeam

Minister of Fire
Dec 28, 2006
17,996
Mt. Rainier Foothills, WA
The ceiling makes as good of an air barrier as you need. Air seal around the edges, tops of walls, penetrations, but dont put plastic on top, just blow in right over the ceiling.

Latex painted plaster ceiling = air seal. I tested this by putting my mouth against the ceiling and trying to blow into my attic. No luck.
 

Hogwildz

Minister of Fire
I'd be putting 2" polyiso board on the bottom of the 2nd floor ceiling. Tape all the inside joints, leave a 1/2" or so gap around the perimeter and can foam the gap closed. Iso is approx R6 per inch, plus with foil faced, there is your barrier and air seal with the taped joints & foamed perimeter. I did that here under 1x8 T&G ciling with nothing but paper backed R30 laying ion top of the T&G. Makes for a tight seal + the added R value. can always blow in more whatever on top of the loose FG later.

Screw 1x3 lathe to the bottom of the iso, and install whatever you want to the lath. I took off the T&G, and reinstalled it to lath under the iso. The batshit showers sucked when removing the T&G.
 
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brenndatomu

Minister of Fire
Aug 21, 2013
5,530
NE Ohio
lan is to fill the small, uneven space between stone walls and wood frame with spray foam, and replace fiberglass with cellulose (both blown in). And, importantly, add a plastic sheet air barrier.
Maybe I misunderstand the plan here, but spray foam is an air barrier, why add the plastic?
I'm hearing that this is getting to be a somewhat common practice, a thin layer of spray foam to air seal, then obtain the rest of the R value with cheap cellulose.
 
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hockeypuck

Feeling the Heat
Sep 6, 2009
347
south central NH
Maybe I misunderstand the plan here, but spray foam is an air barrier, why add the plastic?
I'm hearing that this is getting to be a somewhat common practice, a thin layer of spray foam to air seal, then obtain the rest of the R value with cheap cellulose.
You will have to check with the actual company that makes the foam product, but I think it takes 2" of foam for an official vapor barrier. But I think we are splitting hairs. 1" would probably do it in my book.
 

brenndatomu

Minister of Fire
Aug 21, 2013
5,530
NE Ohio
Same rule as for Isaac - pix or it didn't happen!
Just don't ask him for photo documentation of how he does his OPE winterization! !!! ;lol
 
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John Galt

Member
Oct 22, 2019
64
W Montana
Seems like there are two things happening- walls and ceiling. The foam was for the walls, plastic for an air barrier in the ceiling?

1" of closed cell foam is considered a vapor barrier. I've never seen it sprayed directly on a stone wall but I don't live in that region. Blowing cellulose would require a permeable sheeting to hold in the dense pack, not plastic, since you don't want two vapor barriers with insulation in between. The wall has to dry to the inside or the outside. A wet pack cellulose might work. I think that forms an air barrier and then plastic is stapled inside as a vapor barrier, no foam required.

The ceiling shouldn't need a vapor barrier assuming the attic is vented. Highbeam is right, paint is an air barrier so is plaster. Sealing up light fixtures, outlets, and other penetrations to avoid a stack effect will help more than a plastic sheet under the insulation. The water vapor has to go somewhere, you don't want it condensing in a place where it won't dry.

With a house that age the worst thing you can try to do is attempt to seal it up. You'll never get an air tight house and you could cause some serious rot and mold issues.
 
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Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
Maybe I misunderstand the plan here, but spray foam is an air barrier, why add the plastic?
I'm hearing that this is getting to be a somewhat common practice, a thin layer of spray foam to air seal, then obtain the rest of the R value with cheap cellulose.
Air barrier + cellulose for R value, just as you said.

It's either plastic sheet or thin layer of spray foam, I wouldn't do both. I'll probably go with the plastic because it'll be much cheaper, even though it'll be more work. The joists will be under the air barrier, but I think that'll probably be ok.

EDIT:. I'll probably forget the plastic all together and add another layer of paint to plaster ceiling and foam around the perimeter.
 
Last edited:

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
The ceiling makes as good of an air barrier as you need. Air seal around the edges, tops of walls, penetrations, but dont put plastic on top, just blow in right over the ceiling.

Latex painted plaster ceiling = air seal. I tested this by putting my mouth against the ceiling and trying to blow into my attic. No luck.
I've been wrestling with this in my head. I think you are right. I'll just put another layer of paint on the ceiling, it needs it anyway, and forget about the plastic or foam. That cuts the job by more than half.
 

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
I'd be putting 2" polyiso board on the bottom of the 2nd floor ceiling. Tape all the inside joints, leave a 1/2" or so gap around the perimeter and can foam the gap closed. Iso is approx R6 per inch, plus with foil faced, there is your barrier and air seal with the taped joints & foamed perimeter. I did that here under 1x8 T&G ciling with nothing but paper backed R30 laying ion top of the T&G. Makes for a tight seal + the added R value. can always blow in more whatever on top of the loose FG later.

Screw 1x3 lathe to the bottom of the iso, and install whatever you want to the lath. I took off the T&G, and reinstalled it to lath under the iso. The batshit showers sucked when removing the T&G.
If my ceilings weren't 6' 5", and I wasn't 6' 2", I'd agree. This old house has small dimensions. Plus, your solution is expensive, every sheet of polyiso is $40 around here. I splurged for one sheet to make the hatch to cover the walk-up attic opening, but even that hurt$.

Plus my heat is free, I just want easy 80° rooms in the dead of winter.
 

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
You will have to check with the actual company that makes the foam product, but I think it takes 2" of foam for an official vapor barrier. But I think we are splitting hairs. 1" would probably do it in my book.
I misspoke previously. I meant air barrier, I actually don't want a vapor barrier. I've considered using tyvek or the like, but I think Highbeam is probably right, the multiple layers of paint over the thick plaster should be enough, coupled with some cans of foam around the perimeter.
 

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,732
SEPA
Seems like there are two things happening- walls and ceiling. The foam was for the walls, plastic for an air barrier in the ceiling?

1" of closed cell foam is considered a vapor barrier. I've never seen it sprayed directly on a stone wall but I don't live in that region. Blowing cellulose would require a permeable sheeting to hold in the dense pack, not plastic, since you don't want two vapor barriers with insulation in between. The wall has to dry to the inside or the outside. A wet pack cellulose might work. I think that forms an air barrier and then plastic is stapled inside as a vapor barrier, no foam required.

The ceiling shouldn't need a vapor barrier assuming the attic is vented. Highbeam is right, paint is an air barrier so is plaster. Sealing up light fixtures, outlets, and other penetrations to avoid a stack effect will help more than a plastic sheet under the insulation. The water vapor has to go somewhere, you don't want it condensing in a place where it won't dry.

With a house that age the worst thing you can try to do is attempt to seal it up. You'll never get an air tight house and you could cause some serious rot and mold issues.
Thanks, agree with everything you said.

I am only worrying about the ceilings for now, the walls are actually only the thickness of furring strips, lath, and plaster. The only air gap is the thickness of the furring strips. I will slow the air through that space with foam at the top and bottom, wherever I can get access.
 

johneh

Minister of Fire
Dec 19, 2009
3,125
Eastern Ontario
Plus my heat is free, I just want easy 80° rooms in the dead of winter.
Good thing you don't live here the cost to have my home at 80::F
would bankrupt most also we would be down to our tightey whities.
65 to 70 ::F is warm enough but then I live in the Great White North