We have a one story part of our 3 story house that has a shed roof and no venting in the roof, so this would be known as a hot roof as I understand it.
Our house is 100 years old and recently we gutted this room (which used to be our kitchen) and installed two layers of 2″ poly-iso insulation above a newly installed tongue and groove bead board ceiling (a ceiling with 1-3′ of space above it).
This leaves us with the walls to insulate now. The original room had the blown in cellulose between the sheathing and the plaster/lathe (probably done in the ’70s). All of that is now gone and we are left with a ton of nooks and crannies between many studs and stud sisters applied here and there to accommodate window size changes, uneven walls, electric, etc.
Normally I would think a nice 2″ layer of poly iso would be a good fit for the highest R value per inch, as well as being sort of a vapor barrier under the drywall, but that is going to be a nightmare to get to fit in between all those studs.
Then we were thinking rock wool as it will offer “breathing” of some sort between the sheathing and the drywall.
Many have said to simply hire someone to do the spray-on closed cell foam (or I could do it myself unless it is stated here that what the pros use is better than the A and B kits that I can purchase). I like this idea, but it brings up my main issue: the sheathing on the walls is perfect (over which is an awful concrete layer looking like brick). I have a problem with spraying this permanent foam directly onto the sheathing which has absolutely no rot or moisture intrusion in 100 years btw. But if any occurs, the wood will rot with no escape of the moisture from the other side. Do people put a baffle in between the foam and the sheathing?
So my questions in a nutshell:
1.) What would be my best option?
2.) If spray foam, should a baffle be underneath so it is not in contact with the sheathing?
Thank you everyone for your input! I am needing to get started on this later this week. I so very much appreciate all of the experience any of you may have…
Replies
4" of Poly-iso for roof = R26 not much for a roof. So you may be in a moderate zone 2-3? Yet you want to maximize R-value in the walls. Spray foam will change your vapor barrier plane and force the sheathing to dry outward. This may cause mold where studs meet sheathing because there is no vapor barrier at this now heat transfer highway. Again only a problem with high annual temp swings and poor dew point calculations. Blow-in fiber through a netting will give you a higher R-value than other blown-in, won't settle, and will let your wall continue to dry as it always has. It will fill the tiny cracks/spaces that you haven't caulked beforehand.
So yes, you are verifying what I already read about the spray foams on the sheathing and we're not going to do that. I haven't heard of the blown in fiberglass- will check into it- seems silicosis risk to the installer (me) and any other subsequent exposer would be the primary downside of this?
Thanks
Update: we ultimately went with mineral wool/ rockwook...super easy to cut with a long razor and push between the studs, using cans of spray foam here and there where appropriate. A local home restorer in my neighborhood warned against this saying that the mineral wool R values occur in a laboratory, and not in reality of our old house, and to use the poly-iso sheets cut into shape and spray-foamed along the edges- we decided against that installation nightmare (which we did in our ceiling) and are taking our chances with the mineral wool, despite his claim that "that cold air will penetrate right through it". So be it then- no one lives in an historic house for the winter warmth!