I have an old house built in 1880. Zip code 53545 Southern edge of zone 6. As one would imagine it has major air leaks everywhere. Currently the house uses boiler heat and has a air handler for central air located in the attic. The attic currently has about 3-6 inches of blown in cellulose on the attic floor. The roof is a rafter system with planks that have been sheathed with OSB. The walls are balloon construction and open up into the attic.

We are in the process of planning a major addition and remodel. The asphalt roof is less than 5 years old so we are trying to keep as much as possible however a good amount will be coming off. The cellulose is going to have to be vacuumed out to remove old electrical as well as ceiling repairs.

Does closed cell spray foam seam like a viable option? Ultimately the wall cavities will be opened up and spray foamed as well.

That is what I did in my own 1921 balloon frame farm house. You will see more of this as the energy code will require all the duct-work to be in the envelope or heavily insulated. I love my hot roof.

Did you go with straight spray foam or did you do foam sheathing as well? The roof has a 12/12 pitch so that does make for a lot of attic volume but it does seem like it would be easier to get a proper seal on the roof deck than all of the different ceilings and walls.

To get R50 I would need a little over 8" of closed cell, pricey. Maybe a combination of methods is better?

Write off the roof; you'll end up spending more $$ working around it than dealing with it right. You need to close off the top of the studs & probably the best way to do that is to open the roof at the edge and seal the stud bays with 2x stock. Then figure out how to get the maximum R value in your roof and minimal thermal bridging to avoid ice dams.

Write off the roof; you'll end up spending more $$ working around it than dealing with it right. You need to close off the top of the studs & probably the best way to do that is to open the roof at the edge and seal the stud bays with 2x stock. Then figure out how to get the maximum R value in your roof and minimal thermal bridging to avoid ice dams.

Write off the roof; you'll end up spending more $$ working around it than dealing with it right. You need to close off the top of the studs & probably the best way to do that is to open the roof at the edge and seal the stud bays with 2x stock. Then figure out how to get the maximum R value in your roof and minimal thermal bridging to avoid ice dams.

When you say write the roof off do you mean the shingles and the sheathing or the whole rafter system?

With the balloon framing the rafters are about a foot above where the top plate would be. I wouldn't say there is plenty of room but I've worked in tighter spaces.

I'm sure you can understand the reluctance tear off the whole roof but you may be right that it would cheaper/faster in the end. Definitely something to consider.

Yes. I would use foam to seal the roof covering rafters and deck from below thick enough to cover the dew point, say 3" then blow insulation in the ceiling until you feel good. I used 6" but it is a small attic.

Yes. I would use foam to seal the roof covering rafters and deck from below thick enough to cover the dew point, say 3" then blow insulation in the ceiling until you feel good. I used 6" but it is a small attic.

Closed cell foam is fairly damaging stuff, both due to the high polymer content at 2lbs density but also the VERY high global warming potential of the HFC245fa blowing agent (about 1000x CO2). Use sparingly, and only where necessary. As little as 2" is sufficient for protecting the roof deck, but not sufficient for wintertime dew-point control.

Here's how to get to code-min performance at about the same expense (and twice the performance) of 5" of closed cell foam at much lower environmental impact:

If you have 2x6 rafters you can do just fine with filling the rafter bays with open cell foam (blown with water, seals better than closed cell foam, and half the polymer per R) trimmed flush to the rafter edges which comes out to about R20. Then, staple & seal a layer of MemBrain (about $100 for an 8' x 100' roll online from Menards, if you can't find it locally for less) which is a "smart" vapor retarder, sufficient to protect the roof deck. MemBrain is less than 1 perm when the air is dry (like it is in winter), but in spring when the sun is cooking the moisture out of the roof deck it becomes vapor open (10 perms+) letting the roof dry quickly.

To bring it up to code-min you can then add up to about R12 (3") of rigid high density rigid rock wool to the interior side of the MemBrain, cap-nailed to the rafters. That only ends up being R32 center-cavity, but since it thermally breaks the rafters it ends up being about R37 whole-assembly (after calculating the thermal bridging) or U0.027, which isn't very different from what you'd get out of R49 between joists (code max is U0.026 if doing it on a U-factor basis, so it's not quite, there, but U0.027 is good'nuff for the kind o' girlz I go with. :-) ).  Roxul Comfort Board is available in 3"/R12 thickness. It's more expensive than R12 rigid foam, but it doesn't create a moisture trap the way rigid foam does, and doesn't need an interior side gypsum to meet fire code requirements the way foam does.

From a code point of view you can't count the R-value of the stuff on the attic floor, but it's moisture-safe to leave it there, and adds at least some performance.

Very interesting idea. Just to be clear is this the type of product you're referring to?


"ROXUL® products are mineral wool fibre insulations made from basalt rock and slag. This combination results in a non-combustible product with a melting point of approximately 2150°F (1177°C), which gives it excellent fire resistance properties. ROXUL mineral wool is a water repellent yet vapour permeable material."


It has an R value of 4.2 per inch so would be just a touch better. I need to get back up there and check the rafter depth. To be honest I wouldn't be surprised at all if it's 2x4 rafters, judging by other old houses I've been in I might be more surprised if it isn't 2x4 rafters.

Maybe I'm just easily impressed by all the numbers but I like your idea better than all spray foam even though it would be more labor intensive.

Do you feel going to a hot roof is a reasonable option or should I try to stick with a vented attic?

Roxul is one of the larger vendors of rock wool insulation worldwide. Even though it hasn't eaten into the market for fiberglass batting in the US, it's a marginally better product on a number of axis'.

As long as you don't have ducts & air handlers up there, it's usually easier & cheaper to air-seal & insulate at the attic floor with blown cellulose, and go with a vented attic. But the construction details matter. Sometimes it really IS better to insulate at the roof deck.

Most cooling air handlers belong in the attic so a hot roof is where it's at.

Posted By BadgerBoilerMN on 06 Jul 2015 06:02 PM
Most cooling air handlers belong in the attic so a hot roof is where it's at.

I'm not so convinced that they BELONG there, but that's where they usually end up...

I have an air handler up there now. I would like to move it to the basement and cool the second floor from there but it seems that the only way to get ducks to the second floor is to run them in the exterior walls or a large trunk to the attic and distribute from there. Neither of those seem much better than just leaving the air handler up there and go hot roof.

If you're ducts & air handler are up there and there are no good duct chases for bringing it up elsewhere, go ahead and insulate at the roof deck.