Roof insulation

[Ned Otter]

Hi everyone -

With no insulation in the walls or for the rafters, my shop is absolutely blazing in the summer time (I live in upstate NY). I'm specfically wondering what I might do for insulating the rafter areas. There is no "ceiling" - you can see the inside of the roof area from the shop.

I'd like to keep it open if possible, and I realize that will likely affect how well it can be insulated. To make an actual ceiling would have it be pretty low, not great for moving taller stuff around. Plus my ducting is already at/above the double top plate, complicating that approach. The cavities are not that deep, maybe 7" or so; this is a late-19th century structure.

I've considered insulating the open cavities (the 45* degree areas) with rockwool, and then covering the cavity. Drywall is pretty heavy, not sure this ancient roof structure could handle all that weight. But rockwool is messy stuff, and I don't want it falling all over the place.

Also considered spray foam or foam board. So far, foam board with an air gap seems most palatable, but that might require specific covering due to the fire code, and then I'd be back to the potential roof weight issue.

I've included a couple of images of the roof and cavity area.


All feedback welcomed --

Thx,

Ned

[Ron Selzer]

Have a spray foam contractor out to look it over, discuss it and get a quote.
Ron

[Jim Becker]

Closed cell spray foam will not only insulate, it will also make the structure more rigid as well as sealed and provides a moisture barrier. It can be installed directly on the underside of the roof so you lose no space. It will also brighten up the interior over that old dark wood, even after it "mellows" a bit in color as foam does. If your shop is an outbuilding, it's unlikely leaving it exposed would be an issue. Even in a residence, either using an additive or coating with a special paint will often satisfy code if there is a requirement. It's not inexpensive, but it does a great job with multiple benefits as I mentioned. You will likely get a better price from a small operator...that was the case here with my new shop building. Like about 40% less in my case from the larger companies.

[Cameron Wood]

My approach is to fit rigid foam sheet panels between the rafters, leaving an air gap above.Fiberglass can fill the remaining space, covered in your case even by Tyvek.
Vents are added at the eaves and ridge which exhausts heat and moisture. The shiny foil face helps as a radiant barrier.

I would leave 1/2 or 5/8" space (furring strips), cut the probably 1 1/2" foam into 4' butts with drywall square and snap-off type knife with a long blade, for a press fit into the bays.

I've done this quite a few times. It is DIYable, relatively inexpensive, reversable/modifiable, and does not promote moisture or heat degradation, and is not ugly.

[Maurice Mcmurry]

My approach is to fit rigid foam sheet panels between the rafters, leaving an air gap above.Fiberglass can fill the remaining space, covered in your case even by Tyvek.
Vents are added at the eaves and ridge which exhausts heat and moisture. The shiny foil face helps as a radiant barrier.

I would leave 1/2 or 5/8" space (furring strips), cut the probably 1 1/2" foam into 4' butts with drywall square and snap-off type knife with a long blade, for a press fit into the bays.

I've done this quite a few times. It is DIYable, relatively inexpensive, reversable/modifiable, and does not promote moisture or heat degradation, and is not ugly.

My shop attic is done this way as well. The shingle manufacturer said they require 3 inches of free air under the sheeting (for full warrantee). I cheated a little and have a 2 inch air gap, then 2 inches of Owens Corning Foamular, + 3.5 inches of fiberglass, + sheetrock. I sealed my rigid foam in place with Great Stuff pro foam. My air gap has gotten as hot as 145 degrees which is very close to the limit for both foam board and shingles. The attic room is easy to heat and cool. It has no walls. The slanted roof goes right down to the floor. The ceiling of the shop below and its 2 x 6 walls have dry pack cellulose. Downstairs is also easy to heat and cool.

[Cameron Wood]

My shop attic is done this way as well. The shingle manufacturer said they require 3 inches of free air under the sheeting (for full warrantee). I cheated a little and have a 2 inch air gap, then 2 inches of Owens Corning Foamular, + 3.5 inches of fiberglass, + sheetrock. I sealed my rigid foam in place with Great Stuff pro foam. My air gap has gotten as hot as 145 degrees which is very close to the limit for both foam board and shingles. The attic room is easy to heat and cool. It has no walls. The slanted roof goes right down to the floor. The ceiling of the shop below and its 2 x 6 walls have dry pack cellulose. Downstairs is also easy to heat and cool.

I did an attic conversion like that- 2x4 rafters with additional 2x2 strapping. It became the master bedroom and was (is) amazingly comfortable.

Do you have the venting above and below?

[Maurice Mcmurry]

I do. The aluminum soffit is all perforated and I have 3 big chicken house spinning vents along the ridge. The solar panels shading the south side make a big improvement regarding heat gain during the summer. We feel very fortunate to have been able to build the shop!

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[William Hodge]

Spray foam is a terrible thing to do to a building. It has drawbacks, and is non-reversable. Any trades person I know that has had to work in a spray foam building has had to just saw and chisel the stuff out in order to do any work. For example, an old house had a leak in the flashing around a window. No one knew. The water got in around the spry foam, and started a mold farm. The walls came out, the laborers cam in and hacked out the spray foam.

If the mix isn't right on the spray foam truck, the foam doesn't set up right, and just off gasses. Google "Spray Foam Fail Canada" to read about the failures, illness, and government response to help homeowners out.

A better route would be to frame a ceiling, sheet rock it, air seal it, and use cellulose insulation. The useless space up high won't need heating.

[Bill Dufour]

The polyiso foam board doubled in price during covid and never came down. It is $55 for 2". 4x8 sheet. Cuts well on the table saw. My plan is to do the 16" of foam in the rafters at the little triangle near the eaves of the house then switch to fiberglass for most of the attic floor. Go with r30 or r38 on top of existing r-13. Foil under the roof will be perforated for air flow.
BilL D

[Cameron Wood]

The polyiso foam board doubled in price during covid and never came down. It is $55 for 2". 4x8 sheet. Cuts well on the table saw. My plan is to do the 16" of foam in the rafters at the little triangle near the eaves of the house then switch to fiberglass for most of the attic floor. Go with r30 or r38 on top of existing r-13. Foil under the roof will be perforated for air flow.
BilL D

Ugh, don't saw that stuff. EPS is almost as good R value wise, cheaper, and much more friendly to work with.

[roger wiegand]

Spray foam is way easier albeit more expensive than building some kind of structure and/or fitting rigid foam and will provide much better air sealing. Just depends on how you want to spend your time/money. As with anything there are horror stories with any system you might choose (often trumpeted by the folks selling competing products), but bad experiences are rare with each of these approaches. Check with your local code officials if they would be happy with an intumescent coating in this case vs a rigid fire barrier. If you're willing to run joists across the space then blown in cellulose is a very cost efficient choice for an attic.

Check out the availability of used polyiso (Craigslist is a place to find it). Insulation is commonly replaced when commercial buildings are re-roofed and the panels get recycled in the secondary market, usually cut up into 4 x 4 pieces. Condition can vary a lot, so look for the better quality stuff. I put 12" of polyiso on top of my son's very low pitched roof for pennies on the dollar of what new foam would have cost, and then a new rubber roof over that to solve a difficult insulation situation. The R value of the old foam is slightly lower than new, but the cost of a couple extra inches is very low.

[mike stenson]

My approach is to fit rigid foam sheet panels between the rafters, leaving an air gap above.Fiberglass can fill the remaining space, covered in your case even by Tyvek.
Vents are added at the eaves and ridge which exhausts heat and moisture. The shiny foil face helps as a radiant barrier.

I would leave 1/2 or 5/8" space (furring strips), cut the probably 1 1/2" foam into 4' butts with drywall square and snap-off type knife with a long blade, for a press fit into the bays.

I've done this quite a few times. It is DIYable, relatively inexpensive, reversable/modifiable, and does not promote moisture or heat degradation, and is not ugly.

This is what I did to my shop as well.

[Ned Otter]

Thanks, Cameron --

What's the down side to taking this approach without venting? Guessing it's premature shingle failure, as the temp can/will get too high.

Given there is a double top plate that has no venting, I think it would be difficult to add venting at the eaves, but perhaps I'm incorrect about that. Ridge vent would be possible, but definitely not DIY for me.

Thx,

Ned

My approach is to fit rigid foam sheet panels between the rafters, leaving an air gap above.Fiberglass can fill the remaining space, covered in your case even by Tyvek.
Vents are added at the eaves and ridge which exhausts heat and moisture. The shiny foil face helps as a radiant barrier.

I would leave 1/2 or 5/8" space (furring strips), cut the probably 1 1/2" foam into 4' butts with drywall square and snap-off type knife with a long blade, for a press fit into the bays.

I've done this quite a few times. It is DIYable, relatively inexpensive, reversable/modifiable, and does not promote moisture or heat degradation, and is not ugly.

[Cameron Wood]

Thanks, Cameron --

What's the down side to taking this approach without venting? Guessing it's premature shingle failure, as the temp can/will get too high.

Given there is a double top plate that has no venting, I think it would be difficult to add venting at the eaves, but perhaps I'm incorrect about that. Ridge vent would be possible, but definitely not DIY for me.

Thx,

Ned

It's more about moisture than temperature, 'tho less critical in a shop than living space, which has warmer temps and more water vapor generated.
I would drill the rafter blocks from inside (this is above the plates), and install screened strip or other vents in the eaves if closed, do the foam sheet deal on the sloped roof, and close the flat ceiling at the collar ties with fiberglass batts above, and finally add a couple of turbines near the ridge on the lee or less visible side, whichever makes more sense.

[John Pendery]

In my climate zone the standard practice is open cell spray foam for walls and roof deck, closed cell is used below grade in crawl spaces and basement walls. If spraying roof deck you don’t want to vent your roof. If insulating rafter ties with a vented roof spray foam is a waste. If I were in your shoes spraying the roof deck would be a no brainer. If you dig you’ll find horror stories, but if you were to look statistically at success stories vs failures the failures account for a drop in the ocean.