Roof insulation

[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.

[Maurice Mcmurry]

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.

I did our daughters house this way. 2 inches of DIY Foam It Green to the underside of the roof deck. Then filled the remainder of the 6 inch space with net and fill cellulose. No ventilation. Shingle warrantee voided. I was glad when she sold the house. I am worried because she sold it to her cousin. No complaints from the new owner so far. I have had comments of appreciation about the coziness of the place. If we created a moisture trap I am sure I will find out.

[John Pendery]

I did our daughters house this way. 2 inches of DIY Foam It Green to the underside of the roof deck. Then filled the remainder of the 6 inch space with net and fill cellulose. No ventilation. Shingle warrantee voided. I was glad when she sold the house. I am worried because she sold it to her cousin. No complaints from the new owner so far. I have had comments of appreciation about the coziness of the place. If we created a moisture trap I am sure I will find out.

Hi Maurice, I think I must have not been clear in my comment. That sounds like a hybrid approach that I’m not familiar with. I’m familiar with either foam a roof deck with no ventilation and condition the attic space, or vent the roof and insulate the ceiling/rafter ties. Again I’m speaking from my experience in my area. I have fairly long relationships with roofers, insulation folks, building inspectors etc and with this approach never voided a shingle warranty or run into a problem. This is very common practice but I would never advocate for anyone to do this if it makes them uncomfortable.

[Jim Becker]

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. .

Interesting given that open cell provides no vapor/moisture barrier and has half the insulation performance of closed cell per inch. Our climate here in SE PA is not all that different from where you live in KY (other than maybe more humid here in the Delaware Valley) and open cell is somewhat rarely used.

[Maurice Mcmurry]

These are interesting threads, there have been several over the few years that I have participated. Several acceptable solutions are often discussed with lots of input from various climate zones. It is confusing. I have never heard of anyone getting a warrantee claim settled by a shingle manufacturer. I do read the shingle instructions and try to follow them. Our plan at our daughters house was to replace the roof with metal in a few years. It has been more than a few years since the cousin moved in and the shingles are still fine. No signs of moisture problems either. Jim Becker has referred to the technique we used as "Flash and Batt" (although we used cellulose).

[John Pendery]

Interesting given that open cell provides no vapor/moisture barrier and has half the insulation performance of closed cell per inch. Our climate here in SE PA is not all that different from where you live in KY (other than maybe more humid here in the Delaware Valley) and open cell is somewhat rarely used.

Agreed Jim, open cell is not a moisture vapor barrier and has less R value per inch. The fact that open cell is not a moisture barrier is part of the reason why it’s commonly used here. If there is a leak somewhere it makes itself apparent and can be repaired quickly. Closed cell on a roof deck would conceal problem areas for much longer. Open cell creates an air barrier when applied at any thickness over several inches and current code requires much more than that. It creates an airtight envelope but allows moisture to pass through in the event of some sort of roofing failure. I find differences in building codes from region to region confusing as well. Building science in general has been moving quickly and I think has shown that super tight building envelopes offer far less margins for error than leaky buildings that are hundreds of years old. I guess we can all pick our own poison!

[Jason Roehl]

If you’re just looking to take the edge off an uninsulated structure, an easy remedy would be the rolls of foil-faced bubble wrap. Easy to install, and will provide a reflective barrier between the underside of the roof deck and your work space.