Is it normal for spray foam to go directly against the roof decking?

The idea that your roof deck needs to 'breathe' is one of those old-school construction myths that refuses to die. Wood rot isn't caused by a lack of airflow; it's caused by high moisture content and heat. When you spray closed-cell foam directly to the underside of the decking, you're actually creating an airtight seal that prevents interior moisture from ever reaching that wood. If the shingles are installed correctly, no water gets in from the outside either. I've pulled up shingles on 15-year-old hot roofs and the plywood looks brand new, often better than vented decks that have been exposed to humid attic air for decades.

Regarding the heat, the temperature increase on your shingles is negligible. Studies show that a 'hot roof' only runs about 2 to 10 degrees hotter than a vented one. Shingle manufacturers used to void warranties for this, but most of them—like GAF and Owens Corning—have updated their stance because the science just doesn't support the idea that the foam 'bakes' the shingles. The color of your shingles has a much bigger impact on their lifespan than the insulation type underneath. The real risk isn't the heat, it's a roof leak you can't see. Because the foam is stuck to the wood, a small leak in the shingles won't show up on your ceiling right away. It can sit there and rot the decking from the top down. That's why you need to be 100% sure your flashing and underlayment are perfect before the foam goes in. If you're really worried, you can install **venting baffles** against the deck before spraying, which maintains an air channel from the soffit to the ridge, though it's more labor-intensive and slightly reduces your R-value. If you go the unvented route, make sure the crew cleans that asphalt dust off the rafters and decking first. If the surface is too dirty, the foam won't get a mechanical bond and you'll end up with gaps where condensation can form. But as long as the deck is dry and clean, spraying directly to it is a standard practice that works incredibly well for energy efficiency.

This is so cool to read! I'm just starting to look into insulation for my own place and I'm honestly blown away by how much science goes into a roof. Since everyone is talking about the deck surface, does the type of wood matter for the foam to stick? My shed has some wierd OSB that looks super flaky and I'm wondering if I shoud just sand it down or something before I even think about spraying. Also, has anyone ever tried using those thermal cameras to find leaks trough the foam?

I went thru this three years ago and the main thing I learned is that you have to watch those sprayers like a hawk. I spent two days in my attic with a shop vac and a stiff brush getting that shingle grit off the OSB before they showed up. If you don't, the foam just peels off in sheets later. My energy bills dropped by 40% immediately and my attic is acutally bone dry now. Just make sure you fix any questionable flashing now while the shingles are off, becasue once that foam is cured, you aren't finding a leak til your roof deck is soft.

Don't forget to check your local codes before you pull the trigger. Some spots still require a vapor retarder paint over the foam if you're using open-cell instead of closed-cell. I didn't do my homework on that and the inspector failed me 'cause I missed the ignition barrier coating too. It's an extra step that's a total pain, but you don't wanna have to spray more stuff once the job's supposed to be done. Also, if you've got gas appliances in the attic, you're gonna need to look into sealed combustion units since you're making the space airtight.

If you are worried about that asphalt dust ruining the bond, just spray a thin layer of Froth-Pak 650 first. I've used it on a few deck projects where the wood was less than perfect and that stuff sticks to anything like glue. It gives the professional crew a solid surface to build on so you don't have to worry about delamination later. Plus, it seals those tiny gaps around the rafters better than the big rigs sometimes do.

Quick update—I spent most of yesterday in the attic with a shop vac like you guys suggested. You weren't kidding about the dust! I noticed some of the OSB near the eaves is pretty flaky though. Does the foam acutally help reinforce that, or shoud I be replacing those sheets before the sprayers get here? Also, checking on the ignition barrier requirements now.