Spray foam attic insulation vs regular blown in cellulose (loose) insulation?

I used 2” thick spray foam on the attic floor just to seal off the ceiling penetrations from ceiling fans, lights, etc. Then blown cellulose on top to give the desired R - rating. I also used reflective sheeting on the underside of the roof decking.

My house was built in the early sixties and I had my roof deck spray foamed about 10 years ago. It has been a learning experience but I’m still happy with the results. We had it done instead of new ductwork and replacing existing fiberglass batts. Few things to consider. The permanent seal highlights the amount of moisture in the house due to the reduced amount the A/C will run. To combat this we upgraded the system to a 16sear multistage unit with an additional dehumidifier. It was time to replace anyway but it allowed us to downsize a step. The point about wiring or doing work in the attic is also a good point but only relevant if you intend to do that sort of work.

Aside from the other feedback you have gotten, attic spaces that are spray foamed require sealed combustion gas appliances. So, if you already have a gas furnace and/or other mechanicals that use gas in the attic, chances are they are not sealed combustion and would need to be replaced.

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Imagine if that was the roof.

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I had a lot of trouble with my builder and he cut corners on my roof. There was no ice and water shield, no drip ledge, not starter shingles, and on one of the edges there was a strip with no underlayment. That roof lasted about ten years. I reroofed in 2023. The only damage was due to crappy water management details in the flashing and the issues above. There was only one small part that was over the foam and the roofers were able to cut away the sheathing without damaging the foam underneath too much, then replace the sheathing. That's not ideal, and would be worrisome up North, but not so much at coastal Alabama. I watch that one spot. I did, however, use Ice and Water shield over my entire roof deck instead of underlayment. That means I have a self-adhered, self-healing membrane under all of my shingles. The extra cost was less than $1000, essentially just the material cost difference between underlayment and Ice and Water Sheild.

A lot of roofing philosophy is based on stuff that happens with snow and ice and vapor transmission up North. Down here, we are always most worried about massive water intrusion from outside, plus the sun down here is going to drive any moisture accumulated in your roof sheathing out for most of the year.

I have open cell foam, closed cell is a different dynamic because the closed cell is a vapor barrier and the open cell isn't . The Ice and Water shield is a vapor barrier too, and I believe many roof underlaynemts used here are. There's a lot of things to consider.