Spray Polyurethane Foam (SPF) is a volatile, highly-reactive binary chemical compound. It requires million-dollar heated pressure rigs, severe hazmat suits, and precise stoichiometric mixture ratios. Attempting to foam a roof deck with $800 consumer "Froth-Paks" is a fast track to uninsurability.
1. The "Off-Gassing" Disaster
If the A-Side and B-Side chemicals do not mix at the exact pressure and holding temperature (impossible with consumer tank kits), the foam will not cure properly. It remains "sticky" and will permanently off-gas toxic isocyanates into your breathing air forever. You will be forced to abandon the home.
2. The Flammability Trap
Exposed SPF in an attic must be legally coated with an intumescent thermal barrier (fire-proof paint) or covered with drywall to pass building code. DIY kits rarely include this. A single spark from attic wiring will ignite the raw foam, burning the house down in 3 minutes.
Most standard homeowner's insurance policies explicitly forbid DIY spray foam applications. If an adjuster discovers improperly cured foam covering your rafters, they will flag the property, immediately dropping coverage and severely jeopardizing your mortgage.