The inspector is asking one question: what is between the foam and the room, and is it enough. The answer depends on whether the space is lived in or only entered for service, and on what the foam's evaluation report allows.
The two barriers
| Thermal barrier | Ignition barrier | |
|---|---|---|
| Where | Any space that is occupied: rooms, garages, basements with finished use | Attics and crawlspaces entered only to service utilities, with no storage |
| The standard | 15-minute fire separation; half-inch gypsum is the prescriptive answer | A lighter covering from the code's list, or a coating listed for the foam |
| Prescriptive materials | ½ in gypsum board or an equivalent tested assembly | 1½ in mineral fiber; ¼ in wood structural panel; ⅜ in particleboard; ¼ in hardboard; ⅜ in gypsum; corrosion-resistant steel of at least 0.016 in |
| Coatings | Only a coating the foam's evaluation report lists as a thermal barrier, at the listed thickness, after a full-scale fire test (NFPA 286) | A coating the report lists as an ignition barrier, or none at all when the report passes AC377 Appendix X |
Source: International Residential Code Section R316 and the product evaluation reports below.
What the Thermoseal reports allow
| Foam | Report | Attics and crawlspaces | Thermal barrier by coating |
|---|---|---|---|
| Thermoseal HFO (closed cell) | IAPMO ES ER-0698 | Passes ICC-ES AC377 Appendix X: no prescriptive ignition barrier required | DC315 at 18 mils wet / 12 mils dry (about 1.136 gal per 100 sq ft), or NO-BURN ThB at 14 wet / 9 dry; NFPA 286 |
| Thermoseal 500 (open cell) | IAPMO ES ER-0603 | Ignition barrier by coating: DC315 at 6 mils wet / 4 mils dry (about 0.38 gal per 100 sq ft) | DC315 at 18 mils wet / 12 mils dry (about 1.2 gal per 100 sq ft); NFPA 286 |
| ComfortSeal HFO (closed cell) | ER-975 | see the report | see the report |
| ComfortSeal 500 (open cell) | ER-974 | see the report | see the report |
The reports and data sheets are in the document library. Print the report and leave it on the job; inspectors ask for the report number, not the brand name.
Mils to gallons
Coating rates are given as wet film thickness. One gallon spread at 1 mil wet covers about 1,604 square feet, so:
- 18 mils wet = 1,604 ÷ 18 ≈ 89 square feet per gallon, which is the 1.136 gallons per 100 square feet on the Thermoseal HFO sheet.
- 6 mils wet = 1,604 ÷ 6 ≈ 267 square feet per gallon, the 0.38 gallons per 100 square feet for the Thermoseal 500 ignition barrier.
Measure wet film with a comb gauge as you go. The dry thickness is what the test was run at, and it is reached only if the wet thickness was.
What trips jobs up
- An attic with storage or HVAC that someone works on is still "service only" in most jurisdictions, but an attic the owner plans to floor and use is a living space and needs the thermal barrier. Ask before you spray.
- The coating has to match the foam. A thermal barrier rating is tested on a specific foam. DC315 over Thermoseal HFO is on the report; a different coating or a different foam is not, unless its own report says so.
- Cure before close-up. The coating needs to dry to the listed dry mils before drywall or trim goes over it, and the foam needs its own cure first. Follow the coating maker's recoat and cure times.
- Garages are occupied space. Foam on the garage ceiling under a bedroom gets a thermal barrier, not an ignition barrier.
- Rim joists in a basement are usually treated as part of the occupied basement if the basement is finished, and as a crawlspace condition if it is not. The local inspector decides; the evaluation report gives them the basis.
What to bring to the inspection
- The evaluation report for the foam you sprayed (the number is on the data sheet).
- The coating's product data sheet with the applied wet and dry mils written on it.
- Photos of the comb-gauge readings and of the drums' labels on the job.
Quick answers
What is the difference between an ignition barrier and a thermal barrier?
A thermal barrier is a 15-minute fire separation over the foam, usually half-inch gypsum board, required where people live and work. An ignition barrier is a lighter covering allowed in attics and crawlspaces that are only entered to service utilities. A coating can serve as either if the foam's evaluation report says so, at the thickness it lists.
Does Thermoseal HFO need an ignition barrier in an attic?
The Thermoseal HFO data sheet lists IAPMO ES ER-0698 with a pass under ICC-ES AC377 Appendix X for attics and crawlspaces without a prescriptive ignition barrier. Have the report on hand for the inspector.
How much DC315 does spray foam need for a thermal barrier?
For Thermoseal HFO the data sheet lists DC315 at 18 mils wet, 12 mils dry, about 1.136 gallons per 100 square feet. For Thermoseal 500 it lists 18 wet and 12 dry at about 1.2 gallons per 100 square feet.
Sources
- Thermoseal HFO technical data sheet, V2.0, September 22, 2026 (IAPMO ES ER-0698)
- Thermoseal 500 technical data sheet, V2.7, January 10, 2025 (IAPMO ES ER-0603)
- ComfortSeal HFO technical data sheet (ER-975)
- ComfortSeal 500 technical data sheet (ER-974)
- International Residential Code, Section R316, Foam Plastic