If you are having spray foam installed, at some point the conversation will turn to fire barriers — and it should. Spray polyurethane foam is a combustible material, and building code does not allow it to be left exposed in most situations. DC 315 is how we satisfy that requirement in the spaces where building drywall would be impractical.
Thermal barrier versus ignition barrier
These are two different requirements and they are often confused. A thermal barrier is required between foam and occupied living space — it has to delay temperature rise behind it for 15 minutes. Half-inch drywall is the standard example. An ignition barrier is a lesser requirement that applies in attics and crawl spaces with limited access, where the concern is slowing flame spread across the foam surface rather than protecting an occupied room.
What intumescent means
DC 315 is an intumescent coating, which means it reacts to heat rather than simply resisting it. Under fire exposure the coating expands into a thick, insulating char layer that shields the foam underneath and slows flame spread across the surface. That expansion is the mechanism — a thin sprayed film becomes a protective barrier only when it is needed.
When a coating beats drywall
- Attic slopes, rafters and irregular framing where drywall cannot follow the geometry
- Crawl spaces with limited headroom and no practical way to hang board
- Mechanical rooms and utility spaces around ducts and piping
- Retrofit work where framing out for drywall would consume usable space
- Commercial decks and steel framing with complex profiles
Application thickness is the whole game
DC 315 achieves its rating at a specific tested wet-mil thickness, and that thickness varies depending on the foam type underneath and the rating being met. Applied too thin, it does not perform and it will not pass. This is the most common reason a fire barrier fails inspection, and it is entirely avoidable. We apply to the tested specification and document the coverage so the inspector has what they need on the first visit.
Practical considerations
It is water-based, low odor and low VOC, which means it can be applied in occupied buildings with normal ventilation rather than requiring the whole household to leave. It is typically touch-dry within a few hours and fully cured within about 24 hours. And it does not affect the foam underneath — thermal performance and air sealing are unchanged, because it is a surface treatment.
If another contractor installed your foam
We regularly get called to coat foam somebody else installed, often because a project failed inspection. We will inspect the foam first, and if there is a problem underneath — poor adhesion, insufficient thickness, moisture trapped behind it — we will tell you before we coat over it. Covering a bad installation does not fix it, it just hides it.