How To Use Fireblock Foam
Fireblock foam fills concealed openings in wood-frame construction so fire and smoke can't travel through them. The method is short. The limits matter more.
Fireblock foam goes into concealed gaps in wood-frame construction — the annular space around a pipe, the gap between a top plate and the floor above, the opening where a cable bundle passes through framing. You clean the gap, check the can's application temperature, dispense a bead that fills the opening, let it skin and cut it flush. Hilti CF-AS CJP is the stocked product for this job.
Figuring out where to put fireblock foam, how much to use, and when to pick a different product is the tricky part.
What fireblock foam actually does
Fireblocking seals off hidden routes inside walls and floors. These routes let fire and smoke travel between levels. In wood frame buildings, this means filling holes around pipes or wires, and the spaces between studs.
Hilti CF-AS CJP is rated for Type V-B homes under ICC ES ESR 2179. It tested at UL 723 with a flame spread of 0 and smoke development of 0. An inspector will check for these numbers. The orange foam is easy to see when they check your work.
The maker says one can yields up to 1.6 cubic feet of foam. That's the maximum. How much you get depends on your spray speed and how much the foam expands to fill the gap.
Before you spray
Check the can's temperature before you start spraying. Hilti CF-AS CJP works best between 0 and 35 °C. If it's too cold, the foam won't spray or set properly. If it's too hot, it hardens too quickly. Warm the can up if it's been sitting in a cold truck.
Before the can comes out, clean the gap. Foam won't bond to framing through dust, loose material, oil, or old debris. Brush it out or vacuum it first.
Read the label on the can in your hand and shake it that way. Most one-component foams need a good shake to mix before use. Don't borrow shake steps from another product.
How to apply it
Choose the nozzle for the opening. A straw nozzle handles small holes. For a bigger opening or a line of holes, use a foam gun. It gives you more control over the bead.
Dispense the foam into the gap. Don't fill it to the surface — one-component foam expands after it leaves the nozzle. Start with roughly half the gap depth and let it expand. Overfilling wastes product and can bow framing or push out of the opening.
Let it skin. CF-AS CJP reaches tack-free in about 12 minutes at 74 °F and 50% relative humidity. That's the point where the surface stops being sticky. Don't touch it before then, and don't try to trim it while it's still wet — you'll drag foam out of the gap and make a mess.
Cutting and finishing
Cut time is about 25 minutes at 23 °C and 50% relative humidity. That's when the foam is firm enough to trim flush with the framing. Use a utility knife or a foam saw. Cut it back so the surface is even with the face of the stud or plate — a proud bead can interfere with drywall or sheathing.
You can cut the foam after 25 minutes. But it keeps curing. This cut time doesn't tell you how it handles fire or if paint will stick later.
CF-AS CJP has a temperature resistance range of -40 to 90 °C. That's the service range — the temperatures the cured foam can handle in place. It is not the same as the application temperature range, and it doesn't mean you can apply the foam at -40 °C.
Where fireblock foam doesn't belong
Fireblock foam is not a firestop for rated assemblies. If you're sealing a penetration through a fire-rated wall or floor — a fire separation, a shaft, a rated corridor wall — you need a listed firestop system, not a fireblock foam. The two jobs look similar and the products are not interchangeable. CF-AS CJP is classified as a residential fireblock, not as a firestop for rated construction.
This foam can't replace draftstopping in big hidden areas. It's not for carrying weight or where you need solid backing.
Unsure if a gap needs fireblocking or firestopping? Look at the plans or ask the inspector. Fixing it after drywall is up costs a lot.
Common mistakes
Overfilling is a common mistake. Foam expands. Fill a gap halfway and it can push out from the framing. Start with a little. Add more if you need it.
Another mistake: spraying into a gap that's wet or dirty. The foam won't bond, and later it can pull away. Clean and dry the opening first.
Ignoring the application temperature is the third. A can that's been in a cold truck won't perform. Check the range on the can — for CF-AS CJP it's 0 to 35 °C — and warm or cool the can if you're outside it.
What temperature can you apply fireblock foam?
Hilti CF-AS CJP has an application temperature range of 0 to 35 °C. Below or above that range, the foam may not dispense or cure correctly. This is different from the service temperature range of -40 to 90 °C, which is what the cured foam can handle in place.
How long before you can cut fireblock foam?
CF-AS CJP reaches cut time in about 25 minutes at 23 °C and 50% relative humidity. Tack-free time is about 12 minutes at 74 °F and 50% relative humidity. Cut time is not full cure — the foam continues to cure after you trim it.
Is fireblock foam the same as firestop?
No. Fireblocking stops fire and smoke in hidden wood-frame gaps. Firestopping is a tested system for holes in fire-rated walls or floors. CF-AS CJP is rated as a Type V-B residential fireblock (ICC ES ESR 2179), not for rated construction.
How much foam does one can yield?
Hilti lists CF-AS CJP yield at up to 1.6 ft³. That's a maximum figure. Actual yield depends on how much you dispense and how much the foam expands into the gap.