Closed-cell foam makes its own heat as it cures. With many 2 lb foams a thick lift scorches its own core. Thermoseal HFO is more forgiving: it can be sprayed well past 2 inches in one pass without scorching. We still recommend building thickness in lifts of about 2 inches, because thickness control is about more than heat: it is adhesion, dimensional stability, cell structure, and a yield you can predict.
The two numbers
| 2 lb closed cell (Thermoseal HFO) | |
|---|---|
| Recommended lift | about 2 inches |
| Thicker single passes | possible with Thermoseal HFO without scorching; not recommended |
| Wait between lifts | until tack-free, usually 90 seconds to 2 minutes |
| Substrate temperature | above 45°F summer grade, above 14°F winter grade, and 10°F above dew point |
A lift is one pass over the same area. Three inches of foam is a 2-inch lift and a 1-inch lift, or two 1.5-inch lifts, not one pass.
Why we still say 2 inches
A thick mass of foam holds its heat and cools slowly. Even when it does not scorch, it shrinks more as it cools than a thin lift does, and the stress goes to the weakest line: the bond to the substrate or to the lift under it. The result can be cracks, a hollow sound, or a sheet that lifts off with a putty knife.
On a cold substrate the problem starts at the bottom: the first inch chills against the wall, cures dense and slow, and never bonds, while the top cures normally. By morning the sheet has pulled. Two lifts give the first one time to bond and warm the wall for the second.
Thickness control is also yield control. A gun that hovers stacks 4 inches in one spot and 1 inch beside it; the cavity reads as full, the R-value is uneven, and the set comes up short. Steady 2-inch lifts put the foam where the spec wants it, at the thickness you quoted, without trimming closed cell back.
The tack-free rule
Touch the previous lift with a gloved finger. When it no longer strings or dents, spray the next one. Going sooner bonds the new lift to a soft layer that is still moving; the two cure at different rates and the first one can let go of the substrate. Waiting longer than a few minutes costs nothing but time, and on a cold day the wait is longer.
Pass sequence on a wall
- Check the substrate: temperature, dew point, moisture, dust, oil, frost. Fix it before any foam goes down.
- Spray a thin picture-frame pass around the perimeter of the cavity, the edges and corners where air leaks and bonds fail first.
- Fill the cavity in lifts of about 2 inches, bottom to top, moving at a steady pace. A slow, hovering gun stacks a thick lift in one spot.
- Wait for tack-free, then the next lift.
- Stop short of the stud face. Closed cell is hard and expensive to trim; plan the lifts so the last one lands where the spec wants it.
Open cell is different: it is light, it expands more, and it is normally sprayed to the cavity depth in one pass and trimmed flush. Check the yield card for the maximum pass on the open cell you are spraying.
Reading the result
| What you see | What happened |
|---|---|
| Dark or brown center when you cut a core, burnt smell | Too hot at the gun, or winter grade in the heat; with Thermoseal HFO a thick lift alone rarely does this |
| Foam shrunk off the studs or pulled a seam | Cold substrate, wrong grade for the temperature, or a thick lift that cooled stressed |
| Blisters between lifts, hollow sound | Next lift sprayed before tack-free, or moisture on the surface |
| Peels off in a sheet with the substrate clean | No bond: dust, oil, frost or moisture under the foam |
| Rough, crumbly surface | Cold material, cold substrate, or pressure low for the chamber |
Fix the cause, cut out the bad foam back to sound material, and re-spray in proper lifts. Spraying a fresh lift over a failed one hides it for a season at most.