Spray Foam 101
Closed-cell spray foam starts as two separate liquids in two drums. They’re heated, pumped at exactly one to one, and meet for the first time inside the spray gun. Here’s every step, what happens in the chemistry, and why it has nothing in common with a can from the hardware store.
Scroll to run it. This is a single pass of Heatlok HFO High Lift in a wall cavity, from bare sheathing to cured foam.
Scroll to run the lift. One lift of Heatlok HFO High Lift goes on dark and glossy, reaches full thickness in about four seconds, and cures to a level, even skin. That flat surface is what good work looks like.
Everything upstream of the gun exists to deliver two chemicals at the right ratio, temperature and pressure. Scroll through the six stations.
Compressed air drives the proportioner and the drum pumps and clears the gun after every trigger pull. On its way from the compressor it passes a ½-inch filter that catches oil and dirt and protects the dryer, a refrigerated air dryer (ours is a Schulz ADS) that pulls the moisture out, and a ½-inch regulator that holds the pressure steady. Wet air rusts and freezes air motors, and in the gun it meets the A-side, which reacts with water.
The A-side is isocyanate. The B-side is a resin blend: polyols, the blowing agent, catalysts, flame retardant and surfactants. Each has its own drum and its own air-driven transfer pump feeding its own line to the proportioner. The resin is mixed in the drum before we start so the blowing agent stays evenly spread through it.
The heart of the rig. Two matched pumps push the A and B sides out in equal volumes, stroke for stroke, at spray pressure, commonly well over 1,000 psi. We watch both gauges the whole time; if the pressures drift apart, the ratio is drifting too, and we stop.
Each side runs through its own heater, then through a heated hose that holds temperature for the full run, so the rig can stay outside while we work inside. Warm chemical is thinner, mixes completely and reacts on schedule. Cold chemical does none of those things well.
A plural-component spray gun, nothing like a paint gun. The A and B lines connect to a block under the front of the gun and stay separate right up to its mixing chamber; air comes in through the handle. Pull the trigger and the two sides collide in the chamber at full pressure. Release it and the air purge blows the chamber clean, so mixed foam never sits inside the gun.
The mixed liquid leaves the tip as a fine, even spray. On the wall it starts reacting almost at once, rises to full thickness in about four seconds and forms a firm, even skin.
There is no mixing blade and no stirring. The two sides are mixed by hitting each other at high speed in a chamber smaller than a drinking straw. That is called impingement mixing, and it is why ratio and temperature have to be right before the trigger is pulled.
The moment A meets B, two things happen at once: the liquids start joining into a solid plastic, and the heat from that reaction turns the blowing agent into gas. The race between the two is what makes the foam.
Many closed-cell formulas also use a little water in the resin; it reacts with the isocyanate to make CO₂, which helps the rise. Times are typical for a warm lift of Heatlok HFO High Lift and vary with temperature.
The chemistry is only as good as the conditions it happens in. These are the six things we control on every job, and the reason this is a trade and not a product you buy.
One to one by volume. Too much A-side and foam turns brittle; too much B-side and it stays soft, shrinks or smells. We check pressures constantly and cut test samples.
Heaters and hose are set to the manufacturer’s processing window for the day’s conditions.
Foam only bonds to a clean, dry surface in range. We read surface temperature and moisture before every lift. Heatlok HFO High Lift is rated for substrates down to 15°F.
Every foam has a maximum pass because the reaction makes heat. High Lift is rated for up to 6.5 in. in one pass; older foams needed 2 in. lifts and cooling time.
Right pressure plus a clean tip gives a fine, even fan. Coarse spray means uneven thickness and wasted foam.
Supplied-air respirators and full protection during spraying, and the building kept clear for the manufacturer’s re-occupancy period.
For most people, “spray foam” means a can of gap filler. Same name, very different product.
| Can foam | Professional closed-cell | |
|---|---|---|
| What it is | One component, pre-mixed in a can | Two components that react only when they meet in the gun |
| How it cures | Draws moisture from the air, slowly, from the outside in | Chemical reaction between A and B: full rise in seconds, through the whole lift |
| What it’s for | Filling small gaps and cracks around windows, pipes and wires | Insulating and air-sealing whole walls, roofs, rim joists and buildings |
| Who controls the mix | Nobody; whatever comes out of the straw | Metered 1:1, heated and pressure-checked on every job |
| Tested as insulation | Not intended as a wall or roof insulation layer | Published R-7.4 per inch, code evaluation reports, fire testing |
| Who applies it | Anyone, one bead at a time | Certified installers in supplied-air respirators |
Myth
FactThe can from the hardware store and professional closed-cell share a name and not much else. Can foam is a one-part gap filler that cures from moisture in the air. Closed-cell is a two-part system that reacts in seconds, is metered and heated by a proportioner, and is tested and rated as insulation.
Myth
FactLow-pressure two-part kits are fine for a rim joist or a few gaps. They aren’t heated or pressure-controlled, they’re sensitive to temperature, and on a whole building they cost far more per board foot than professional foam, with no ratio check if the output drifts.
Myth
FactEvery foam has a maximum pass because the reaction makes heat. Go past it and the foam can scorch, crack or pull away. The skill is hitting the right total thickness within the rated lift, and High Lift lets us do it in fewer passes safely.
Myth
FactCorrectly mixed and applied foam cures into an inert plastic. The building is kept clear during spraying and for the manufacturer’s re-occupancy period. Lasting odor problems trace back to off-ratio or poorly applied foam, which is exactly what ratio checks and sample cuts prevent.
Myth
FactThe blowing agent changed. HFO has a global warming potential of about 1; the HFC blowing agents it replaced were in the hundreds to thousands. Performance didn’t go backward: Heatlok HFO High Lift is rated R-7.4 per inch.
We install both. Here is how they differ and where each belongs.
Best for: metal buildings, pole barns, crawlspaces, rim joists, roof decks, anywhere moisture or condensation is a risk.
Best for: sound control in interior walls and deep, dry attic cavities where thickness is free and moisture is controlled elsewhere.
R-values are nominal manufacturer values (aged, ASTM C518). Open-cell figures are typical of half-pound foams. Neither is a guarantee of energy savings. Read the full comparison.
Sixty seconds in the estimator gets you a range. A walkthrough gets you a fixed price, the thickness and the lot numbers in writing.
