Winter Foam Jobs Start with the Right Heat — Here’s How to Get It Right

Your target substrate temps should match your product’s TDS — not your gut feeling.

Cold weather doesn’t have to kill your season — but bad heating practices will.

Every winter, we see it. Crews roll up to uninsulated metal buildings or pole barns with the right equipment, the right foam, and the right game plan. But the job still fails. The foam collapses. Fish-eyes everywhere. Delamination. Weird shrinkage. And nobody knows why.

The answer is almost always the same: you tried to fight winter with the wrong kind of heat — or the right kind, used the wrong way.

Getting a structure spray-ready in cold months isn’t about how many BTUs you can throw at it. It’s about control. Controlled heat, controlled humidity, and controlled environment. If you’re not dialed in on all three, you're gambling every time you pull the trigger.

Let’s break down what really matters — and what separates pros from people hoping for the best.

It's Not About the Air — It's About the Surface

Step one: you’re not spraying foam into air. You’re bonding foam to a substrate.

And that substrate — whether it’s OSB, steel, CMU block, or poured concrete — is what determines your success. It doesn’t matter if your laser thermometer reads 68°F in the center of the room if the walls are still sitting at 38°F. That foam is going to collapse, shear off, or underperform.

Your target substrate temps should match your product’s TDS — not your gut feeling.

Typical guidance:

• Open-cell foam: Minimum substrate temp 40°F, ideal 50°F+

• Closed-cell foam: Minimum substrate temp 50°F, ideal 60°F+

• Steel buildings: Add 5–10°F buffer to compensate for thermal conductivity

Spray onto cold steel or cold block, and you’ll find out real quick how forgiving your foam isn’t.

Moisture Is the Hidden Killer

When the substrate is cold and the heater is dirty, humidity happens fast.

Combustion-based heaters (like diesel torpedoes or propane salamanders) don’t just generate heat — they generate water vapor. Every gallon of diesel burned can release almost a gallon of water into the air, depending on efficiency. That moisture doesn’t just hang in the air — it condenses on your cold substrates.

Once you’ve got condensation on metal or masonry, your job just became a science experiment. Even if it looks dry, the microfilm of moisture can ruin adhesion, disrupt rise, or cause blistering, discoloration, and soft cells.

Humidity above 60% in a winter spray job is asking for failure. Even 50% can be risky in enclosed structures with poor airflow.

Not All Heat Is Created Equal: The Indirect-Fired Advantage

If you remember nothing else from this article, remember this:

Indirect-fired heaters are the only heaters that give you clean, dry, safe, spray-compatible heat.

They burn fuel in a sealed chamber and pass the thermal energy through a heat exchanger to deliver moisture-free warm air into the structure. No fumes, no soot, no water vapor, no risk to your foam.

Compare that with a kerosene torpedo or propane cannon dumping moist, oxygen-depleted air directly into your spray zone, and it’s easy to see why so many winter jobs go sideways.

Here's the breakdown:

Heater Type	Adds Moisture?	Safe for Spraying?	Power Source	Notes
Indirect-Fired	✅ No	✅ Yes	Diesel, Propane	Clean, dry air via heat exchanger. Best for enclosed buildings. Can run during spraying.
Electric Resistance	✅ No	✅ Yes	Electricity	Clean but often underpowered for large spaces. Good supplement.
Torpedo (Kero/Diesel)	❌Yes	❌ No	Diesel, Kerosene	High moisture and fume output. Good only for early preheat and vented zones.
Propane Salamander	❌ Yes	❌ No	Propane	Easy to use, but risky. Adds substantial moisture. Avoid if structure is sealed.
Open Flame	❌ Yes + 🔥	❌ No	Any fuel	Should never be used in spray zone. Fire hazard + chemistry hazard.

Bottom line: Only indirect-fired or clean electric heat gives you the conditions spray foam needs to cure correctly in cold weather.

Can You Leave an Indirect-Fired Heater Running Overnight?

Yes — and in cold-weather spray foam prep, you often should.

Unlike open-flame or direct-fired units, indirect-fired heaters are designed for safe, long-duration operation in enclosed or semi-enclosed spaces. If you want steel purlins or concrete floors to hit sprayable temps by morning, that heat needs to run through the night — not just a few hours before your crew shows up.

But overnight heat isn’t just a plug-it-in-and-pray situation. You still need to manage it wisely.

Here’s how to do it right:

• Use a thermostat or inline temperature controller. Letting temps spike to 90°F overnight doesn’t help. It can cause excess drying, substrate expansion, or condensation swings. Target a stable 60–65°F holding temp to warm the mass gradually.

• Fuel up ahead of time. Most indirect units run 8–12 hours on a full tank — but don’t chance it. Refill before sundown. Cold starts in the morning waste time and money.

• Vent properly. The heater itself exhausts combustion gases outside via flue or duct. Double-check that your vent line is:

  • Properly connected

  • Not blocked by snow, tarps, or equipment

  • Aimed away from air intakes, windows, or plastic sheeting

• Secure power and placement. Place the heater on level, non-combustible ground. Make sure extension cords (if using for control units or fans) are rated for outdoor winter use. Don’t run cords under doors that get closed and crushed overnight.

• Don’t rely on propane indoors. If your indirect unit is propane-fired, keep the tanks outdoors and run hoses in. Never store cylinders inside the building overnight — it’s a serious fire code violation and a potential explosion risk.

• Use CO detectors if in doubt. While indirect heaters are designed not to leak combustion air, stuff happens — especially on chaotic jobsites. A battery-powered CO detector near your unit is cheap insurance.

• Walk the perimeter before lights out. Do one last loop:

  • Make sure doors are sealed

  • Plastic sheeting hasn’t come loose

  • No debris is blocking intakes or ducts

  • Fuel is topped off

  • Thermostat is working

How Many BTUs Do You Really Need?

There’s no magic number — but there is solid math.

To estimate the BTUs required to heat a structure in winter, consider:

• Volume of air (cubic feet)

• Desired temp rise (°F)

• Building envelope (tight vs leaky)

• Timeframe (overnight vs fast ramp-up)

Basic formula:

BTUs/hour = (Cubic Feet of Air Volume) × (Temp Rise in °F) × 0.133

Let’s break that down with an example:

• A 40×60×14 pole barn = 33,600 cubic feet

• You want to raise temp from 20°F → 60°F = 40°F delta

• Plug it in:33,600 × 40 × 0.133 = ≈178,000 BTUs/hour

That’s assuming a reasonably tight structure and continuous runtime. Add 10–20% extra if the structure is leaky or poorly enclosed.

So you’ll need one 200,000 BTU/hr indirect-fired heater — or multiple smaller units — to get you there and hold the line.

Building Size	BTUs Needed (40°F rise)	Recommended Heater
30×40×12 (14,400 CF)	~76,500 BTU/hr	85–100k BTU unit
40×60×14 (33,600 CF)	~178,000 BTU/hr	200k BTU unit
60×80×16 (76,800 CF)	~408,000 BTU/hr	Two 225k BTU units or larger

If you’re in extreme cold, or heating steel that sucks heat away, upsize accordingly.

Tips for Effective Preheat (and Holding Temp)

1. Start Early - Turn on your heaters the night before. Preheating takes time — especially if the slab or steel is well below freezing.

2. Seal the Envelope - Use plastic, foam board, or even heavy tarps to close eaves, roofline gaps, garage doors, and soffits. Heat escapes fast without a barrier.

3. Mix Air, Not Moisture - Use low-speed fans to circulate heat, not blow on your spray zone. Airflow evens temps but should never cross your spray gun.

4. Monitor Everything

   • Use IR thermometers to log substrate temps

   • Use hygrometers to track RH

   • Consider using data loggers for job documentation

5. Shut Down Combustion Heaters Before Spraying - If you used torpedoes or salamanders for preheat, shut them off during application. Let the building hold temp, and switch to indirect-fired or electric only.

Final Word: Dry Heat Is the Only Heat That Counts

In the winter spray game, shortcuts cost you twice: once when the foam fails, and again when you eat the rework.

Don’t let a “warm enough” gut check replace real numbers. Don’t let combustion moisture sabotage your spray zone. And don’t settle for just any heater.

Use indirect-fired heat. Monitor your substrate temps. Control your humidity. That’s how real pros spray foam in February — and why their jobs hold up in July.

by Gage Jaeger, Owner and Founder of Foambid