Spraying Foam in Summer Heat: Hot Steel Limits and Early-Morning Starts

Can foam go on hot steel?

Only while the steel is under the product's own upper limit, and only if that is measured before each pass. That is the whole answer to spray foam in hot weather on a metal roof, and the rest of this page explains where the limit comes from and what the sheets print.

Cold gets the attention, because cold stops a job outright. Summer has a limit too. A steel roof in direct sun can run far hotter than the air, and a hot surface changes how closed-cell foam bonds and forms its cells. PolyJacks sprays closed-cell foam only, inside buildings, within about 35 miles of Spring Hill, KS, so the question that matters here is a roof deck or wall panel that has been baking since morning. The cold-side limits are on cold-weather spray foam limits. This page is the other end of the thermometer.

What the makers print for spray foam in hot weather

Nobody publishes one summer number for all foam. These are the limits and instructions on the documents we opened. They belong to the products named, not to ours.

What the table says in plain terms: the sheets disagree, and they are not even measuring the same thing. One recommends a substrate up to 120 F. One winter grade tops out at 100 F. One sell sheet gives 105 F as an application temperature without saying whether that is air or surface. One data sheet cares about 80 F because that is where its formula changes. A distributor's article puts the surface target at 60 F to 80 F. Take the gap as the finding: the right upper limit is whatever is printed for the exact product and grade in the drums, and it can be well below what a hot roof reaches.

We opened the Gaco guide and the UPC sheet as documents. The Huntsman sheet is a Canadian sell sheet, and the sprayfoamsys article is a seller's page, so treat its range as a seller's account, not a spec. None of these makers is in Kansas or Missouri. How hot a surface can be before foam bonds badly is a matter of chemistry and heat, and it is the same wherever the building stands.

The sheets name two different heat problems

Keep these apart, because the fix differs.

• The foam itself runs too hot. UPC ties stretched, elongated cells, poor yield, quick curing and later cracking to foam applied too hot, and says to lower temperatures. Gaco ties a pass over 2 in to charring. These are mostly about the chemical temperature and the pass thickness the crew sets.
• The surface is too hot. The hot-weather article we read ties a hot surface to adhesion trouble. The substrate ranges above are how the makers express it.

Both show up in summer, and the roof makes the second one worse than the weather report suggests.

Hot steel and what it does to foam

The air at a roof is not the temperature of the roof. Researchers at Oak Ridge National Laboratory tracked twelve painted and unpainted standing-seam metal roofs at an outdoor test facility (Kriner, Miller and Desjarlais, ASHRAE, two years of weathering). In a clear August week, the daily maximum air temperature there ran about 85 F to 95 F. The paper reports that the low-reflectance sections, including an asphalt-shingle one, all exceeded 160 F on the roof surface, with some days at 165 F. The paper says roof temperature depends strongly on the surface's solar reflectance and infrared emittance.

Read that with care, for three reasons.

1. It is the outer face. Foam on a metal roof goes on the underside, from inside the building. The underside of a deck is not the same temperature as the sunlit side, and we have no measurement of the gap.
2. It is a test site in Tennessee, not a Kansas building. The sun heating dark metal is physics, so it applies here, but the number is theirs, not ours.
3. It is a low-reflectance result. The paper says reflectance and emittance drive roof temperature, so a light-colored or reflective roof will not read the same. The color of your roof is a question for the visit.

What the paper does show is the size of the gap. A week of ordinary August air produced sunlit roof surfaces more than 60 F above it. A weather report cannot tell a crew what the steel reads, which is why the reading has to come from the steel.

For the Kansas City area, the National Weather Service gives normal highs of 87 F to 89 F in July (Kansas City/Pleasant Hill, MO, 1991 to 2020), with a record of 109 F on July 15, 1934. Set against the sheets above, the normal air temperature is inside most of the printed ranges, and the record is above the 105 F and 100 F figures on the sheets above. A normal summer afternoon is not the problem. A roof that has been in the sun since early morning, in an afternoon that runs above normal, is.

How summer roof work is scheduled

This is the method a crew uses to decide whether a roof pass can start. The limits come from the sheet of the product in the drums, so this page prints no limit of ours.

1. Read the sheet's upper substrate limit for the grade in the drums. Check whether the product has a summer grade, as the UPC sheet does.
2. Measure the steel, not the air. UPC tells crews to use a laser thermometer for drum temperatures, and the same style of non-contact reading is the simple way to get a surface number. Take readings at several spots, including the sunniest panels.
3. If the steel reads over the limit, wait or shade it. The usual answer is to start early, before the sun has loaded the roof, or to pick a cooler or overcast day.
4. Cap the pass. Keep each pass within the sheet's maximum thickness. Gaco and UPC both set 2 in for their products, and both tie thicker or hotter passes to charring or shrinkage.
5. Read again before the next pass. A roof warms through the morning, so a reading that passed at seven can fail at noon. The foam from the previous pass is also warm; the Gaco guide's winter tip is to let a pass cool to 110 F before the next, and the same caution fits a hot day, though the guide gives it for winter.
6. Stop when the reading says stop. A day that begins inside the limits can end outside them. The honest schedule is a window, and sometimes the roof work moves to another day.

Until this is confirmed, the list above is the general method, not a promise about our crews.

The wall and ceiling surfaces inside a building are usually not in direct sun, so they run closer to the air. That is why the early-start rule matters most on roofs. How the foam goes on a roof deck from the inside is on spraying under a metal roof. The full sequence from checklist to cleanup is on what happens on spray day.

Crew heat in protective gear

Heat is a limit on people too. NIOSH describes heat stress as the combined load from the body's own heat, the environment, and clothing and protective equipment. Its page lists protective equipment and lack of acclimatization among the risk factors, and says employers should reduce heat stress through engineering and work-practice controls.

That bears on scheduling. Spray crews work in protective clothing, so an early start protects the crew as well as the foam. A roof that reads over the product's limit at one in the afternoon is also a bad place to be then.

Best time of year to spray foam

There is no season that guarantees a good day. Any day works if the steel, the air and the chemical are inside the product's limits at the moment of spraying. What the calendar changes is how often that happens.

Here are National Weather Service daily normals for 1991 to 2020 at the Kansas City office's station.

Sources: NWS Kansas City monthly normals pages for April, May, July, August, September and October.

How to use it. These are outdoor air normals, not steel readings. In the April, May and October rows the normal high and low both sit inside the 40 F to 120 F range printed on the Gaco sheet for its regular grade, and the highs stay under the 80 F where the UPC sheet switches formula. The September 15 high is right at 80 F. In July and August the highs run 8 to 9 F above that switch point, and a sunlit roof can run far above the air. In the coldest weeks the problem flips, as the winter page shows.

That is the basis for saying spring and fall are easier. It is not a promise. A warm spring afternoon can still heat a dark roof, and a cool August morning can be a fine start. Summer work on roofs is mostly a question of which hour of the day.

If your building has a dark roof and you want it insulated in July, the plan is the early-morning start. If timing is flexible, ask for the visit now and the work for the week the readings favor.

FAQ

Can you spray foam on a hot metal roof?

Only while the steel reads under the product's maximum substrate temperature, taken before each pass. The sheets we opened set recommended upper limits from 100 F to 120 F and warn about foam applied too hot or too thick. Early starts and cooler days are how roof work stays inside the limit.

Is spray foam affected by hot weather?

Yes. The sheets tie heat to formula changes, frothing, stretched cells, poor yield and later shrinkage or cracking. The makers' fixes are a summer formula, lower temperatures and thinner passes.

What is the best time of year to spray foam a metal building?

Any day when the steel and the air are inside the product's limits at both ends. Spring and fall give more of those days in the Kansas City normals, and summer roof work usually means an early start. No season is guaranteed.

Can foam go on steel that has been in the sun all day?

Not until it has been read. Sunlit steel can be much hotter than the air, and the foam's bond depends on the surface it lands on. See spraying under a metal roof for the work and the before-we-spray checklist for what we ask of you.

Book a free on-site measurement

At the visit we measure the building, look at the roof and its color, ask how the space is used and talk through timing. A hot building is a reason to plan the season with us, and the readings on your steel decide the day. The visit is free, and a written estimate follows. A hot, stuffy building and what foam does about it is on a metal building that is too hot in summer. How to book is on the free estimate visit.

Book a free on-site measurement

Pick "Spray foam insulation" on the booking form. We work within about 35 miles of Spring Hill, KS.

Spray foam insulation is a new PolyJacks service line run by its own crew and rig.

Who wrote this: PolyJacks spray foam team. How it was made: drafted with AI assistance from the sources below, then checked against each source.

Sources

• Gaco, "Spray Application Guide, Gaco 183M-CAN Regular and Winter," October 2015: gaco.com
• UPC (Universal Polymers), "UPC 2.0 HL HFO Technical Data Sheet and Spray Guidelines," code UPC-TDS-2.0-HFO 091720EA, hosted by Sprayworks Equipment: sprayworksequipment.com
• Huntsman Building Solutions, Heatlok Soya HP commercial sell sheet (Canada): huntsmanbuildingsolutions.com
• sprayfoamsys.com, "Things to know before using spray foam in hot spaces" (undated): sprayfoamsys.com
• Scott A. Kriner, William A. Miller and Andre O. Desjarlais, "The Thermal Performance of Painted and Unpainted Standing-Seam Metal Roofing Systems Exposed to Two Years of Weathering," Oak Ridge National Laboratory Buildings Technology Center: web.ornl.gov
• NIOSH, Heat Stress (CDC): cdc.gov
• National Weather Service Kansas City, daily climate extremes and normals (1991-2020): weather.gov , weather.gov , weather.gov , weather.gov , weather.gov , weather.gov