Summer grade or winter grade spray foam: when to switch

The grade is the B-side. The A-side drum of ISO is the same; the winter B-side carries a heavier catalyst load so the foam reacts hot and fast on a cold wall. The summer B-side is dialed back so the foam has time to rise before it sets in a hot attic. Spray the wrong one and the foam tells you the next morning.

The temperatures on the data sheets

Product	Grade	Substrate and air temperature	Also required
Thermoseal HFO (closed cell, 2 lb)	Summer	above 45°F	substrate 10°F above dew point
Thermoseal HFO (closed cell, 2 lb)	Winter	above 14°F	substrate 10°F above dew point
Thermoseal 5G (closed cell, commercial)	Summer / Winter	above 45°F / above 14°F	substrate 10°F above dew point
Thermoseal 500 (open cell, 0.5 lb)	One grade	above 32°F	substrate 10°F above dew point
Thermoseal 800 (open cell, 0.75 lb)	One grade	above 32°F	substrate 10°F above dew point

Those are substrate temperatures. The air in the building at 10 am is not the wall. A metal building or a slab that sat at 20°F overnight is still a 20°F substrate at 7 am even when the heater says 50.

When to switch

Switch when the substrate you will spray that day will be under about 45°F for closed cell. In practice:

Fall: the first week the overnight low stays under 40°F, winter grade goes on the truck. Keep summer grade for daytime attics that are still hot.
Spring: go back to summer grade when the morning substrate is reliably above 45°F and attics are climbing past 90°F.
Shoulder months: carry both. The job decides, not the month. A crawlspace in March is a winter-grade job; the attic above it on the same afternoon may be a summer-grade job.
Open cell: no switch to make. Below 32°F at the substrate, wait, heat the space, or spray closed cell.

The dew point rule applies all year: the substrate must be at least 10°F above the dew point, or the foam goes down on a film of moisture and the bond is gone.

What goes wrong with the wrong grade

Mistake	What you see	Why
Summer grade on a cold substrate	Slow rise, dense foam, shrink-back or pulling off the studs by morning	Not enough catalyst for the cold; the reaction stalls and the foam cures stressed
Winter grade in the heat	Dark or burnt core, strong smell, rough surface, cracking as it cools	Too much catalyst for the heat; the lift cooks
Either grade on a wet or frosty wall	Blisters, poor adhesion, foam that peels in sheets	Moisture in the reaction and no bond to the substrate

Drum temperature is a separate question

The grade handles the wall. The drums still have to be warm. Thermoseal HFO drums are brought gradually to 77°F before spraying and are not recirculated; Thermoseal 500 recirculates at 77 to 90°F. A winter-grade set sprayed from 40°F drums is still a cold set. The drum storage guide covers warming safely.

How to order it

The winter and summer sets are separate items in the catalog, named Summer Version and Winter Version. Stock at each branch shows on the product page and on the branch page, so a crew can check before driving over. Both versions are priced at the same account rate.

Quick answers

What temperature is winter grade spray foam rated for?

Thermoseal HFO winter grade is rated for substrate and ambient temperatures above 14°F. The summer grade is for above 45°F. Both still need the substrate at least 10°F above the dew point.

Is there a winter grade for open-cell spray foam?

No. Thermoseal 500 and 800 open cell are one grade and need the substrate and air above 32°F.

What happens if I spray winter grade in warm weather?

The winter B-side carries more catalyst and reacts hotter and faster. In warm conditions it can scorch the core of a lift, trap heat and shrink, and the surface can get rough. In the heat, use the summer grade.