Best by project · Based on the 2021 IRC; your local code may differ
The first inch is bought to stop condensation. R-value is what you buy after that.
Product examples · not ranked
DuPont Froth-Pak 630 Foam SystemSee it on Amazon 
HandiFoam FR (HFO) II-200 and II-600See it on Amazon 
Akfix ThermCoat 24-PackSee it on Amazon 
Magic Chems Spray Foam Insulation Kit 24-PackSee it on Amazon 
TYTAN Professional Thermospray KitSee it on Amazon As an Amazon Associate I earn from qualifying purchases.
This page is about a pre-engineered steel building: a steel frame with steel purlins and girts, and the panel screwed straight to them, so there is no cavity at all and nothing between the inside air and the outside metal. A post-frame pole barn is wood framing under the same kind of skin and the arithmetic is different enough to be its own page, so it has one. Here the foam goes on the inside face of the skin, sprayed between the framing and, if the budget allows, over it.
Condensation is why foam wins this job rather than batts, and it is worth being exact about when it happens. Water forms where the panel is colder than the dew point of the air touching it. Two things get it there. On a clear, still night the roof radiates to the sky and falls several degrees below the outside air, which is the case everyone describes. The one that actually soaks most buildings is duller: an unsealed slab, bare ground, a vehicle brought in wet or an unvented propane heater puts moisture into the air, and it meets a cold panel on any winter day. Dry air against a cold panel does nothing at all, which is why sealing the floor matters as much as anything you spray.
The traditional answer is a faced fiberglass blanket laid over the purlins, and it fails in ways that have little to do with someone tearing it. It is punctured by the roof screws at installation, by design. It is crushed to almost no R-value where it is pinched between the purlin and the panel, which is itself part of why the cold lines appear along the framing. And air leaks at the laps. Closed cell foam sprayed onto the panel removes the air path and lifts the inside face of the assembly above the dew point, which is two mechanisms rather than one. It does that where it is continuous and actually stuck: a panel that is oily, rusty or loosely coated is where adhesion fails and the path comes back. How much vapor moves through after that is a number the maker publishes, and HandiFoam FR prints 0.9 perm at 1 in on its data sheet.
Then there is the size of it. A modest 30 by 40 ft building with 14 ft walls has about 3,000 sq ft of surface to cover. That is ten times a rim joist and three times a basement, and it is the number that decides whether you are buying kits or calling a contractor.
Assembly, moisture and code planning for this job is at our sister publication under common ownership: Metal-building condensation and enclosure planning, at Spray Foam Guides. This page stays with exact products and the records their makers publish.
The assembly decides the product
The first inch or two of closed cell foam is bought to keep humid air off cold steel, not to hit an R-value. How thick that has to be depends on how warm and humid you keep the inside and how cold it gets outside, so it is a calculation rather than a number someone can hand you. What the makers publish, and what you compare, is the perm rating at a stated thickness.
In a pre-engineered building the purlins and girts are steel and they run from the inside face to the outside skin. Spraying between them insulates the bays and leaves the frame conducting straight through, which is what draws the cold lines people photograph on a metal ceiling in winter. Covering the framing means spraying over it, which adds depth across the whole building and is where the cost goes. A post-frame barn with wood girts does not have this problem to the same degree, because wood conducts a small fraction of what steel does.
The working ranges on these sheets are ambient and can temperatures, not substrate temperatures, and they are not one number. Most one-component sheets we hold top out around 86 F, Loctite goes to 95 F, Great Stuff states 60 to 90 F, DAP reaches 120 F and the BEEST kit asks for 68 to 77 F. On the canister side HandiFoam asks for core chemical at 75 to 85 F while Froth-Pak asks only for tank contents at 75 F or warmer. What none of them publishes is a limit for the surface you are spraying onto, and a south-facing steel wall in July is hot enough to change how the foam rises and sticks. Read the range on the product you buy, and treat a hot skin as a reason to move to the shaded side or the early morning.
The IRC governs one- and two-family dwellings. A barn, a shop or a storage building is not a dwelling, so the code that applies is the IBC, and many states exempt agricultural buildings from the building code entirely while still applying zoning and electrical rules. Ask the official which one covers your building before you plan around any section on this page. The answer changes the day the building becomes living or working space.
What the code says
Section numbers are from the 2021 International Residential Code. Your city or county adopts one edition of the code and changes parts of it, and your local building official has the final word. A thermal barrier and an ignition barrier are different things, and the glossary explains both.
Products that document this job
Listed by product family. Each line says why the product is on this list, always from something the maker published. Follow the link for the documents, the questions still open, and dated prices.
Quantity, worked once
Board feet are square feet times inches. The kit figures are the makers’ theoretical yields, which every maker that defines the term says are laboratory numbers; plan on less.
Eleven to fourteen kits at the makers’ own theoretical yields, before any allowance for the loss every one of them warns about. That is the number worth sitting with before you order anything. At this scale, get a contractor quote alongside it, because the comparison stops being obvious the way it is for one wall.
Run your own numbers in the calculatorQuestions
Usually not a separate one, and adding sheet plastic on the warm side of foam that is already low perm can trap water between the two layers. Closed cell foam at enough thickness is the vapor retarder, and the number that says so is the perm rating the maker publishes at the thickness you sprayed. HandiFoam FR prints 0.9 perm at 1 in, which its own sheet calls a Class II vapor retarder. Read that figure on your product, then ask the official what your adopted code asks for.
Nobody can give you one number, and be careful of anyone who does. It depends on how warm and humid you keep the inside, how cold your design temperature is, and the perm rating of the product. What is true generally is that the first inch or two does most of the condensation work and the inches after that are buying R-value. If the building will be conditioned, size the total from the energy code and treat the condensation layer as the floor, not the target.
You can, and the arithmetic above is the reason most people do not. A 30 by 40 building at 2 in, gable ends included, is 6,420 board feet, which is fourteen 24-can kits at their claimed yield and more once you allow for loss. Cans make sense for a bay or two, a shop wall, or the penetrations. For the whole envelope the canister kits are not cheaper per claimed board foot, they are roughly double, but they are the only ones whose yield figure the maker defines and they go on far faster. At this scale get a contractor quote too.
That depends on which code covers your building and how it is used, which is why the constraint above says to ask first. Where the building is a dwelling the answer is yes, a thermal barrier. Where it is not, the IBC foam plastic rules apply with their own exceptions, and some of those turn on the occupancy and the height. Exposed foam in a working shop is the case to get confirmed before you spray, not after.
From the job to the finished wall
This page answers it for metal buildings. The same five questions run for every other part of the house.
Board feet are the area times the thickness. Price the kits that cover them against hiring the job out.
A kit, the gun and cleaner it works with, something to cover what stays clean, and the gear you wear.
Check the temperature, prepare the surface, spray in passes and cover the foam where people live.
The sheets give a tack-free time in minutes and a full cure in hours; only some say when people can go back in.
We have not sprayed or tested any product on this page, and nothing here stands in for the label, the safety data sheet or what your local inspector tells you.