Best by project · Based on the 2021 IRC; your local code may differ
The drips come off the underside of the roof panel. That is the surface the first inch is bought for.
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 
TYTAN Professional Thermospray KitSee it on Amazon As an Amazon Associate I earn from qualifying purchases.
A post-frame barn is wood framing under a steel skin. Posts stand on 8 to 10 ft centers, horizontal girts run between them, and the panel is screwed straight to the girts. That is not a stud wall: the spaces between girts are shallow and the posts interrupt them, so there is no continuous cavity running floor to eave the way there is in a framed house. Overhead it is purlins over trusses with the roof panel screwed on and, in most barns, nothing under it. The inside surface of the building is the underside of the steel.
Which is why the drips are a roof problem before they are an insulation problem. On a clear, still night the panel radiates to the sky and falls below the temperature of the air under it; any moisture in that air condenses on the metal and falls. A dirt or gravel floor, a vehicle brought in wet, livestock, or an unvented propane heater keeps that air loaded all winter. Foam sprayed onto the underside of the panel takes away the air path and lifts the surface the air touches above the dew point, which is two mechanisms rather than one, and it only does either where it is continuous and actually stuck to clean metal.
The wood frame changes the arithmetic that makes a pre-engineered steel building expensive to insulate. Steel purlins carry heat from the inside face to the outside skin, which is why those buildings are sprayed over the framing rather than between it. Wood girts and posts conduct a small fraction of what steel does, so foam between the framing is not undone by the frame beside it. That is the one thing a pole barn makes easier.
Then the size. A 30 by 40 barn with 10 ft walls and a 4:12 roof has about 2,650 sq ft of surface to cover, which is two and a half times a basement and ten times a rim joist. The number below is worth working out before anything is ordered, because at that scale the comparison with a contractor quote stops being obvious.
Assembly, moisture and code planning for this job is at our sister publication under common ownership: Pole-barn 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
In an unheated barn the foam is bought to stop water forming on the panel, and the first inch or two does most of that work. R-value is what the inches after that buy, and they are only worth buying if the building is going to be heated or cooled. How thick is a calculation from your inside conditions and your design temperature, not a number anyone can hand you; what the makers publish, and what you compare, is the perm rating at a stated thickness.
There is no ceiling and no sheathing in most barns, so the foam goes onto the metal itself. Where the panel is oily from rolling, dusty from the floor, rusted at a screw line or wet from last night, the foam does not bond and the air path comes back under it. A barn that has stood open for ten years needs the panel cleaned before anything is sprayed, and the sheets state the surface conditions their adhesion claim assumes.
The IRC governs dwellings, so it does not cover a barn. Many states exempt agricultural buildings from the building code entirely while still applying zoning and electrical rules, and others exempt them only while the use stays agricultural. The IBC foam plastic rules cover what the IRC does not. Which one your building falls under is a question for the building official and the answer changes the day the barn becomes a shop, an office or a home.
A barn with livestock in it has a moisture source that never stops, and sealing the building does not remove it. Foam takes away the air leaks that were doing the ventilating by accident, so a barn that houses animals needs a ventilation plan after it is insulated rather than fewer gaps. That is a decision to make with whoever advises on the stock, not from an insulation page.
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.
Five to six kits for the first inch at the makers’ own theoretical yields, before any allowance for the loss every one of them warns about. If the barn is unheated and the problem is drips, that first inch is the whole job. If it is going to be a heated shop, price the second inch and get a contractor quote beside it.
Run your own numbers in the calculatorQuestions
If the barn is being built, a condensation layer under the panel is decided at that point and it is a different purchase. If it is already standing with bare steel overhead, closed cell foam sprayed onto the panel is the vapor control as well as the insulation, 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. Adding sheet plastic under foam that is already low perm can trap water between the two layers.
If the problem is water dripping off the steel, the roof is the whole problem and the walls can wait. If the barn is going to be heated, the walls are where most of the heat leaves and both get sprayed. The order follows which of the two you are actually buying.
The arithmetic above is the answer: six 24-can kits for one inch over a 30 by 40 barn, at the makers’ claimed yields. It can be done and people do it. The canister systems are not cheaper per claimed board foot, roughly double, but they are the only ones whose yield figure the maker defines and they go on far faster. Read the ventilation and respirator conditions on the exact product either way.
In many states an agricultural building is, and the exemption is written into the state adoption rather than the model code. It usually turns on use, so it ends when the barn becomes a shop, an office or living space, and zoning and electrical rules are separate from it in either case. The building official is the only source for your address.
From the job to the finished wall
This page answers it for pole barns. 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.