Is spray foam worth it? An installer's honest answer
Every guide on this question is written by someone selling something — foam, buildings, or a competing product. This one is different: it’s the actual judgment of a man who sprayed urethane foam for a living for 30 years across Idaho and Wyoming, and who has nothing to sell you today. Which means you get both halves: the sales pitch he genuinely believed, and the situations where he’d tell you to slow down.
The case for foam, from the man who sprayed it
Asked for his pitch against conventional insulation, he gave three points, and they hold up:
- Roughly double the insulation per inch. Closed-cell foam runs about R-7 per inch against roughly R-3.5 for fiberglass. Where space is tight — thin walls, shallow roofs — foam fits insulation where batts simply can’t.
- It attaches itself. On a pole barn or metal building there’s often nothing convenient to fasten conventional insulation to — foam sprays on and stays, no framing-out, no pins, no sagging in ten years. On metal buildings this often makes foam cheaper installed than products that need a whole attachment system built first.
- It stops all air infiltration. Not “reduces” — stops. The air seal is half of what you’re buying, it’s the half no batt can match, and in a windy climate it’s often worth more than the R-value. (Side benefit he noted: sealed buildings also don’t invite rodents — foam has no food value and critters generally leave it alone, though a determined one can chew through.)
His overall verdict after 30 years: for the right building, it’s simply the best product — “it pays for itself” over the building’s life.
Where he’d hesitate
The honest half. Three situations gave him pause, and each is a mechanism, not a mood:
- Old, rusty metal roofs. The one failure he calls catastrophic: foam over already-rusted metal traps condensation against the rust and, in his words, “rusts the roof right off” — the fate of Idaho potato cellars sprayed over old tin. The full mechanism is here. New metal, fine. Rust you can see, walk away or fix the metal first.
- An old building you want to keep a long time. Asked whether he’d foam a 20-year-old pole barn, his answer was “it matters how long I was going to be there” — expecting an already-aging metal skin to serve another 40–50 years under foam is asking a lot. Foam is permanent; make sure what’s under it is worth being permanent about.
- High humidity with no ventilation plan. His real answer to “when would you tell someone not to foam it?” was: almost never — instead, “you have to convince them to vent it right.” Encapsulated buildings need mechanical fresh air, humid buildings need dehumidification, and open-cell never goes on a roof deck. The full playbook is in the condensation and venting guide. The buildings that turn into horror stories are almost never wrong-product — they’re right-product, no moisture plan.
So the honest reframe: the question is rarely foam or no foam. It’s is this building — its metal, its age, its humidity, its ventilation — set up for foam to succeed?
Protecting yourself: the installer’s own checklist
What the man who did the work tells his own family to check:
The needle trick. You’re paying by the inch, and thickness is the easiest thing to shave. Take a long, thin needle — a syringe needle works — and in any spot that looks thin, push it in and measure. If you paid for two inches, average two inches is what should be there: foam can’t be perfectly smooth, but it should never be one inch here and three there. His words: if he says two inches, “make sure it’s two inches.” Walk the job before final payment, and physically look at everything — the spots nobody crawls up to see are exactly where corners get cut.
Vet the crew like it matters, because it does. Years in business first, reviews second — and watch how they spray. Modern rigs lay foam down smooth; lumpy, cratered work means the equipment heat wasn’t right, and equipment control is the skill that separates pros from the off-ratio disasters on the problems page. One more tell: ask what ratio their machine sprays and what they do when it drifts. A pro has an immediate answer; a shrug means find another crew.
Ask about the fire barrier — it’s a real requirement, not an upsell. Under the residential code, foam plastic insulation must be separated from interior living space by a 15-minute thermal barrier — half-inch drywall is the standard example (IRC R316.4 / IBC 2603.4; see the polyurethane industry’s own code guidance). For shops and pole barns, coatings exist to meet the requirement — and our installer’s blunt field observation was that plenty of barn jobs skip it. A contractor who raises the thermal barrier question unprompted is showing you they work to code; one who’s never heard of it is telling you something too. (Exact requirements vary by occupancy and your local adopted code — ask your building department what applies.)
The truth about DIY kits. His math: a kit big enough for a real job costs about the same as hiring the work done — “you don’t save anything” — and you supply the inexperience for free. He used the small kits himself for exactly one thing: patching, where hauling out a rig made no sense. Patch a spot with a kit; insulate a building with a crew. And don’t buy thickness you don’t need either — past two to three inches the heating savings never pay you back unless code requires it.
The bottom line
Worth it? On a sound building with a ventilation plan and an experienced crew: yes, and he’d say emphatically yes — double the R per inch, self-attaching, airtight. On rusty metal, on a building at the end of its life, or with a crew that can’t answer the ratio question: the same product becomes an expensive mistake you’ll pay again to remove.
Start with the pole barn insulation guide if you’re comparing options, the cost mechanics if you’re holding a quote — or describe your building below and we’ll tell you honestly whether foam fits it.