Pole Barn Insulation: Spray Foam vs Blanket vs Foil vs Board

What pole barn insulation has to do

Most pole barn insulation questions start with R-value, but the usual complaint is water: drips off the purlins, frost on the wall panels, rust around the fasteners.

Steel is thin and conducts heat well. Take a shop held at 60°F and 50% relative humidity. That air has a dew point of about 41°F. On a 0°F night, a simple calculation with standard air-film values puts the inside face of bare steel near 12°F. Moisture in the air condenses or freezes on it, and when the sun hits the roof the next morning, it rains indoors.

With 2 inches of closed-cell foam bonded to the panel, at about R-7 per inch (a typical aged value for HFO closed-cell), the same calculation puts the foam's room-side surface near 57°F, well above the dew point. The steel behind it is still about 1°F, but room air can't get to it.

So the insulation has to slow heat loss and keep inside air off the cold metal. NAIMA, the fiberglass insulation trade group, makes the same point for metal buildings: vapor retarder on the warm side, and enough insulation to keep it above the inside air's dew point.

The four options side by side

Closed-cell spray foam

Closed-cell foam is sprayed straight onto the back of the steel, where it bonds and cures into a rigid layer with no gap for air. Manufacturers' data sheets list air permeance below 0.02 L/s·m² at 1 inch, and Class II vapor retarder performance at about 1¼ to 1⅞ inches, depending on the product. That is why we use closed-cell on metal buildings, never open-cell; see closed-cell vs open-cell in Climate Zone 5.

On pole barn walls and roof decks we usually quote 2 to 3 inches, because the hot-box thickness data shows most of the heat-loss reduction comes in the first few inches. At about R-7 per inch, two inches is about R-14 and three inches about R-21. How thick spray foam should be goes further.

The trade-offs are cost and one fire-safety detail. Manufacturers require the foam to be separated from the building interior by an approved thermal barrier, such as 1/2-inch gypsum, or by a coating tested and listed for that foam. Plan for it in any shop where people work; our thermal barrier guide explains the options.

Vinyl-backed fiberglass blankets

These are the faced fiberglass rolls pulled over the girts and purlins of many new pole barns, with the steel screwed on top. NAIMA's metal-building standard lists common blankets from R-10 to R-30.

The facing does the moisture work. Plain vinyl rates about 1.0 perm, right at the line between a vapor retarder and not. Foil and polypropylene-scrim-kraft facings rate 0.02 to 0.09 perm, and NAIMA recommends 0.02-perm facings where interior humidity is high. Tabs have to be stapled and folded or glued, and rips covered with facing tape. Overstretching the blanket compresses it and lowers its R-value, and it is squeezed thin at every girt and purlin regardless.

The bigger limitation is timing: the blanket goes in while the building is sheeted, so adding one later means taking the steel off. And once a tear or open seam lets humid air behind the facing, condensation forms on the steel where you can't see it. NAIMA notes that water or ice in the insulation seriously degrades its performance.

Reflective bubble foil

Bubble foil works mainly by reflecting radiant heat, not by resisting conduction. The Department of Energy says the reflective surface must face an air space to work, that radiant barriers are more effective in hot climates than cool ones, and that in cooler areas adding more thermal insulation is usually more cost-effective.

The R-values printed on reflective products depend on that air space. Under the FTC's R-value rule, the material is tested by itself and any air-space credit is figured separately. Press the foil tight to the steel and the space on that side is gone.

Because it is thin, the room side of bubble foil stays cold in January. In a heated, humid building the condensation just moves from the steel to the foil. Foil makes sense for cutting summer roof heat in an unheated building; it is not winter insulation on its own.

Rigid foam board

Rigid board (EPS, XPS or foil-faced polyiso) is fastened between or across the girts on the inside, at the per-inch values from PNNL shown in the table. Board performs well when every joint and edge is taped or foamed and the layer runs continuously over the girts so the framing isn't a shortcut for heat.

The weak spot is the space behind the board: a gap at a corner, door frame or girt lets inside air into the cavity against the steel. The Department of Energy notes that foam board used indoors must be covered with 1/2-inch gypsum or another code-approved material, so budget for a liner.

Which option fits which building

• Cold storage, no heat. You may not need wall insulation at all. The usual problem is the roof sweating onto equipment. A well-sealed blanket installed as the roof goes on, or closed-cell on the roof deck only, often solves it. Foam on the walls of a building that is never heated is hard to justify.
• Heated shop or garage. Closed-cell on walls, gables and roof deck is the simplest route to a dry, warm building. A blanket or board system with an interior liner can also work if the vapor control is detailed carefully. See our metal shop project and the heated garage and workshop guide.
• Livestock and horse barns. Animals put a lot of moisture into the air, so ventilation comes first; insulation without ventilation still sweats. Use low-perm vapor control, and put a liner over any insulation animals can reach. Our West Middlesex horse barn and Blair County metal building show how we sized those jobs.
• Living space. A house in a steel shell has its own code rules; see barndominium insulation.

What it costs

Our public guidance for closed-cell is roughly $1.50 to $2.25 per board foot installed, depending on the job. A board foot is one square foot, one inch thick, so 2 inches works out to about $3.00 to $4.50 per square foot of wall or roof, and 3 inches to about $4.50 to $6.75.

We don't publish prices for blankets, foil or board; material prices move too often. When you compare, price the whole assembly that controls condensation: the insulation, tape and sealants, the interior liner or thermal barrier, and labor, including removing and reinstalling steel if the building is already sheeted. The gap between options narrows or widens depending on how much of that extra work your building needs. For how foam quotes are built, see spray foam insulation cost.

Want a number for your own building? Price My Project or call (814) 208-3478.