Vapor Barriers in Metal Buildings and Pole Barns: When Closed-Cell Foam Is the Retarder

A vapor barrier for metal building walls and roofs is a layer that slows water vapor from moving through a wall or roof, so damp inside air cannot reach cold steel and turn to water. In a building that gets closed-cell spray foam, the foam itself can be that layer, but only at a thickness where its data sheet says so, and only if nothing is added that traps moisture between two tight layers. This page sorts out the terms, the perm classes, the case for foam as the retarder in a metal building or pole barn, and the double-barrier trap.

If your problem is a roof that drips, start with why a metal building sweats. This page answers one narrower question: does the foam need a separate sheet over it?

Barrier or retarder: what the words mean

"Vapor barrier" is the everyday word. The code word is "vapor retarder", and it is a graded term, not a yes or no. A building-science technical note defines a vapor retarder as a material that "controls the migration of moisture due to vapor diffusion", and says a lower perm rating means a material lets less vapor through (TechNote on vapor retarders, December 2015). A perm is the unit of that measure. A "barrier" in daily talk is the tightest end of the range.

Two things get lumped together that are not the same:

• Vapor diffusion is water vapor passing through the material itself, molecule by molecule. Perm ratings describe this.
• Air leakage is damp air moving through gaps and seams. The same technote treats it as a separate path that has to be handled separately.

In a metal building the second path is often the bigger one: laps, eaves, fastener holes and door openings. A sheet that is a fine barrier on paper does little if air bypasses it through the seams. A tight, bonded layer of foam handles both at once, which is why foam gets treated differently from a loose plastic sheet.

Retarder classes by perm rating

The model codes sort vapor retarders into three classes by permeance. The residential code definitions, as summarized in two Department of Energy and industry documents, are below. Permeance is measured by the dry-cup method of ASTM E96, according to the Building America summary (DOE Building America, vapor retarder classification).

The same technote lists closed-cell spray foam (polyurethane) as typically about 2 perms at 1 inch and 0.8 perm at 2.5 inches, so it sits in Class III or Class II depending on thickness. Open-cell spray foam is listed at about 25 perms at 3.5 inches, which makes it permeable, not a retarder (TechNote, Table 2). We spray closed-cell only, so the open-cell row is background, not something we offer.

Two honest limits on that table:

1. It comes from the residential code. Pole barns and steel shops are often built under other rules, or under none (see spray foam for pole barns for how agricultural buildings are treated). We use the classes as a shared vocabulary for perm ratings, which are a property of a material and are measured the same way in Kansas as anywhere. Whether any class is required in your building is a question for your building official.
2. Generic numbers are not our product's numbers. They show the shape of the range. The numbers that count come from the data sheet of the product that goes on your building.

Closed-cell foam as the vapor barrier for metal building and pole barn use

Closed-cell foam is a vapor retarder by thickness. The Building America guide that the closed-cell product page quotes says closed-cell foam at its typically applied thickness is defined as a vapor retarder, and permeance falls as the layer gets thicker. A thin coat may not reach Class II. A thicker one does. Where the line falls is different for every product, so one maker's table is a pattern and not a promise; the worked case with one maker's published permeance values is on how thick spray foam should be.

What this means for the two building types:

• Steel building with foam on the underside of the metal. The steel on the outside is itself very tight to vapor, and the 2009 code list names sheet metal among Class I examples. Water vapor that gets between foam and steel has almost nowhere to go. That is the reason for the rule that the foam must be fully bonded, with no air gap against the steel: vapor and damp air must not be able to reach cold metal. How foam is laid on a roof is on foam under a metal roof.
• Pole barn with steel over wood framing. Same steel cladding, plus wood. Wood framing takes on and gives off moisture, so the details differ; the drip case is on why a pole barn drips.

The Pole Barn Guru, a pole-building supplier's column, warns against a barrier product placed under roof steel because moisture "gets trapped between it and roof steel", and advises against open-cell foam against steel cladding (Hansen Pole Buildings, April 2024). That is a supplier's opinion, not a test, but it points the same way as the double-barrier problem below.

Where our product lands. The perm values of the product we will spray, at each thickness we would apply, come from its technical data sheet. This page does not state a thickness at which our foam is a retarder; read that from the data sheet of the product you are quoted.

Where a retarder belongs in climate zone 4

The Kansas City area is in climate zone 4A, mixed-humid. A Department of Energy laboratory report names Jackson County, Missouri, as zone 4A (PNNL-20263, March 2011). That is a Missouri county in a national lab report, used for the zone designation only. The code book your office uses decides an exact address.

For zone 4A and frame walls, the technote says the residential code sets no vapor retarder requirement for those walls, and recommends a Class II retarder in the walls it covers, with none needed where there is continuous exterior insulation (TechNote, Table 3). In a mixed-humid zone, vapor moves in more than one direction over the year: outward in heating season, and inward in the cooling season. That is why the same technote warns that a Class I product on the interior can trap moisture that arrives from outdoors in the cooling season.

A frame-wall recommendation is not a rule for a steel building. It does explain the general principle: in a mixed climate, a layer that is tight on both sides of the assembly leaves moisture nowhere to go.

The double-barrier trap

A double vapor barrier is a wall with vapor-impermeable material on both the interior and exterior surfaces. The technote names it as the condition to avoid and says at least one face should be moderately to highly permeable so the assembly can dry (TechNote, December 2015).

A steel building starts with one tight face already: the steel. Add these to the inside and you have built the trap:

We say "can" because this depends on the assembly and the moisture sources. It is not a rule that a second layer always fails. It is a reason to ask before adding one. The first questions are whether the foam is already thick enough to be the retarder, and what is feeding moisture into the building (see the moisture source table). A moisture source left alone, such as a damp slab or a hay loft, can overload any layer.

Fire rules are a separate matter. A cover over foam, such as a thermal barrier, is often required by code for its fire behavior, not for vapor. That is not a vapor barrier question and is covered with the code for metal building insulation options. Never skip a fire-rated cover because it is also a tight layer; ask the building official which layers apply and how they should be detailed.

A cooler room is held colder than the building around it, so vapor keeps moving into the room; see spray foam for a walk-in cooler.

Spray-on vapor barrier: what the phrase can mean

"Spray on vapor barrier" covers a few different products: sprayed closed-cell foam, a liquid-applied membrane, and coatings. They are not interchangeable. A liquid membrane is a thin film with its own perm rating and its own fire behavior and does no insulating. Closed-cell foam insulates, adds air sealing, and acts as a retarder at its design thickness. We do closed-cell foam only; we do not apply liquid membranes or coatings.

FAQ

Do I need a vapor barrier in a pole barn?

With closed-cell foam at the thickness its data sheet lists as a retarder, the foam is the retarder, and a second plastic layer can trap moisture. Whether your building needs any retarder, and which class, is a question for the building official and, for agricultural buildings, may not be a code question at all. The class table above shows what the perm ratings mean.

Will a vapor barrier stop my metal building from dripping?

Only if it is continuous on the warm side and the moisture source is handled. Gaps, laps and the wrong side defeat it, and a barrier does nothing about water that arrives as damp air through openings. The sources and fixes are on why a metal building sweats; you can check your own numbers with the dew point and frost point tool.

Is closed-cell foam a vapor barrier or a vapor retarder?

By the code definitions, a retarder. A "barrier" in the strict sense is Class I, at 0.1 perm or less. Closed-cell foam reaches Class II or Class III depending on thickness, and whether a given product ever reaches Class I is on its data sheet.

Can I spray foam over an existing plastic vapor barrier?

Foam bonded to a plastic sheet is not bonded to the steel, and the sheet can hold water. Which layers stay is a site question, and the old layers usually have to be looked at before foam goes on; see metal building insulation options. We do not remove old insulation. If it has to come out, someone else does that first.

Book a free on-site measurement

At the visit we measure the building and look at the layers already in it: sheets, facings, old insulation, the slab, and the sources of moisture. If foam is part of the answer you get a written estimate that states the thickness. The visit is free. More on the process is on the free estimate visit.

Book a free on-site measurement

Pick "Spray foam insulation" on the booking form. We work within about 35 miles of Spring Hill, KS.

Spray foam insulation is a new PolyJacks service line run by its own crew and rig.

Sources

• "Vapor Retarders: Reducing Moisture Risk in Frame Walls," TechNote, December 2015 (hosted by ICC): iccsafe.org
• U.S. Department of Energy, Building America, "Vapor Retarder Classification" innovation summary: energy.gov
• Pacific Northwest National Laboratory, PNNL-20263, "Comparison of 2006 IECC and 2009 IECC Commercial Energy Code Requirements for Kansas City, MO," March 2011: pnnl.gov
• Hansen Pole Buildings, "Vapor Barriers, Treated Columns and Spray Foam," April 2024: hansenpolebuildings.com
• DOE Building America, "Which Spray Foam Is Right For You?": www1.eere.energy.gov