Foam Closure Strips and Why a Steel Building Needs Them

You finish the roof, step inside, and see daylight in a neat zigzag along the eave. A week later there are wasp nests in the ribs and a drift of fine snow on the slab after the first storm. That is what missing or wrong foam closure strips look like, and it is one of the most common punch list items I find when I walk a building someone else put up.

Foam closure strips are cheap and they get skipped because the roof looks finished without them. Here is what they do, how to match them, and where each kind goes.

What foam closure strips actually do

Ribbed steel panels are not flat. A PBR or R panel has major ribs about 1 1/4 inches tall every 12 inches, with smaller stiffening ribs between them. When that panel sits on an eave strut or meets a ridge cap, the trim is flat and the panel is not, so you get a row of open triangles and trapezoids across the whole length of the wall.

A closure strip is closed cell foam cut to the exact panel profile. On a 40 foot sidewall it fills roughly 40 openings per eave, each big enough for a bird or a lot of wind driven rain.

Done right, the strips do four jobs at once:

• Stop wind driven rain and snow from blowing up under the ridge cap and eave trim.
• Keep birds, wasps, mice, and bats out of the panel ribs and the building.
• Cut drafts, which matters a lot once you have paid for insulation.

Inside closures, outside closures, and vented closures

There are two basic shapes, and ordering the wrong one is the classic mistake.

Inside closures

An inside closure matches the underside of the panel. It sits between the panel and the structural member below it, so it goes at the eave, on the eave strut, and at the base of a wall panel where it meets the base trim. Its profile follows the low flats of the sheet.

Outside closures

An outside closure matches the top of the panel. It sits between the panel and a piece of trim laid over it, which means under the ridge cap, under the high side of a lean-to roof, and under rake or transition trim. Its profile follows the raised ribs.

Vented closures

Vented closures have a mesh or a perforated face so air can move but bugs and most water cannot. They usually go at the ridge. If you are counting on a continuous ridge vent to clear moist air, a solid closure there will defeat it. Our guide on how roof ventilation keeps a steel building dry explains why that airflow matters more in winter than in summer.

Matching foam closure strips to your panel profile

Closure strips are profile specific. A strip cut for one brand's R panel may be off by a quarter inch on another, which leaves a gap at every rib.

The safest move is to order closures from the same supplier as your panels, by the panel's exact name. If you are still deciding between profiles, read PBR vs R panel and which profile your steel building needs first, because the answer changes which closures you buy.

On quantities, a 36 inch strip covers one 36 inch panel width. A 30x40 building with a gable roof needs closures at two eaves and both sides of the ridge, so about 160 linear feet, or 54 strips before waste. Order 10 percent extra. Strips run about $1.50 to $4 each depending on profile and whether they are vented, so the whole job is often under $250.

How to install them without leaks

The foam alone is not a seal. The seal comes from butyl tape mastic on both faces of the closure, compressed by fasteners.

1. Clean the eave strut or panel surface. Oil and dust are why tape lets go.
2. Run a continuous line of butyl tape along the strut, about 1 inch in from the edge.
3. Press the inside closure onto the tape, lining its ribs up with where the panel ribs will land. Butt joints tight, no gaps.
4. Run a second line of tape on top of the closure.
5. Set the panel and fasten through the flats into the strut, typically at 12 inches on center at the eave, so the screws squeeze the tape-closure-tape stack.
6. At the ridge, repeat with outside closures on top of the panels before the ridge cap goes on.

A good test: stand inside on a sunny afternoon with the lights off. You should see no light along the eave or ridge.

Where common advice gets it wrong

You will hear that spray foam from a can does the same job for less. I disagree, and I have pulled enough of it out to have a reason.

Canned foam expands unevenly, cures with a crust that cracks as the steel moves through hot and cold cycles, and breaks down under UV where it is exposed at the eave. Steel panels on a dark roof can swing 80 degrees or more between a winter night and a summer afternoon, and every one of those cycles pulls a little at a rigid foam plug. Within two or three seasons you have gaps again. Cut closures are closed cell polyethylene that stays flexible and are shaped to stay put under the tape.

The other bit of common advice is that closures are optional on a carport or an open shed. Partly true. You can skip inside closures on a three sided equipment shed where airflow is the point. You should still put outside closures under the ridge cap, because that is where blown rain gets in and runs down the inside of the panels onto whatever you parked there.

Signs your closures have failed

Closures wear out, mostly from sun exposure at the eave and from birds pecking at them. Look for:

• Light showing at the eave or ridge from inside.
• Nesting material, droppings, or wasp activity in the ribs.
• Rust streaks starting at the eave strut, which often mean water has been getting past.

Replacing eave closures on an existing roof means backing out the eave fasteners a few panels at a time, sliding new tape-backed strips in, and refastening with fresh screws. On the ridge, the cap has to come off. Our walkthrough on steel building flashing repair covers removing and resetting trim without bending it.

Frequently asked questions

Do I need foam closure strips on a metal roof?

On any enclosed building, yes. Without them the gaps at the eave and ridge let in rain, snow, birds, and insects, and they let conditioned air out. On an open sided shed you can skip the inside ones, but keep the outside closures at the ridge.

What is the difference between inside and outside closure strips?

Inside closures go under the panel, between the sheet and the eave strut or base trim. Outside closures go on top of the panel, under the ridge cap or other trim. They are mirror images of each other, shaped to the bottom or top of the profile.

Can I use expanding foam instead of closure strips?

It works for a season or two, but it cracks as the steel expands and contracts and breaks down in sunlight. Profile matched closures with butyl tape cost a few dollars a strip and last far longer. Save canned foam for small odd shaped gaps where no strip fits.