Vapor barrier calculator

Rolls of 6-mil poly for a wall or ceiling. The overlap is counted as linear seams, not a flat percentage, and the climate-zone check that decides whether the sheet belongs on the inside at all is built in.

01 How the math works

Start with the surface area: run × height for a wall, length × width for a ceiling or floor. Then decide how the roll lands on it. Vapor-barrier poly comes in wide rolls — commonly 10 ft (3.05 m) or 20 ft (6.10 m) face width by 100 ft (30.48 m) long. The face width is almost always enough to span a standard wall height in one pass.

The part most calculators get wrong is the overlap. A seam lap is a linear loss — it only happens where two sheets meet. Run a 10 ft roll along an 8 ft wall and the face width covers the full height in a single course, so there are no horizontal field seams: the only waste is the 2 ft of roll width you trim or lap at the floor. Treating that as a flat percentage is where the area-times-1.10 approach drifts. We count the courses and the laps, then round up to whole rolls.

02 Worked example

Take a cold-climate basement wall where the interior vapor retarder is required: a run of 40 ft × 8 ft high (12.19 m × 2.44 m), which is 320 ft² (29.73 m²). Cover it with a 10 ft × 100 ft (3.05 m × 30.48 m) roll of 6-mil poly and a 6 in (152 mm) lap.

Run the roll horizontally. The 10 ft face width spans the 8 ft height in one course — no horizontal seam — and the 40 ft run is under the 100 ft roll length, so no end-lap either. Material off the roll is 10 ft × 40 ft = 400 ft² (37.16 m²) to cover 320 ft² of wall. The 80 ft² (7.43 m²) difference is roll-width-versus-wall-height trim — 20% here, not the 10% a flat factor assumes.

One 10 ft × 100 ft roll holds 1,000 ft² (92.90 m²), so a single roll covers this wall and two more like it. Order one roll; the leftover is the handling margin.

03 When this calculator is wrong

The roll count is only half the question. The other half is whether a polyethylene sheet belongs on that surface at all, and here the honest answer depends on where the building is.

• Climate zone decides the side — and whether to use poly. IRC R702.7 requires a Class I or II vapor retarder on the interior side of frame walls in Climate Zones 5, 6, 7, 8, and Marine 4; interior 6-mil poly is the usual way to meet it. In Zones 1 through 3 an interior Class I barrier is not permitted on frame walls — in a hot-humid climate the sheet traps moisture driven inward by solar-heated outdoor air, and the wall can't dry inward. A roll count is the easy part; putting poly on the wrong wall is the expensive part.
• A flat waste percentage double-counts the overlap. When the span fits one roll width, the only loss is the width-versus-height trim. On the worked-example wall that's 20%; on a 10 ft roll over a 9 ft wall it's about 10%; stack two courses and the lap adds its own strip. The seam model here tracks that; a bolted-on 10% doesn't.
• Under-slab barriers are a different product. Sheeting in contact with soil under a slab is governed by ASTM E1745 — Class A is at least 10 mil (0.254 mm) and tests below 0.01 perm, far tighter and tougher than the 6-mil (0.15 mm) wall grade. Don't size an under-slab liner from a wall roll count.
• It doesn't deduct penetrations. Windows, doors, outlets, and pipe boots aren't subtracted — you run the sheet past them and cut and tape after, which is why the trim allowance exists. On a wall full of openings, keep the waste setting where it is rather than netting them out.

04 What to do with the result

Round up to whole rolls — poly is cheap and a short sheet means an untaped seam or a pieced-in patch where you least want one. The standard 10% cut-and-handling allowance holds for a straightforward wall or ceiling. For a small, cut-up job — a short run broken by windows, or a ceiling with a lot of fixtures — bump it to 15%, because small sheets have more edge waste and less margin to work with. The exception is a large, open, rectangular surface: there 10% is generous and you can trust the lower number.

Lap every seam 6 in (152 mm) over a framing member and seal it — an unsealed lap is a hole in the barrier, not a barrier. Confirm your climate zone against the zone check above before you buy: if it reads "not permitted," the roll count is moot and the money is better spent on air-sealing and a vapor-open finish.

05 Common questions

Which side of the wall does the vapor barrier go on?

The warm-in-winter side. In cold climates (IRC Zones 5, 6, 7, 8, and Marine 4) that's the interior face of the framing, under the drywall — which is where R702.7 requires it. Putting it on the cold side in those zones, or on the interior in a hot-humid climate, is how walls get wet.

Do I need a plastic vapor barrier in a hot climate?

Not on the interior of a frame wall. In Zones 1 through 3 an interior Class I poly sheet is not permitted — it traps moisture the humid outdoor air drives inward. Those walls use a vapor-open interior finish and rely on air-sealing instead. Poly is still correct under slabs and over crawlspace soil in any zone.

How much should vapor barrier overlap at the seams?

6 in (152 mm) is the common minimum — the figure ASTM E1643 sets for under-slab joints and the one most installers use on walls and ceilings too. Lap over a stud or joist and seal the seam with tape or sealant.

What thickness of poly is a vapor barrier?

6-mil (0.15 mm) polyethylene is the common construction grade for walls, ceilings, and crawlspaces. It tests well inside the IRC Class I range (0.1 perm or less). Under a slab, step up to a 10-mil-plus ASTM E1745 Class A product — thicker for puncture resistance and a tighter perm rating.

How many square feet are in a roll of vapor barrier?

Depends on the roll. A 10 ft × 100 ft roll is 1,000 ft² (92.90 m²); a 20 ft × 100 ft roll is 2,000 ft² (185.81 m²). That's the face area, before you subtract the overlap and trim — which is what the calculator above does.