Mil (Thickness)
A unit of measurement equal to one thousandth of an inch (0.001"). Used to specify the thickness of vapor barriers, paint films, membranes, and sheet materials.
In Detail
Common mil measurements in construction: 6 mil poly (standard vapor barrier), 10–20 mil (heavy-duty vapor barriers, crawl space liners), 4–6 mil (paint dry film thickness per coat), 40 mil (peel-and-stick roofing membranes). For reference: a standard sheet of paper is about 4 mil. Thicker poly vapor barriers (10+ mil) are more durable during construction but cost more.