What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingdamp-proof course (DPC)
also damp-proof membrane (DPM)
DPC is the strip in a wall, DPM the sheet under a floor. Both resist LIQUID rising by capillarity — neither is a vapour control layer, and British practice names that separately.
Australia
Same word, different thingdamp-proof course
also DPC, vapour barrier (under slab)
As the UK for the wall component. The under-slab sheet is commonly called a vapour barrier in Australian practice even though its main job is liquid and capillary break — the naming, not the function, differs.
United Statesyours
Same word, different thingcapillary break
also vapor barrier, vapor retarder, under-slab membrane
American practice has no everyday equivalent of DPC in a wall, and "vapor barrier" names a component defined by its PERMEANCE rather than by stopping rising liquid — so the words map but the specifications do not.
Canada
Same word, different thingvapour barrier
also air barrier, capillary break
Canadian codes distinguish air barrier, vapour barrier and capillary break as three separate functions, which is a finer division than DPC/DPM and does not line up one-to-one with it.
The governing standard, by market
United Kingdom
BS 8215
Design and installation of damp-proof courses in masonry.
United States
ASTM E1745
Plastic water vapor retarders used in contact with soil under concrete slabs.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is a damp-proof course the same as a vapour barrier?
- No, and treating them as the same is one of the more expensive translation errors in building. A damp-proof course resists LIQUID water rising through masonry by capillary action — water climbing through the pores of brick and mortar from the ground — and it works by being impermeable and continuous in a horizontal plane near the base of a wall. A vapour barrier or vapour retarder resists water VAPOUR moving by diffusion through an assembly, and it is specified by its permeance and by its position relative to the insulation and the dew point. A material can do one and not the other, and putting a vapour barrier in the plane where a DPC belongs, or vice versa, solves neither problem.
- What does a specification mean by DPM?
- The sheet beneath a ground-bearing floor slab, as distinct from the strip in a wall. In British usage a damp-proof membrane is laid under or within a floor construction to stop moisture rising from the ground into the slab and the finish above it, and it is lapped and sealed to the wall's damp-proof course so that the two form one continuous barrier. That continuity is the detail that matters and the one most often missed: a perfect DPM and a perfect DPC with no lap between them leaves a path at the junction. American practice covers the same function with an under-slab membrane or capillary break, specified to a vapour transmission standard rather than to a damp-proofing one.
- Why does British practice have a DPC at all?
- Because of the building stock and the wall construction the word grew up around. Solid and cavity masonry walls built directly off the ground will wick moisture upward through mortar and brick unless the path is interrupted, and in a climate with persistently wet ground that rise is visible as damp staining, salt deposits and decayed plaster a metre or so up the wall. A horizontal impermeable course near the base interrupts it. North American light-frame construction sits on a foundation wall with the timber held clear of the ground and detailed differently, so the problem presents differently and the vocabulary developed around vapour and air control instead. Neither tradition is missing the concept; they divide it differently.