What are the vapour barrier requirements for Calgary basement insulation?

The Alberta Building Code requires a vapour barrier on the warm (interior) side of insulated basement walls, typically 6-mil polyethylene sheeting, unless you are using closed-cell spray foam at 2 inches or greater — which acts as its own vapour barrier. Getting the vapour barrier right is absolutely critical in Calgary basements, where the temperature differential between a -30-degree winter night and your heated basement interior can exceed 50 degrees Celsius, creating enormous vapour drive toward the cold concrete.

The purpose of the vapour barrier is to prevent warm, moisture-laden indoor air from reaching the cold foundation wall surface where it would condense into liquid water. Without a properly installed vapour barrier, moisture migrates through your insulation and drywall, condenses on the concrete, and creates a hidden mould factory behind your finished walls. This is the number one cause of mould in Calgary basement developments, and remediation costs $10,000 to $30,000 or more once you factor in demolition, mould removal, and rebuilding.

Vapour Barrier Options by Insulation Type

If you are using a standard framed wall with fiberglass or mineral wool batts, the 6-mil poly sheet goes on the room side of the studs, stapled to the framing with all seams overlapped by at least 4 inches and sealed with acoustic sealant or red tuck tape. The poly must be continuous — every penetration for electrical boxes, plumbing, and HVAC needs to be sealed with poly pads and acoustic sealant. Your Safety Codes Officer will inspect this carefully, and gaps or tears will require repair before you can proceed to drywall.

If you are using closed-cell spray foam at 2 inches or greater (R-13+), you do not need a separate poly vapour barrier. The spray foam itself has a vapour permeance low enough to meet code requirements, and adding poly over spray foam actually creates a double vapour barrier that can trap moisture between layers — which is counterproductive. This is one of the major advantages of closed-cell spray foam in basement applications: it simplifies the assembly and eliminates the risk of improperly sealed poly.

For the increasingly popular hybrid approach — rigid XPS foam board against the foundation wall with batt insulation in the stud cavities — the requirements depend on the foam thickness. If the XPS is 2 inches or thicker (R-10), many building officials accept it as the vapour retarder and do not require additional poly. However, practices vary by inspector in Calgary, so confirm with your local Safety Codes Officer before closing up walls. When in doubt, installing 6-mil poly on the warm side of a hybrid assembly is never wrong.

One common mistake is installing the vapour barrier on the wrong side — between the concrete and the insulation rather than on the room side. This traps moisture inside the wall cavity with no path to dry out, guaranteeing mould growth. Another error is using vapour barrier paint as a substitute for 6-mil poly; while vapour-retarding primers exist, they are not accepted as a code-compliant vapour barrier for below-grade wall assemblies in Alberta. If you are unsure about your vapour barrier strategy, a qualified basement contractor can ensure your assembly meets code. Find local professionals through the Calgary Construction Network at calgaryconstructionnetwork.com.