Installation of Insulation, Vapour Barriers, and Air Barriers
This domain tests how the building envelope controls heat, air, and moisture. The exam separates three functions that are often confused: insulation slows heat flow, the air barrier stops air movement, and the vapour barrier limits vapour diffusion.
Core rules and concepts
- Insulation: batts must fill the cavity with no gaps or compression; split batts around wiring and pipes. Rigid insulation, blown insulation, and spray foam each have installation rules. Compressed insulation loses R-value.
- Vapour barrier: in cold climates it goes on the warm side of the insulation, typically behind the interior finish. Use the specified material and perm rating, lap joints, and seal with the specified sealant. Avoid two vapour barriers in one assembly.
- Air barrier: must be continuous across walls, ceilings, floors, windows, doors, and penetrations. Seal top plates, rim joists, electrical boxes, and service penetrations. The air barrier may be the sheathing membrane, poly, or a spray-foam layer, depending on the design.
- Weather barrier: sheds water and must lap shingle-fashion; it is not automatically the air barrier or vapour barrier.
- Code and climate: the National Building Code and provincial codes set minimum thermal performance and require effective vapour and air barriers. Placement depends on climate; in cooling-dominated climates the vapour control layer can move.
Worked example
A 2x6 wall in a cold climate has exterior sheathing, weather barrier, batt insulation, and interior gypsum board. The vapour barrier goes on the warm side, behind the gypsum board, sealed to the top and bottom plates and around every penetration with acoustical sealant. The air barrier is made continuous by sealing the sheathing joints, the rim joist, and the window perimeter. The reasoning is that warm moist interior air must not reach a cold surface where it can condense.
Traps that cost marks
Installing two vapour barriers; placing poly on the cold side; compressing batts; interrupting the air barrier at the ceiling or rim joist; and relying on tape where a flexible sealant is specified.