How the insulation calculator works
It works out the wall area, subtracts doors and windows, and divides by the coverage printed on the bag. Bag coverage depends on the product’s R-value and your stud spacing, so it’s an input rather than a guess. The vapour barrier is the full wall area plus 10% for overlaps, since poly goes over openings and gets cut out afterwards.
The formula
area = wall length × wall height − doors × 20 ft² − windows × 15 ft²
bags = area ÷ coverage per bag, rounded up
poly = wall length × wall height × 1.1
Tips
- Match the batt to the cavity. 2×4 walls take batts made for 3½ in cavities, 2×6 walls take 5½ in. Batts are also made for 16 in or 24 in stud spacing, so buy the right width.
- Required R-values depend on your province and climate zone, and some areas have energy step codes. Check what applies before you buy.
- Vapour barrier: in most of Canada it goes on the warm (interior) side, with seams overlapped and sealed with acoustic sealant or tape. Basement walls and some wall assemblies are different, so check local practice.
- Wear a mask, gloves and long sleeves when handling fibreglass or mineral wool.