Insulation and air sealing are different problems
Insulation slows the flow of heat through solid material. Air sealing stops air from moving through gaps entirely. A thick layer of insulation does almost nothing to stop air movement — and air movement carries heat many times faster than conduction through a material. In most existing homes, the attic has both problems, and air sealing needs to come first.
Adding insulation on top of unsealed bypasses is a common and expensive mistake. You end up with a well-insulated attic that still has unconditioned air flowing freely between your living space and the outside. Air seal the ceiling plane first, then top up insulation.
Does adding more insulation still make sense?
It depends on where you're starting. The energy savings from insulation follow a law of diminishing returns — going from R-10 to R-20 cuts heat loss through the ceiling by roughly half. Going from R-40 to R-50 cuts it by about 20%. The cost-per-dollar-saved gets worse with each additional layer.
In BC, attic insulation is typically cost-effective up to around R-40 to R-50 (approximately 300–380 mm of blown cellulose or fibreglass). Beyond that, the payback period extends considerably, especially if the attic already has air sealing done.
What a Home Performance Report tells you
A report models your home's ceiling heat loss at your current insulation level and compares it to the projected savings from topping up to various target R-values. This gives you an actual cost-benefit number — not a rule of thumb — so you can decide whether the upgrade makes financial sense at your specific starting point.
Not sure whether your attic insulation is worth topping up? A Home Performance Report will calculate your current ceiling heat loss and tell you exactly what additional insulation is worth at your specific R-value.
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