Insulation is one of the cheapest upgrades for energy savings, and one of the easiest to under-order. The quantity depends on two things: the area to cover, and the depth or performance you're aiming for. Get the target right first, then the material count is simple.
R-value targets
R-value measures resistance to heat flow: the higher, the better. Provincial energy codes set the minimums, and they vary with climate. In Ontario, for example, SB-12 calls for roughly R-50 to R-60 in attics and about R-22 in 2×6 walls (or R-20 plus continuous insulation outside), and colder provinces and northern zones ask for more. Batts are labelled with both the imperial R-value and the metric RSI value (RSI = R ÷ 5.68).
Batts vs blown-in
Batts and rolls are easy to fit between regularly spaced framing and simple to count: area ÷ coverage per bag. They're the usual choice for walls and floors. Blown cellulose or fibreglass suits attics with obstructions and irregular joists, and fills around wiring and pipes better. Count blown insulation from the settled depth needed for your target R-value and the bag coverage chart printed on the bag.
Settled depth
Blown insulation settles over time. Cellulose gives roughly R-3.6 per inch and loose fibreglass about R-2.5. For R-49, that's around 13½" of cellulose or nearly 20" of fibreglass. Install to the installed (initial) depth on the maker's chart, not the settled depth, and fix depth rulers to the joists so the inspector can check.
Counting batts
Bags = area × (1 + waste) ÷ coverage per bag. Coverage varies with R-value and width, for example roughly 40 sq ft for an R-13 bag and around 50–60 sq ft for some R-30 bags, so read the bag. Allow 5% for cutting around framing, blocking and penetrations.
Things that go wrong
- Blocking eaves ventilation. Fit baffles first, so air can flow from the eaves.
- Squashing insulation under boards. Compressed insulation loses much of its value. Raise storage boards on legs.
- Covering recessed lights that aren't rated for contact with insulation.
- Forgetting the attic hatch: insulate and weatherstrip it too.
- Leaving water pipes and tanks above the insulation unprotected. Once the attic is colder, they can freeze.
- Not sealing air leaks first. Gaps around pipes, cables and light fittings let warm, moist air into the cold space.
Walls and floors
Exterior walls in new construction are insulated between studs with batts, blown cellulose or spray foam, often with continuous rigid insulation outside. Floors over crawl spaces and garages need batts held up with supports, or rigid board. Each of these has its own detailing, so treat them as separate items on the quote.
Safety and access
Wear gloves, long sleeves, eye protection and a dust mask. Work from boards across the joists, never on the ceiling between them, and keep good lighting. Check for old wiring before covering it, and keep insulation clear of chimneys and flues.
A worked example
A 40 × 28 ft attic (1,120 sq ft) blown to R-49 with cellulose: about 13.6" settled, roughly 1.8 lb per sq ft, so around 2,030 lb: about 81 bags of 25 lb, or 85 with 5% waste. Check against the bag chart, which allows for the product's own density.
The insulation calculator works out batts or blown-in bags for any R-value.
Common questions
Can I put too much insulation in a attic?
Not in terms of heat loss, but you must keep the ventilation paths at the eaves open, and leave access to junction boxes, water tanks and hatches.
Does insulation need a vapour barrier?
In almost all of Canada, yes. The building code requires a vapour barrier, usually 6 mil polyethylene, on the warm side of the insulation in walls and ceilings, sealed at the joints and around boxes to stop air leaks as well.
Pricing the job
Materials are only part of the price. Clearing stored items out of the attic, fitting baffles, moving or protecting cables, insulating pipes and tanks, boarding for storage, and disposing of old insulation all take time. List them separately, so the client can see what they're paying for and decide on the optional items.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.