Insulation or Heat Pump First? Why You Seal and Insulate Before You Buy the Equipment, What It Does to the Size of the Heat Pump and the Heat-Loss Number That Decides It

How a heat pump gets its size

A furnace is bought by rule of thumb and oversized by habit; a 100,000 BTU furnace in a house that needs 45,000 just cycles more and nobody notices. A heat pump cannot be bought that way. It delivers less heat as the outdoor temperature drops, so it is chosen for the coldest design day, around minus 22 C in Simcoe County, and the choice comes from a room-by-room heat-loss calculation under CSA F280. Too small and the backup strips run all January. Too big and it short-cycles, dehumidifies badly in July and costs more to buy. The right size is the one that matches the heat loss, and the heat loss is the sum of the envelope: the attic R, the wall R, the windows, the basement, and the air leakage, which on an older house is the biggest single number on the sheet.

That is the whole argument in one sentence: change the envelope and the number changes. The heat-loss calculation page covers when the code requires one, and the cold-climate heat pump page covers the equipment. This page is about the order.

A 1975 bungalow worked through

1,200 sq ft bungalow near Midland, attic at R-20, basement unfinished, original windows, tests at about 10 ACH50, which is normal for its age. Run the heat loss as it stands and it comes out around 55,000 BTU per hour on the design day; the attic and the air leakage between them are close to half of it. The HVAC contractor, sizing to that number, proposes a 5-ton cold-climate heat pump with electric backup, and the quote is in the $22,000 to $28,000 range installed.

Now do the attic first. Top-up R-20 to R-60, air seal the attic plane, baffles, hatch: $4,820 before HST, $5,447 with, off the cost page. Add the rim joists, 128 lf at $5.50, $704, and the basement sill and penetrations in the whole-house air sealing package, $2,400. Call it $8,000 before HST and the house now tests around 6 ACH50 with an R-60 lid. Run the heat loss again and it lands around 38,000 to 40,000 BTU per hour. That is a 3-ton unit, not a 5-ton, and the difference between those two at install is $4,000 to $7,000, with a smaller electrical service upgrade behind it and a machine that spends more of the winter in its efficient range. The insulation paid for most of itself before the first heating bill arrived, and then it keeps paying every January.

What happens when the heat pump goes in first

The 5-ton unit goes in. Next year the attic gets done. The heat loss is now 39,000 and the equipment is sized for 55,000. In shoulder seasons it cycles on and off instead of running low and steady, which is the worst way for a variable-speed compressor to live; in summer it cools the house so fast it never wrings the humidity out, and the basement smells like a basement. The owner paid for 2 tons of capacity that will never be used, and a 5-ton outdoor unit is not something you trade in. I have seen this house. The owner did everything right except the order.

The other failure is the one nobody calls about: the heat pump sized to the leaky house works fine, so the attic never gets done, and the backup strips carry the cold weeks at three times the cost per kilowatt-hour of the compressor. The insulation money went into the hydro bill, a little every month, forever.

The order, as a list

1. Blower door test, $300 to $450, or an EnerGuide evaluation at $500 to $800 if you want the assessment-path rebates on the table. Either one gives you the leakage number the heat-loss design needs.
2. Air seal: attic plane, rim and sill, penetrations, weatherstripping. The package is $1,500 to $3,500. Fibreglass is not an air barrier, and blowing more of it over a leaky ceiling fixes nothing; a batt in a leaky wall is a wool sweater in the wind.
3. Attic to R-60. The cheapest R on the house, $1.50 to $3.50 a square foot.
4. Rim joists in foam, basement walls if they are bare. Bundled with the attic to get past the spray foam minimum.
5. Windows if they are single-pane or failed; otherwise leave them. Windows are expensive R.
6. Then the heat-loss calculation, on the house as it now stands, and the heat pump sized to that number and nothing else. The stamped design that says what the house needs is the job of OntarioHeatLoss.ca; an HVAC quote without a heat-loss sheet behind it is a guess with a compressor attached.
7. Have the ventilation looked at the same month as the sealing. A tighter house needs its bath fans, range hood and any gas or oil appliance checked for backdrafting, and a house under about 3 ACH50 wants an HRV. The airtightness section has the list.

The two exceptions

If the furnace is dead in January, buy the equipment; nobody insulates an attic at minus 20 with the family in a motel. Ask the contractor to size it to a heat loss that assumes the attic at R-60 and the rim sealed, write that assumption on the quote, and do the attic in April. It is not ideal, and a variable-speed unit has enough range to cover the gap if the assumption is written down rather than hoped for.

And if the house is ICF, the wall and the basement are already done, the attic is the only envelope question, and the order collapses to: ceiling to R-60, blower door, heat-loss, equipment. The houses we build test under 2 ACH50 and the heat pumps in them are a size smaller than the HVAC contractors expect; that is the arithmetic on this page working in the other direction.

Questions people ask

Sources and further reading

• Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
• EnerGuide home evaluations: Natural Resources Canada: what a registered energy advisor does, the blower door test and the rating that the assessment-path rebates depend on.
• OntarioHeatLoss.ca: the stamped heat-loss design that insulation and airtightness change, and where the HRV and equipment sizing come from.