Workspace
The calculator's own fields, action and results arrive with the verified pack when you load it. Nothing is computed in this page.
Home efficiency · the draught priced
Air-sealing: the annual saving
Price the draught: fewer air changes per hour shrink the infiltration conductance, and degree-days, COP and tariff turn the tightness into an annual saving.
Verified engine
What the engine returns
The engine returns the annual saving with the conductance reduction and the electricity never purchased beside it. Read the reduction first — it is the physical claim the money rests on — then the kilowatt-hours against the household’s annual consumption, which is the headline that survives every future price change.
Inputs the result is built from
Heated volume
Existing air changes per hour
Air changes per hour after sealing
Volumetric heat factor of air
Annual heating degree-days
Electricity tariff
Heating system COP
How the result is derived
MethodThe heat factor of air, the air-change rate and the heated volume multiply into the infiltration conductance before and after sealing; the difference, multiplied by the annual heating degree-days with the day-to-hours and kilowatt scaling carried in the relation, yields the heat kept, divided by the heating system’s coefficient to yield the electricity never purchased, and priced at the tariff.
StandardInfiltration heat loss by the air-change method: the volumetric heat factor of air times air changes times volume, with savings by the degree-day method
GuardA sealed rate at or above the existing rate is refused — the pack ships the refusal as a declared test vector — because sealing that loosens a house is a contradiction, and the two rate fields swapped is the data-entry error the refusal is really naming.
How the saving moves with the tightness achieved
The energy behind the saving
What a tighter blower-door number is worth each winter
Every hour a house exchanges some fraction of its air with the outdoors — through cracks, service penetrations, loft hatches and the gaps around frames — and every exchange dumps heated air outside and draws in cold air to be heated again. The air-change relation turns that traffic into a conductance: the rate of exchange, times the heated volume being exchanged, times the modest heat a unit of air carries per degree, gives watts per kelvin exactly as a wall or roof does. The engine builds that conductance for the house before sealing and after, and the difference is the measure’s physical worth.
The two rates deserve more care than any other input. A natural air-change rate is not what a blower-door test prints — test figures are taken with the house held at an artificial pressure and must be converted to natural conditions before they belong in these fields, and an unconverted figure inflates everything downstream. The after rate is a promise as much as a measurement: it is what the sealing round is expected to achieve, best taken from a post-work retest rather than a contractor’s optimism.
From the conductance reduction onward this page walks the identical degree-day chain the published insulation saving page walks — accumulated cold, the heating machine’s coefficient, the tariff — and it should, because a watt per kelvin lost through moving air costs exactly what a watt per kelvin lost through a thin loft costs. What is distinct is everything above the chain: insulation slows heat through the fabric, sealing stops the fabric being bypassed, and a house can be excellent at one while quietly leaking the other’s worth away.
The air factor is physics rather than preference — the sensible heat a volume of air surrenders per degree of cooling, held by the pack to the narrow band the properties of air allow. What the relation does not police is breathability: it will happily price tightness beyond what healthy indoor air permits. Ventilation standards, moisture and radon set a floor under the after rate that no saving justifies crossing — seal first, then ventilate deliberately.
The heat factor of air, the air-change rate and the heated volume multiply into the infiltration conductance before and after sealing; the difference, multiplied by the annual heating degree-days with the day-to-hours and kilowatt scaling carried in the relation, yields the heat kept, divided by the heating system’s coefficient to yield the electricity never purchased, and priced at the tariff.
When this calculation is used
- Pricing a professional sealing round before commissioning it, with the existing rate from a blower-door report converted to natural conditions.
- Reading a post-work retest: the before and after rates from the two reports, turned into the annual figure the work actually earned.
- Ranking sealing against loft or wall insulation for the same house — this page and the insulation saving sibling price their reductions through the same chain, so the annual figures compare honestly.
- Feeding the payback sibling, where the sealing quote is divided by this annual figure.
Worked example
Run the pack’s declared anchor case: a mid-sized house’s heated volume, its natural air-change rate cut in half by a thorough sealing round, in a mid-severity heating climate at a mid-range residential tariff, with electric-resistance heat converting one-for-one and the air factor left at its physical default.
The engine returns the annual saving with the conductance reduction and the electricity never purchased beside it. Read the reduction first — it is the physical claim the money rests on — then the kilowatt-hours against the household’s annual consumption, which is the headline that survives every future price change.
Every figure in this example is produced by the certified engine when the calculator loads, checked against the signed pack’s declared test vectors; nothing on this page stores an answer. Halve the after rate again and the saving grows by less than the first halving earned — each further air change removed is smaller than the last.
What each input represents
Heated volume
The volume the exchanged air fills, in cubic metres — heated floor area times average ceiling height, counting only the spaces kept warm. It scales the conductance directly: the same rate in a larger house moves more air.
Existing air changes per hour
How many times each hour the house currently swaps its full volume of air with the outdoors, at natural conditions — a blower-door figure after conversion, never the raw test printout.
Air changes per hour after sealing
The rate the sealing round is expected to achieve, in the same natural-conditions terms. The guard insists it sits below the existing rate; ventilation standards say how far below it should be allowed to go.
Volumetric heat factor of air
The sensible heat a cubic metre of air carries per degree — a physical property with a pack default, held to the narrow band air itself occupies. It is rarely worth touching.
Annual heating degree-days
The site’s accumulated cold, as Celsius degree-days from published climate normals — the same climate input every saving page in this cluster shares, and the multiplier that makes the identical tightness worth more in a harsher winter.
Electricity tariff
The per-kilowatt-hour price the avoided heating would have been billed at, from the household’s own bill — blended, for time-of-use plans, toward the hours the heating actually runs.
Heating system COP
Unity for electric resistance, the seasonal coefficient for a heat pump. It divides the money: an efficient machine had already made the escaping heat cheap.
Assumptions and limits
- The exchange is modelled as one constant rate through a single-zone house for the whole season, though real infiltration rises with wind and with the indoor–outdoor temperature difference — strongest in exactly the weather that costs most.
- Only sensible heat is priced: the latent load of conditioning incoming air sits outside the relation, as does any humidification the drier winter air demands.
- The chain prices electrically heated homes — resistance or heat pump at one seasonal coefficient; a combustion-heated house needs its avoided heat priced through fuel and efficiency instead.
- One flat tariff prices every avoided kilowatt-hour, presumed neither escalating nor discounted, and the sealed rate is presumed to hold — ageing sealant and new penetrations erode it over the years.
What the guards protect against
- A sealed rate at or above the existing rate is refused — the pack ships the refusal as a declared test vector — because sealing that loosens a house is a contradiction, and the two rate fields swapped is the data-entry error the refusal is really naming.
- The heated volume must be positive, and the air-change bands are capped above anything a habitable building leaks — a figure beyond them is usually a per-day rate, or a pressurised test figure entered without conversion.
- The air factor is held to the narrow band physics allows for the heat capacity of air — a value outside it describes a different fluid, not a different assumption — and the tariff floor turns away prices entered in cents as if they were dollars.
Provenance
Infiltration heat loss by the air-change method: the volumetric heat factor of air times air changes times volume, with savings by the degree-day method
The heat factor of air, the air-change rate and the heated volume multiply into the infiltration conductance before and after sealing; the difference, multiplied by the annual heating degree-days with the day-to-hours and kilowatt scaling carried in the relation, yields the heat kept, divided by the heating system’s coefficient to yield the electricity never purchased, and priced at the tariff.
Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, which displays from the verified leaf when the calculator loads.