Air-sealing simple payback

When a round of sealant has earned its money back

This page runs the infiltration chain in one pass so the decision can be made from the quote and two blower-door figures alone: the drop in air changes over the heated volume, through the heat factor of air, becomes a conductance reduction; the climate turns the reduction into heat kept, the heating machine’s coefficient into electricity never bought, the tariff into money; and the installed cost divided by that money is the horizon in years. The annual figure is returned alongside, so the verdict can be audited against the saving sibling that owns each step.

Sealing screens well because its economics are lopsided: the materials are among the cheapest in the retrofit trade while the conductance a leaky house gives up is substantial, so horizons come out short where insulation waits years and glazing waits decades. The same cheapness makes quotes wildly inconsistent — a round of sealing has no standard scope the way a loft roll does — which is exactly why competing bids compare more honestly as horizons than as totals.

The ratio carries the usual omissions of simple payback, undiscounted and unescalated, and one of its own: what ages here is the measure itself. Sealant shrinks, gaskets compress, new penetrations appear, and the tested tightness drifts back toward the old rate — so the horizon must clear the reseal interval, not merely please against the building’s life the way the insulation payback sibling can. Draught comfort, the commonest reason households actually seal, never enters the arithmetic at all.

The declared reverse workflow turns the ratio into a negotiating ceiling: fix the horizon the household will tolerate and the engine solves for the largest installed cost that still meets it. Held against a spread of inconsistent sealing bids, that ceiling converts each one into a distance from defensible — and flags the quote priced like construction for work priced like caulk.

When this calculation is used

• Judging a sealing contractor’s quote: the invoice total and the promised after rate, converted into the horizon the household decides on.
• Comparing a professional round against a weekend of do-it-yourself materials — two costs over the same physical saving, two horizons.
• Ranking sealing against the insulation and glazing paybacks in this cluster for one budget, horizon against horizon.
• Setting a ceiling with the declared reverse workflow: a tolerable horizon in, the most a quote may cost and still qualify out.

Worked example

Run the pack’s declared anchor case: the same mid-sized house whose natural air-change rate the saving sibling cuts in half, priced at the modest installed cost a professional sealing round commands, in a mid-severity climate at a mid-range tariff with electric-resistance heat.

The engine returns the horizon in years with the annual saving beside it. Read the years against the life of the work rather than the life of the house: sealant and gaskets age faster than mineral wool, so the horizon must clear the reseal interval, not merely the decade.

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. Then raise the coefficient from resistance to a heat pump’s and watch the horizon stretch — the machine had already banked most of what the sealing would have saved.

What each input represents

Assumptions and limits

• The horizon is undiscounted and static: this year’s saving is presumed to repeat identically, with no energy-price escalation, no discount rate, and no value on draught comfort or air quality.
• The quoted cost is taken as complete, and the promised after rate as achieved and durable — ageing sealant and new penetrations stretch the real horizon after the fact.
• Infiltration is modelled as one constant rate in a single-zone house, though real leakage rises with wind and cold in exactly the weather that matters most.
• The chain prices electrically heated homes — resistance or heat pump; a combustion-heated house needs its avoided heat priced through fuel and efficiency instead.

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 work that loosens the house has no saving to divide into, and the horizon would explode toward infinity instead of naming the swapped rate fields.
• The installed cost must be positive: with nothing spent there is no payback question, and a zeroed field is a missing quote rather than free labour.
• The air-change bands are capped above anything a habitable building leaks, catching unconverted test figures, and the air factor is held to the narrow band the physics of air allows — a value outside it describes a different fluid, not a different assumption.

Provenance

Simple payback screening: installed cost over annual saving, with the saving estimated by the air-change and degree-day methods

The heat factor of air, the drop in air changes and the heated volume multiply into the conductance reduction; the degree-day relation prices it through the system coefficient and tariff into an annual saving, and the installed cost is divided by that figure; the declared reverse workflow solves for the cost a target horizon allows.

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.