Window U-value upgrade: the UA reduction

Why windows measure the same leak upside down

The vocabulary split is not an accident of trade jargon but a fit to the product. Insulation is bought by thickness, and resistances add as layers stack, so an R-value is the natural currency of something you pile up. A window is a finished assembly whose worth is how little heat crosses it per unit of area and degree of difference — a transmittance — and the two currencies are reciprocals of one another: a high resistance IS a low conductance, read from opposite ends. The practical consequence runs through every field on this page: for glazing, the smaller rating is the better product, and an upgrade is a rating that goes down.

The reciprocal vocabulary also makes the arithmetic kinder here than on the insulation side. Because a U-value is already a conductance per area, the engine multiplies each rating by the glazed area and subtracts — no difference of reciprocals, no whole-stack bookkeeping. The before and after conductances are returned beside the reduction, in the same watts per kelvin the walls, roof and infiltration speak, so the result drops straight into the whole-house ledger.

Which rating to enter matters as much as the arithmetic. A window is rated as a whole assembly — glass, frame, spacer and edge together — and it is that whole-window figure the fields want. Centre-of-glass values, which some literature leads with, describe the best square of the product and flatter every upgrade computed from them. For the existing windows, the field help carries typical figures by glazing generation; a survey label or manufacturer datasheet beats all of them.

A transmittance names one channel only: conduction through the assembly. Old windows also leak air around worn sashes, admit and reject solar gain, and radiate discomfort that furniture arrangements quietly obey — real reasons to replace glazing that no U-value difference can see. This page deliberately stops at the conductance; what the climate pays for it is the saving sibling’s story, and the quote is the payback sibling’s.

When this calculation is used

• Converting a replacement-window quote’s rated U-value into the conductance the upgrade actually removes, before deciding whether to price it.
• Feeding the roll-up: the before and after conductances slot straight into the whole-house ledger beside walls, roof and infiltration.
• Comparing two candidate windows over the same openings — run each upgraded rating and compare reductions before anyone mentions money.
• Translating between dialects: reading a glazing upgrade in the same watts-per-kelvin currency the insulation pages produce from R-values.

Worked example

Run the pack’s declared reference upgrade: a house’s ordinary complement of glazing moving from an ageing double-glazed rating to a modern one.

The engine returns the reduction with the before and after conductances beside it. Read the pair, not just the difference: how much of the glazing’s conductance the upgrade removes says more about the product than the raw reduction does, and what remains is what the roll-up ledger will carry forward.

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. Swap the upgraded rating between two candidate products and the reduction moves with the label — this is the one page in the cluster where the label is the whole story.

What each input represents

Assumptions and limits

• Both ratings are whole-window transmittances — glass, frame, spacer and edge together; centre-of-glass figures overstate the assembly and flatter the upgrade.
• The glazed area is the same before and after; a replacement that changes opening sizes changes both conductances beyond what the ratings say.
• A U-value speaks only of conduction: solar gain, the air leakage of worn sashes and comfort near the glass are real reasons to replace windows that this difference cannot see.
• Ratings are treated as delivered: installation quality — the seal between frame and wall above all — decides whether the labelled transmittance is what the house receives.

What the guards protect against

• An upgraded rating at or above the existing one is refused — the pack ships the refusal as a declared test vector — because glass that conducts no less is no upgrade, and with windows the mistake is usually the two ratings swapped, or an R-value pasted where its reciprocal belongs.
• The glazed area must be positive: with no glass changing there is no conductance to reduce, and a zero here is a survey not yet done.
• Both ratings are held to the band manufactured glazing occupies — refused above the worst bare single pane and below anything a catalogue sells — the territory where imperial ratings and resistance figures entered as transmittances tend to land.

Provenance

Envelope conductance from thermal transmittance: conductance equals glazed area times U-value, with transmittance the reciprocal of thermal resistance

The glazed area multiplied by the existing transmittance gives the conductance before, the same area by the upgraded transmittance the conductance after, and their difference is the reduction — reported beside both so the share removed can be judged.

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.