How to Choose Glazing for a Low Energy or Passive House Build
More new build and self-build clients are coming to us with low energy targets — not necessarily full Passivhaus certification, but a fabric-first approach that treats the building envelope as the primary energy strategy. Glazing sits at the centre of that decision.
Choosing windows for a low energy build involves more trade-offs than standard residential glazing. You need to balance U-value, g-value, and Lt-value against each other — and against your budget. Get the balance wrong and you can meet your thermal target on paper while creating a house that's too dark, too cold in winter, or prone to overheating in summer.
This guide explains the three key performance values, when to specify timber versus aluminium-clad frames, and how to approach the specification conversation.
The three values that matter
U-value (Uw) — heat loss through the whole window (frame + glass). The lower the value, the more thermally efficient. Passivhaus requires a whole-window U-value of 0.8 W/m²K or below. Standard double glazing achieves around 1.2 W/m²K; high-performance triple glazing in a thermally broken frame achieves 0.8 or below.
g-value (solar heat transmittance) — the proportion of solar energy (heat) that passes through the glass into the room. A higher g-value means more passive solar gain, which helps heat south-facing rooms in winter. A lower g-value reduces overheating risk but also reduces free heat from the sun. For a Passivhaus with south glazing, maintaining a reasonably high g-value is part of the energy model.
Lt-value (light transmittance) — the proportion of visible daylight that comes through the glass. Adding coatings to improve U-value and g-value control typically reduces Lt-value. There is a practical lower limit — rooms that feel gloomy to live in have been over-specified on thermal performance at the expense of daylight.
Triple glazing for low energy builds
For any project targeting Passivhaus certification or equivalent low energy performance, triple glazing is effectively required. The thermal improvement over double glazing — going from around Uw 1.2 down to Uw 0.8 or below — is significant enough to affect the energy model materially.
The additional weight of triple glazing (a 28mm triple unit weighs roughly 50% more than a comparable double) affects hardware specification, particularly on large opening sashes. We account for this in our frame and hardware selection — it's not a problem, but it's something to be aware of if you're comparing our quote with a double-glazed alternative.
Frame material: timber or aluminium-clad?
Both our timber and aluminium-clad frames are available with the same glazing specifications. The choice between them is largely one of aesthetics, maintenance preference, and context rather than thermal performance — our aluminium-clad frames use a thermally broken aluminium outer section over a timber core, so the frame U-value is comparable.
Timber frames are typically chosen for traditional or heritage contexts, and by clients who prefer natural materials. Aluminium-clad frames suit contemporary builds and require almost no maintenance over their design life. Both are available in any colour from our standard range.
How to approach the specification
The most useful thing you can bring to a conversation with us is your thermal performance target — either a U-value target from your energy assessor, your SAP or PHPP model, or simply the energy standard you're building to (Passivhaus, EnerPHit, Part L compliance plus, etc.).
From there, we'll work through the glazing combination that hits your target without over-specifying — there's no point in specifying a very low g-value on north-facing windows where solar gain is minimal, or in reducing Lt-value below what's comfortable to live with in order to squeeze the last fraction off the U-value.
We supply to architects and self-builders across the UK. Call us to discuss your project, or send us your schedule and we'll come back with a full specification and quotation.