Is triple glazing always necessary?

In a UK cool-temperate Passive House, effectively yes — double glazing rarely reaches the Uw ≤ 0.80 target or the comfort criterion. For a less ambitious retrofit, high-quality double glazing with a warm-edge spacer and an insulated frame can still be a large improvement, but it won't deliver Passive House comfort or performance.

Won't triple glazing make rooms dark?

Modern low-iron triple glazing transmits plenty of light; the visual difference versus double glazing is small. Any reduction in light or solar gain is accounted for in the PHPP model when sizing the glazing area, so it's a designed trade-off, not an accident.

My new windows still get condensation on the inside — why?

Condensation on the room-side of a genuinely high-performance window almost always indicates high indoor humidity and/or inadequate ventilation, not a faulty window — the warm inner pane should stay above dewpoint. Condensation on the outside of the glass in the morning is actually a sign the glazing is performing well (the outer pane is cold because so little heat is escaping).

Passive House Windows: U-values, Triple Glazing & g-value

A window has to do contradictory jobs: keep heat in during winter, let useful sunlight (and its warmth) in, keep summer heat out, provide light and view, and seal airtight. Passive House windows are engineered to resolve those tensions, and they are specified by numbers, not brand names.

The three U-values you need to understand

A window's thermal performance is described by three related U-values. Confusing them is the most common mistake homeowners make when comparing quotes:

Window U-value components

Symbol	Refers to	Passive House target
Ug	Centre-of-glass only (the glazing unit)	≤ 0.70 W/m²K (typically ~0.50–0.70)
Uf	Frame only	Low — insulated, thermally broken frame
Uw	Whole window (glass + frame + spacer)	≤ 0.80 W/m²K
Uw,installed	Whole window including the fixing detail	≤ 0.85 W/m²K

Triple glazing, coatings and gas fill

Passive House glazing in a cool-temperate climate is almost always triple-glazed. The performance comes from three things working together:

Two low-emissivity (low-E) coatings — microscopically-thin metallic layers that reflect long-wave heat radiation back into the room while letting short-wave sunlight through.
Two cavities filled with an inert gas (argon, or krypton where cavity widths are constrained) — gases that conduct heat far less readily than air.
Optimised cavity widths (typically ~16 mm with argon) to minimise convective heat transfer within the unit.

The result is a glazing unit (Ug ~0.5–0.7 W/m²K) that loses roughly a quarter of the heat of old double glazing and a tenth of single glazing — while the warm inner pane eliminates the cold downdraught and radiant chill you feel beside an old window.

Warm-edge spacers and the frame

The edge of the glazing unit and the frame are where windows traditionally fail thermally. Two details matter:

Warm-edge spacers

The spacer holds the panes apart at the edge of the unit. Traditional aluminium spacers are highly conductive — a thermal bridge running right around every pane, producing a cold strip at the glass edge where condensation and edge-mould form. 'Warm-edge' spacers (stainless steel, or composite/polymer) cut that edge heat loss dramatically and keep the glass edge warm.

Insulated, thermally-broken frames

Passive House frames — whether timber, timber-aluminium composite or insulated PVCu — contain insulation within the frame profile and a thermal break separating inside from outside. A bare aluminium frame, by contrast, is a continuous metal bridge and is unsuitable unless heavily thermally broken.

g-value: the solar gain balancing act

The g-value (or Solar Heat Gain Coefficient, SHGC) is the fraction of solar energy that passes through the glazing — from 0 (none) to 1 (all). It is a deliberate trade-off:

A higher g-value (~0.50–0.60) lets in useful free winter heat — valuable on south-facing glazing in a heating-dominated climate.
A lower g-value reduces summer solar gain and overheating risk — valuable on east/west glazing and large areas.
Triple glazing inevitably lowers g-value slightly (an extra pane and coating to pass through), which is one reason orientation and area must be modelled, not guessed.

Installation position — the detail most people miss

A Passive House window is only as good as its installation. Two principles govern it:

Set the window in the insulation plane. A window fixed in the middle of a masonry reveal, surrounded by cold structure, creates a thermal bridge around its whole perimeter regardless of how good the window is. In Passive House detailing the frame is positioned within (or overlapped by) the wall insulation so the insulation wraps onto the frame.
Seal it airtight, inside and out. The 'airtight inside, weathertight outside' rule applies: an internal airtight seal (tape or membrane bonded to the air barrier) and an external weather seal that still allows any incidental moisture to escape. The gap is filled with insulation, not just expanding foam slapped in as an afterthought.

This is why Uw,installed (≤ 0.85 W/m²K) exists as a separate, slightly relaxed target — it captures the small additional loss of the fixing detail and forces the installer to detail it properly.

The comfort criterion

Underlying the 0.80 W/m²K Uw target is human comfort, not just energy. PHI derives the standard so that, on the coldest design day, the internal glass surface stays warm enough that (a) you feel no radiant chill sitting beside the window, and (b) the air beside the glass doesn't cool enough to form a cold downdraught across the floor. A Passive House lets you place a sofa or desk right against the window — something an old single-glazed bay never allowed in winter.

High-performance windows — U-values, glazing and the comfort criterion.