A Passivhaus window in our climate needs a whole-window U-value of 0.80 W/m²K, verified again once it sits in the wall. That figure is not a brochure number. It combines glazing, frame, spacer and the installation junction, and it exists so the inside pane stays warm enough to sit beside in January.
Why 0.80, and why does “installed” matter?
The limit is derived from comfort, not energy. The Passive House Institute’s certification criteria for transparent components require that the surface temperature of a window deviates no more than 4.2 K from a 22°C operative room temperature. Beyond that gap, cold air slides off the glass and pools at your ankles, and the window radiates chill at anyone nearby. Work the arithmetic back through a design winter day and, for the cool-temperate zone that covers Ireland and the UK, you land on a whole-window U-value of 0.80 W/m²K. The value is checked again in the installed state, where the criteria allow up to 0.85 once the installation bridge is included.
The practical consequence is large: a window that meets the criterion needs no radiator beneath it. Seating can go against the glass. The 26-storey House at Cornell Tech in New York, the world’s tallest Passivhaus at completion, holds comfortable temperatures beside floor-to-ceiling glazing through a New York winter for exactly this reason.
The word “installed” is where projects go wrong. A manufacturer’s Uw describes a standard test size in free air. Build that same window into a wall with the frame proud of the insulation layer and the installation psi-value quietly claws back a slice of the performance you paid for. PHPP wants the window as built, not as advertised.
What makes up a window U-value?
A window is four thermal components pretending to be one product.
| Part | Symbol | What it covers | What drives it |
|---|---|---|---|
| Glazing | Ug | Centre-pane performance to EN 673 | Number of panes, low-e coatings, argon or krypton fill |
| Frame | Uf | Heat loss through the frame section | Material, chamber design, insulation inserts, frame depth |
| Glass edge | Ψg | The spacer bar bonding the panes | Warm-edge composite versus aluminium spacer |
| Installation | Ψinstall | The junction between frame and wall | Frame position relative to the insulation layer, overlap at reveals |
The whole-window value weighs Ug and Uf by their areas, then adds the spacer psi along the glass perimeter. The installed value adds the installation psi along the frame perimeter. Every term is calculated, the frames by finite element modelling under DIN EN ISO 10077, which is the same family of thermal modelling used for any other junction.
Two consequences follow. Small windows are worse than big ones, because frame and edge dominate as the glass area shrinks. And a poor spacer drags down excellent glazing: in the PHPP files we review, an aluminium spacer under triple glazing is one of the most common silent losses, cheap to fix at order stage and impossible after.
Why triple glazing?
Because two panes cannot get there. The reference glazing in the PHI criteria for the cool-temperate zone is Ug 0.70 W/m²K, which needs three panes, two low-e coatings and gas-filled cavities. No double-glazed unit gets near that figure, and since the whole-window limit of 0.80 includes the frame and edges, double glazing fails before the frame is even considered.
The third pane also lifts the internal surface temperature, which is the comfort criterion again, and it is checked for hygiene too: the criteria require a temperature factor f_Rsi of at least 0.70 at the coldest point of the frame for our climate, calculated with an internal surface resistance of 0.25 m²K/W, so that condensation and mould have no cold corner to start from. The physics is the same surface-temperature problem covered by our condensation and dew point tool.
Is a window a heat loss or a heat source?
Both, and the balance is designable. The g-value states what fraction of the solar energy striking the glass gets through. The U-value governs what leaks back out. Over a heating season, glazing that faces the sun can collect more energy than it loses: the PHI’s component database puts it directly, noting that Passivhaus windows “can even gain more heat in winter than they lose”, and that narrow frames matter because the sun only enters through the glass.
This is why window specification is an energy-balance exercise, not a shopping exercise. Chasing the lowest possible Ug usually trades away g-value, and on a south elevation that trade can make the building worse. PHPP weighs every window by orientation, shading and glazing properties month by month, which is the job our PHPP energy modelling service does on live projects. On site the result reads as a pattern: generous glazing towards the sun, modest openings to the north, and frames as slim as the structure allows.
What does a certified component actually tell you?
The PHI component database lists windows whose values were verified by independent calculation rather than declared by the manufacturer, each carrying an efficiency class from phC up to phA+ based on the heat loss through the opaque parts of the window. Certified products also come with the spacer and installation details already modelled, which removes the guesswork from the PHPP entry.
You do not have to use certified components, and good uncertified windows exist. The difference is evidence. With an uncertified product the burden of proof moves to the design team, and we have seen the gap between a declared Uw and a calculated one decide whether a building certifies.
A window quotation is worth ten minutes of scrutiny before it is signed:
- Ask for Uw to EN ISO 10077 at the actual window sizes, not the standard test size, since small windows perform worse than the brochure.
- Check the spacer specification. “Warm edge” should be named and its psi-value stated.
- Check Uf separately. A frame above roughly 1.0 W/m²K cannot rescue any glazing.
- Ask for the g-value and check it against the orientation the windows will face.
- Agree the installation detail before ordering, with the frame overlapping the insulation layer, because Ψinstall is decided by the wall, not the window factory.
Where Mosart fits
Mosart’s certifiers review window schedules, installation details and the PHPP window sheets on every project we certify, and calculate installation psi-values where no certified detail exists. Check what a specification change does to an element with our free U-value calculator, or bring the whole window schedule to our PHPP energy modelling team.