Uw is the number that tells you how much heat escapes through a window. The lower it is, the less energy you lose in winter. A good window today has a Uw of around 0.7–0.9 W/m²K; windows from fifteen years ago could be at 1.5 or even 2.0.
The difference looks innocent, but across a whole house — a dozen windows, an entire heating season — it is real money on the fuel or electricity bill.
The window's Uw is not the glass's Ug
The Uw of the whole window depends not only on the glass but also on the frame profile and the way it is joined. Two windows with the same glass unit can end up with different results. When asking for a quote, always ask for the Uw of the whole assembly, not just the glass. Ug is always lower — and some sellers quote only that.
Polish building regulations require Uw of no more than 0.9 W/m²K for vertical windows and 1.1 for roof windows.
Spacer bar: aluminium or warm edge
The spacer is the profile between the panes that holds the gap and keeps the argon-filled cavity sealed. Its parameter is Ψ (psi), the linear heat-loss coefficient at the glass edge, in W/mK. The lower it is, the less heat escapes around the perimeter and the warmer the edge of the glass on the inside.
Manufacturer's figures for a 4-12-4-12-4 triple unit in a uPVC profile: aluminium spacer 0.078, Swisspacer Advance 0.037, Swisspacer Ultimate 0.030. In aluminium frames the gap is wider: 0.120 versus 0.042 and 0.031.
What does that mean for Uw? Take a 1.5 × 1.5 m window. The glass perimeter is about 5.5 m. The difference between aluminium and Advance is 0.041 W/mK times 5.5 m, i.e. 0.23 W/K over 2.25 m². That is roughly 0.1 W/m²K on the Uw of the whole window. On a 0.8 window that is more than 10 percent.
Condensation often matters more. A cold aluminium spacer creates the coldest point at the edge of the glass. That is where water vapour condenses first — hence the dew at the bottom of the pane on winter mornings. A warm-edge spacer raises the edge temperature and delays that moment.
Colours available: white, black, brown, caramel, grey, light grey. Dark spacer for dark profiles, white or light grey for white ones. With a triple unit an aluminium spacer makes no sense — it is like investing in 0.5 glass and leaving a thermal bridge around the whole perimeter.
A warm window and a cool house are not the same thing
Customers ask for a window "that is warm in winter and cool in summer". Two different technologies are responsible for those two things, and one does not replace the other.
Uw and Ug measure how much heat escapes outwards when there is a temperature difference — they work in winter. Low-emissivity coating, argon and a warm spacer take care of that.
The g-value measures how much sun comes in. A normal triple unit with a low-e coating has g of about 0.5. That means that through 6 m² of south-facing window on a July afternoon more than two kilowatts of heat come in. A Uw of 0.7 does nothing about that, because it works on temperature difference, and the sun comes in regardless.
The regulations know this. The same annex to the Polish building regulations requires that in summer g does not exceed 0.35, calculated as glass g times the reduction factor of the shading. Without external shading, ordinary glass will not meet that. Exceptions: north-facing windows and windows shaded from outside.
The catch the other way: glass with g 0.3 lets in less sun all year round, including in January, when those gains would be welcome. So sometimes ordinary glass plus an external venetian blind is the better solution than solar-control glass without shading.
What this means when ordering
- Compare the Uw of the whole window for the specific configuration, not the Ug of the glass and not the "Uw of the system".
- With a triple unit, take a warm-edge spacer — the price difference is small, the comfort difference is large.
- If you have large south-facing windows, think about external shading separately from Uw.
In the configurator Uw is calculated live for the chosen system, glass and spacer — you can see how each change affects the result.