Two identical units were made with the same glass, the same 16 mm argon cavity and the same gas fill. One carried an aluminium box spacer and one carried a thermoplastic warm edge. Only the edge changed, and the edge is where the difference is.
One unit, two edges: warm edge against aluminium
rev Bwritten 2026-08-27issue 04rev Bdrawn by The Transom Set (a named set, not a person)
What moved and what did not
The centre-of-glass U-value was the same to two decimal places, because the glass and the cavity were identical. The edge-of-glass U-value moved by about a quarter. The whole-unit U-value moved by a little over five per cent. The condensation index moved by eleven points, which is the largest change of the four.
| Quantity | Aluminium box | Warm edge | Change |
|---|---|---|---|
| centre of glass | 2.61 W/m²K | 2.61 W/m²K | none |
| edge of glass | 2.34 W/m²K | 1.79 W/m²K | − 23 % |
| whole unit | 2.02 W/m²K | 1.90 W/m²K | − 6 % |
| condensation index | 52 | 63 | + 11 points |
The largest change is the one that is not on the energy model
The condensation index moved further than the U-value, and it is the number that decides whether the occupant ever notices. That is a general pattern with edge improvements: the energy effect is small and the comfort effect is not. A specifier reading only the U-value will conclude that the upgrade is marginal, and the conclusion will be right about the energy and wrong about the window.
The cost side is worth stating plainly. A warm-edge spacer adds a small amount to the unit price, and it adds nothing to the frame or the installation. Per unit of comfort improvement it is one of the cheapest changes available in a glazing package.
A warm-edge spacer is not a fix for a cold frame. It moves the coldest surface from the glass edge toward the frame, and if the frame is unbroken aluminium the coldest surface is now the frame. The next revision records exactly that outcome.
What this changes
It changes which figure a specification should quote. If the concern is energy, the whole-unit U-value is the relevant number and the change is small. If the concern is condensation on the glass, the edge and the index are the relevant numbers and the change is large. Quoting one when the concern is the other is how a good upgrade gets rejected.
Documents this sheet is drawn from
- 1Information, Criteria, and Algorithms for Certified Passive House Components: Insulated Glazing Units and Solar Control Glazingssupports the certified component criteria that an improved edge is assessed against
- 2PHI Criteria for Transparent Components V5.2 (Passive House Institute)supports the transparent component criteria used to judge the unit as a whole
- 3AAMA 1503: Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections (Intertek)supports the thermal transmittance and condensation resistance test that produced the two figures
Each line points at one specific document, with its own title as the link text. No line is a home page and no line is a search result.
Sign the sheet and stop Sign the sheet and stop once you know which of the four quantities your concern actually maps onto. Then read the right column.