The question 'double or triple glazing?' is often asked as if it were the whole of thermal performance. It is not. The glazing unit is one part of a window's U-value; the frame, the edge effects and the spacer bar all contribute, and the figure that actually describes the installed door is the whole-window U-value, Uw. Choosing glazing is really about choosing the Uw you need and the frame that can carry the glazing that delivers it.
Uw and Ug are not the same thing
Ug is the centre-of-glass U-value — the insulating performance of the glazing unit alone, ignoring the frame. Uw is the whole-window U-value — the glass, the frame and the edge effects combined. A door with a Ug of 0.6 W/m²K can have a Uw of 1.0 W/m²K or higher once the frame is included, because the frame is usually the weakest thermal link in a minimal-frame door.
When a system is described as 'passive house suitable', it is the Uw that matters, not the Ug. The Solarlux cero III, for example, publishes a Uw of ≤0.80 W/m²K with triple glazing, which is what makes it passive-house certified. The MFP18-H50 publishes a Uw down to 1.05 W/m²K with 48 mm triple glazing. These are the figures to compare, not the glass thickness.
What the glazing thickness changes
Thicker glazing generally means a lower Ug, because there is more insulating cavity and, in triple glazing, a second low-e coating. But thicker glazing also means a heavier panel and a frame engineered to carry it. A 28 mm double-glazed unit is standard across most of the range; a 50 mm triple-glazed unit needs a frame designed for it, and not every system takes one.
The glazing thickness also sets the maximum panel size, because the heavier the glass, the larger the carriage and track needed. This is why the systems with the largest panels — Vitrocsa V56 with 56 mm glazing, MFP18-H40 with 38 mm — are engineered around the weight, and why a triple-glazed door is not always available in the same panel sizes as a double-glazed one.
Where a Uw is not published
Not every system publishes a Uw. Vitrocsa, for example, does not publish a whole-window Uw for the V56 or V32 in the same way Solarlux does; the TH+ publishes a MINERGIE-frame Uw of 0.97 W/m²K. Where a Uw is not published, we will not state one, and any thermal comparison should be made on the documented figures, not assumed. If a project needs a certified Uw for Building Regulations or planning, ask for the certificate before specifying.
This is one of the honest trade-offs in the range. The Vitrocsa systems are thermally broken and perform well, but they are not documented to the same passive-house standard as the Solarlux cero III. If the project needs a documented Uw below 1.0, the cero III and the MFP18-H50 are the systems with the published figures.
So, double or triple?
For most UK homes, double glazing with a Uw around 1.3 W/m²K is adequate and is what most of the range delivers as standard. Triple glazing — and the wider, heavier frames that carry it — is worth it where the project is high-performance (passive house, low-energy), exposed, or where the building regulations or the client demand a lower Uw. The glazing choice follows the performance target, not the other way around.
- Compare Uw, not Ug — the frame matters.
- Thicker glazing needs a frame engineered for it.
- Where a Uw is not published, do not assume one.
- Triple glazing is worth it for passive house, exposed, or low-energy projects.
- Ask for the certified Uw if Building Regulations need a specific value.
