Triple glazing
Triple glazing is an insulated glass unit with three panes of glass and two gas-filled spaces instead of two panes and one. It lowers the whole-window U-factor by roughly 0.06-0.15 on European systems, at a price premium and a weight penalty.
An insulated glass unit has a spacer, gas fill and a coating. Double glazing is two panes with one gas space. Triple glazing adds a third pane and a second gas space, so heat has two barriers to cross instead of one.
A typical triple unit is about 1-3/4 in. thick (4/16/4/16/4 mm, 44 mm overall), against about 1 in. (24 mm) for a standard double. That thickness is why triple needs a deeper sash and heavier hardware.
What it does to U-factor
Published whole-window U-factors for systems I quote, converted to US units. Lower is better.
| System | Double glazed | Triple glazed | Improvement |
|---|---|---|---|
| uPVC tilt-turn (Aluplast Ideal Neo) | U-0.22 | U-0.13 | 0.09 |
| Aluminum (Aliplast Genesis 75) | U-0.22 | U-0.15 | 0.07 |
| uPVC sliding door (Aluplast Smart-Slide) | U-0.32 | U-0.17 | 0.15 |
These are catalog values from the system providers, not NFRC ratings. Sliding doors gain the most because they carry the most glass.
What it costs
In my estimator, triple glazing adds about 11-14% to the supplied window price on aluminum and about 15% on uPVC. It also adds weight, roughly 30-40% on the glass, which matters most on large lift-and-slide doors.
Where it earns its cost
- IECC zones 5-8. Heating load drops and the interior glass stays warmer, which also cuts condensation.
- Passive House. The target of about U-0.14 is very hard to reach with double glazing.
- Zones 1-3 and large sliders. Usually a well-specified double with a strong low-E coating is the better buy.
The full zone-by-zone case is in triple vs. double glazing for US projects. To price both glass options on your own schedule, run it through the estimator. Triple units are quoted with a warm-edge spacer and gas fill as part of the package.
Common questions
- How much more does triple glazing cost?
- On European windows in my estimator, triple glazing adds about 11-14% to the supplied price on aluminum and about 15% on uPVC. The percentage is higher on uPVC because glass is a larger share of the unit cost.
- Is triple glazing worth it in a warm climate?
- Usually not. In IECC zones 1-3 the building is not fighting heat loss, so SHGC and air tightness matter more than an extra pane. Triple adds cost and weight there without much payoff. It earns its place in zones 5-8 and on Passive House projects.
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