Main Features of Efficient Windows in a Passive House
The windows are one of the most sensitive points of a building’s envelope and represent about 22% of heat loss — 12% through the frame and 10% through the glass.
Therefore, it is essential to carefully select the windows to minimize thermal losses without compromising indoor comfort.
In addition to limiting energy losses, windows also ensure natural lighting, ventilation, solar control, and visual connection with the outside.
The ideal glass should reduce heat loss while simultaneously allowing sufficient solar radiation to warm the space. The goal is to find the balance between solar gains and thermal insulation.
Types of Glass (Ug coefficient):
- Single Glass → Ug = 5.5 W/m²K
- Double Glass with Air → Ug = 3 W/m²K
- Double Glass with Noble Gas → Ug = 2.2 W/m²K
- Double Glass with Noble Gas and Low Emissivity → Ug = 1.3 to 0.9 W/m²K
- Triple Glass with Air → Ug = 2.3 W/m²K
- Triple Glass with Noble Gas and Low Emissivity → Ug = 0.8 to 0.5 W/m²K
Frames and Materials
The frame is the most fragile element, as it blocks part of the solar radiation (on average, only 30% of the total surface of the window is glass).
The Passivhaus standard does not impose specific materials, allowing for the use of solid wood, PVC, or aluminum — as long as, in the latter case, the profile has thermal break.
Recommendations by Climate Zone:
Central Europe (temperate climate):
- Value U between 0.85 and 0.70 W/m²K
- Triple Glass
- Insulated Frame
- Solar Factor (g) ≈ 50% — retains heat in winter and helps prevent overheating in summer.
Portugal (milder climate):
- Value U up to 1.70 W/m²K
- Double Glass
- Insulated Frame
- Solar Factor (g) ≈ 60%
Finally, the proper installation of windows is crucial:
they must be aligned with the exterior insulation to minimize thermal losses and ensure good air tightness.