Windows are a major heat loss weakness, even at night when the sun is gone. With thermally insulating curtains, you reduce heat flow through your window at night by up to 25%. Especially effective in cold regions.
Step by step
Regular curtains slow air circulation but don't help against radiation. Thermal curtains have multiple layers: often a reflective metal layer inside, heat-storing material (polyester, cotton) in the middle, and durable fabric outside. The air between window and curtain becomes insulated, and the metal layer reflects heat radiation back into the room. At night, when outside temperature drops, less condensation forms on the cold window glass, and room heat stays longer.
Source: Fraunhofer-Institut für Bauphysik IBP, Thermal curtain improves U-value of a standard window by up to 30%
Step by step
- Measure your window and curtain rodMeasure the window width, height, and distance between frame and wall. Thermal curtains should start at least 20 cm above the window and end 10 cm below the windowsill, this creates an airtight curtain tunnel.
- Buy the right thermal curtainsChoose curtains with thick lining (at least 200 g/m², better 300+ g/m²). A reflective layer (metallic backing) is important. Size them so the curtains overlap slightly when closed.
- Mount the rod highAttach the curtain rod as high as possible on the wall, not to the window frame itself. This prevents air circulation between window and curtain.
- Hang the curtain properlyHang the curtains so they close completely at night with no gaps. Big folds in the fabric are fine, they actually help store more air.
- Close at night, open during the dayMaximum effect happens at night when it's coldest outside. During the day (especially sunny days) keep curtains open so free solar heat comes in.
Worked example
Before: 3-room old building with 6 large windows. At night lots of heat radiation escapes, residents close rooms at night for heat saving. Heating costs €2,200/year.
After: 4 windows with good thermal curtains (costs €300–400, about €100 per window for good quality). Night heat loss drops by 20–25%. Heating system runs less at night → 12–15% savings over the year (about €260–330/year). Pays for itself in 1.5–2 years.