Blocking Heat from a Bay Window in Summer: A Comprehensive Guide

Why the Bay Window is the #1 Hot Spot in Your Home

A bay window poses a thermal problem that classic windows do not, due to three cumulative factors:

• Surface area: A standard bay window measures 2 to 5 m² of glazing. A classic window is 0.5 to 1.5 m². With the same thermal coefficient, a 4 m² bay lets in 3 to 4 times more heat than a 1 m² window. This is a simple surface effect.
• Orientation: Bay windows are generally oriented south or west to take advantage of light. These are precisely the orientations that receive the most intense and prolonged solar radiation in summer (4 to 8 hours of direct sun per day).
• Greenhouse effect: Glass lets visible light and short infrared rays (sun's heat) pass through, but blocks long infrared rays (heat re-emitted by interior surfaces). Heat enters but does not exit. It's the same principle as an agricultural greenhouse, applied to your living room.

According to ADEME, windows and bay windows are responsible for 30 to 50% of solar heat gain in a home in summer. For a 4 m² south-facing bay, this can represent 1,500 to 2,500 watts of incoming heat during peak hours, more than a living room electric heater.

Comparison Table: Solutions to Block Heat from a Bay Window

Key point: The outdoor blind is technically the most effective (it blocks heat before the glass) but it costs 5 to 10 times more, requires a professional and co-ownership authorization. The indoor thermal curtain offers the best efficiency/cost/simplicity ratio for a tenant or owner without a renovation budget.

Main Solution: Custom-Made Thermal Curtain for Bay Window

A high-density thermal curtain placed in front of the bay window creates a barrier between the hot glazing and the room's air. The fabric absorbs and blocks infrared radiation, preventing heat from spreading into the living room.

Why custom-made is essential for a bay window:

• Atypical dimensions: Bay windows measure 200 to 400 cm wide and 220 to 270 cm high. No standard ready-made curtain covers these dimensions without leaving gaps.
• Central overlap: For a bay over 200 cm, two panels meeting in the center must overlap by 10 to 15 cm. Without this overlap, heat, light, and noise pass through the central gap.
• Side overhang: The curtain must extend 15 to 20 cm beyond each side of the bay. On a large glass surface, every uncovered centimeter lets in a proportional amount of heat.

Kurtens custom-made bay window curtains are manufactured to the nearest centimeter, in free width and up to 270 cm in height. The 620 g/m² multi-layer fabric blocks 60 to 80% of solar radiation, with 100%* blackout and 22 dB* sound attenuation. To properly measure your bay window, consult the size guide.

How to Measure and Install a Curtain on a Bay Window

Bay windows are the trickiest surfaces to measure. Walls are not always parallel and the floor is not always level. Here's the method:

Width measurement:

• Measure the width at three points: top, middle, and bottom of the bay.
• Take the largest measurement.
• Add 30 to 40 cm (15 to 20 cm overhang on each side).
• For two panels: divide by two and add 10 to 15 cm overlap per panel.

Height measurement:

• Measure the height at three points: left, center, and right.
• Take the largest measurement.
• Differences can reach 1 to 3 cm, enough to create a visible gap if you take the wrong measurement.

The rod: For a bay over 200 cm, a single-piece rod is preferable (no joint in the middle that creates a weak point). The rod must support the weight: two Kurtens panels of 150x250 cm at 620 g/m² weigh approximately 4.6 kg in total. Plan for a solid wall-mounted rod with an intermediate support if the length exceeds 200 cm. Consult the installation guide.

Sliding Bay Window: How to Install a Curtain in Front

Sliding bay windows pose a specific constraint: the handle and opening mechanism protrude from the plane of the glass. The curtain cannot be placed directly against the glazing.

The solution: Install the rod on the wall above the bay (10 to 15 cm above the frame), with a projection of 10 to 15 cm from the wall. The curtain hangs freely in front of the entire bay, without touching the handle or the sliding rail. You can open and close the bay without unhooking the curtain; just push it aside.

For pocket sliding doors (where the sashes disappear into the wall), the curtain should only cover the visible glass surface, not the wall parts. Measure the width of the glass part when the bay is closed.

The Complete Summer Strategy for a Bay Window

The most effective combination to keep a living room cool behind a large bay window, without air conditioning:

1. In the morning (before 10 am): Curtain open, windows open if the outside air is still cool (below 25°C). Enjoy natural light and ventilation.
2. As soon as the sun hits the bay: Close the thermal curtain. Close windows if the outside temperature exceeds the inside temperature. The curtain blocks 60 to 80% of solar heat and 100% of direct light.
3. In the late afternoon (after 6-7 pm): Open the curtain and windows to ventilate and release accumulated heat.
4. At night: Cross-ventilation (windows open on both sides of the home). The accumulated coolness is trapped the next morning by the closed curtain.

This daily routine, systematically applied, keeps a living room behind a bay window 5 to 10°C below the outdoor temperature during heat peaks, with no electricity consumption.

What to Remember

The bay window is the primary source of overheating in a home in summer, due to its large surface area, orientation, and the greenhouse effect. A custom-made high-density (620 g/m²) blackout thermal curtain, sized to the exact dimensions of the bay, blocks 60 to 80% of solar radiation and keeps the room 5 to 7°C cooler.

Discover our custom-made bay window curtains: free width, height up to 270 cm, 620 g/m² fabric, 100% blackout*, thermal insulation up to 7°C*, and sound attenuation 22 dB*. 7 colors, free delivery in 2 to 3 weeks. For thermal protection on all your windows, consult our custom-made thermal curtain collection.

*Data from tests conducted under optimal conditions.

Frequently Asked Questions

A large window is the number-one hot spot of a home, but heat comes in through every sun-exposed window. For the full room-by-room strategy, see our guide on how to cool a room without air conditioning.