Should double glazed windows feel hot on the inside?

No, double glazed windows should not feel hot on the inside surface in normal weather conditions. The internal pane of a properly functioning double glazed unit should feel close to room temperature, not hot. According to the Energy Saving Trust, a well-installed double glazed window with a low-emissivity coating and gas fill will have an internal surface temperature within 2-3°C of the room air temperature (Energy Saving Trust, 2026). If the glass feels hot, it indicates a problem with solar gain, a faulty seal, or a poor-quality unit.

The key variable is the solar heat gain coefficient (SHGC) of the glazing. In winter or on overcast days, the internal pane should feel cool but not hot. On a sunny day, the glass may feel warm to the touch if the window has a high SHGC, which allows more solar radiation to pass through. However, a hot feeling, especially if it persists into the evening, suggests the sealed unit has failed. This allows heat to transfer directly from the outside. This typically affects south- or west-facing windows.

Failed seals cause heat transfer

If the internal pane feels hot, the most common cause is a broken seal. Double glazed units rely on a gas-filled gap (usually argon or krypton) to reduce heat transfer. When the seal fails, moisture enters, the gas escapes, and the window becomes a single-glazed unit. The inside pane then heats up as outside temperatures rise. The Glass and Glazing Federation notes that a failed seal reduces thermal efficiency by up to 50% and can cause condensation between panes (BRE, 2026). If you feel heat, check for misting or water droplets inside the unit, these confirm seal failure and mean replacement is needed.

Solar gain can warm the glass

On a sunny day, especially with south-facing windows, the internal pane may feel warm to the touch. This is normal if the glass has a high solar heat gain coefficient, common in older or budget double glazing. The Energy Saving Trust states that modern low-emissivity (low-E) glass reduces this effect. This reflects more solar heat back outside (Energy Saving Trust, 2026). If the warmth is only noticeable in direct sunlight and disappears quickly when the sun moves, it is not a defect. However, if the glass stays hot for hours, check the window’s U-value, aim for 1.4 W/m²K or lower for modern units.

High U-value indicates poor insulation

A hot internal pane can also mean the window has a poor U-value. The U-value measures how quickly heat passes through the glass, lower is better. Building Regulations in England and Wales require replacement windows to have a U-value of 1.4 W/m²K or less (GOV.UK, 2026). If your windows are older than 20 years, they may have a U-value above 2.0 W/m²K, meaning they transfer heat readily. In summer, this makes the inside pane feel hot. Upgrading to double glazing with a U-value of 1.2 W/m²K or lower will keep the glass cooler and reduce energy bills by around £195 per year for a typical semi-detached house.

A worked example

Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates this upgrade cuts heat loss through the windows by up to 50%, saving around £195 per year on heating bills. With a 25-year lifespan, the total energy saving reaches approximately £4,875, meaning the investment pays for itself within 14 years. If the existing windows have a broken seal and the glass feels hot even on cool days, the payback accelerates because the heat loss is worse. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.

What homeowners often get wrong

The most common mistake is assuming any warmth on the glass means the window is working well and keeping heat inside. Three specific errors lead to wasted money and missed savings.

1. Confusing solar warmth with seal failure Many homeowners think a warm pane on a sunny afternoon proves the window is efficient. In reality, the glass should feel close to room temperature within an hour of sunset. If it stays hot into the evening, the argon gas has likely leaked out and the seal is broken, costing you around £100 extra per year in lost heat.
2. Ignoring fog or mist between panes A hazy appearance between the glass layers is often dismissed as condensation that will dry out. This visible mist means the seal has failed completely and the insulating gas is gone. Replacing the sealed unit promptly saves up to £250 annually compared to running heating through a failed window.
3. Choosing the cheapest replacement unit Opting for a budget double glazed unit with no low-E coating or a poor solar heat gain coefficient can leave the glass feeling hot in summer and cold in winter. Spending an extra £150 per window on A-rated glazing cuts heat loss by a third and prevents the hot-glass problem entirely.

Quick reference

• An internal pane temperature more than 3°C above room air temperature signals a failed seal or poor glazing according to the Energy Saving Trust.
• South- and west-facing windows with a solar heat gain coefficient above 0.6 may feel warm in direct sun but should cool down within one hour of the sun moving off the glass.
• You can check for seal failure by looking for condensation, mist, or a greasy film between the two panes of glass.
• Replacing a failed double glazed unit with A-rated argon-filled glazing saves roughly £195 per year on a typical semi-detached home in the UK.
• Installing new windows without checking the Energy Saving Trust recommended product database risks buying units that feel hot and waste heat for decades.