Cold Spots and Window Downdraught: The Draught That Isn't a Draught
You seal every gap, the draught excluder is on, and the seat by the window is still unusable in January. That is because what you are feeling is not infiltration — it is a convection loop driven by cold glass.
How to tell convection from infiltration
Hold a lit match or a thin tissue strip an inch from the glass at head height, then at ankle height. Infiltration moves the flame sideways at a specific point, usually a joint. Downdraught pulls it downwards along the whole face of the glass. If it is the second, no amount of sealing will help.
Why a big single-glazed window is worse than a small one
The convection loop scales with glass area and with the temperature difference. A tall Victorian sash or a 1960s picture window presents a large, very cold plane; room air touching it cools, becomes denser, drops to the floor and flows across the room at ankle level. That is why people describe a cold draught coming from the middle of a sealed window.
Raising the surface temperature is the only fix
Secondary glazing with a Low-E soft-coat pane reflects long-wave radiant heat back into the room and keeps the room-side surface much closer to air temperature. The temperature difference driving the loop collapses, and the loop with it. Most clients notice this within the first heating cycle — considerably faster than they notice the change on the heating bill.
- Low-E soft-coat glass on the secondary pane
- U-values from 1.868 W/m²K achievable
- Works on windows too large or too protected to replace
Frequently asked questions
Curtains help overnight but trap the cold air behind them, which then spills into the room when you open them. They also do nothing while you are sat in the room using it.
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