Cold downdraught is the downward air current that forms at a cold window or door and is felt as a draught across the floor. It is one of the most common comfort complaints in existing homes — and it is almost always mistaken for a gap somewhere.
How it works
There does not have to be a single gap. The effect is pure air circulation:
- Warm indoor air rises off the heating and spreads along the ceiling.
- That air reaches the window and cools against the cold glass.
- Cooled air is heavier and falls down the face of the glass.
- At the bottom, that cold current spreads out across the floor.
So what you feel is not outside air but your own indoor air, chilled against the glass. With floor-to-ceiling glazing on a north elevation the effect is strongest: the cold surface is large and the drop is long.
Common causes
- Single glazing, or old, poorly performing double glazing
- Failed double glazing (often visible as misting between the panes)
- Poorly insulated or uninsulated frames, spandrel panels and window boards
- A radiator sited on an internal wall rather than under the window
- Insufficient heat output, or underfloor heating running at too low a flow temperature
- A genuine thermal bridge or air leak at the frame-to-wall junction
Solutions, in order of effect
- Better glazing. The heart of the problem is the surface temperature of the pane. Modern low-e double or triple glazing raises it enough that the downdraught largely disappears. It is the only measure that removes the cause.
- Heat in the right place. A radiator directly under the window catches the falling cold air and sends it back up. That is exactly why radiators have traditionally sat beneath windows. With underfloor heating, a trench convector along the glazing line does the same job.
- Insulate frame, spandrel and window board. The glass is rarely the only cold surface.
- Heavy full-length curtains or close-fitting blinds. They separate the cold air layer from the room — but they must reach the floor, and must not hang over the radiator, or the radiator stops working.
- Draught seals. They solve a gap, not a downdraught — but if there is also a gap, they help.
Point to note
Measure before you spend. An inexpensive infrared thermometer shows immediately whether the complaint comes from the glass, the frame, the spandrel or a leak at the junction. That determines which of the measures above is worth doing.
Related terms
- Thermal bridge
- Glazing
- Double glazing
- Radiator
- Convector
- Indoor climate
Want to know more about construction terms? Visit our knowledge base at fredsdiyplans.com.