Will Secondary Glazing Stop Condensation?

Why it forms in the first place

Warm air holds more moisture than cold air. When room air touches a surface below its dew point, the moisture condenses out. Single glazing is the coldest surface in most rooms by a wide margin, so it is where the water appears. Cooking, showering, drying clothes indoors and simply breathing all add moisture, and a modern draught proofed house lets less of it escape than an old leaky one did.

How secondary glazing changes it

The room side pane sits in a warmer position, insulated from the cold outer pane by the cavity, so it stays above dew point in normal conditions and stays dry. That is why streaming bedroom windows usually stop after installation. Low E glass raises the inner surface temperature further and makes the effect more reliable in very cold weather.

• Inner pane stays warmer and above dew point
• Low E glass improves the margin further
• Draughts and cold down draught also stop
• Curtains no longer trap cold against the glass

The detail that matters: vent outwards, seal inwards

The room side unit should be sealed as tightly as possible and the cavity should be able to breathe slightly to the outside, usually through the natural leakage of the original window. Detailed that way, any moisture that reaches the cavity escapes outwards. Reverse it, by sealing the primary window tight and leaving the secondary leaky, and you will get condensation between the two panes, which is the single most common complaint about badly fitted secondary glazing.

When glazing will not fix it

If the room has a moisture source that nothing controls, such as an unvented tumble dryer, a bathroom with no working extract, drying washing on radiators, or a genuine damp problem in the structure, the water will simply find the next coldest surface, often a corner or behind a wardrobe on an external wall. Fix the moisture source and ventilation alongside the glazing, not instead of it.