Draught Proofing vs Secondary Glazing: Which Do Your Windows Need?
Dr Sarah Chen
Building Physics Consultant
If your period windows rattle, whistle and let cold air in, you have probably been offered two answers: draught proofing or secondary glazing. They are often presented as rivals, but they fix different things. Here is how to decide which your windows need, and when it makes sense to do both.
The Short Answer
- Choose draught proofing if your main complaint is cold air leaking around the frames, the windows are in good repair and noise is not a problem.
- Choose secondary glazing if you also want to cut heat lost through the glass, reduce outside noise or stop condensation, or if the windows are too worn to seal well.
- Choose both if you want the best result from original sash windows you intend to keep for decades.
What Draught Proofing Does
Draught proofing seals the gaps between the moving parts of a window and its frame. On sash windows this usually means routing grooves into the sashes and fitting brush or compression seals, plus a seal at the meeting rails. On casements it is typically compression seals around the opening edge.
Well-done sash window draught proofing stops rattles, reduces cold air movement and can make the sashes easier to slide. It is often carried out alongside overhaul work such as new sash cords and beads. Read Victorian sash window draughts: solutions and our sash window restoration guide.
What it does not do: the glass stays single glazed, so heat still conducts through it, and noise reduction is modest. Draught proofing typically improves sound insulation by only a few decibels.
What Secondary Glazing Does
Secondary glazing fits a complete second window inside the original, with its own frame and seals. It stops draughts from reaching the room, because any air leaking past the old sash is trapped in the cavity. It also adds an insulating air gap that greatly reduces heat loss through the glass, and with acoustic glass it is the most effective way to reduce outside noise in a period home. See secondary glazing for sash windows.
Comparison
| Factor | Draught proofing | Secondary glazing |
|---|---|---|
| Stops draughts | Yes | Yes |
| Heat loss through glass | No change | Cut by more than half |
| Noise reduction | A few dB | Up to 45–54dB combined |
| Condensation | Little change | Usually reduced |
| Changes original window | Small routed grooves | No |
| Listed buildings | Often needs consent | Usually acceptable |
| Typical cost | Lower per window | Higher per window |
Listed Buildings: A Point People Miss
Routing grooves into historic sashes is an alteration to the fabric, so in a listed building it may need listed building consent. Secondary glazing is fitted independently and is usually reversible, which is why conservation officers often prefer it. See secondary glazing for listed buildings.
Why Doing Both Can Make Sense
If you are overhauling sash windows anyway, draught proofing keeps the outer window sound and stops it rattling, while secondary glazing handles heat and noise. Sealing the outer window well also reduces the chance of condensation in the cavity, because less warm, damp air reaches the cold outer pane.
Frequently Asked Questions
Is draught proofing as good as secondary glazing?
For draughts alone, good draught proofing can be very effective. For heat loss through the glass and for noise, secondary glazing performs far better.
Do I need draught proofing if I have secondary glazing?
Not to stop draughts in the room, as the secondary unit already does that. It can still help keep the original window from rattling and reduce cavity condensation.
Which is cheaper, draught proofing or secondary glazing?
Draught proofing costs less per window. Secondary glazing costs more but solves heat, noise and draughts together. See our secondary glazing prices.
