Where a draught at the window really comes from
A draught at a window almost never comes from the glazing: it comes from the junctions, the sash seal, the threshold and the roller shutter box. That is why a double-glazed window can blow, and why new glazing does not always settle the feeling.
The marker is simple: cold through the glazing feels like a cold, still surface, while a draught is felt moving, often at ankle level or with a hand run along the frame.
Telling cold and draught apart
They are two distinct phenomena, and they are not corrected the same way. Confusing them is the most common reason a purchase disappoints.
| What you feel | Likely cause | What corrects it |
|---|---|---|
| A still coolness near the window, with no blow | The glazed surface itself, colder than the room's air | A dense fabric covering the glass and overlapping the frame |
| A moving thread of air, often low down | The sash seal, the threshold, a junction or the shutter box | Draught-proofing, adjusting the hardware, a door draught excluder |
| A diffuse blow all around the frame | The installation of the joinery, between frame and wall | Repointing, outside a curtain's reach |
How to locate the leak in five minutes
The simplest method needs nothing more than a low flame or a thin piece of paper. With window and door closed, run slowly along the frame: the paper moves, or the flame leans, where air passes. Repeat at the top, on both jambs, at the bottom and under the shutter box.
The result often surprises: the leak is rarely where it was expected, and it is more often low down and at the shutter box than on the jambs.
What the curtain settles, and what it masks
A dense curtain stops the air circulating along the glazing and removes the cold-surface sensation. It does not plug a leak: it puts it behind a layer of fabric, which makes it less noticeable without treating it. On a clear leak, draught-proofing comes first.
The Union des Fabricants de Menuiseries makes a point that clarifies the whole subject: the ADEME estimate, which attributes 10 to 15% of losses to windows, does not account for air leakage around joinery already installed. The real share of leaks is therefore understated in the most quoted figure.
The condition that makes a curtain effective
It fits in one sentence, and it is almost always the explanation for a curtain that changes nothing: the panel must overlap the frame, not stop at the glass. A standard-width curtain leaves gaps along the sides and at the top, and air passes through them whatever the fabric weight.
The full reasoning is in insulating your windows from the cold without building work, and the particular case of the shutter box in fitting curtains with a roller shutter box.
Frequently asked questions
Can double glazing cause draughts?
Yes, and it is common. Glazing addresses conduction through the glass, not the air-tightness of the frame or of the installation. Recent glazing badly adjusted or badly fitted blows as much as an old one.
Does a curtain stop a draught?
It attenuates it markedly but does not remove it. It slows the air circulating along the window and cuts the sensation of a blow. The leak itself still has to be treated by draught-proofing.
Where does the draught at the bottom of a window come from?
Most often from the threshold or the sash seal, sometimes from the joint between frame and sill. That is the place to test first, because cold air, being heavier, naturally goes down there.
Should you draught-proof before buying a curtain?
If you feel a clear blow, yes: draught-proofing costs little and treats the cause. If the sensation is one of still coolness, the glazing is at fault, and the curtain is the right answer.
The season is gathered on our winter curtains page, and our made-to-measure thermal curtains are cut to overlap the frame, which a standard curtain does not.
More curtain tips
Explore our latest articles
