How to find window air leaks: the incense stick and candle trick

Finding window air leaks: incense, a candle and common sense

An incense stick, a round candle with an aluminium base and a lighter. That's all you need to know whether your window really closes properly or whether the outside air is getting in rent-free. The trick is so simple it's almost annoying: if the smoke moves when you bring it close to the frame, there's a draught; if the flame leans, air is coming in from outside. And if outside air is getting in, your heating is working to warm up the street.

Diagnosing the problem, however, is the easy part. The hard part comes afterwards: what to do about that leak and why it exists.

Why air comes in even when the window seems closed

The problem rarely lies with the glass. It lies in the perimeter, in the roller shutter box, in the metal sill, in the joint between the panels of a sliding window or in the terrace door. Sliding windows, by design, leave gaps at the top and bottom that no weatherstrip can fully seal. Some point the finger directly at the installation: if, when fitting the window, the fitter didn't shim it to avoid the thermal bridge, didn't seal it with polyurethane foam and didn't finish it with silicone, even the best frame on the market is useless.

The installation recipe discussed here is mechanical: level, shim, screw, foam the perimeter, trim the excess and apply silicone. Skipping the foam step is standard practice in a botched end-of-job installation, and you can detect it by removing the shutter's tape cover with a screwdriver: if the plastic box comes out with no resistance, there's no insulation behind it.

Does uPVC insulate better than aluminium?

Yes, and for a physical reason that no sales pitch can argue with: aluminium is a metal, and metals conduct. A thermal break mitigates the problem, but it doesn't turn the frame into an insulator. uPVC, on the other hand, withstands cold and noise better. As for the glass, double glazing with an air gap and low-emissivity coating adds to both comfort and your bill; 6-4-6 glazing is cited as a specific solution for reducing outside noise.

The double window alternative —a second sash on the inside— works and costs less than replacing the whole opening, but there's a catch: it's awkward to clean, and the joint between the two windows remains a point where air sneaks in. A patch, not a solution.

Tilt-and-turn windows and the rest of the house

A tilt-and-turn window seals against the frame and lets you ventilate without taking up space, with the added advantage of opening the whole sash at once. But in winter you don't sleep with it open: air, noise and dirt come in. And there lies the most repeated mistake in approach: replacing the window without touching the rest doesn't fix the home. The façade, the air cavity, the walls, the roller shutter box, the front door, the ventilation ducts and the metal sill all form a system. Improving one part of a broken engine doesn't get it running.

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The practical conclusion is uncomfortable: the incense detects the leak, but it doesn't explain it. And most leaks aren't where the owner is looking. They're where the installer skimped on a bit of foam.