Your suit isn't the problem. The lining is.

Every summer the same conversation happens at the bench, three or four times a week. A man says he cannot wear his suit in this weather, that he sweats through it by lunch, that wool is simply too hot for a Toronto July.

Then he opens the jacket and there it is: a shiny, slippery, faintly plastic-looking lining, because that is exactly what it is.

Wool is not the villain here

Wool is one of the better performing fibres in heat. It is hygroscopic, meaning it takes moisture into the fibre itself and releases it slowly, which is why a wool suit can absorb a surprising amount before it ever feels wet. Desert clothing has been wool for a very long time and not by accident.

What wool cannot do is breathe through a barrier. And a full polyester lining is a barrier: a woven plastic film covering the entire inside of the jacket, from shoulder to hem, sleeve to sleeve. Whatever the suiting does, the moisture reaches that layer and stops.

So a man in a beautifully open high-twist wool with a cheap polyester lining is, thermally speaking, wearing a bin liner with a nice coat over it.

The four-second test

Open your jacket. Look at the inside body fabric, not the suiting.

• Does it look shiny and slick, almost wet-looking, and feel cool and slippery like a windbreaker? That is polyester.
• Does it look matte, softer, slightly dry to the touch, more like a fine cotton? That is likely cupro or viscose, and you are fine.

Check the sleeve lining too. Some makers put a decent lining in the body and polyester in the sleeves, which is where a lot of the heat actually builds.

Why so many suits have it

Cost, and only cost. Polyester lining is cheap, it is dimensionally stable so it is easy to sew fast, and it is completely invisible in a photograph and on a hanger. Nobody rejects a suit in a shop because of the lining. They just quietly stop wearing it two summers later and never quite work out why.

It is one of the clearest tells of where a maker decided to save, which is why it belongs alongside the other tells in how to tell a good suit and canvassed versus fused. Fusing and polyester lining tend to travel together.

What Sam lines in

Cupro, usually sold as Bemberg. It is regenerated cellulose, so a plant-derived fibre rather than a plastic one, and it behaves accordingly: it breathes, it moves moisture, and it does not hold heat against you. It also happens to feel considerably better going on over a shirt.

The full account, including half-lining for the hottest months and what can be done about a jacket you already own, is on the linings page.

If you already own the suit

You are not stuck with it. Relining an existing jacket is ordinary work, and a good suit whose only real fault is its lining is very much worth rescuing. Bring it in and you will be told honestly whether the shell justifies it, because sometimes it does not and there is no sense pretending otherwise.

That conversation costs nothing, and it is a considerably better use of an afternoon than buying another suit that will do exactly the same thing to you next July.