Quick Answer: Warmth comes from trapped air, not from the outer fabric, so the lining and any interlining are what determine how warm a coat is. A single smooth polyester lining means the coat is as warm as its shell and no warmer. A quilted lining, a flannel-backed lining or a separate interlining adds real insulation. Check whether the lining is full or half, whether it is quilted, and whether the sleeves are lined smoothly - and check all three before you look at the price.

Two coats in the same wool at the same weight can differ enormously in warmth.

The difference is entirely on the inside, and it is never on the label.

Why does the lining decide the warmth?

Insulation is trapped air, not material. Whatever holds still air between you and the outside is what keeps you warm.

A smooth polyester lining traps almost none, so the coat is exactly as warm as its shell fabric and no more.

A quilted lining traps a great deal, because the wadding between the layers is mostly air held in place.

Flannel-backed and brushed linings sit in between, adding a genuine but modest amount.

Interlining is the invisible one - a separate layer between shell and lining, common in good coats and almost never mentioned in the description.

Which is why coat descriptions are so unhelpful: they list the shell composition and say nothing about the part that determines the answer.

What should you physically check?

Open it and look at the whole inside. A half-lining that stops at the waist means the lower body is shell only.

Feel for quilting, which is obvious immediately and is the single strongest warmth signal.

Check the sleeves are lined, and lined smoothly - an unlined coat sleeve drags over knitwear and makes the coat annoying to put on.

Look at the back vent and the shoulders, since a lining that stops short in either place is a cost saving with a comfort consequence.

Hold it up to the light - if you can see through the shell, the lining is doing all of the work and you should judge it accordingly.

And weigh it in your hands. Weight is a crude proxy for warmth and it is more reliable than any description on a website.

What else affects warmth besides the lining?

Coverage, more than anything. A longer coat is warmer than a warmer short coat, because the area covered dominates.

Closure design. A double-breasted front has two layers across the chest and no gap; a single-breasted one has a seam right where the wind is.

The collar, since an upright collar that closes at the throat is worth more than most people credit.

Cuff design. An open cuff lets air in with every arm swing; a tab or a knitted inner cuff stops it.

Fit over layers, because a coat too tight to wear a jumper under is a cold coat regardless of its construction.

And wind resistance, which is where tightly-woven and coated shells beat loose ones of the same weight.

Myth vs Fact: Coat Warmth

Myth: wool percentage is the main indicator.

Fact: it matters, and it is not decisive. A 70% wool coat with a quilted lining and a proper collar will out-perform a 100% wool coat with a thin polyester lining and an open neck. Composition is the number that is available, not the number that determines the answer.

Myth: heavier always means warmer.

Fact: weight correlates with warmth reasonably well, which is why picking a coat up is a useful test - but a heavy tightly-woven shell with no insulation is heavy and cold, and a light quilted coat can be considerably warmer. Weight is a proxy and proxies fail at the edges.

Myth: a lining is only there for comfort.

Fact: linings do three jobs - they let the coat slide over other clothes, they protect the shell from wear, and they insulate. Coats economise on the third one constantly, since it is the one you cannot see in a photograph.

Myth: you can add warmth by layering underneath.

Fact: only if the coat has room for it. A coat bought to fit over a shirt is a cold coat in December, and this is the single most common autumn buying error - trying the coat on in August over a tee.

Myth: quilted linings look cheap.

Fact: quilted linings appear in expensive coats routinely and are a mark of a coat designed to be worn in actual winter. What looks cheap is a thin shiny polyester lining, and that is the one people are less likely to notice.

Myth: the shop is the place to judge warmth.

Fact: you are indoors, you are acclimatised to the current season, and you have been walking around. Judge construction - lining, length, closure, cuffs - rather than how warm it feels, which in August tells you nothing at all.

FAQ: Coat Lining Questions, Answered

What is the fastest way to judge coat warmth in a shop?

Open it, feel for quilting, check the lining reaches the hem and the sleeves, then pick the whole thing up. Four seconds.

Is a half-lined coat ever a good buy?

For a transitional coat, yes - it is deliberately less warm and that is the point. For winter outerwear, no.

Can a lining be added afterwards?

A tailor can add or replace a lining, and on a coat it is expensive enough that it is rarely worth doing unless the coat is exceptional.

What is interlining?

A separate insulating layer between shell and lining. It is common in well-made coats and almost never listed in the description.

Why do my coat sleeves catch on my jumper?

Unlined or partially lined sleeves. It is the most common corner cut and the most annoying one day to day.

Should I buy a coat in August?

Stock and sizes are complete, so yes for shapes you are sure of - but try it over a jumper and judge construction rather than how warm it feels.

TL;DR:

• Warmth is trapped air, so the lining and interlining decide it, not the shell composition.
• Smooth polyester lining means the coat is as warm as its shell and no warmer; quilting means real insulation.
• Check the lining reaches the hem and the sleeves, then pick the coat up - weight is a crude but useful proxy.
• Coverage, closure, collar and cuffs matter as much as material.
• Try it over the jumper you will actually wear, since a coat too tight to layer under is a cold coat.

The part you are buying is the outside. The part that decides is the inside, and it takes four seconds to check.