Quick Answer: Buildings switch heating on by calendar and by the coldest morning of the week, not by the daily forecast, so from mid-September onward the office is frequently warmer than the street. Dress the base for the warmer of the two environments - indoors - and treat the outdoor layer as entirely removable. A sleeveless or short-sleeved base under a jacket beats a long-sleeved top under the same jacket, because the long sleeve cannot be taken off.

The air conditioning fight ends and the heating fight begins about three weeks later.

And the heating fight is harder, because you cannot remove a layer you are wearing as your base.

Why is the office suddenly warmer than outside?

Heating runs on schedules and thresholds, typically set once for the season rather than adjusted daily.

Building systems respond to the coldest recent morning, which in September is well below the afternoon high.

Thermal mass lags badly. A building that heated overnight is still warm at two in the afternoon regardless of what happened outside.

Occupancy adds heat, and offices refill in September after the summer gaps.

And thermostats are usually set for winter clothing, which nobody is wearing yet.

Which means indoor and outdoor requirements diverge for about six weeks, and the divergence runs in the opposite direction to what your instinct expects.

What should the base layer be?

Sleeveless or short-sleeved, because that is the only base you can be comfortable in at 23 degrees indoors.

A long-sleeved base is the trap, since it cannot be removed and you will spend the afternoon in it.

Breathable fabric matters more indoors than outdoors, as still warm air is considerably less comfortable than moving cool air.

Avoid heavy knits as a base entirely until the building settles, usually around late October.

And keep the base neutral, since it is what will be seen for most of the working day once the layer is off.

The base is the office outfit. The layer is the commute outfit.

How do you handle the commute at each end?

One layer that comes off completely and lives on the back of a chair or in a bag.

A jacket rather than a jumper, since a jumper has to go over your head and a jacket does not.

A large scarf for the morning, which is the highest warmth-per-gram item and packs into any bag.

Closed shoes, which handle a cold morning and an overheated office equally well.

And keep a spare layer at your desk if the building is genuinely unpredictable, which is a low-effort solution to a recurring problem.

Dress for the eight hours, carry for the ninety minutes, which is the same principle as autumn dressing generally.

Myth vs Fact: Indoor Temperature Dressing

Myth: you should dress for the outdoor temperature.

Fact: dress for wherever you spend the most hours, which for most people is indoors. The commute is ninety minutes and the office is eight, and yet almost everyone dresses for the ninety and adapts for the eight.

Myth: layering solves it.

Fact: layering solves it only if every layer above the base is removable. A long-sleeved top under a cardigan is one removable layer and one permanent one, which is why so many people are still uncomfortable after taking the cardigan off.

Myth: office thermostats are set for comfort.

Fact: they are set for a building, on a schedule, based on assumptions about what occupants are wearing in deep winter. Individual comfort is not really an input, which is why arguing with the setting is less productive than dressing around it.

Myth: natural fibres are always more comfortable indoors.

Fact: in still warm air, breathability and weave matter more than fibre origin. A dense cotton top can be warmer than a fine viscose one, and heavy knits of any fibre are the actual problem.

Myth: it evens out once winter properly arrives.

Fact: it does, largely - the outdoor temperature drops to meet the indoor one and everyone starts wearing winter clothes. The problem is specific to the six-week transition at each end of the year, which is exactly when nobody plans for it.

Myth: a desk fan is an overreaction in September.

Fact: it is a rational response to a building running on a calendar. Nobody who keeps one in September regrets it, and it costs less than any of the clothing solutions.

FAQ: Office Temperature Questions, Answered

Should I dress for indoors or outdoors?

Indoors, since that is where the hours are. Carry a removable layer for the commute at each end.

Why is the office warmer than the street in September?

Heating runs on schedules and responds to the coldest recent morning, not to today’s afternoon high.

What is the best base layer?

Sleeveless or short-sleeved in a breathable fabric. A long-sleeved base cannot be removed, which is the whole problem.

Jacket or cardigan?

Jacket. It comes off without going over your head and it hangs on a chair without creasing badly.

When does this settle down?

Late October in most places, when the outdoor temperature drops to meet the indoor setting.

Is it worth keeping a spare layer at my desk?

Yes, for genuinely unpredictable buildings. It is the lowest-effort fix for a problem that recurs daily for six weeks.

TL;DR:

• Heating runs on a calendar, so from mid-September the office is frequently warmer than the street.
• Dress the base for indoors, where the hours are, and treat the outdoor layer as fully removable.
• A sleeveless base under a jacket beats a long-sleeved base, because the long sleeve cannot come off.
• A jacket beats a jumper for a layer, and a large scarf covers the morning at almost no weight.
• Keep a spare layer at your desk if the building is unpredictable - it is the cheapest fix available.

You are dressing for two climates now, and only one of them is on the forecast.