A U-value describes how quickly heat passes through part of a building, such as a wall, roof or window. A low U-value means good insulation and slow heat loss. A high U-value means poor insulation and fast heat loss.

Two buildings can look similar from the outside but behave very differently once you start heating or cooling them. Here is what changes.

Heating a poorly insulated building

In a building with high U-values, heat escapes almost as fast as it is put in. Think of an old warehouse with single glazing and an uninsulated metal roof.

It needs a large heating system to keep up on cold days.
It cools down quickly when the heating goes off, so it is often cold first thing in the morning.
Rooms can feel uncomfortable even when the air is warm, because cold walls and windows draw heat from your body.
Cold surfaces can lead to condensation and damp.

For a heat pump, this is the hardest situation. To push enough heat in, it may need to run at higher water temperatures. Heat pumps are less efficient at higher temperatures, so running costs go up.

Heating a well-insulated building

In a building with low U-values, heat stays in much longer.

A smaller heating system can do the job.
Temperatures stay steady, with less difference between morning and afternoon.
Walls and windows are warmer to the touch, so the space feels comfortable at a lower air temperature.
A heat pump can run gently at low temperatures for long periods, which is exactly how it works best.

Cooling: the other side of the story

In summer, a poorly insulated building lets heat in quickly through the roof and walls. It warms up fast on a hot day, but it also loses that heat fairly quickly at night.

A well-insulated building keeps outside heat out, which helps. But it also keeps heat in. Heat from the sun through windows, from people, lighting and equipment can build up during the day and has nowhere to go. Without good shading, ventilation or cooling, a well-insulated building can overheat.

So insulation does not remove the need to think about cooling. It changes where the heat comes from.

Insulation and thermal mass are different things

Insulation slows heat moving through a material. Thermal mass is the ability to store heat, as heavy materials such as concrete, brick and stone do. A building with both good insulation and plenty of thermal mass changes temperature slowly, which helps smooth out daily highs and lows. A lightweight, poorly insulated building reacts quickly to every change in the weather.

What this means for system design

Start with a proper heat loss calculation based on the actual U-values, not a rule of thumb.
Where practical, improve the building fabric first. Every bit of heat you stop losing is heat you do not have to make.
Size the heat pump and emitters to suit the building, so the system can run at the lowest sensible temperature.
In well-insulated buildings, plan for summer as well as winter: shading, ventilation and cooling.
Match the control strategy to the building. Slow-reacting buildings suit steady running rather than switching on and off.

The short version

Poorly insulated buildings need more heat and are harder to keep comfortable. Well-insulated buildings need less heat but can overheat in summer. Understanding how a building holds and loses heat is what lets us design heating and cooling that suits it.