Why Extensions Are Too Hot in Summer and Too Cold in Winter

The modern rear extension has a recognisable shape: bifold or sliding doors across the back, a roof lantern or a run of rooflights, and as few solid walls as the structure will allow. It is bright, it connects the house to the garden, and it is the reason most people build one.

It is also why the room is uncomfortable for a good part of the year.

Glass is the problem in both directions

In summer, a roof lantern faces the sky. The sun is at its highest and strongest in the middle of the day, which is exactly when a horizontal or shallow-pitched piece of glass is pointing straight at it. Even good glazing passes a substantial amount of solar energy, and the room behaves like a conservatory because, thermally, that is roughly what it is.

In winter the same glass is the weakest part of the building. Even a high-performance triple-glazed unit loses several times more heat per square metre than the insulated wall it replaced. A room that is mostly glass loses heat quickly, and keeps losing it all night.

So an extension has two opposite problems in the same space. Radiators address one of them.

What people try first

Blinds help, and most people resent them, because blocking the view is the opposite of why the extension was built. Lantern blinds in particular are expensive and awkward to operate.

Solar control film works better than people expect and is cheap, at the cost of a permanent slight tint and some loss of winter solar gain. It is a reasonable first step and it will not, on its own, make a south-facing glass box usable in August.

Underfloor heating is extremely common in extensions and it is good at what it does. It is also slow to respond, which suits a room in constant use and suits an occasionally-used room badly, and it does precisely nothing in summer.

The sizing mistake

Here is the practical thing to take away. If someone sizes a system for your extension by floor area alone, they will undersize it.

A four by five metre room with one window and a four by five metre room with bifolds across one side and a three metre lantern overhead have the same floor area and nothing like the same cooling load. Glazing area, orientation, the type of glass and the size of the lantern all have to be in the calculation.

It matters because an undersized unit does not simply underperform — it runs flat out continuously, never reaches the temperature you asked for, costs more to run than a correctly sized one and wears out sooner. When you get quotes, ask what the sizing was based on. The answer tells you a great deal about who you are dealing with.

One device, both problems

A reverse-cycle wall unit removes heat in summer and delivers it in winter, responds in minutes rather than hours, and heats or cools only the room it is in. For a space with a lot of glass and intermittent use, that combination is hard to beat with anything else — and it is one install, not two.