Insulated garden rooms, warm through a Newcastle winter

A U-value tells you how easily heat passes through part of a building, such as a wall, roof or window. The lower the number, the better it keeps heat in.

The figure depends on the whole build-up of the wall, roof or floor. As well as the insulation, timber studs, cladding, air gaps and linings all play a part, and so does the detailing at corners and around windows. Two rooms with the same thickness of insulation can perform quite differently.

When you compare quotes, ask what insulation goes in the floor, walls and roof, how thick it is and how well the glazing performs. Be wary of one headline figure with no explanation behind it.

Where heat escapes

• Roof: warm air rises, so the roof needs the most insulation.
• Glass: even good glazing loses more heat than an insulated wall.
• Floor: heat is lost downwards unless the floor is well insulated.
• Gaps: unsealed joints let warm air out and cold air in.

The key is to give moisture a way out. People breathe out water vapour, wet coats bring it in, and a gym or studio can produce a lot. If it can't escape, it settles on the coldest surface, which is usually the glass. Newcastle's damp winters make this especially important.

Trickle vents in the window frames, plus windows you can open, handle most rooms. A garden gym, a room with a sink or a room used for long spells with the doors shut may need an extract fan or a heat recovery unit, which ventilates without losing much warmth.

The floor needs care too. A raised floor should have a clear, ventilated space beneath it, and a concrete slab needs a damp-proof membrane. Our installation guide explains how we choose between the two. In cold, damp spells, a little background heat keeps the room fresh.