Insulated Garden Rooms: Warm in Winter, Cool in Summer

Every supplier says their garden rooms are insulated. Very few will tell you the U-value, and fewer still will talk about airtightness - which is where most comfort is won or lost.

Four things working together

• Insulation - continuous through floor, walls and roof, with junctions detailed to avoid cold bridges
• Airtightness - a taped, continuous membrane so warm air is not simply leaking out around openings
• Ventilation - controlled fresh air so an airtight room does not get stuffy or damp
• Shading - overhangs, pergolas or solar-control glass to stop summer overheating

Get all four right and a twenty square metre room can be warmed in minutes and stays comfortable through a heatwave. Get only the first right and you have a well-stuffed box that is draughty in January and unbearable in July.

Condensation and the vapour barrier

Warm, moist internal air meeting a cold surface is the mechanism behind most garden room failures. A vapour control layer on the warm side, insulation without gaps, and a ventilated cavity on the cold side gives moisture nowhere to condense inside the structure. This is invisible work, and it is the work that matters.

Floors get forgotten

A cold floor makes a warm room feel cold. Insulating the floor deck to the same standard as the walls, and lifting the structure clear of the ground with proper ventilation beneath, changes how the room feels on your feet in winter. Underfloor heating is optional; an insulated floor is not.

Running costs

A well-built room used as a daily office typically costs very little to heat because it is small and the fabric holds the heat. That is the argument for spending on the envelope rather than on a larger heater: you pay once for insulation and every month for kilowatt hours.