Garden room insulation: building one you can use in January

Target U-values

Aim for a build that would satisfy Part L if it were habitable, even where regulations do not require it. In practice that means roughly 0.18 W/m²K in the roof, 0.20 in the walls, 0.22 in the floor and 1.4 or better in the glazing.

• Roof: 120–150mm rigid PIR between and over joists
• Walls: 100mm PIR in a 140mm stud, or mineral wool plus insulated sheathing
• Floor: 100mm PIR over the deck, with a continuous vapour control layer
• Glazing: argon-filled double glazing with a warm-edge spacer; triple only if the room is very exposed

Thermal bridging is what actually gets you

A room can hit its headline U-values and still feel cold if the frame is uninterrupted timber at every corner, around every opening and at the wall-to-roof junction. Insist on continuous insulation over the studs, or an engineered I-joist frame, so heat is not shortcutting through the timber.

Breather membranes, VCL and condensation

The order of layers matters: vapour control layer on the warm side, breather membrane on the cold side, and a ventilated cavity behind the cladding. Get this backwards and moisture from inside the room condenses inside the wall build-up. That is what causes the musty smell and mould people blame on the cladding.

Heating, cooling and ventilation

In a properly insulated 15m² room, a single 2.5kW air-source air-conditioning unit heats and cools the space for pennies per hour and dehumidifies in the process. Add trickle ventilation or a small heat-recovery unit so the air changes when the doors are shut all day — a sealed, well-insulated room with two people in it gets stuffy fast.