Insulation

Insulation is where many extensions quietly lose performance. The usual cause is not the product on the drawings but what happens on site: boards cut loosely, joints left untaped, or a cheaper product substituted that does not reach the U-value the building regulations require. Once the plasterboard goes on, you cannot check it.

These guides cover the insulation products used in a domestic extension and the smaller materials that decide whether they perform: PIR rigid boards for floors, roofs and walls where space is tight; mineral wool and sheep's wool for stud walls, lofts and acoustic separation; cavity batts that must sit tight without mortar drops bridging the cavity; EPS for ground floors; and insulated plasterboard for upgrading solid walls. Each page explains the lambda value, the thickness needed for Part L, and the installation details that make or break thermal performance.

Where to start

Read 100mm PIR insulation, cavity wall insulation batts and aluminium foil tape first. Between them they cover the floor, the walls and the joints, which is where most of the heat in a new extension is kept or lost. Then read Marmox Thermoblock, because the cold bridge at the wall-floor junction is decided at foundation level, long before anyone is thinking about insulation.

The guides, grouped by job

Rigid boards. PIR in 50mm, 75mm and 120mm as well as 100mm, with EPS as the budget floor board and grey graphite EPS where a thinner board helps, such as external wall insulation. For cold solid walls there is insulated plasterboard.

Quilts and batts. 100mm mineral wool for lofts and stud walls, and sheep's wool where you want a natural fibre and are prepared to pay for it.

Sealing joints and gaps. Low-expansion foam for gaps around PIR boards, compressible expanding tape around window frames, which has a direct bearing on your air test, and a vapour control membrane on the warm side where the specification calls for one.

Details that protect the insulation. Perimeter edge strip around screed floors, loft legs so boarding a loft does not crush the quilt, and pipe lagging for hot and cold pipework.

Where these fit in the build

The thermal break and floor insulation go in with the ground floor oversite and floor build-up, and cavity batts go in as the walls and blockwork rise. Most of the rest is fitted at the first fix insulation stage, before plasterboard, and the edge strip goes down just before screeding. The insulation calculator helps you estimate how much you need.