Retrofit insulation is insulation added to an existing building, as opposed to insulation built in from the start. In houses it usually means the cavity wall, the ground floor, the roof and the glazing. The difficulty is that the structure already exists: the space available is limited, and the existing moisture balance must not be upset.
The four main measures
- Cavity wall insulation — mineral wool, EPS beads or foam is blown into the existing cavity through drilled holes. Quick, usually a single day’s work, and the cheapest measure per m²
- Floor insulation — boards, quilts or sprayed foam fixed to the underside of the ground floor, worked from the crawl space or cellar. Mostly removes cold feet and draughts through joints
- Roof insulation — between and under the rafters from inside, or above the rafters under a new covering. Working from outside costs more but gives a continuous, largely thermal-bridge-free layer
- Better glazing — double or triple glazed units in place of single or early double glazing; check that the existing frame can carry the heavier unit
Costs and returns
As a rough guide, and heavily dependent on the property: cavity wall insulation typically runs between £15 and £35 per m², floor insulation between £30 and £70 per m², and roof insulation from around £25 per m² (between rafters, DIY) to well over £100 per m² for insulation above the rafters with a new covering. Cavity wall insulation usually pays back within a few years; floor and roof insulation take longer. Grant schemes exist in most countries and change frequently — always check the current terms with the relevant national body before budgeting on them.
Points to watch
- Seal gaps first, then insulate. Insulation in a draughty envelope delivers far less than the calculation suggests
- Survey the cavity first. A borescope inspection should show the cavity width, debris, mortar snots, wall ties and any damp. A wall that lets driving rain through, or a cavity full of mortar droppings, is not suitable
- Ventilation has to keep up. A better insulated, more airtight house needs more ventilation, not less
- Vapour control. Insulating a roof from the inside requires a continuous, properly sealed vapour control layer on the warm side; without it, moisture condenses inside the build-up
- Work top down. Roof and glazing generally give the most comfort per unit spent
Related terms
- Cavity wall insulation
- Floor insulation
- Roof insulation
- Vapour control layer
- Thermal bridge
- Airtightness
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