Insulating an Extension Properly

Why extensions run cold

An extension is a small room wearing a lot of outside. A rear kitchen extension often has three exposed walls, an exposed roof and an exposed floor — far more heat-loss surface per square metre of room than any room in the main house. Add the fact that many were built decades ago to whatever the regulations then demanded (or by a builder who treated insulation as optional), and the “cold extension” is not a mystery. It is geometry plus history.

Age is the quickest first clue. A pre-war lean-to, a 1970s flat-roofed kitchen and a 2010s rear extension were built to entirely different standards, and each fails in its own characteristic way — the older the extension, the more likely the walls themselves are the problem; the newer it is, the more likely the fault sits at the junction or in the heating layout rather than the fabric.

The good news is that the same geometry makes extensions rewarding to fix: treat a small number of surfaces and the whole room changes character. The key is diagnosing which of the four classic weaknesses you have — because they need different work, and paying for the wrong one is the expensive way to stay cold.

Failure one: the junction with the original house

Where the extension meets the old building, two structures of different ages, thicknesses and materials join — and insulation, if it exists at all, usually stops short of the joint on both sides. The result is a cold seam of masonry running floor to ceiling at the junction, and it behaves exactly like the cold bridges described in our cold bridging guide: the corner runs cold, condensation forms there first, and a grey-black mould stripe appears in the corner of the extension where it meets the original rear wall.

The fix is continuity. If the extension gets external wall insulation, the boards must return onto the original house wall — or the junction must be closed with a properly detailed stop bead and sealed joint where the neighbouring wall cannot be treated. Internally, the insulation carries round the corner onto the old wall for at least a short return, so there is no bare cold strip left at the joint. This is a few boards’ worth of material and a disproportionate share of the thinking on the job.

Failure two: the flat roof

Many extensions carry a flat roof, and older flat roofs were built with little or no insulation — sometimes a token quilt laid between joists above the ceiling. Heat rises into the roof void, meets a cold deck, and the void condenses; meanwhile the room below loses heat through what is effectively an uninsulated lid.

The right moment to fix a flat roof is when the covering is renewed: rigid insulation goes above the deck as a warm-roof build-up, keeping the whole structure on the warm side and eliminating the condensation risk in one move. We installed exactly this on a real job — a 17m² warm flat roof in Crystal Palace (SE19) at £6,735, one of the 35 installed jobs published in our 2026 London price report. If the covering has years of life left, insulating from inside is possible but must be designed around ventilation and vapour control — stuffing the joist void full and hoping is how flat roofs rot from the inside.

Failure three: single-skin and thin walls

Older lean-tos, converted outbuildings and cheaply built extensions can have walls of a single skin of brick — around 110mm of masonry between the kitchen and January. Even where there are two skins, extension cavities are often narrow, uninsulated or bridged with mortar droppings. First establish what you have — our cavity vs solid wall guide shows how to tell from brick pattern and thickness.

Single-skin walls get treated as solid walls: external wall insulation where the outside face is accessible and planning allows (often easy on a rear extension), or internal insulation where it is not — the trade-offs are the same as for any solid wall, covered in EWI vs IWI. On single-skin walls we lean heavily towards breathable or moisture-tolerant build-ups, because a single brick skin gets wet through in driven rain and the insulation system has to live with that.

Failure four: the cold floor

Extension floors are either a solid slab — often laid with no insulation under it — or suspended timber over a shallow, draughty void. A cold floor drags the whole room down: your feet read the floor temperature, not the air temperature, and a kitchen with an uninsulated slab feels cold at thermostat settings that keep the rest of the house comfortable.

Suspended timber floors can be insulated between the joists — typically £76–£110/m² across our priced floor jobs. Solid slabs are harder retrofits: rigid insulation over the slab raises the floor level, which collides with door thresholds and kitchen units, so the realistic moment is during a refit. Where a floor cannot sensibly be insulated yet, sealing the perimeter gap where floor meets wall still removes the worst draught for almost no money. Full options are on our floor insulation service page.

The honest caveat: sometimes the extension is fine

Not every cold extension needs its fabric treated. We survey extensions — particularly ones built in recent decades — where the walls, roof and floor are adequately insulated and the real faults are one bad junction with the original house, an undersized or badly placed radiator, or a large glazed door area that no wall insulation will change. In those cases the honest advice is small: detail the junction, fix the heating layout, and spend the rest of the budget where the house actually loses heat — often the original building, not the extension bolted to it.

That is what the survey is for. We measure, we tell you which of the four weaknesses your extension has — and if the answer is “none, really”, we tell you that too. Get a free, fixed, itemised quote and you’ll see exactly which elements we propose to treat, and why.

Extension insulation FAQs