Retrofitting Insulation to Improve Energy Performance

Why Retrofit Insulation Is Different From Insulating a New Build

Fitting insulation into an existing British home is rarely a case of simply stuffing mineral wool into a cavity and walking away. New builds are designed from the outset with a continuous thermal envelope, a controlled ventilation strategy and materials chosen to work together. Older houses were built to breathe, to dry outwards, and to rely on a certain amount of incidental air movement. If you seal them up without thinking about moisture, you create a problem rather than solving one.

The goal of a good retrofit is straightforward: reduce heat loss through walls, roofs and floors while keeping the building fabric dry. Get that balance right and you will feel the difference in both comfort and fuel bills. Get it wrong and you risk trapped damp, rotting timbers and interstitial condensation that only shows itself years later.

Understand Your Building Before You Insulate

Before specifying any material, work out what you are actually dealing with. The age of the property, the construction of its walls, and whether it already has a damp problem will determine almost everything that follows.

• Solid walls are typical of pre-1920s housing and have no cavity. They absorb and release moisture readily, so they generally need breathable insulation that allows vapour to pass through.
• Cavity walls from the 1930s onwards can often take blown insulation, provided the cavity is clear, the brickwork is sound and there is no evidence of driving rain penetration.
• Timber frame and steel frame homes need careful assessment of the structural frame and existing sheathing before anything is added.
• Existing damp, leaking gutters and high ground levels must be fixed first. Insulating over a wet wall traps the problem inside.

A simple moisture meter reading and a careful look at the loft, sub-floor and any cold corners will tell you a great deal. If in doubt, a thermographic survey in winter reveals exactly where heat is escaping and where cold bridges sit.

Walls: The Biggest Single Source of Heat Loss

Uninsulated solid walls can lose more heat than any other part of the house. Internal insulation is often the practical choice because it does not alter the external appearance, though it does reduce room sizes slightly and requires careful detailing at floors, ceilings and window reveals.

For solid masonry, breathable options such as wood fibre, cork, hemp-lime or calcium silicate boards work well. They allow moisture to migrate through the wall rather than condensing behind a plastic vapour barrier. A typical build-up might be a parge coat or lime render to level the wall, a breathable adhesive, then 60–100mm of wood fibre insulation, finished with a lime or clay plaster. This keeps the wall able to dry inwards when conditions demand it.

External wall insulation is another route, adding a weatherproof skin of insulation and render outside the existing wall. It eliminates cold bridging at floor and ceiling junctions, keeps internal floor space intact and can transform a drab elevation. It is more expensive, needs planning consent in some cases, and demands meticulous detailing around windows, sills and rainwater goods.

Cavity wall insulation remains one of the cheapest wins where it is suitable. Mineral wool, polystyrene beads or blown bead systems all perform well when the cavity is clean and the exposure is not extreme. On very exposed western coasts, beads with a water-repellent treatment are usually the safer bet.

Roofs and Lofts: Quick Wins and Careful Detailing

Loft insulation is the easiest place to start and usually the best value for money. If you currently have 100mm between the joists, topping up to 270–300mm of mineral wool will cut heat loss dramatically. Keep the eaves ventilation path clear with proprietary rafter trays or baffles, and never compress insulation into the gap under the roof covering.

Where you are converting a loft or insulating a pitched roof between and above the rafters, aim for a continuous layer of breathable insulation, with a vapour-permeable underlay and a ventilated void above. Rigid wood fibre or sheep's wool boards sit naturally in older roofs, whereas PIR boards perform well but need an airtight, vapour-tight installation to avoid trapping moisture against the rafters.

Flat roofs are trickier. Warm deck construction places insulation above the structure, keeping the timbers warm and dry. Cold deck roofs rely on ventilation that is easily blocked or omitted in practice, which is why warm deck is generally the more reliable approach on a retrofit.

Floors and the Cold Bridge Problem

Ground floors over a ventilated void are a common source of draughts and cold feet. If you can access the sub-floor, mineral wool or wood fibre batts held in place with netting or breathable membrane make a noticeable difference. Suspended timber floors benefit from sealing gaps between boards before insulation is added, as air leakage often matters as much as conduction.

Solid concrete floors are harder to insulate without raising the finished level, but where headroom allows, a breathable insulation board topped with a limecrete or timber floating floor works well. Don't forget the junctions: where walls meet floors, ceilings and window reveals, a continuous layer of insulation or a thermal break keeps cold bridges at bay.

Getting the Details Right

Insulation only performs as well as its installation. Gaps, compression and poorly fitted edges can knock a third off the real-world performance of a wall or roof. Whenever you add insulation, think about airtightness at the same time, but never seal a breathable structure with non-breathable materials unless you have a designed ventilation strategy to match.

• Use breathable membranes and tapes that suit the wall build-up.
• Keep ventilation paths at eaves, sub-floor and roof voids clear.
• Detail window reveals and lintels carefully to avoid cold spots and condensation.
• Fix leaks, gutters and drainage before you insulate anything.
• Consider mechanical ventilation with heat recovery if you are making the house significantly more airtight.

Do this thoughtfully and a retrofit will reward you for decades: a warmer, quieter, drier home that costs less to run and protects the fabric it was built from.