How to plan internal insulation for a house built in the 1990s

Where necessary, open up selected areas under controlled conditions. Check the thickness and continuity of old insulation and the condition of any cavity. Record deviations from the drawings. Thermal imaging can identify areas for inspection but cannot reliably identify every layer. Describe each wall zone separately.

Investigate leaking sills and gutters and damp near the floor before covering the wall. Internal insulation makes the old masonry colder. Its moisture content and drying potential therefore matter more than in a simple redecoration. Assess the substrate before setting a date to close the lining.

Check ventilation and heating too. Replacement doors or windows may have changed conditions, and established airing habits do not prove the current system works. Consider how the rooms are used, especially kitchens, bathrooms and bedrooms with extensive fitted furniture.

Include existing layers and the chosen PIR properties in the calculation. Assess moisture separately from the thermal target. Adding a vapour-tight lining to masonry with existing external insulation may significantly alter drying. Do not transfer results to an extension built differently.

Show ring beams, reveals and floor junctions in each option. In a two-storey house, continuity at the intermediate floor can be harder to achieve than across the wall face. A thermal bridge assessment may be needed there. Where data are uncertain, also calculate a less favourable case to see how sensitive the result is.

Choose stages that can be sealed at their boundaries. When renovating one wall, allow for its future junction with the adjoining room. Mark areas that will remain temporarily uninsulated and how they will be protected. Specify the end detail even behind a wardrobe or at a ceiling.

For example, two adjoining rooms along one façade could be insulated first if the design allows continuous work. This is not a universal sequence. A staircase thermal bridge or repairs on the side exposed to rain may take priority. Base the schedule on the survey and on how the house can be used during the work.

Before ordering boards, determine the new positions of sockets, radiators and sills. Allow for finishes and any service void. In older houses, cables and pipes may follow unexpected routes. Locating them reduces accidental penetrations and repairs during installation.

Heavy fittings need fixings into a suitable substrate. Detail the seal at every penetration. Check that windows still open once the reveals are thicker and that valves remain accessible. The complete lining may require a deeper sill even if the PIR board itself fits.

Photograph substrates, fixings and junctions before covering them. Keep material records and note any changes. At handover inspect perimeter seals, finishes and ventilation. If the proposed assembly changed during construction, reassess the aspects affected.

Monitor corners, reveals and areas behind furniture. When comparing energy consumption, account for the weather, room temperatures and number of occupants. A single mild winter does not prove all details are sound. Use observations from the first stage to correct problems before moving on to other rooms.