Can a 1990s house be insulated internally while keeping the existing façade?

Older insulation may have gaps or differ in thickness from what the owner remembers. Give those uncertainties a place in the calculation. If inspection leaves questions open, compare several plausible conditions. This shows whether internal insulation is likely to make an appreciable difference in the room.

In a cold bedroom the gable wall is one possible cause. Also check the window, the roof above and the heating controls. Record when the room feels cold and precisely where. Surface temperature readings and checks for draughts can help narrow down which parts need work. Insulating every wall may be unnecessary.

Look for damp at the plinth, balcony junctions and window sills. Repair leaks before lining the room. If inadequate ventilation allows water vapour to build up, insulation alone will not remove it. Plan repairs around both the construction and the way the house is occupied.

Adding insulation on the inside changes the temperature of the masonry between the two insulation layers. Assess how the masonry can dry and how airtight the room-side junctions will be. Before adding foil-faced PIR as a second vapour-resistant layer, assess heat and moisture movement through the actual wall construction.

Consider a wall with a reliably established U-value of 0.50 W/(m²·K). Its total resistance is 2.00 m²K/W. Adding 50 mm of insulation at an assumed λ of 0.022 W/(m·K) gives a theoretical U-value of about 0.23 W/(m²·K). This simplified example omits fixings and thermal bridges. The actual retrofit needs its own calculation.

Replacing flooring, services or fitted furniture is a convenient time to install internal insulation. The edges of the wall are accessible, the radiator can be moved and the reveals prepared. Keeping furniture in the room throughout limits access and makes it harder to form and inspect the junctions.

Allow for the full lining thickness. An insulated plasterboard combines some installation steps, but still needs space for substrate levelling, adhesive and finishing. A fitted wardrobe may need resizing and the sill adapting. Include these items in the estimate before settling on an insulation thickness.

Before closing the wall, locate old cables and patches where services were altered. Photograph the exposed surface using fixed points in the room as references. The record will help future installers locate routes without damaging the new lining.

Ring beams and balcony slabs can form significant thermal bridges in houses of this period. Insulating the flat wall area does not in itself resolve them. Check junctions with ceilings, floors and intersecting walls. Determine any additional insulation needed and how far it must extend from the geometry of each junction.

At windows, check sash clearance, reveal temperature and the seal to the frame. A finishing strip alone does not provide the insulation the detail requires. Even with a good U-value at the centre of the wall, a narrow, poorly insulated reveal can remain the coldest part of the room.

After the work, adjust the heating controls and monitor humidity. Keep the layer specification, photographs and calculations. If external insulation is added later, the designer will need these details to account for the internal layer already in place.

Assess energy use alongside outdoor conditions, indoor temperatures and heating hours. A single-room retrofit is unlikely to explain the whole change in the household heating bill. A warmer wall surface and a more comfortable room can be meaningful gains even where a percentage saving cannot be established.