Insulating interior walls: how to go about it

When is insulating interior walls a good choice?

Many homes built before the 1920s have solid walls without a cavity. Cavity wall insulation is not possible there. On listed buildings or in a protected townscape, the outside often may not be changed either. That leaves the inside. The same applies to a garage or shed you want to use as a workspace or hobby room.

Internal wall insulation generally counts as one of the ISDE measures, provided you meet the requirements. Read more under ISDE subsidy for insulation. For the labour of insulating a home older than two years, 9% VAT generally applies.

Insulating interior walls with a lining wall: the build-up

Battens of timber or metal stud are fixed against the wall. Insulation goes in between, for example mineral wool or insulation boards. On the warm side comes a vapour control membrane, and plasterboard over that. That membrane is crucial: without it, warm, moist air from the room passes through the insulation to the cold wall and condenses there.

That is also how we did it when insulating a garage: mineral wool between timber battens against the brick wall, followed by a vapour control membrane and the lining wall. The difference between a furring wall and a separate partition wall is explained in furring wall or partition wall.

• Battens against the wall, timber or metal stud
• Insulation between the battens
• Vapour control membrane on the warm side, joints taped
• Plasterboard, then plastered or painted

Damp, cold bridges and loss of space

The weak points are at the junctions: where the lining wall meets the floor, the ceiling, a window frame or an internal wall. Cold can leak into the home there and condensation sometimes forms. By continuing the insulation a little way along the window reveals and taping the membrane properly, you limit that. Also check in advance that the wall itself is dry; penetrating or rising damp has to be solved first.

Expect to lose about 8 to 15 centimetres per wall, depending on the insulation thickness. In a small room that is noticeable, but the gain in comfort usually outweighs it by far. Sockets and radiators move along with it, so plan in the electrician.

If you want to save space, you can choose thinner boards with a higher insulation value per centimetre. They cost more, but in a narrow room or hallway that can be worthwhile. We discuss in advance what works best in your situation.

• Wall dry and free of damp problems first
• Continue the insulation along frames and reveals
• Tape the membrane airtight at joints and penetrations
• Plan electrics and radiators in advance

Insulating internal walls against noise

Sometimes it is not about heat but about sound, for example between a bedroom and the bathroom or between office spaces. In that case we fill the wall with acoustic insulation and sometimes use a double layer of plasterboard. Read more about walls under interior and partition walls.

Shahiro is happy to come and see which approach suits your home. If you are insulating the floor at the same time, see insulating a floor. You always receive a tailored quote.

Frequently asked questions