How can I add internal insulation to a solid or 'hard-to-treat' wall?

Insulation can be fitted to the inside of solid masonry walls or 'hard-to-treat' cavity walls. This will of course make the rooms smaller, and there will need to be careful detailing (for example at windows and doors) to ensure 'cold bridges' (material 'bridging' from the cold side to the warm side of the insulation) are not created - as these could lead to mould growth and then perhaps damage to the building in the long term.

Conventional internal insulation is often with plastic foam boards, but natural & renewable alternatives are available - such as wood fibre boards that are easy to fit to a flat wall. Another option is to fit timber studwork to the walls to hold an non-rigid insulation material such as cellulose (for example recycled newspaper), and then finish with board & plaster (which can be clay or lime based). The details will vary for different materials, and vapour control layers may well be needed. the natural materials will not give as much insulation value as plastic foam, but a thicker layer could be used if possible.

If pushed for space, then using the less environmentally friendly plastic foam insulation in order to meet a decent standard will be better than no insulation. Where space is extremely limited, or around the reveals at windows & doors, a very high performance (& higher cost) insulation such as aerogel or vacuum panels could be used to avoid having cold uninsulated sections around windows.

Some practical tips:

The standard 'dot and dab' method of attaching sheet materials (e.g. insulation) to internal walls leaves lots of air movement in the space, which is not ideal. It is better to have a continuous parge coat on the wall to aid air tightness and put the insulation up against this. Or if not, at least have strips of adhesive rather than dots, so that air movement is restricted - i.e. adhesive around the perimeter of each board. If the existing internal finish is gypsum plaster, then you'd really need to remove this before adding insulation, and then add a basic breathable parge coat (e.g. just a basic application with a large brush). If the existing finish is lime plaster (and breathable) then it's OK to leave this.

Check if the insulation is certified for use without a vapour control layer (VCL), or if one must be used. Wood fibre insulation should be certified and so OK without a VCL. Foil-backed plasterboard is sometimes used, to give a vapour control layer (VCL) on the warm side of the insulation. However, it is hard to get a continuous layer with all the joints required - and this could then give problems. It may well be better to have a separate membrane as the VCL, and then a standard plasterboard or other finish.

Achieving air tightness by taping edges is important. The best quality (e.g. 'Pro Clima') tape is expensive (maybe £23 for 30 metres) but is strong, with strong adhesive. So it is useful for where the greatest strength is needed - at window reveals, corners, etc. Also where joists go into a wall - tape to the wall, then add the insulation layer (with gap cut for joist) and then tape insulation to the joists. At some flat junctions you could use a cheaper foil tape - such as the type used for sealing insulation to pipes.

Adding a continuous insulation layer to internal walls will mean cutting away a little of the ceiling finish (e.g. plasterboard) where it meets the wall. See taping of joists, above. You can get air leaks and heat loss between floors, from gaps where joists enter the wall - especially if there is a cavity.

The location of doors and windows and stairs can pose a problem when adding internal insulation. For example if the door or window is right up against a corner where two external walls meet, or if a staircase runs alongside an external wall. A window could perhaps be replaced with a smaller one (if windows need to be upgraded anyway), but reducing the width of a door or staircase is not so easy, so it will need careful thought about what approach and material will be suitable to achieve a decent insulation value. Some terraced houses have passages through, so there may be a need to insulate the walls along these passages as well.

In a kitchen, you're likely to want a finish that units can be fixed to. So a timber board (e.g. OSB) could be used to give a more solid finish over internal insulation.

Where an external wall meets an internal wall (e.g. for terraces/semis), internal insulation will need to be returned in a short way to avoid a cold bridge at the corner. Depending on the insulation being used, bring it in between 300 and 600mm. As this will leave a step in the wall, you could add a bookcase or something else at this point.

Click here for a few examples of internal insulation with natural/renewable materials. Please contact us if you need to find suppliers of some of these materials.

Further information:

Our short course on eco-refurbishment gives a lot more advice and hands-on experience for those interested in renovating a house and adding good levels of insulation.

If you want to find a builder who is knowledgeable about 'eco' building materials, try the Association for Environment Conscious Building (AECB) to find someone based near to you.

There is some more information on these webpages:

These websites also have information and examples of renovation projects:


AECB: the Sustainable Building Association -
Network of builders, architects, manufacturers, and organisations. Aims to develop, share and promote best practice in environmentally sustainable building. There is a list of members on the website.

Study at CAT: Our University Courses

Graduate School of the Environment - Sustainability Masters Courses

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