Solid Wall Insulation

This page answers questions about solid wall insulation. This is for walls that don't have a cavity, or which do have a cavity but the cavity cannot be filled (for example if it is too narrow). For cavity walls see Cavity Wall Insulation

Solid walls are usually made from brick or stone but they can also be steel-frame, timber-frame or made from pre-fabricated concrete. Generally, the same advice applies but there can be special issues. For timber frame buildings you need to be particularly concerned about condensation so a hygroscopic insulation that absorbs moisture might be appropriate. These are usually natural materials and you will need a thicker layer for the same performance. For steel frame buildings the insulation needs to accommodate movement due to changes in temperature. If your wall is not a standard masonry wall, you should consider getting specialist advice.

For other types of insulation, go to the InsulationFAQ.

Why should I insulate my solid walls?

Solid walls are much more heat leaky than cavity walls even before they are filled. However, insulation can reduce heat leakage by 90%. You will be more comfortable, have lower fuel bills and you will have reduced your carbon footprint.

Unfortunately, insulating solid walls is far more disruptive than filling cavity walls and also a lot more expensive so you may elect to insulate just a part of the house, at least at first. It is a good idea to start with the north side as that is usually the coldest, or on the side that you use the most and need the warmth.

Solid walls can be insulated, either on the inside or on the outside. Internal insulation can be done piecemeal, one room at a time but external insulation needs to be done at least one wall at a time.

Should I insulate inside or out? Can I mix the two?

Internal wall insulation is done by fitting rigid insulation boards to the wall, or by building a stud wall filled in with a fibre insulation, natural or mineral wool.

External wall insulation involves fixing a layer of insulation material to the wall, then covering it with a special type of render (plasterwork) or cladding. The finish can be smooth, textured, painted, tiled, panelled, pebble-dashed, or finished with brick slips.

There are pros and cons to each and the right solution for you can depend on your future plans and the age and design of your house.

External insulation is usually less disruptive and keeps the walls warm and dry so there is less concern about moisture. However you may need to extend your eaves, window sills and reveals and it is disruptive to external plants, as well as pipework. External insulation transforms the appearance of your home (though you have a wide choice of finishes) and you may need planning permission. External insulation also gives you a high thermal mass – see below.

Internal wall insulation is typically cheaper to install, loses some floor space and is disruptive to internal cabling, internal fixtures such as skirting boards and door frames and pipework but does not change the external appearance of the house. Moisture problems can be more of an issue but are minimised by using an experienced professional. If your home is timber frame or has lime plaster then you should consider using a natural building material which absorbs moisture.

It is also possible to mix both internal and external for example using internal insulation at the front but external at the back. This may be a good solution if your home is of a heritage construction and has period features at the front but not at the back. Alternatively if you have plans to carry out other work e.g. redo the roof you can bring costs down by tying the two together. If you are carrying out major refurbishment work inside internal insulation may be the obvious choice.

If you do mix internal and external insulation, where they join you must make sure there is an overlap. Otherwise it is easy for the heat to wriggle between and you get a cold bridge.

Do I require Planning Permission or Building Regs?

Planning permission is about the look of your property and how it fits into the neighbourhood. Building regulations are about standards for safety, durability, heat management and so on. In most cases you only need planning permission if you are changing the external appearance. There is some guidance here on If your property is listed or is in a conservation area you should consult your local planning authority.

However, in all cases you do need to comply with the current Building Regulations. The main condition to meet is the thermal performance of the insulated wall - if you live in England or Wales then it must have a U-value of no more than 0.30 W/m2K. As a rough guide you will need around 60mm to 120mm of insulation to achieve this, depending on what insulation material you use. However, if this is impractical then the regulations allow a lower standard. The rule is '.. if such an upgrade is not technically or functionally feasible, the element should be upgraded to the best standard which can be achieved within a simple payback of no greater than 15 years.' (

Normally your installer will ensure that the insulation is up to standard and will arrange approval from the local Building Control Office for you. If they are not going to do this, you should contact Building Control at an early stage to make sure you comply.

If you are planning to remove and replace more than half of the internal plaster or external render of a wall, or if you are dry lining a wall, then you have to insulate to this standard whether you were planning to insulate or not.

What is thermal mass/inertia and why does it matter?

If you have external insulation then your heating system will be heating your walls, which takes a lot of energy. This means they have a high thermal inertia and it will take a while for your heating system to heat your house – but the house also will retain that energy for a long time. In summer, the insulation and thermal inertia will help keep your house cool.

If you have a low thermal inertia, for example if you insulate on the inside, then your house will heat up quickly in winter, saving on fuel bills. But it will also warm quickly in summer.

How much insulation do I need on my solid brick wall?

That depends on the material. For example, 100mm Celotex or Kingspan type material would reduce the U-value of a solid wall down to around 0.2. Alternatively you could use 170mm Thermafleece (mainly wool). If space is at a premium and building regulations allow you could accept a higher U-value e.g. 40mm ThermaLine Plus (for internal insulation, including plasterboard) should get you down to 0.65. Manufacturers (Celotex) will provide U-Values for different wall constructions.

Very high performance materials such as Spacetherm can be useful for small but critical areas such as window reveals. However, it is too expensive for most people to use on large areas.

Should I worry about moisture?

Moisture is bad for walls – especially if it freezes inside the wall, and it is bad for insulation too, as wet insulation is not very effective. Wet insulation will grow mould and can even rot.

Walls get wet from rain and damp on the outside and are exposed to moisture on the inside from people breathing as well as cooking, bathing and drying clothes. However uninsulated solid walls normally dry out naturally because the heating in the house warms the walls as well. Water evaporates away. If you already have a damp problem it is essential that this is fixed before you install insulation – whether internal or external. If it is not then your problem is likely to get worse.

External insulation on a solid wall can protect against penetrating damp from rain and provide a new weather proof layer to your home. There is a risk of bridging the existing damproofing layer causing rising damp (that did not exist before) and it is important that recessed windows are insulated effectively as otherwise you can get condensation there. However a reputable and experienced installer will know how to avoid these problems.

Internal insulation should not be overdone: with the insulation in place, the solid wall should still have some heat coming through to prevent internal condensation and avoid having freezing temperatures deep into the wall, as both can damage the brickwork. In most cases, going below an U value of about 0.4 needs careful consideration. In addition, the quality of the workmanship is key. The insulation must be installed evenly, with particular attention to edges of internal walls and sills that can easily get missed. If this happens you can get pockets where moisture builds up due to the difference in temperature. A careful workman will take extra care in these high risk areas.

However, no installation is perfect and in case moisture does get into the walls problems can develop which are hidden and hard to treat. Timber frame buildings are of concern because if the timber gets wet it can rot and you won't see it until it is too late.

The industry does not have as much experience with solid wall insulation as with cavity wall insulation and it is hard to be sure how rarely these problems will arise. Therefore some experts recommend that we should use breathable insulation and finishes to ensure that the moisture can dry out naturally. High performance polyisocyanurate or ‘PIR’ foam insulation such as Kingspan or Celotex is not breathable: rockwool, wool and wood fibre insulations are breathable but you will need a thicker layer to achieve the same U-value. Also, you will need a breathable finish such as lime, hemp or clay plaster on the inside or a breathable render on the outside, and if you use paint it must be breathable too.

How long will external insulation last?

External installation suppliers will be able to give you the details for the specific external insulation of interest, for example, Weber External Wall Insulation has a 30 year design life on existing properties, 60 year design life on new properties.

Can I still attach things to walls which have been insulated?

The simple answer is yes! However, in all cases when going through the insulation into the wall, you need to avoid creating a thermal bridge to the wall. That means avoiding metal fixings, if at all possible.

For internal insulation, the method you use to hang things will depend on the type of insulation you have opted for. Stud walls are strong enough to hold heavy fittings such as kitchen units, radiators or wash basins and are therefore the preferred option when refurbishing a kitchen or bathroom for example and you are taking the opportunity to insulate. If insulation board is used you need to ensure that your fixtures and fittings are long enough to go through the insulation board and into the wall behind. For hanging pictures, you can attach a picture rail to the studs or you can fix hooks into plaster board.

For external insulation, light and medium loads can be fixed with spiral anchors that screw into the insulation layer. Heavier loads can be fixed using plastic drill & hammer fixings with nylon centre pins. Ideally, fixings should be combined with stronger types of insulation materials at the time of installation, however this is only possible when you know where the strength will be needed. It is important that the insulation is protected from rain so be sure to seal around the fixings.

Where electrical cables are to be buried in polystyrene (EPS) insulation, make sure the cables are enclosed in conduits. PVC cable insulation in contact with polystyrene can go brittle over time due to plasticiser leech, which could result in sparks or short circuits within the flammable polystyrene!

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