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 wall insulation 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 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.

External wall insulation going on to the back of a house

External wall insulation finished

Why should I insulate my solid walls?

Solid walls are much more prone to heat leakage than cavity walls (even unfilled cavity walls). 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. For more detail on the layers - see Greenspec on internal insulatiion

External wall insulation (EWI) involves fixing a layer of insulation material to the wall, then covering it with render or cladding. The finish can be smooth, textured, painted, tiled, panelled, pebble-dashed, or finished with brick slips. See Greenspec on insulated render for more detail and diagrams.

EWI is generally best if you can do it, but there are pros and cons to each. EWI is generally cheaper for large homes with continuous walls but has a higher fixed cost. IWI can be cheaper for small homes. The right solution for you can depend on your future plans and the age and design of your house.

External wall insulation

  • Advantages
    • Usually less disruptive to install
    • Keeps the walls warm and dry so there is less concern about moisture
    • Gives a continuous layer of insulation which is a good thing: where there are gaps in insulation there will be more heat loss and potentially a risk of damp and mould
    • Gives you a high thermal mass which helps you keep cool in summer
  • Disadvantages
    • You may need to extend your eaves, window sills and reveals, adjust guttering and so on.
    • Disruptive to plants that are close to the wall.
    • Ideally you should move the windows into the insulation layer, to avoid thermal bridges.
    • Increases the size of your house - which can be awkward for example against a narrow alleyway.
    • Changes the appearance of your home (though you have a wide choice of finishes) and you may need planning permission.

Internal wall insulation

  • Advantages
    • Does not change the external appearance of the house.
    • Does not increase the size of your house.
    • Can be done a room at a time, for example starting with the room you use most often or is most cold.
  • Disadvantages
    • Loses floor space - you can minimise this with thin insulation, see How thick should my insulation be .
    • Messy to install - requires refitting electric cables, radiators and associated plumbing, other internal fixtures such as skirting boards and door frames.
    • Cornices can be reconstructed but this requires special skills.
    • Moisture problems are more of an issue but these can be minimised.

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

If you have plans to carry out other work e.g. redo the roof you can bring costs down by doing EWI at the same time - one lot of scaffolding and you can extend the eaves while doing the roof. If you are carrying out major refurbishment work inside internal insulation may be an easier choice.

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 30mm to 60mm of insulation to achieve this, depending on what insulation material you use. However, if this is impractical then the regulations allow a lower standard 0.7 W/m2K should normally be achievable. This is from the building regulations Part L1A, section 4 on renovated and retained elements).

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 happens at the fiddly bits - walls, windows, holes ...?

The ideal is to get a continuous layer of insulation around the whole house. Double glazed windows have their own insulation and the frames should have thermal breaks in them to minimise heat loss there too. It is the junctions which are hard to get right. If not done carefully you get pathways for the heat to bypass the insulation. Here are some diagrams showing the problems and some possible solutions. You can find a lot more examples in the Retrofit Pattern Book EWI details and IWI

These diagram shows a window set into a wall in plan view. In each case the inside of the room is on the left. Starting from the left we have:

  1. IWI leaving a thermal bridge (red arrow).
  2. IWI with thin insulation to minimise the thermal bridge
  3. EWI with the window moved into the insulation layer. The problem is analogous to that with IWI and you can fix it the same way as (2) by putting thin insulation on the reveals on the outside of the window; this is a different fix which gives you more light.

Internal and external wall insulation options around a window

These diagrams show the problem with the wall floor junction assuming a suspended floor. It is normal practice to stop the insulation above the DPC but below the floor. However this leaves a thermal bridge. You can fix this by insulating the floor (not shown), or by taking the insulation below ground level. You would need to use expanded polystyrene for the part below ground and make sure you do not block air vents to the underfloor void.

One option for detailing the wall/floor junction

You will have other holes in your wall for various reasons - water overflow pipes, cable services, electric services ... The smaller the hole the less impact overall in terms of heat loss. If there is room, the holes should be filled with something insulating such as a foam filler. Also they must be sealed to be airtight and watertight with tape and/or sealant. You do not want draughts coming through or your insulation getting wet.

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

If you have external insulation then your heating system will be heating your walls. They can store a lot of heat (they have a high thermal inertia) so it will take a while for your heating system to warm the house up – but the house will stay warm 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 during hot weather which is increasingly a problem due to climate change (see also Keeping Cool in Summer). It will also heat up quickly in winter which means you can take advantage of gas heating or other high temperature heating to only heat your house when you are in it. However, if you plan to use a heat pump in the future for low carbon heating, you are usually advised to run it steadily to get a better efficiency.

What sort of insulation should I use?

You would be wise to consult a retrofit adviser on this as there are several different kinds which are used in different circumstances.

For external wall insulation, expanded polystyrene (EPS) is normally the cheapest. It is also resilient to getting wet so it can be used below ground level which is idea for reducing thermal bridging at the wall/floor boundary. Phenolic foams or PIR may be used if space is an issue, as they are thinner for the same performance. If there is a concern about vapour transport through the wall - for example if you have lime plaster - then mineral wool is also possible, or various organic materials.

For internal wall insulation, phenolic foams are much more common than EPS because of the thickness issue. However, many people prefer to use natural materials such as wood fibre. This is particularly important for rooms where there will be a lot of humidity such as bathrooms and kitchens.

Where thickness is at a premium, very high performance insulation such as aerogel may be used but this is expensive. For example you might use it on window reveals or in places where there is very little room between a door and a wall. Aerogel is vapour permeable so you can use it alongside natural materials. The chart compares the thickness of different materials with the same overall thermal resistance. Aerogel and PIR are considerably better than the others.

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

Greater thickness means less heat loss, which is measured by the U-value. High values mean high heat loss so low values are good. A typical solid brick wall will be about 1.8 and you should normally be able to reduce this to 0.3 - a savings of 85%. To get there with phenolic foam you would need about 60mm. Add in a top layer of plasterboard and you are looking at about 75mm. However, there are diminishing returns. With 40mm of insulation - total about 55mm - you would get to 0.44 which still saves 76% of your heat loss.

If you use natural materials, you will need a greater thickness for the same heat loss savings see What sort of insulation should I use.

You do not have to use the same thickness everywhere, though if the thin places are too thin there is a risk of condensation and mould. Because of the diminishing returns of adding more insulation, you can often get by with just half the thickness. So if you have awkward places where space is limited, such as a wall against a staircase, then you could halve the thickness there accepting the higher heat loss or - or splash out on high performance insulation on those places.

If your rooms are small, you might choose a higher U-value such as 0.5 allowing thinner insulation to save on space. However, do not think you might add some more later. Of course you can, but you almost certainly will not. Having reduced your U-value to 0.5, the energy savings from adding a second layer of insulation are much less than the first time round while the cost and disruption will be the same.

Should I worry about moisture?

Moisture is bad for walls – especially if it freezes inside the wall. 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 often 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. 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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