2.2.5 Using insulation

Loft insulation

The most familiar use of wool-type insulation is in loft insulation. Since its first introduction into the UK Building Regulations for housing, the recommended thickness has increased from 25 mm in 1974 to 270 mm at present (2023) for both new build and for existing houses. Figure 10 shows the U-values resulting from different thicknesses.

Figure 10 Loft insulation thicknesses and U-values (Source: EST, 2002)

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This diagram shows a labelled ceiling supported from above by a number of labelled ceiling joists. The ceiling joists have a vertical thickness of about 100 millimetres. The ceiling is seen in section from the side. Different insulation thicknesses are shown in the loft above the ceiling with their U-values shown above in units of watts per square metre per kelvin. Vertical orange arrows of different thicknesses also indicate the relative heat losses. At the left an uninsulated ceiling has a broad orange arrow and has a U-value of 1.9. To the right a layer of insulation 25 millimetres thick is shown with a thinner orange arrow and a U-value of 1.0. To the right of this a layer of insulation 80 millimetres thick is shown with a thinner heat loss arrow and a U-value of 0.48. To the right of this a double layer of insulation is shown. The bottom layer is 100 millimetres thick and reaches to the top of the ceiling joist. A further layer 150 millimetres thick covers both the first layer and top of the ceiling joist. It has a thin heat loss arrow and a U-value of 0.16. At the far right a similar arrangement is shown but with the thickness of the top layer increased to 200 millimetres. This has a thin heat loss arrow and a U-value of 0.14.

Figure 10 Loft insulation thicknesses and U-values (Source: EST, 2002)

Loft insulation is usually sold in rolls. A ‘base layer’ of 100 thickness is rolled out between the wooden ceiling joists (Figure 11) and then a further layer 170 mm thick is added over the top, blocking the upward heat loss through the ceiling joists. This is likely to achieve a U-value of about 0.15 W m^-2 K⁻¹, a twelve-fold improvement compared to an uninsulated loft.

Figure 11 Rolling out a loft insulation base layer between the ceiling joists

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This is a drawing of a man kneeling inside a loft area on a board laid across two ceiling joists. He is wearing a face mask and rubber gloves. He is unrolling a roll of loft insulation and placing it between the ceiling joists.

Figure 11 Rolling out a loft insulation base layer between the ceiling joists

It is important to make sure that any insulation does not block gaps at the roof eaves, to allow air movement within the loft space (see Section 2.3). Insulation should be omitted under any water tanks in the loft space to allow heat from the house to reach them. They, and any water pipes in the loft space, should be properly insulated to make sure they don’t freeze in winter.

Cavity wall insulation in existing buildings

The walls of buildings can be insulated in many ways. In existing buildings with cavity walls insulation can be inserted into the cavity as long as the building isn’t exposed to high winds with driving rain, since the main function of the cavity is to stop damp penetration through the wall.

All brick walls may at first sight look the same, but on closer inspection the outer skin of a cavity wall, as shown in Figure 12, will be seen to be made up of bricks all laid side-on (stretchers). A solid brick wall will also include bricks laid end-on at right angles (headers).

Figure 12 Solid brick and cavity walls

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This diagram shows the difference between a solid brick and a cavity wall. Drawing ‘a’ at the left shows a solid brick wall. It shows that some bricks labelled ‘stretchers’ are laid parallel to the direction of the wall. Other bricks labelled ‘headers’ run right through the wall. Diagram (b) at the right shows a cavity wall. It has two skins or layers. All of the bricks are laid running parallel to the direction of the wall. They are labelled ‘all stretchers’. There is a labelled ‘brick cavity’ between the two skins.

Figure 12 Solid brick and cavity walls

Holes can be drilled in the outer brick skin and foam insulation can be injected into the gap, or rock wool or polystyrene beads can be blown into it. Typically this will improve the U-value from about 1.5 W m^–2 K^–1 to about 0.6 W m^–2 K^–1.

Internal insulation of existing solid walls

A typical solid brick wall two bricks (about 230 mm or 9 inches) thick has a relatively poor U-value of 1.4 – 2 W m^-2 K^-1. Such solid walls can be ‘dry lined’. This involves putting a layer of insulation on the inside faced with plasterboard. This is fairly simple to do (as long as the occupants don’t mind the disturbance to the interior of the house). Sheets of foam-backed plasterboard can be glued to the wall (Figure 13(a)), or alternatively insulated battens can be screwed to the wall with a layer of insulation between them and covered with a surface layer of plasterboard screwed to the battens (Figure 13(b)).

Figure 13 (a) Internally insulating a wall with foam-backed plasterboard glued to the wall (b) Internal insulation using mineral wool fitted between insulating battens and covered with plasterboard

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This figure has two parts. At the left part ‘a’ shows two people in a room holding up a sheet of plasterboard with a foam backing. It is to be glued to the wall surface. The wall has a window in it and the foam-backed plasterboard has a cut-out to accommodate the shape of the window. At the right part ‘b’ of the figure shows a photograph of the interior surface of a wall containing a window. It shows that insulated battens approximately 30 millimetres thick have been screwed to the wall. A layer of brown rockwool insulation has been fitted between them. Finally at the right a layer of plasterboard is shown screwed to the insulation over the battens. The battens and insulation are carefully shaped to fit the window opening.

Figure 13 (a) Internally insulating a wall with foam-backed plasterboard glued to the wall (b) Internal insulation using mineral wool ...

The U-values that can be achieved are mainly dependent on how much reduction in the interior room sizes can be tolerated. For example the 2013 UK Building Regulations suggest a target value of 0.3 W m^-2 K^-1. This would require the use of 75 mm of polyisocyanurate foam.

External insulation of solid walls

Alternatively solid walls can be externally insulated, usually with a thick layer of polystyrene foam which is then either covered with a layer of cement render or a special cladding layer (see Figures 14(a) and 14(b)).

Figure 14 (a) details of external insulation (b) external insulation advertised at a trade fair. The outer surface can be rendered as in (a) or even clad in a thin layer of imitation bricks

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This figure has two parts. At the left part ‘a’ is a drawing of details of external insulation. It shows the corner of a brick-built building. A labelled sheet of polystyrene is shown covering the brick. At the corner of the building a labelled vertical ‘metal corner strip’ is shown. A sheet of labelled ‘mesh reinforcement’ is shown covering the polystyrene insulation. The mesh is secured to the wall by labelled ‘nails’. At the bottom a layer of labelled ‘render’ is shown covering the mesh reinforcement. The diagram shows that the bottom of the brick wall is not insulated. This is labelled ‘damp-proof level’. At the right part ‘b’ of the figure is a photograph of such an insulation system. It shows a wall with an inset window viewed from the outside. At the top right, thin sheets of pink foam insulation are shown attached to the wall surface with special fixing clips. At the bottom right a mesh reinforcement is shown covering the insulation. Immediately below the window the outer surface is shown covered with a thin layer of imitation bricks.

Figure 14 (a) details of external insulation (b) external insulation advertised at a trade fair. The outer surface can be rendered as in ...

External insulation is commonly used in the refurbishment of tower blocks of flats. Although it is relatively expensive, it is possible to achieve good U-values of better than 0.3 W m^–2 K^–1 with 100 mm or more of insulation. The disastrous fire in the Grenfell Tower block in London, in summer 2017, has stressed the need for the overall design of such external insulation arrangements to be properly fireproof. Also, insulation materials should not give off toxic fumes when burning or in normal use (see Latif, E. et al., 2019).

New buildings

In new construction, brick walls can be built with cavities and insulation batts built in. If the building is in an area of driving rain then an air gap may also have to be included. Insulating aerated concrete blockwork can also be used to build the inner leaf (see Figure 15).

Figure 15 Insulation inserted into the cavity of a new wall (Source: EST, 2005)

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This is a drawing of a cavity wall. To the left the outside layer is shown as a skin of brick just one brick thick. To the right is a cavity shown filled with labelled ‘insulation’. To the right is a wall skin of labelled ‘aerated concrete’. Finally on the inside of the wall is a labelled layer of ‘plaster’.

Figure 15 Insulation inserted into the cavity of a new wall (Source: EST, 2005)

A wall U-value of 0.26 W m^-2 K^-1 is currently (2023) suggested by the UK Building Regulations. This can be bettered by making the cavity as wide as necessary (200 mm to 300 mm) to incorporate more insulation and, if necessary, to retain an air gap to prevent damp penetrating the wall. Timber frame construction can also use considerable thicknesses of insulation; 200 mm or more of wall insulation is commonly used in Scandinavia and Germany.

Floors

The floors of buildings can also be well insulated. Modern UK construction often uses thick sheets of polystyrene or polyurethane foam. In older buildings with suspended timber floors, sheets of insulation material can be inserted under the floorboards between the joists. The Building Regulations for England (HM Gov, 2021) suggest a floor U-value of 0.18 W m-2 K-1 for new buildings and for refurbishment projects. This is likely to require the use of polystyrene insulation more than 100 mm thick.