Solid wall insulation
It is only relatively recently that the external walls of buildings have been insulated, and only since the 1920’s or 30’s that they have even included cavities between the inner and outer layers that could accept insulation. Before this, walls were largely uninsulated and solid. Buildings built in this way they tend to lose more heat from the inside to the outside and so can be cold, can suffer condensation and can be expensive to heat.
Typically the U-values (a measure thermal conductance) of walls are:
Insulating walls reduces heat loss and so the cost of heating. It also tends to increase internal surface temperatures, so reducing the likelihood of condensation. Insulating cavity walls is a relatively straight-forward process that involves filling the cavity with insulation. Insulating solid walls is more complicated, and so more expensive, involving insulation of either the inner or outer face of the wall.
 Internal insulation
External walls can be insulated by fixing insulation boards to their internal surface, or by building an internal stud-wall adjacent to their surface and then insulating the cavity between the stud wall and the external wall. Internal insulation is generally less expensive than external insulation, although it may still take more than 10 years to recover the cost in terms of reductions in energy bills (ref uswitch).
Installing internal insulation can be very disruptive, involving the replacement of skirting boards, door frames, electrical fittings, shelves, radiators, pipe work, coving and cornices. It also reduces the habitable space within the building. Metal or wooden stud wall constructions require the greatest depth and so result in the greatest loss of habitable space. For example, if the stud and insulation construction is 120mm thick, in a 3m x 3m corner room this would result in a loss of habitable floor area of more than 7%. Insulation boards are thinner, but more difficult to attach fixtures and fittings to.
Internal insulation can conceal damp problems, and so these must be dealt with before the insulation is fitted. In addition, where there are any gaps in the insulation, condensation problems can occur (see cold bridge). This means that particular attention must be given to the standard of workmanship, and care taken with joints and to insulate areas around windows and where internal walls meet external walls.
Some condensation issues can be mitigated by adding a vapour membrane to the inner surface of the insulation to prevent damp air reaching the cold external wall, however, care must then be taken with jointing panels (if the membrane is integral to the panel) and if holes are subsequently made in the membrane, these must be re-sealed.
 External insulation
External insulation is generally less disruptive than internal insulation and it does not reduce the habitable area. It can also improve durability, weatherproofing and acoustic insulation and can reduce condensation problems. However, it may need planning permission, and tends to be more expensive. The cost of external insulation may take more than twenty years to recover through savings in energy bills (ref uswitch).
As with internal insulation, external insulation may require repositioning of pipework and other fittings, as well as window sills and sometimes even roof overhangs. External passageways or driveways may be made narrower, and there is the possibility of elements of the building protruding beyond the boundaries of the site.
Care must be taken to insulate window recesses to prevent cold bridging, and as with internal insulation, any existing damp problems should be resolved prior to installation. Where there is a damp-proof layer, insulation should not begin until above that layer.
 Hansford report
In November 2015, the chief construction adviser to the government Peter Hansford published his recommendations for restoring the credibility of solid wall insulation.
He claimed that numerous examples of inadequate installation and poor workmanship have led to solid wall insulation developing something of a poor reputation.
His recommendations for restoring the credibility of solid wall insulation include:
- More robust accreditation of assessors and qualified installers.
- Development of guidance for assessors, designers, supervisors and customers.
- Establishment of a retrofit co-ordinator.
You can read about the report here.
 Related articles on Designing Buildings Wiki
- BRE and Willmott Dixon project to retrofit of a 1920s semi-detached house.
- Cavity wall insulation.
- Cold bridge.
- Defects in brickwork.
- Dry lining.
- Floor insulation.
- Green deal.
- Insulation specification.
- Roof insulation.
- Solid Wall Insulation: Unlocking demand and driving up standards.
- Thermal comfort.
 External references
- uswitch, wall insulation types.
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