Internal glasswool wall insulation between studs. Timber itself can be a thermal bridge in a super-insulated home.
compatibility with door fittings must also be checked.
Applying internal wall insulation in between studwork. A vapour control layer is vital.
a single nail driven by a residents into the dry lining or vapour control layer behind the plasterboard can ruin it. One solution is a service zone separate from the plasterboard but this takes up more space. Can you guarantee residents will always locate and bang nails into the vertical studs?
External wall insulation has improved the appearance and added weather protection to the house on the left making it more energy efficient.
External wall insulation involves applying an insulating layer and a decorative weatherproof finish to the outside wall of a building. The external cladding or render is generally the major cost, so you want to maximize the amount of insulation to maximize the benefits.
Cross section of external wall insulation.
any dew point, where water vapour leaving the inside condenses onto a cold surface in the wall, will be nearer the outside.
it might be more expensive (£8,000 to £13,000), especially if scaffolding is factored in, unless carried out at the same time as other external work.
The Steico external wall insulation system uses interlocking waterproofed wood fibreboard over studs infilled with wood fibreboard batts, protected by two applications of a proprietary lime render.
When taking insulation into window and door reveals, whether internally or externally, use the maximum width allowed by the amount of frame.
External insulation is covered by some kind of render to protect it from the elements. Lime render is preferable as it is breathable. Acrylic renders are also widely used, but these, being plastic, are not breathable. Both are flexible to a point and can be coloured in any number of tints.
The path of the airtightness barrier around the skin of the building should be mapped during planning of the work, so that it joins up around all edges of the wall.
Airtight membranes with variable vapour resistance are known as intelligent membranes. These have the almost magical property of being able to resist vapour migrating into structural elements, particularly in timber frame constructions, to fight interstitial condensation over the lifetime of the building. They are recommended for solid walls whether the insulation is internal or external.
They consist of a sheet with various layers that allow or obstruct moisture-laid air from passing through it according to the relative humidity, temperatures and pressures on either side. For example, if a room has high temperature and humidity and it is cold on the outside it would prevent the moisture from travelling outwards and risk condensing on cold surfaces.
Conversely, when it is cooler on the inside and there is less humidity, moisture can gradually return to help dry out the interstitial space.
Andy Simmonds’ Grove Cottage, Hereford, the first Passivhaus standard retrofit in the UK. Andy is CEO of the AECB. The thermal image shows the benefit. Andy used a foot of expanded polystyrene to achieve this effect, which is literally “outstanding” from the houses either side.
Walls should have a U-value of less than 0.30 W/m2K to meet Building Regulations. Around 120mm of insulation will usually achieve this. Half of this value, i.e. double this depth, should on average be aimed for to achieve around Passivhaus standard.
Insulating external walls can save on average between £450 and £500 a year on heating costs, and around 1.8-2 tonnes of carbon dioxide emissions for a typical dwelling.
The payback on internal insulation is around 10-11 years, and on external insulation around 15-17 years on average, but this really depends on the amount of insulation applied, the way the building is occupied and the state of the rest of the dwelling, so this figure should be confirmed in individual cases.
In an earlier article, I compared and ranked the best thermal insulation materials. Of relevance to this topic is that walls should be able to ‘breathe’ in order to prevent condensation. This means that the best choice is natural materials, including wood fibreboard, mineral wool batts and rolls. Polyurethane, polystyrene and phenolic foam boards do not ‘breathe’.
So, is it best to insulate on the inside or the outside of an external wall? There is no one definitive answer to this question, as it depends on the particular case. However, all other things being equal, external will give better results than internal insulation.
You can find out more about external and internal wall insulation at green open house events in September. Speak to real homeowners as they share their personal experience of refurbishing their homes as part of SuperHome Open Days. SuperHomes are older homes refurbished by their owners for greater comfort, lower bills and far fewer carbon emissions – at least 60% less! Entry is free. Book now.
1. In conservation areas it is important to preserve the original facade of buildings. This rules out external wall insulation at the front. However, it may still be possible to install it to the rear or the side. Quite a few refurbished Victorian SuperHomes have both internal and external wall insulation installed in order to maintain the original facade – take these examples   .