It is very cheap and easy to build in wall insulation at the beginning of a self build project compared with the complicated job of adding more later. It is only a few decades ago that insulation was not bothered with at all (cavity walls only had cavities to stop driving rain getting through the wall – they give practically no insulation).
Conventional wisdom does not yet exist on the subject because every few years the advantages of yet thicker insulation become apparent. So at what point does increasing the thickness become silly? As a minimum, to satisfy the current building regulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place) you will need approximately 125mm of a high grade insulation such as wood cellulose or mineral fibre (or some kind of equivalent to that if you are getting some of the insulation from insulating blockwork). However this looks small beer compared with the 300mm of high grade insulation which is used in PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. buildings providing a U valuemeasurement of how much heat escapes (or gets in). The units are W/sq.m./°c. see Insulation properties of about 0.1. This is the sort of insulation required to attain near zero carbonbit of a slippery fish. It tends to mean that a building uses no carbon (oil, coal, etc) to heat it (meaning in a 'net' way). It usually ignores the carbon which goes into building it (the embodied energy). See the page on Zero Carbon? standards which meet the EPBDEnergy Performance of Buildings Directive. This is the EU directive that all new buildings shall be "Nearly Zero-Energy Buildings’ from 2021.. The Building Regulations cover the minimum allowable U values in part L1A (for new dwellings) and part L1B (for existing dwellings)
Different insulating materials have different values. See Insulation properties.
Going beyond that sort of level of wall insulation produces diminishing returns and it becomes more important to consider air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard and the insulation values of other elements such as windows and doors.
There are six main factors which have bearing on how much insulation it is sensible to go for –
- the importance of limiting global warming and pollution from fuel
- the future costs of fuel
- relative cheapness vs. diminishing returns
- loss of space (this mainly applies to thick wall insulation rather than roofs or floors)
- other savings to be made from high insulation such as better use of space (on the basis that cold damp areas get underused) and less risk of damage through condensation.
- loss of light and view from windows (this mainly applies to wall and roof insulation)
Global warming – Whatever your views about the dangers of global warming, house building, unlike, say car manufacturing, is a very long term undertaking and therefore it makes sense to think in terms of hundreds of years rather than a decade or two. All extra insulation will contribute to lower fuel use in the future. This would be an argument for very heavy insulation.
Fuel prices – Although fuel prices may well increase substantially over the next few decades there may be a more distant future with cheaper clean fuel from new technologies such as nuclear fusion. But don’t hold your breath. Still an argument for heavy insulation!
Relative cheapness – Compared with the overall price of building materials and the price of energy, insulation is a minor cost so this is an argument for heavy insulation. The view taken with the Passivhaus standard is that a thickness of over about 300mm of a high grade insulation in walls starts giving diminishing returns. Rather than more insulation it is important to look at the form of the building and air tightness.
Loss of space – If buildable ground area is at a premium, which is the case with an awful lot of UK property then the cost of this space needs weighing against the benefits of extra insulation. This may be a major factor concerning the internal insulation of walls. See the section on wall insulation
Other savings –
With sufficient insulation it is often not necessary to have radiators in certain areas, particularly ‘open plan’ areas and circulation areas such as stairs. The extreme case of this is with Passivhaus design which is so energy efficient that no central heating system is required, with the consequent savings of probably £4000 – £5000. This usually means insulation thicknesses in the range of 300mm of cellulose or mineral fibre or somewhat more if insulating blockwork is used.
Cold areas attract condensation on surfaces which then tend to need constant decoration. Articles like books and fabric become mouldy.
There is an effective saving in the size of a well insulated house. People tend to avoid badly heated areas, especially if they are sitting for long periods and this makes parts of houses less usable. Most of us can think of a big old rambling house that is hardly habitable in winter.
Window problems – Very thick wall and roof insulation can considerably cut down on the amount of light which gets in through windows (and the view angle available). Overcoming this problem may involve having splays or reveals to the walls round the windows.
An online U-value calculator is available on this page by Kingspan