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Wall insulation

reasons to build ecologicallyExisting buildings

Obviously if you have a cavity wall in an existing house (which became fairly standard after the 1950s) you can have insulation blown into the cavity and there are grants to help with this which you can get through your energy supplier. See the EST web site.

However this is only going to provide 50 mm. of insulation which would not bring you up to present day building regulation standards, never mind any kind of eco house standards. If you want to go further then you may want to consider external wall insulation or internal lining.

Whichever route you take it is important to make sure that you don’t create conditions which could cause interstitial condensation where moisture traveling through the wall condenses out within the thickness of the wall and creates damp patches or rot.

The other important thing is to consider the junctions between elements such as at the eaves. The insulation should carry on continuously at these places and any potential drafts should be sealed. This can get quite complicated if several types of building materials meet at complicated angles and configerations. It is often difficult to get good information on how best to handle this kind of thing and you may need professional help.

hemp insulation

hemp insulation

A fairly simple and effective way of cladding the walls of an existing building is with the Larsen Truss. This is basically a timber framework attached to the outside of the building and filled with insulation. It is then covered with an external cladding. Google images has a good set of examples, mainly from North America.

External retrofit insulation

There are several reasons you might consider external insulation on existing walls:

  • to avoid losing space internally
  • to avoid disturbing internal surfaces
  • to use the walls as a heat store/buffer to achieve a decrement delay
  • to avoid the ‘cooling fin’ effect caused where internal walls connect to external ones.

The above plan shows how an internal wall between two room, connecting with an external solid wall, causes a direct pathway for heat to escape out. External insulation prevents this but internal insulation has little effect. Of course cavity wall insulation isolates internal walls from external ones but 50mm gives a very low level of insulation in the first place.


External wall insulation is not straight forward. You probably need planning permission to add insulation to the outside of a building. Contact the local planning department at the council. Considering the work involved in insulating the outside of a building it makes sense to add as much insulation as is feasible while you are doing the job. 100mm would seem like a minimum, 200mm sensible and 300mm would mean you might be able to achieve Passivhaus standard. Of course the thickness you use depends to some extent on how effective the insulation is. (see insulation properties). This of course will most likely entail the repositioning of gutters, fall pipes and possibly gullies.

It will probably necessitate extending the roof downwards and outwards to cover the extra wall thickness and it may mean some fairly nifty detailing round openings, particularly windows. This may also have implications for the eaves level in relation to the tops of windows. If you are contemplating doing a loft conversion then combining it with external wall insulation might make a lot of sense.  You also need to give some design consideration to how the roof or loft insulation meets the new wall insulation to avoid cold bridging. Avoiding condensation and allowing traditional walls to breathe are also crucial.

So far very few already existing houses in the UK have been upgraded with external insulation though on the continent there is plenty of activity in this area. One of the few books on the subject is The Complete Guide to External Wall Insulation by Christopher Pearson.

New buildings

The minimum standards required by the Building Regulations for new dwellings are in Approved Documents part L1A

How much insulation?

It is very cheap and easy to build in wall insulation at the beginning 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 regulations 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 Passivhaus buildings providing a ‘U’ value of about 0.1. This is the sort of insulation required to attain zero carbon standards which the EU will require by 2020

Going beyond that sort of level of wall insulation produces diminishing returns and it becomes more important to consider air tightness 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 –

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 relatively 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 – (this mainly applies to thick wall insulation rather than roofs or floors) 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.

The density of the UK population combined with the tendency to increase the density of housing on brownfield sites (to preserve the greenbelt) means that building land is at a huge premium. Typically the cost of a building plot may be roughly the same as the cost of the building materials. When planning permission is given there are very tight restrictions on how much of the site can be built on (particularly regarding the building line at the front and how far you can extend back). The result is that the outside perimeter line of a house is restricted and if you want thicker wall insulation you have to make the rooms smaller. This reduces the floor area considerably.

Taking the example of a detached three storey 150 sq. m. house on a square plan. The loss of internal floor space (due to thicker insulation in order to achieve Zero Carbon or Passivhaus or AECB gold standard) would be about 7.5 sq. m. or 5%. This would represent wastage of about £7,500 in terms of how much space you end up with. (This is assuming a plot value of £150,000. which might not be untypical for much of the UK but could be much higher in the South East).

This factor alone (loss of space), may have a considerable bearing on the type of construction to use. A timber frame or SIPs approach ensures the maximum available space for insulation

A really well insulated Passivhaus wall will have about 300mm. of insulation (and consequently no need for a central heating system)

If you work to the latest Building Regulations using a traditional cavity wall you will probably end up with something like the following:

  • 112mm outer skin of brick
  • 125mm of cavity insulation
  • 100mm insulating breeze block inner wall

A total thickness of 350mm if you include plaster. Although this is better insulation than was the case till recent changes to the building regulations it is hardly eco-house standard. To bring this up toPassivhaus standard would require about another 125mm of insulation, giving a total thickness of 475mm. (when plaster is included). This is a very thick wall.

Compare this with a timber framed wall using an outer rain screen .

  • Rain screen of about 45mm
  • insulation of 300mm
  • internal lining 50mm. (allowing for services cavity behind an internal lining board)

A total of 405mm. This is 70mm thinner than a masonry wall to achieve Passivhaus standard. And of course this applies to all the external walls on all storeys so there is a considerable effective increase in useable floor area. There is also the issue of windows being less effective in very thick walls in terms of light and views.

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.

Problems fitting insulation

Poor fitting of insulation is a notorious problem on building sites.  There are five main types of situation where poor fitting can happen

  • poor jointing
  • gaps on the face of the insulation
  • areas missing
  • slump and settlement
  • insulation causing bridging of cavities

Poor jointing and gaps on the face

this is a particular problem where solid sheets of insulation such as polystyrene are used. In some cases it only takes a gap of a few millimetres between sheets to completely spoil the efficacy of the insulation. Take for example a traditional cavity wall which gets insulated using expanded polystyrene sheets and imagine that the sheets fit closely but have slight gaps because they were not cut perfectly or mortar snots hold them away from the inner leaf. 

As can be seen, the inner leaf is subjected to a stream of cold outside air and has virtually no insulation. Something similar happens with mineral fibre, especially at joints and edges. If a number of such cavities link up then this can form a huge source of escaping heat.

Areas missing

this tends to happen when there are areas which are inaccessible or where a spray in product such as warmcell is being used and it is physically impossible to reach the area. This can easily happen at the eaves or in complex areas where there are extra noggins. This problem needs addressing at the design stage.

Slump and settlement

this can occasionally happen with insulation such as Warmcell, polystyrene bead and any other poured or pumped insulation. In the case of Warmcell in floors it should be packed in quite tightly so that if it settles slightly then it is still in contact with the lower surface of the floor itself.

Bridging of cavities

if the wall design includes cavities then it is most important that they don’t get accidentally bridged by the insulation. This can happen particularly with mineral wool being badly fitted and the result is that moisture may find its way over from the outer leaf to the inner one.

The Building Regulations on cavity walls are in Approved Documents

see also – How much insulation?, | Insulation properties | Calculating insulation | Table of insulation valuesDecrement delay |

2 comments to Wall insulation

  • bobthebuilder

    Probably not but it depends on other factors. Under the current building regulations the walls need a certain amount of insulation but it might have been included somewhere else such as in the inner leaf of the wall. You could ask to see the drawings that the builder worked to or you could contact the building inspector to find out if it conforms

  • Eric

    I have just purchased a brand new brick built house and no cavity wall insulation has been used, is this allowed?

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