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Insulation and wall thickness

See also – How much insulation?

With increasing levels of insulation, the total thickness of a wall becomes an issue in relation to land values, especially in densely built urban areas where space is restricted. The total footprint of the external walls of a house can easily be around 15 – 20% of the ground floor area and this becomes significant.

Even with present insulation standards walls can be very thick.

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

  • 112m m outer skin of brick (or 200 of stone)
  • 50 mm cavity
  • 75mm of insulation
  • 100mm insulating breeze block inner wall
  • 25 mm plaster

A total thickness of about 360mm 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 to Passivhaus standard would require about another 225 mm of insulation, giving a total thickness of 585mm. This is a very thick wall.

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

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 395mm. This is 190mm thinner than a (stone) 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 usable floor area. There is also the issue of windows being less effective in very thick walls in terms of light and views.

There is a peculiar and not instantly obvious relationship between wall insulation, population density and planning regulations.

Cost of land

The density of the UK population combined with recent decisions 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 (this includes the building line at the front and how far you can extend back along with how close you can build to the side boundaries). 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.

Loss of floor space

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).

Existing 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.

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 travelling 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 configurations. It is often difficult to get good information on how best to handle this kind of thing and you may need professional help. A nice example of upgrading internal insulation at an eaves and a window on an old stone building can be seen on a study of Gibson Mill

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

2 comments to Insulation and wall thickness

  • Michael Ballard

    Hi I am building a rear porch to accommodate a small toilet and a small butler sink. with a washing macine
    Clearly I am limited to 3 Square Metres externally. What is the minimum size wall that with an outside render that will meet the Regs.
    I am thinking of 150mm Solar Celcon aerated Blocks with Thermal XP Board Plasterboard on the inside, will this meet the Regs?

    Thank You


  • Mark

    I am trying to get planning permission at the moment and have submitted several pre-apps’ but keep getting knocked back on the basis of too big & overbearing. The planners keep making reference to the surrounding houses suggesting I make my plans the same size as those. The fact that I intend to build to Passive House standards with 500mm thick walls seems to have no bearing at all. It looks like I will have to build to their sizes or not at all and either accept smaller floor areas (than even the neighbours) to be eco or just build to current BR.

    There should be some policy guidance on this matter from central government. Not the wooly nonse published at the moment that can be interpreted any way the planners chose but a clear statement – if you build to standard X you can be X% larger!

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