Sunday, 4 December 2011

Ecohouse lesson 1 - Build Tight, Ventilate Right

A while back I borrowed a book called Ecohouse: A Design Guide from my friends at Pennine Camphill Community. I opened it at the beginning of the chapter on ventilation, and read:
The control of ventilation is one of the most subtle and yet the most important concerns of the building designer. How to make air move about the building in a way that satisfies, and even delights, the occupant.
I really liked this idea, and the suggestion that you should let the wind do the ventilating - and design your house around the prevailing wind conditions. I have heard however of things like the German Passivhaus standard where the house is completely sealed and airtight (and they say you can heat a whole house in the cold north for a year for £25 and the indoor air quality is excellent - there's even less dust!). So before I start thinking too carefully about ventilation it seems my first lesson is in an earlier chapter of the book, called 'Pushing the building envelope'.

Sure enough, on p.94 we read 'a sustainable building must be an airtight building'. Further:
It is apparently normal for up to 50% of the heat loss from new buildings in the UK to come from uncontrolled air leakage (Ecohouse p.92 quoting research from 2000) 
air passing through the external envelope - walls, floor and roof - of a building carries moisture with it. This causes significant damage in buildings, particularly to timber, and can substantially shorten the life of a building. 
Infiltration is the technical term ... for the uncontrolled movement of air through buildings. ... Excess air movement through infiltration:
  • Makes heating systems inadequate
  • Gives rise to cold draughts and discomfort
  • Increases fuel bills and CO2 emissions
  • Allows potentially damaging moisture to penetrate our building fabric
  • Reduces the effectiveness of insulation
The biggest pollutant we have to deal with in our dwellings is moisture - plain old H2O. Minimising the build-up of moisture helps prevent condensation and mould growth, and discourages dust mites. ... If we have open-flued appliances, such as wood-burning stoves, we need more ventilation for safety...
Mainstream UK construction has become much more aware of airtightness over the past five years, and the latest revision to the Building Regulations (England & Wales), which came into force in April 2006, will accelerate this trend. ... Anyone interested in sustainable construction should appreciate that the targets for airtightness ... are very lax. [They] set a maximum air permeability target of 10 for all new buildings... [A]ll you have to do to meet this Building Regulations target is to build a dwelling where heated air remains inside for 7 minutes in the hour when a 20mph wind is blowing! this is neither energy-efficient nor sustainable, by a substantial margin. Hence the advanced energy standards set by the Association for Environment Conscious Building, AECB, of an air permeability of 3.0 for their silver standard and 0.75 for their gold standard. The first of these is the lowest air permeability we would recommend for naturally ventilated buildings, whilst the higher gold standard is comparable to the German Passivhaus standard...
There's a lot more in this chapter. There's a lot about insulation (use 500mm in the roof and 300mm in the walls - or 'think of a number, then double it'; insulate the roof, not the top ceiling), about cold bridges and special constructions for avoiding them (window frame designs, nylon wall ties, timber sub-frames around doors and between lintels, etc), and about condensation (ideally install a vapour barrier on the inside wall and a breather membrane on the outside).

Other notes:
  • you need to really closely supervise builders if you want to eliminate air leakage as standard workmanship accounts for lots of problems (even simply dropping excess mortar inside wall cavities)...
  • self-build has advantages here :-)
  • need to carefully construct and seal every point where a cable or pipe goes through a wall or ceiling
  • draughts get in behind dry-lining, so better to apply wet plaster
  • pay particular attention to insulating corners: there are illustrations that show how teperatures drop more rapidly in corners; and additionally it's harder to insulate them because of inaccessibility - need to ensure any insulation properly installed - could think about thickening walls at corners...
  • use thermalite blocks between concrete floors and walls to avoid cold bridging


  1. Mmm... Wonder where you got this book from!

    1. Hey Raph. Yes it was you, thanks - good book x