Air Tightness and Ventilation in UK Dwellings - A sensitive balance!

In the near future it will be interesting to see how our buildings are reacting to increased air tightness, together with ventilation.  In truth only time will tell if this is working when these buildings have been occupied and used for a number of years

Source: Source: http://www.brookvent.co.uk/

In order to tackle climate change the UK government have made commitments under the Climate Change Act 2008 to reducing greenhouse gas emissions by at least 80% (from the 1990 baseline) by 2050.  Buildings in particular have been identified as a means making significant reductions to meeting these targets: According to BBC News (2006), ‘Transport consistently grabs the headlines on climate change emissions but buildings pour out about half of the UK's CO2 - 30% from homes, 20% from commercial buildings’.  The Climate Change Risk Assessment (CCRA) completed an assessment of a variety of impacts of various sectors may need to prepare, which included the Built Environment: ‘The UK’s built environment includes: 27 million homes, commercial and industrial properties, hospitals, schools, other buildings and the wider urban environment. At the current replacement rate, around 70% of buildings that will be in use in the 2050s already exist.

Reduction of greenhouse gas emissions within buildings, particularly carbon dioxide, can be achieved in two broad categories, the first is the way in which we create energy and the second is the way in which we can conserve or limit waste of energy.  In an earlier article, why should we both with renewable technologies? (Link) I considered the former and emphasised the importance of installing and utilising renewable technologies within buildings, which will significantly reduce our dependency of burning fossil fuels and subsequently reduce carbon dioxide emissions.  For the purposes of this article however I want to briefly focus on conserving energy in buildings and in particular the important balance between air tightness and ventilation.

Source: Source: http://www.airtightbuilding.com/

In order to try to make buildings more energy efficient UK Building Regulation, particularly Approved Document L has evolved beyond all recognition of the last 15 to 20 years.  Those like myself who were in practice prior to this will remember a much shorter, single Approved Document (now there are four parts, L1A, L1B, L2A & L2B), where the most complex issue was how to navigate your way through a SAP calculation.   Nowadays the focus on external envelope is only part of the story and there is now a requirement to consider carbon dioxide emissions for example: under Approved Document L1A , all dwellings must be designed and built in a manner that their Dwelling Carbon Dioxide Emission Rate (DER) measured in kg of CO² produced annually per m² floor area is no worse than a defined Target Carbon Dioxide Emission Rate (TER), as well as air permeability (or air leakage).

Since 2006, Building Regulations in England & Wales and Northern Ireland have required mandatory air leakage testing of new homes. These regulations were further revised in England and Wales in October 2010. ‘Air leakage, air permeability and air tightness are all terms that refer to the uncontrolled loss of air from inside a building to the outside and the infiltration of air coming from outside to inside. This loss or gain of air through cracks, holes or gaps in the fabric of the building is often felt to us as draughts.

Achieving a good level of air tightness is important for the energy efficiency of the building. The benefits of improved insulation and more energy efficient heating systems are lost if warm air can leak out of the building and cold air can leak in. Poor air tightness can be responsible for up to 40% of heat loss from buildings’ Source: http://www.nhbc.co.uk/

The above statement makes a good point in relation to heat loss in those buildings with poor air tightness and that by ‘plugging the gaps’, so to speak, we can ensure that our buildings remain warmer for longer.  This in turn will mean that we will not need as much heating in our buildings and consequently reduce carbon dioxide emissions in the process.  The added other benefit should be a reduction in our energy bills.  Constructing or upgrading a building to meet these air tightness standards is all well and good but we must also consider a balance with ventilating a building.

Ventilation is an important consideration in buildings as it helps to control the internal environment in respect of health & comfort, control of condensation and humidity, discharge of emissions from fuel burning appliances and removal of any airborne pollutants.  Building Regulations Approved Document F provides requirements for ventilation rates in various rooms such as kitchens, bathrooms etc as well as rates for whole house ventilation. The Approved Document also provides examples of different forms of ventilation such as mechanical, background, passive stack etc.  Consideration of the Approved Documents will be crucial in firstly complying with Building Regulations and secondly ensuring that there is an adequate balance between air tightness and ventilation. It would be a mistake to consider each of these in isolation.  It is important to realise that the level of air tightness achieved within a building will have an important influence on the overall ventilation rates that will be achieved (higher levels of air tightness, lower ventilation rates) and the type of ventilation strategy that should be adopted.

In the near future it will be interesting to see how our buildings are reacting to increased air tightness together with ventilation.  In truth only time will tell if this is working when these buildings have been occupied and used for a number of years.  As an example; with the increased use of construction methods such as timber frame which is nicely concealed within a highly air tight environment it is easy to foresee problems in the future with timber decay and other common defects if the balance between air tightness, ventilation and indeed detailing is not adequately considered.  This is also the case for control of both surface and interstitial condensation (condensation that occurs within the fabric of a building), which are both influenced by air tightness and ventilation in conjunction with other factors. We will no doubt find out within the near future if we have got this balance right.

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