Higher standards of airtightness
Tackling the loss of heat through unintended (adventitious) ventilation has become one of the principal challenges for the house-building industry in recent years. Successive changes to Approved Document L of the Building Regulations (setting more ambitious energy and CO2 targets), more strictly defined ventilation provisions introduced through Approved Document F and the introduction of mandatory sample air permeability testing have all encouraged homes to be built to a higher standard of airtightness. The positive effects that improved airtightness should deliver on energy efficiency and reduction of CO2 emissions do, however, need to be balanced against the potential for a reduction in indoor air quality.
The trend towards MVHR
The transition towards airtight homes means that purpose-provided ventilation is now more necessary than ever before. Approved Document F was revised in 2010 specifically to cater even for homes that are completely airtight and which would need larger purpose-provided ventilation openings, with the potential to cause substantial heat loss. For this reason, ventilation options that are able to recover heat from the outgoing ventilation (exhaust) air have an obvious attraction. Evidence from a few studies point to the fact that, working correctly, MVHR is able to have a positive effect on IAQ and health, but this can only be expected to be realised in practice if the system is functioning correctly. The Zero Carbon Hub Ventilation and Indoor Air Quality Task Group considers that examples of failures in typical design, installation and commissioning practice are all too common and these will have the effect of reducing the performance of systems. Badly performing systems may not deliver the anticipated carbon savings and may result in degraded IAQ with related consequences for health.
Indoor air quality (IAQ)
Appropriate indoor air quality can be defined as the absence of air contaminants/pollution which may impair the comfort or health of building occupants and a principal reason for the ventilation required by Approved Document F is to control chemical, physical or biological contaminants in the air that people breathe. Those contaminants that may be present in homes include moisture, combustion by-products, emissions from building materials and furnishings, allergens including mould spores and particulates from cooking and cleaning products.
Previous desk research by the NHBC Foundation in 2009 identified a range of studies from the UK and other countries which point to a link between IAQ and health of occupants. The health effects include a range of serious conditions such as allergic and asthma symptoms, lung cancer, chronic obstructive pulmonary disease, airborne respiratory infections, cardiovascular disease. The report also noted the prevalence of ‘sick building syndrome’, symptoms of which include respiratory complaints, irritation and fatigue. Amongst the conclusions of a subsequent report by the World Health Organisation is that ‘sufficient epidemiological evidence is available from studies conducted in different countries and under different climatic conditions to show that the occupants of damp or mouldy buildings, both houses and public buildings, are at increased risk of respiratory symptoms, respiratory infections and exacerbation of asthma. Some evidence suggests increased risks of allergic rhinitis and asthma’.
There is currently very limited evidence in the UK of existing studies of IAQ in homes. Only a few homes built to contemporary standards of airtightness have been studied but, worryingly, these studies identified high levels of relative humidity and nitrogen dioxide in a significant minority of the homes surveyed and high total volatile organic compound levels in over half of the homes. Evidence from other countries was also reviewed and the Zero Carbon Hub Task Group concluded that many pollutants are present within the internal environment of homes and that these tend to be at their highest in new homes or homes that have been recently refurbished.
The interim report from the Zero Carbon Hub led Ventilation and Indoor Air Quality Task Group -Mechanical ventilation with heat recovery in new homes
Part F 2010 – where to start: An introduction for house builders and designers (NF37)