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How well-designed buildings are both airtight and breathable

Words:
Sarah Lewis

There's a difference between airtightness and breathability and achieving both is what produces the best indoor air quality – as Passivhaus standards show

Old Holloway interior, straw bale Passivhaus.
Old Holloway interior, straw bale Passivhaus. Credit: © Juraj Mikurcik

Airtightness and breathability in buildings are two very different things. Buildings can be both airtight and breathable. A RIBA Journal article, Scotland’s new heat standard ups the zero carbon ante, contained a quote that suggested that the current focus on air tightness should shift to a ‘focus on principles of healthy building, such as making walls vapour-open and using natural insulation like hemp or sheep’s wool’.

Airtightness is about reducing unwanted draughts and heat loss through gaps in the building fabric, while breathability is about the diffusion of moisture through the building fabric. There are many Passivhaus projects built using natural, breathable vapour-open materials such as straw, clay, hemp, wood fibre or sheep's wool. The Passivhaus standard underscores the critical role of understanding building physics in the development of healthy, low-energy homes.

Build tight, ventilate right

All buildings should be airtight, but this is not the same as being moisture-closed, non-breathable or vapour impermeable. There are three main reasons why we need to make buildings airtight:

1)  It’s key to low operational energy in buildings, reducing energy bills and carbon emissions

Once the insulation levels in a building have been increased and the thermal bridges dealt with, heat losses from uncontrolled ventilation (ie air gaps in the building fabric) become significant. 

2) It prevents hidden condensation and mould in the fabric of the building, creating healthier buildings

Airtightness protects all the building fabric from moisture in the air. Movement of moisture by bulk air movement can carry far more moisture than vapour diffusion and if this air enters the building fabric, interstitial condensation may occur.

3) It helps prevent unwanted drafts, creating a more comfortable environment

Infiltration of air through a leaky building fabric is uncontrolled ventilation and can lead to drafts. When the air outside is colder than inside, this leakage can be very uncomfortable. Air velocity is one of the basic indicators of thermal comfort.

  • Parc y Rhodyn straw bale Passivhaus construction.
    Parc y Rhodyn straw bale Passivhaus construction. Credit: Hawland
  • Parc y Rhodyn straw bale Passivhaus
    Parc y Rhodyn straw bale Passivhaus
12

Natural ‘breathable’ materials

There are many benefits from using natural, breathable (vapour-open) materials – see www.asbp.org.uk for further information. The Passivhaus Trust website contains many examples of Passivhaus projects built using breathable vapour-open materials such as straw, hemp, timber fibre,  sheep's wool etc.

Not hermetically-sealed boxes

And of course you can open the windows in a Passivhaus. While occupants have this option, exceptional indoor air quality is primarily maintained by the mechanical ventilation with heat recovery (MVHR) system, eliminating the need for window opening in winter. With natural ventilation, bringing in sub-zero temperature air for ventilation is the only option on a cold winter’s day. In a Passivhaus, heat is recovered from exhaust air, providing fresh, filtered air at around 18°C without active heating.

An MVHR system is relatively easy to repair and maintain. In the event of an MVHR system breaking down, the house can be ventilated using windows until the MVHR system is repaired. There is never any danger to occupants as there would always be sufficient air to breathe, even with the high levels of airtightness of a Passivhaus.

Indoor air quality – 30 years of data

Indoor air quality is a key comfort criteria for the Passivhaus standard and the evidence of over 30 years of the standard bears this out:

  • A review of international research into the indoor environmental quality of Passivhaus homes found: ‘The quality of the indoor environment in newly built Passivhaus dwellings is comparable or better than other new low-energy homes, especially in relation to indoor air quality (IAQ), as buildings achieved higher air change rates. Concentrations of total volatile organic compounds (VOCs), particulate matter (PM2.5) and formaldehyde were found to be lower in Passivhaus dwellings.’
  • Analysis of reported CO2 measurements in international research shows that in homes which use the Passivhaus ventilation approach, peak concentrations are in general substantially lower than in conventional homes relying on natural ventilation and/or window opening.
  • In the UK, CO2 concentrations have generally been found to be lower in Passivhaus than in non-Passivhaus homes, and usually within recommended limits. In a 2016 study looking at homes with a variety of ventilation systems, the highest peak bedroom CO2 levels were seen in the homes without MVHR – almost one third peaked above 3000ppm (see graphic). Bedroom CO2 in most of the Passivhaus homes meanwhile remained below 1250 ppm even at the peaks; all averaged below 1000ppm, and most averaged below 900ppm through the winter.
C02 levels in bedrooms with and without MVHR. Source: Characteristics and performance of MVHR systems: a meta study of MVHR systems used in the Innovate UK building performance evaluation Programme. / Sharpe, Tim; McGill, Grainne; Gupta, Rajat et al. 2016. 103 p.
C02 levels in bedrooms with and without MVHR. Source: Characteristics and performance of MVHR systems: a meta study of MVHR systems used in the Innovate UK building performance evaluation Programme. / Sharpe, Tim; McGill, Grainne; Gupta, Rajat et al. 2016. 103 p.
  • Measurements of relative humidity data from a number of UK and Irish Passivhaus homes suggest around 45% RH as a typical level, in a range generally between 35-55% RH – that is, within the range usually recommended.
  • The combination of indoor relative humidity within the recommended range, steady, affordable warmth, and lack of cold surfaces indoors, greatly reduces the risk of condensation and mould in Passivhaus – this is an important principle underlying the standard. To date, no case of mould growth in Passivhaus certified dwellings has been reported in the research literature.

See the references in Health, wellbeing and people performance Passivhaus benefits supplementary paper May 2023, Passivhaus Trust (pages 34-5).

Sarah Lewis is research and policy director, UK Passivhaus Trust

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