A well written article in the RIBA Journal on air quality in Low energy buildings, by Justin Bere. Air quality in Passive Houses | RIBAJ
By their very nature Passive House buildings are well insulated with high levels of airtightness.
To avoid condensation build-up in such airtight buildings, stale humid air generated by the occupants must be extracted and replaced with fresh external air by way of a heat exchanger (part of a Mechanically Ventilated Heat exchange unit or MVHR).
Low energy buildings should be considered simple machines. All machines are an assembly of components working together. Remove a single component and the machine will fail.
If we are to reduce the carbon footprint of our buildings we need to understand this concept. Removal of the ventilation component of the machine will result in humid internal conditions. Left unchecked the building fabric will suffer interstitial condensation, lose its insulation properties and at worst, the building will decay.
MVHR as part of a low energy strategy. Tim Pullen has written a useful article in Homebuilding and Renovating magazine.
the article suggests the following savings as a basic rule of thumb:
20% energy saving: if you are looking to use MVHR as an energy saving measure, an airtightness of below 3m³/hr/m2 for a 20% saving. This level of airtightness can be difficult to achieve for older houses, requiring a deep retrofit.
5% energy saving: 5m³/hr/m2 for new build houses is the accepted threshold level for effective MVHR installation (the article suggests the saving of around 5%). It is worth considering that by 2025 the Building Regulations in England will require an improvement to the minimum airtightness to this figure of 5m³/hr/m2. you may consider exceeding this Building Regs requirement by a small amount in order to improve energy savings.
Given MVHR systems are relatively inexpensive to install and operate, an well designed MVHR system looks like a sensible strategy for hew houses, although self-builders should be aware there are hidden costs associated with making a building airtight (additional detailing and special attention is required on site) to achieve these levels. The 2025 Buildings Regulations will also require all new build houses to be pressure tested to show compliance.
There are still many benefits of installing MVHR on less airtight buildings; the air filters improve air quality, significantly reduces moisture (and associated mould), reduces allergies such as asthma and even allows clothes to be dried naturally in a designated airing room (normally located in the plant room where air is extracted from).
By continually replacing moist, polluted air from wet areas such as bathrooms and kitchens with drier fresh air from outside you are also reducing the risk of damage to the fabric of the property (roof, walls, ceiling and doors). This has been proven to increase the longevity of your home, reducing maintenance costs.
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