To understand the importance of MVHR heat recovery ventilation systems in low energy buildings it is first important to be clear about relationship between insulation and airtightness. While this may appear obvious, it is our experience that there is a great deal of confusion about the contribution the two make to energy efficient buildings, and their interrelationship.
The thermal envelope of a building is the shell which separates the inside climate from the outside. This is not necessarily comprised of the outside walls/roof of the building. This envelope is considered from two main aspects with regards to the energy efficiency of a building: insulation and airtightness. Though related, these are separate considerations and when building energy efficient houses both should considered at every stage of the build. While it is obvious that high levels of insulation of the thermal envelope will contribute building energy efficiency, the role of airtightness is often not fully appreciated. In more efficient buildings increasing airtightness can be as significant as increasing levels of insulation. It is therefore important that an airtightness strategy is adopted at the very early stages of the design process and implemented right through to the completion of the build.
Historically houses have been so leaky and poorly insulated that the effect of putting heat into a building was to create a temperature difference between inside and out resulting in a chimney or stack effect. The lighter and less dense warm air in the building rises, leaking out from the top of the building and drawing cold air through the bottom. Result: extremely good ventilation (cold and draughty), and extremely high energy consumption.
We then started paying attention to insulation and to some extent airtightness. Result: condensation on interior walls and mildew. Why? Because people and their activities create humidity, restricted air flow prevented the humidity from escaping, poor building techniques creating uneven insulation otherwise known as cold spots or thermal bridging. These cold spots provided places for the high levels of humidity to condense into water.
Recognising this problem the building regulations stipulated mandatory ventilation levels relieving the symptoms but resulting in greater energy loss.
Coming up to date and keeping the focus on ventilation we are now at the point that we have the knowledge of how to build houses that are both highly insulated and airtight, but people continue to produce humidity and odours that need to be dealt with, and of course people need air to breathe. Having gone to all the trouble of sealing and insulating the building an alternative to trickle vents and extractor fans was needed that could ventilate houses effectively without significant energy loss. Heat Recovery Ventilation (or MVHR) offers an effective solution.