If I achieve good Airtightness levels, will I need MVHR?

By Iain Fairnington – Technical Director, A.Proctor Group Ltd

This is an increasing question as to whether you require MVHR if your building is airtight. In a word, No, but it can be advantageous and to use a Technical phrase ‘it depends’!!

The levels of airtightness in UK construction regulations is constantly being lowered, arguably too slowly. Many people are now understanding that good levels of airtightness is a thermal improvement method, but are there knock on effects?
Iain Fairnington, Technical Director at the A. Proctor Group sits on BS 5250 committee which deals with Moisture Management and explained that the committee generally agree that the easiest way to avoid moisture build up is to put the heating up and open the windows. This allows any moisture to escape and avoids cold spots – Simple but not environmentally friendly. So, we need to look at how we manage the balance of Heat, Air and Moisture Movement.
When you have high levels of airtightness installation (low level airtightness) then this could lead to stale moist air being trapped inside a building, almost to “sweat” levels if the heating is not controlled. By introducing ventilation this moves the air out and replaces it with fresh air. Is this throwing out the baby with the bathwater when you have paid to heat that air and you are letting that warm air to escape while bringing in cooler air that needs heated up?

There are various ways to look at this including-

No ventilation system – here the airtightness is above 3 generally and only reliance is on trickle passive ventilation and mechanical ventilation in kitchens and bathrooms

MEV– Mechanical extract Ventilation- Here the ventilation system extracts moisture from areas such as kitchens, bathrooms, shower rooms etc., to the outside

MVHR– Mechanical Ventilation with Heat recovery- Here the air is extracted and any heat from this air is recovered

All have pros and cons and the ultimate decision will depend on a number of issues such as the need to filter air/reduce energy consumption or budget.
There is an old saying of “Build tight/ventilate right” which explains a lot in terms of the balance of getting this right. A self-build project may prefer to have an MVHR system and the owners be happy to change the filters every 6 moths or so, however a developer may not want to put this on the owner so may choose MEV. The MEV can be quieter and runs at a low pressure. The MEV can also be cheaper to install as not as much pipework is required.

Which ever system is chosen by the architect/developer or home owner, there is no need to be scared of airtightness levels being to low. There are solutions to ensure the building is healthy and efficient.

At the A Proctor Group we support the fabric first philosophy. We provide many solutions to build in energy efficiency and the moisture management. This can be seen in our Reflective membranes both external (Reflectashield TF) and internal (Reflectatherm) as well as our Wraptite which is an airtight vapour permeable externally applied self-adhered membrane for both walls and roofs. The Group supply high performance thin insulation systems for walls , floors and roofs which can also be used to reduce cold bridging in critical areas.

As with all decisions in life, it is all a balance of what’s important to you and others.

www.proctorgroup.com