We love upgrading our homes to make them nicer, cosier, and easier to heat. But one issue is often overlooked, which can backfire and spoil the works: Research shows that upgrading a home through new glazing, insulation and draft-proofing triples the risk of mould — along with other health hazards that are less visible: increased dust mites, high CO2 levels above 1000ppm and harmful volatile organic compounds (chemicals).
The same counts for new build homes, which are now by default quite air tight.
It is therefore vitally important to have a fresh look at the ventilation strategy as part of any renovation works. Trickle vents are in most cases not enough to keep the air healthy and remove pollution and excess water vapour, unless the occupants keep their bedroom windows tilted all night and cross-ventilate regularly during the day. This might suit some, but the more popular solutions are based on constant operating fans that keep everything fresh: Either whole house extract systems, like MEV and dMEV or balanced systems like MVHR.
dMEV systems have become very popular with mass house builders due to their low cost. They are simple, constant operating extract fans in wet rooms and the kitchen. With the mass house builders embracing this cheap methodology, the Scottish Government instigated research into their effect, only to be surprised by the outcome: About half of them leave the occupants dwell in unacceptably poor indoor air quality.
The main reason for this downfall is quite simple: Whilst MEV and dMEV systems are designed to deal with moisture in wet rooms, the ventilation path through the habitable rooms is subject to change, depending if internal doors or trickle vents are open or closed.
Whist it is possible to specify dMEV systems better than the industry currently does, MVHR systems are the only form of ventilation where the indoor air quality can be safeguarded to a certain level. As MVHR systems not only extract stale air from wet rooms, but also supply fresh air to habitable rooms, the amount of air movement can be specified and adjusted to the needs of each room.
Surprisingly, there is currently no Building Standard in the UK that quantifies ventilation rates to individual habitable rooms to ensure a good outcome. Especially where it matters most, that means where people spend most of their time, a healthy living environment is vital: In master bedrooms, children’s bedrooms, home offices and living rooms. In reality, MVHR systems will offer a significant improvement of the indoor climate, but many will not achieve the levels that would be deemed optimal.
In search for better outcomes for the end-user, Stefan Huber of PAUL Heat Recovery has been looking at the way systems are specified by the time-proven Passive House methodology. “It’s the outcome that counts, not just the tick in the box. I can’t understand why most companies in the UK don’t look any further than achieving the minimum statutory requirements. In most cases a much better outcome can be achieved, even without any cost implication or only marginal ones.”
“I have retrofitted an MVHR system in my house, which was quite an undertaking, as I had to core through a few of brick walls. I could keep the layout quite simple with most ducting hidden in a suspended ceiling in the bathroom and a custom-made wall cabinet. Despite the effort, I don’t regret it one single bit. There is no mould any more in the bathroom or condensation on the windows and we can feel the difference that clean air has made to health and wellbeing.”
The question that is often asked is: “Does MVHR make sense in my house, as we are far from achieving Passive House air tightness?” There are four aspects to answering the question:
- Monetary-wise, MVHR will start to make sense, when the house is relatively airtight, i.e. with pressure test result is 5 m3/(h m2) @ 50Pa or less. Depending on the building fabric, that can be achieved in many retrofit scenarios.
- Health-wise, MVHR systems can make a difference, even if the house is not that airtight. Good systems with pollen filters have proven to be a great health-benefit for some people. In midge-infested regions, there is an additional benefit: These little beasties will stay outdoors, instead of having you for dinner.
- Humidity-wise, MVHR systems will powerfully tackle excess humidity in three ways: They extract at source, they create slow, but constant air movement through the dwelling and they will passively de-humidify at no extra cost.
- Noise-wise: If there is heavy traffic noise around the property, well-specified MVHR systems can significantly reduce indoor noise levels.
Whilst MVHR systems with ducting to all rooms is more suited for new builds, retrofitting these is not impossible. Whole house MVHR is more easily achievable in bungalows. What is important here, is that all supply and extract ducting in the cold loft is fully covered by the loft insulation, so that no heat is being lost.
The industry has also come up with innovative solutions that keep ducting to the minimum, such as de-central MVHR systems and Cascade MVHR.
In terms of ducting, we see a strong trend in Europe to use good quality semi-rigid ducting, with either 75 or 90mm outer diameter. These come in 50m rolls and are flexible enough to bend as needed.
PVC ducting, both round and rectangular is still the predominant ducting type in the UK, as it is relatively cheap, but it has some major disadvantages: Their sharp bends increase the resistance substantially and with it the energy use and noise. Also duct-cleaning, which might be needed after 10 years is almost impossible with such ducting.
As varying as the size and layout of various dwellings is, so is the potential solution for a ventilation system. The secret is finding a good ventilation expert, like PAUL Heat Recovery, that can advise you, specify and design systems professionally and offers a good range of good systems and ducting solutions.