A breath of fresh air

Eliot Warrington of Solarcrest explains how airtightness will affect any building, whether clients want to build a certified Passivhaus or just meet Building Regulations

UK Building Regulations are really promoting airtightness and it’s now mandatory to pressure test every new home as it has clear benefits for energy efficiency, heat conservation and therefore carbon reduction, the underlying driver of change.

But this march towards airtightness has come at a cost to those properties with poor ventilation beyond unsightly damp and mould patches. With black mould now linked to triggering and exacerbating asthma in children, it’s not just the health of the home that is under threat.

In terms of minimum ventilation requirements, the inclusion of window ‘trickle vents’ and extract-only fans in bathrooms and kitchens is still permitted. The problem is, in an average three-bed property the total ‘free area’ of these vents is equivalent to around one square foot. It’s like having a window wide open all year round. Close the vents to stop cold draughts, odours, outdoor pollution or street noise and you stop your ventilation. Leave the vents open and what’s the point of building an airtight house? Trickle vents and extract fans are not compatible with low energy, healthy housing.

MVHR is a simple concept. Working as a central extractor fan to draw damp air out from every wet room, an MVHR recovers over 90 per cent of the heat from that stale air and uses it to warm fresh air, which it supplies to every habitable room. That way, in a well-sealed property the heating only needs to lift the air temp from maybe 19°C to 21°C, instead of from zero to 21°C. Best of all, with all incoming air passing through a single duct instead of dozens of trickle vents, filtration can be incorporated to clean the supply air to stop outdoor pollutants from entering the home.

Every year in the UK around 40,000 early deaths are attributed to poor air quality. Opening windows to let in ‘fresh air’ is a myth if you live in a town or city as the air outside is anything but fresh. Even in the countryside the outdoor air is dusty, pollinated, damp and more often than not cold. With every other kind of ventilation, whatever’s outside gets dragged inside. Only MVHR can clean, warm and dehumidify it. Sadly, Building Regulations allow developers to ‘get away’ with unfiltered trickle vents, even on busy roads, regardless of the associated health problems. Not every local authority has a pro-active Environmental Health Officer with the power to overrule building control and planning when needed, although the trend is changing now the lawsuits are starting to come through.

But no matter how bad outdoor air quality gets, indoor air quality is always worse with carpets, furniture and manufactured timber products all off-gassing Volatile Organic Compounds. Taking indoor air pollutants out is as important as stopping outdoor pollutants from coming in. MVHR takes it out without replacing it with outdoor pollution.

So, an MVHR provides quality air at near room temperature for no more energy cost than running a fridge, forever. In fact, the running costs are less than zero when you deduct the value of the warm air it recovers. MVHR in a three-bed home will typically save over £100 pa on the heating bills. But what else can it do? In two words… “acoustic attenuation.”

More and more councils are imposing sound mitigation conditions on planning applications, particularly in towns and cities or for properties by busy roads. Approved Document E ‘Resistance To Sound’ is the official regulation. Typically, flanking sound from the road enters the property through gaps and cracks in the fabric, through window trickle vents and in particular, through the eaves then into the roof space and then through the ceiling of top floor rooms. Mitigating unwanted sound ingress is another good reason to aim for airtightness.

MVHR ducting can contribute towards sound issues or it can attenuate them depending on the system. Branch ducting is the cheapest and therefore the most common type of ducting, but it’s also the least effective when it comes to attenuation. It’s almost impossible to clean and is the most difficult to hide given its size, especially if you need to include inline silencers which can be 250 mm diameter or more. Branch ducting tends to be full of 90° bends, which cause turbulence in the air, creating an unwanted low pitch hum, especially in the rooms closest to the MVHR plant where air velocity is greatest. Sharp bends also add static pressure to the system, meaning the fan unit needs to work harder to force air through the pipes. Another acoustic drawback with a branch system is that each room tee’s off a single branch pipe, which allows crosstalk between rooms. Fortunately, there is a solution and it’s called radial ducting.

Radial ‘semi-rigid’ ducting has a much smaller diameter, typically 75 mm, so is easy to hide. Silencers are not necessary. Each room has its own dedicated pipe rather than teeing off a branch, thus preventing crosstalk. Each pipe connects to a manifold normally located close to the MVHR unit, ideally an attenuating manifold. Turbulence is eliminated by the avoidance of sharp 90° bends, instead radial bends tend to sweep around corners. Air movement noise is avoided too because the air velocity in each pipe is much lower, just enough to serve the room at the end of the pipe and not the whole building. Static pressure on a radial system is typically around half the equivalent branch system, meaning the MVHR unit can more easily circulate air. Finally, the best radial ducting has an anti-static anti-bacterial lining to prevent dust and grime building up.

As with anything, with ventilation you get what you pay for. There is certainly a big difference between a budget system designed to get through the regulations and a premier league system designed to get through the next 20 years, quietly without failure. You wouldn’t think about cutting corners on the aesthetics of a building, so why cut corners on something as important as air quality?

Eliot Warrington is managing director at Solarcrest