Mark Siddall from LEAP (Low Energy Architectural Practice) explains the circumstances under which poor ventilation can occur and why the control strategy becomes increasingly important the more airtight the building is.

Interview with Mark Siddall

Mark Siddall trained as an architectural technologist before training as an architect.

He established LEAP in 2011 and he's also written over 30 articles and papers on building performance and Passivhaus. Get free access to his research.

Poor Ventilation Can Lead to Poor Health

In very poorly ventilated buildings mould and fungus can grow on the surfaces and in carpets, etc. This can cause respiratory problems for the occupants.

Humidity can also have an impact. In a low humidity environment immune systems can become depleted and there may be increased incidents of asthma. If humidity gets too high then dust mites can populate a home quite quickly and that can increase the likelihood of asthma, too.

Occupants Only React When They Perceive a Problem

Mark says there's a reasonable amount of evidence to suggest that unless occupants perceive a problem with ventilation they just get on with their lives.

In older homes that are particularly leaky anyway few issues tend to arise. However the more airtight a building is, the more important the control strategy becomes (whether it's a natural or mechanical ventilation system).

There Are Four Different Ventilation Strategies

Within the (UK) building regulations there are four primary systems:

1. Windows with trickle vents and intermittent mechanical extract ventilation.
2. Windows with trickle vents and passive stack ventilation.
3. Continuous mechanical extract ventilation.
4. Mechanical ventilation with heat recovery.

Mechanical extract ventilation with trickle vents is by far the most common ventilation strategy (in the UK) but with a focus on lower energy buildings there is a move towards mechanical ventilation with heat recovery (MVHR).

 A ‘good practice' installation of MVHR

Mechanical Ventilation Can Get Switched Off Unnecessarily or Because of Problems

Mark describes some of the circumstances that have led to occupants switching off their mechanical ventilation systems. For example, if people are conscious of their finances they can perceive that a mechanical ventilation strategy is more costly. This need not necessarily be the case if it's a well-designed system with some low energy fans.

In a similar way people who are environmentally aware might think that by switching off the ventilation system they are reducing their carbon emissions. 

Another trigger can be when the ventilation system is too noisy. 

Unless there is a natural ventilation strategy, switching off the mechanical ventilation will have an impact on the indoor air quality.

There Will Always be Some Noise Associated with the Ventilation Strategy

Whether it's a natural ventilation strategy through trickle ventilators or a mechanical ventilation system there is likely to be some level of noise.

The question is really what should be considered an appropriate level.

Mark says: “The World Health Organisation defines an appropriate noise standard as being 35dBA . Then for mechanical ventilation systems there's a range of different acoustic targets that you might try and conform to depending on whether you're referring to UK Building Regulations and its recommendations or whether you're referring to good practice.”

There is research suggesting that the World Health Organisation noise level may not be appropriate for mechanically ventilated buildings.

A duct attenuator

Attenuators Can Reduce Noise

Similar to an attenuator for a car exhaust there are attenuators for ventilation systems.

They can be situated at appropriate locations within the house to prevent break out noise from the ventilation system into the rooms and also to help reduce the risk of noise transfer between rooms.

Is There Drier Air Inside Buildings with MVHR?

Mark explains why some of us might perceive drier air inside buildings that have mechanical ventilation with heat recovery.

If it's cold outside, for example zero degrees, then the air can only hold so much moisture. When that moisture is brought inside the building, its temperature is raised and the relative humidity of that air drops. It's that dry air that is then created that might be perceived through dry noses, dry eyes and the likes. 

It's important to note that this is irrespective of whether it's a mechanical system or a natural system, assuming that they're both supplying the same amount of fresh air.

Mark continues: “What you tend to find though is that where there are reports of drier air it tends to be in buildings that have got more mechanical ventilation systems and that is because, I would argue, the systems are constantly providing that ventilation whereas natural ventilation in itself is inconsistent.“

Apex Acoustics doing some Passivhaus sound testing 

Clocks and Fridges Are Often Noisier than Ventilation Systems!

Mark reflects on some research he was doing with Jack Harvie-Clarke from Apex Acoustics, where they were trying to identify whether there were potential noise issues from ventilation systems.

After sound testing at the Racecourse Passivhaus Development they found that the alarm clock, kitchen clock and the fridge were noisier than the MVHR.

This is exactly what they were hoping to find and it means that the system had been designed properly, the contractor had completed the work appropriately and the system had also been commissioned well.

The User has Perfect Control Over the Ventilation  

In a Passivhaus – just like any other build – it's down to the user on how to operate the building. Different fan speeds allow the occupant to control the rate at which the building is ventilated and of course the windows can be opened at any time of the year!

External Noise or Concerns Over Security Can Deter People from Opening Windows

Mark also points out that despite having tilt-and-turn windows, which are arguably more secure, some occupants at the Racecourse bungalows were still reluctant to keep windows open on summer nights because they perceived it as a security risk. This could therefore make the building more likely to overheat.

Similarly there are associations between noise and ventilation, so people keep their windows closed up to the point that the threshold of discomfort becomes too great and then they'd rather put up with the outside noise and open the windows!

 Capped ducts

Rigid Ductwork is Preferable

If there are kinks in the ductwork then a higher fan power will be necessary to get air to the right places. This in turn leads to more noise and using more electricity energy.

Rigid ductwork therefore ensures that there are no kinks and bends.

Keep Duct Runs Short

Using long duct runs not only means more material is being used, but there's more labour and more cost.

Prevent Ductwork From Getting Dirty During Construction

Having dirty ductwork can lead to occupants getting drier eyes, sore throats and other things that are associated with Sick Building Syndrome.

So it's important to ensure that the ductwork is clean when it's designed and constructed because by far the most dirt will get into the system during the construction process.

 Bad practice – dirty ductwork!

Mark even got Wolfgang Feist to take a look inside his ventilation heat recovery system! It's been in place for 10 years and he reported back that it's as clean as the day it was finished.

As long as no dirt got into the system initially and the filters have been changed as required, there should be no cause for concern.

One of Mark's Projects: Steel Farm

Find Out More

• Download Mark's 30+ articles and papers on energy performance and Passivhaus

Transcript

Download a transcript of the interview with Mark Siddall.

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