Andy Simmonds from Simmonds Mills Architects explains ventilation in the home, why it's important and what the options are if you're looking to retrofit a property for energy efficiency.
Interview with Andy Simmonds
Aged 12, Andy got his first taste of sustainable construction taking a few courses at the Centre for Alternative Technology. After attending architecture college, Andy worked with engineer Chris Wallis, son of Barnes Wallis, on numerous interesting projects including stonework, timber frame construction, repairing windmills, etc. With a love of nature and the countryside, he developed an awareness of the importance of ecology when designing and building. As well as running Simmonds Mills Architects he is part time Chief Executive of the AECB (the sustainable building association).
The AECB is a Network of People with a Common Aim of Promoting Sustainable Building
Andy gives some background about the AECB. It was set up by a builder and his wife in 1989, focussing on construction but with an awareness of ecology and the environment. It’s got a very broad mix of members and there are many resources, such as the low energy buildings database, the technical discussion forums, the annual conference, local groups and so on. With about 1500 members, this vibrant group of people share their practical experience and expertise with each other. Andy says it’s a very fast way of learning, getting to grips with things and improving your business as well.
Ventilation is Necessary to Get The Air to Breathe, But Also to Remove The Pollutants
Andy uses an example of the International Space Station to illustrate the fundamentals of ventilation. They are in a sealed unit (although not completely airtight) and need air to breathe, but also a way to flush away pollutants. These could be carbon dioxide or other gases created by equipment, etc.
Early Buildings Just Let The Wind Blow Through
Although early buildings had a lot of natural ventilation, some stone and brick buildings with solid floors were actually pretty airtight. As people found new ways of building, some constructions (from 60s, 70s and 80s, for example) were potentially leakier and more open to the elements than some of the older buildings. Draught proofing without thinking of ventilation can create quite unhealthy environments.
Over Ventilating and Under Ventilating Are The Two Extremes
A house with all its windows open could be an example of over ventilating. This might be useful to clear a fug but if it's cold outside, a lot of heat energy from the house will be wasted during ventilation. The reverse situation could be keeping all the windows shut and draught proofing well, so any pollutants from off-gassing carpets (for example) or from cooking with gas hob, etc., cannot escape. In the heating season neither of these extremes is an effective way of ventilating a house and getting good air quality.
In a Retrofit, Mechanical Ventilation Lets You Control The Situation
Mechanical ventilation is an approach that's becoming used much more often. Two systems worth considering at are extract-only or mechanical ventilation with heat recovery (MVHR).
Extract-only Mechanical Ventilation
This approach extracts air from wet rooms, kitchens, bathrooms and so on, and then the fresh air is drawn in through trickle vents in the windows. These are slots in the windows, which will provide a background level of fresh air coming into the house. However, with the extract ventilation system, which is basically a box with a fan in it, the air is continually pulled through at a very low level. Even with a peak load during cooking, for example, this background ventilation can cope without turning up the fan.
Incoming Air From Trickle Vents Will be at The Outdoor Temperature
Andy explains that on a cold winter’s day, you might feel that incoming air as a cold draught if you’re sitting near the window. This is why radiators have often been positioned underneath the windows, so as the cold air comes in it falls down and to some extent warms up. Andy outlines a good approach used in Scandinavia, where they put an air inlet through the wall, maybe behind the radiator, so as you pull in air from outside it comes and warms itself on the radiator and doesn’t feel like chilled air.
Mechanical Ventilation with Heat Recovery Provides Fresh Air and Keeps The Heat
Mechanical ventilation with heat recovery both extracts the air from the wet rooms (kitchens, bathrooms and so on) but also puts it back in via ducts and air terminals throughout the house. This is therefore a balanced system. Although the heat is taken out of the outgoing air and put back into the incoming air, the air itself doesn’t mix. The heat is transferred as the air crosses through a matrix, a plastic honeycomb for example. So just the heat is transferred. The warmed incoming air is fresh.
In a Low Energy Retrofit, Ventilation Air and Air Leakage are Considered Separate Issues
Andy points out that until recently ‘ventilation air' and ‘air leakage' (the air that finds its way in through cracks, gaps and holes in the building) were much the same thing. However, in a low energy retrofit where the objective is to make the building as airtight as possible, the issues are considered separately. Creating and designing a system specifically for ventilation is vital.
Think About Ventilation From The Outset
Your ventilation strategy will depend a lot on the building you're hoping to retrofit. Once you have a professional assessment of how airtight you can make the building, then you can make an appropriate strategy. British houses from the 1970s and 1980s can be difficult to make airtight, whereas solid-walled properties with solid floors are often much easier. This year the AECB aims to release a retrofit guidance programme to help in this process.
Grove Cottage meets the EnerPHit standard
Larger MVHR Units Are More Efficient
Andy talks through the MVHR unit he has at Grove Cottage, which has two low voltage, low wattage fans, one for extracting air from the house and one for pulling air into the house. The largest part of the box is the heat exchanger. Andy's unit is situated behind cupboard doors in a utility room. While it does take up a fair amount of space, Andy points out that the larger the unit, the more efficient. This is because the more space the air has to move through, the less work the fans have to do, so very small units tend to be much less efficient than Passivhaus certified units.
Mechanical Ventilation Only Makes Sense With Certain Levels of Airtightness
Andy generally only recommends mechanical ventilation with heat recovery when the air change rate for the house is less than 1.5 air changes per hour. This is because the savings must warrant the investment in electrical energy. However, for the extract-only system an air change rate of 3 would be acceptable. Part of the difficulty is in planning for the air change rate when you’re deciding what sort of ventilation system to have, because after construction you may not have achieved the level of airtightness you had hoped for!
Externally Insulating Grove Cottage Was an Easy Route
Andy explains his approach to retrofitting Grove Cottage, which is a solid brick house. As the brickwork was of no particular architectural quality – it was painted white – externally insulating was the easier option (compared to internally insulating). The starting point was to mark on the plans in a red pen where the continuous air tightness plane goes, forming a strategy of how to deal with weak points. Andy describes how he sealed the brickwork to the roof membrane (which is basically a plastic sheet) with bitumen tape, which proved to be very successful. He emphasises the importance of a clear strategy and not leaving it to the last minute.
The Right Advice Can Save You Money
It is essential to spend time early on thinking through the detail of the retrofit. This could stop all sorts of problems arising on site and prevent a poor result at the end of the job. Andy reflects on how people can be reluctant to spend money on these services, preferring to invest in the measures instead. However, this upfront thinking could actually save a lot of money.
Maintaining a Ventilation System is Not Onerous
Andy's MVHR unit runs throughout the year and supplies fresh air which is slightly warmed by the outgoing air (although it's not a heating system) to about 21 degrees. The system has been set so that the fans go up to a higher speed around mealtimes, etc. While you can adjust it manually on the control panel as required, most of the time no intervention is necessary. The one exception to this is when the cardboard and fleece filters need changing. A light on the unit may even indicate when to do this.
Make Sure You Have Easy Access to Your Ventilation Unit
Think carefully about where the ventilation system is located. If it's positioned in the loft or screwed down behind a panel, this will make the simple maintenance jobs more of a pain. Equally, in a retrofitted house like Grove Cottage (that has been well-insulated and has triple glazing, etc.) and is therefore peaceful, you want to locate the ventilation units away from quiet spaces. There will be a slight hum, so keep it away from sleeping areas, for example.
Ventilation Systems Must be Properly Commissioned and Balanced
When these systems are put in, it's very important that they are commissioned and balanced properly. There’s a number of reasons for that, but in terms of comfort, over ventilation could mean that you start to notice draughts!
Not Considering Ventilation Could be a Health Risk
Andy outlines his concerns about the choices some people are making with their homes. He gives an example of putting in a wood stove. Leaving aside the combustion products and air pollution, Andy suggests that without an external air supply vent for the wood stove, you will be pulling in cold air up through your basement, through the floorboards and cracks and gaps. This will create cold draughts, which will add to discomfort, but the whole reason the stove was installed was to be more comfortable!
MVHR Systems For Retrofit Likely to be a Small Niche
Andy believes that we need to be much more conscious of ventilation because air quality is very important. He sees mechanical ventilation with heat recovery as being a relatively small niche for retrofit, but that extract-only ventilation could have a bigger role to play.
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