Sarah Halford

Architecture and Ceramics

build tight, ventilate right

This is a long one, only because I’m quoting a huge chunk from the RIAS practice note on airtightness. This is always a hot topic in our practice, as the building regs are giving designers higher and higher airtightness levels to work to. To cut the technical chat; if your house is new, it will be airtight. There will be intermittent fans in the bathroom and kitchen, often linked to the light switch.

It is essential that air moves through a home, to reduce pathogens from furniture, allergens from pets, CO2 from respiration, and most importantly moisture. If there is insufficient ventilation, stale odours and an unhealthy living environment can build up, with mould appearing. This isn’t from the ground, or coming through the walls, it can be just from the bathroom, kitchen and clothes drying indoors.

Older homes avoid the need for controlled ventilation by having… uncontrolled ventilation. Wind whistling across the chimney creates a passive stack effect, drawing air into the home through gaps around the windows, airbricks, and doors. We call them draughts out here. Of course, you can open the windows too, but that’s obviously seasonal.

Because we now build houses to higher levels of airtightness – using timber kit technology, tapes and membranes – we often have to use mechanical ventilation to safely ventilate the rooms. The article below is by a representative of PassivHaus, a method of construction which pairs incredibly airtight building with whole house ventilation with heat recovery. It gives a good overview of the issues, with a bias towards building to Passivhaus standards.

A fresh breeze of air, but energy efficient please

This Practice Note has been prepared by Stephen Huber of the Scottish PassivHaus Centre (SPHC) which provides professional advice on Passive House technology.

Ventilation has always been an important issue for a healthy indoor climate and for the maintaining of the building fabric.

With the increase of fuel prices, various approaches were tried to make buildings more efficient. Good insulation together with increased air tightness reduced the heating demand greatly. But when in the 1980’s dwellings in central europe were built more air tight and were double glazed, unwanted effects of lack of ventilation appeared which caused unhealthy living conditions and harmed the building structure:

Many people are not aware how many materials used in buildings off-gas chemicals, which can build up poisonous concentration levels within the dwelling. We are talking mainly about chemicals from carpets, but also furniture, paints, etc. In the Scottish Highlands, we also experience raised levels of Radon in the ground, which is carcinogenic. It finds its way into buildings through tiny gaps in floor slabs and can only be avoided by special membranes or good ventilation. On the other hand, moisture from respiration, cooking, evaporation and drying of clothes, etc. builds up and causes the growth of mould, dust mites and bacteria. These conditions are a breeding ground for respiratory diseases, such as asthma, as well as allergies, skin irritation, fatigue and headache.

Most Scottish dwellings are well ventilated, not to say, too well ventilated. At least it is good for the health one might think. However especially in social housing we find a number of problems related to lack of air tightness and poor insulation, which make it very expensive to heat the building with the result that more people fall into fuel poverty. Simulations show that the energy demand of a 1970’s semi-detached no-fines building amounts up to 560kWh/(m2 yr). As a consequence parts of the house often stay unheated in the cold season, which leads to an increase in levels of indoor humidity with all the negative side effects mentioned above. Sometimes gas stoves are also used, which emit additional moisture.

To combat mould growth, dust mites and bacteria, indoor humidity levels should be kept on average below 50%. Also attention should be paid to the choice of materials.

In answer to these issues, building regulations are tightening up on insulation and air tightness, with pressure tests becoming mandatory as of October (in Scotland). This is a welcome trend towards more energy efficiency. We see how much more can be achieved when we look at the Passive House methodology, which reduces the heating demand down to 15kWh/(m2 yr). Compared with the figure mentioned above, the potential savings are dramatic. Such results are based on the three principles of energy efficiency in buildings:

  • an effective design optimising solar gain and minimising losses,
  • good insulation avoiding thermal bridging,
  • excellent air tightness (e.g. Passive House requires about 10 times better air tightness than building standards).

Since 1991 over 30,000 residential and commercial buildings have been built to this passive standard all across Europe and other continents in different climatic zones.

Now, how do we survive in such sealed buildings?

The challenge was to find a controlled way of ventilation, which provided fresh air without the heat losses that are associated with most other forms of ventilation. Such whole house ventilation systems that recovered the heat from the exhaust air and thus minimised the heat losses have been in operation for 20 years and have proved to effectively reduce the heating demand, on average by about 30%, compared to dwellings with uncontrolled ventilation. By that I mean the introduction of fresh air through opening of windows, gaps in the building fabric and trickle vents, which depends on greatly varying wind speeds and temperature differentials.

Mechanical Ventilation with Heat Recovery (MVHR) is designed to provide the right amount of fresh air where it is needed (in bedrooms, living rooms, studies) and extracts where moisture and smells build up (kitchens, bathrooms, utility rooms). It operates on slow air movements all around the clock with different speed levels during the day and at night. The beauty is, that apart from the modest expenditure for the operation of the fans, the heat recovery is passive i.e free of charge. In terms of energy use, we are talking about 29-40W for a normal 3 bedroom dwelling for efficient systems.

I don’t actually like the word “mechanical” as it is associated with rattling extractor fans and life saving machinery in hospitals; that’s why I prefer to call it Heat Recovery Ventilation (HRV). Practice has proven that well designed and correctly installed systems are hardly noticeable in terms of noise and air movements, but the air quality is consistently good. Humidity levels are kept low in the cold season, as then the heat exchange process actively de-humidifies. Thus it provides an optimised indoor climate.

In the warm months some systems have a summer bypass or they can be switched off to ventilate by the opening of windows.

To secure the success of HRV systems, careful attention has to be paid to design, duct layout and insulation as well the choice of system. There are great differences in performance which are not easily detected in the claims of manufacturers as test methodologies vary. Heat recovery rate, energy consumption, insulation and noise levels are important factors, which can cost energy and make the difference between an inaudible system and an annoying one.

The most secure way of choosing a system which performs well is to compare certification results on the Passivhaus Institute’s website.

I would like to end with the comments of a tenant in a Passive House in Fife:

…so far very impressive. There are no smells lingering in the house ever, whether it be a good or bad smell! Air is noticeably always fresh, clean feeling. House always feels dust free. Air never feels humid or dry just consistently pure. This has made a real difference in our every day life and that’s just three weeks in.

Stephen Huber started the Scottish Passive House Centre in Rosyth with a business partner. The SPHC is an exhibition and training facility centre concentrating on energy efficiency and the Passive House methodology. They organised the First Zero Carbon Housing Conference in Edinburgh in 2007 and deliver seminars and training to promote highly energy efficient buildings. Stephen is also a lecturer at the University of Strathclyde on Passive House ventilation.



This week a rep from a local manufacturer gave us a CPD talk about ventilation, and recommended that where heat recovery units are used, they are coupled with a passive stack system, which cuts out the wasted energy from the recovery unit during the summer months. I feel that heat recovery technology is going to improve and become more efficient, and that all houses will use this technology, but i could be hugely wrong. I was also surprised to find that the rep recommended the use of passive stacks whereever possible, as there are new products which react to moisture in the air to open and close the stacks, basically preventing all the heat sailing up through the ceiling. I was under the impression that a passive stack had to be over 15m high, but this is wrong, and stacks can be used on a two story house.


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This entry was posted on June 17, 2011 by in architecture.
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