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Dynamic insulation - insulation that acts as a heat exchanger - is part of the McLaren leisure centre at Callander, Scotland A breath of warm air

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CI/SfB (M2) (H6) Uniclass N35:N331

The McLaren Community Leisure Centre at Callander in Scotland was designed by Gaia Architects in association with mcm Architects from Edinburgh. It brings a refreshing breath of air to the confused and sometimes stifling debate about breathing construction. It is not only the first major building in the uk to incorporate the most radical form of breathing construction - dynamic insulation - but it is also the largest dynamically insulated building in the world, and the first to use the technique in a wet environment.

Breathing construction

A desire for greater energy efficiency has led increasingly to the sealing of buildings in order to control air leakage and so reduce heat loss, while also preventing moisture permeation with the consequence of interstitial condensation. This has led to an increasing reliance on space-consuming, expensive mechanical systems to deal with internal air quality and moisture.

Ecological designers are concerned about the potential health effects of sealed buildings full of synthetic materials, and about the indoor air-quality problems associated with inadequate maintenance of supply- side ductwork. They have reacted by adopting breathing construction, natural ventilation and careful materials specification.

Despite increasing adoption of breathing construction, there is still an alarming amount of misunderstanding about what it entails. There are two main types of so-called breathing construction, which are often confused - moisture-permeable and air-permeable.

The timber-framed breathing wall, which is what most people understand by breathing construction, is actually moisture-permeable, not air-permeable. Its construction aims to eliminate the risk of interstitial condensation by allowing water vapour to diffuse naturally through the structure. What gets in, gets out.

It exploits the physics of a vapour pressure gradient - a higher pressure inside the building than out - to encourage moisture flow from within the building outward into a ventilated cavity. In the process it dispenses with the need for a vapour barrier.

To work correctly, the inside layer needs a vapour resistance of at least five times that of the outside sheathing. While it is true that all timber walls breathe water vapour, some walls clearly breathe more than others, depending on the vapour resistance of the materials used (aj 15.10.98). The effect can be enhanced by the use of hygroscopic materials such as cellulose-fibre insulation, which, combined with bitumen-softboard sheathing and plasterboard internal lining, is the only form of breathing timber construction with an Agrement certificate.

Despite some claims to the contrary, this construction does not allow air to breathe through it. In fact, tests by Gwalia Housing Society showed the construction to be more air-tight and therefore more energy-efficient than traditional sealed timber construction. It may, however, aid internal comfort by providing some passive control of internal relative humidity.

Air-permeable breathing

Dynamic insulation, on the other hand, is an air-permeable form of breathing construction. Some of the heat usually lost by conduction to the outside is reclaimed by ventilation air being drawn into the building through the insulation. The building fabric becomes a counter-flow heat exchanger through which the movement of air from outside to inside is controlled by careful balance of internal and external air pressures. This pressure difference is normally created by negative pressure induced internally, either through natural (stack, for example) or mechanical (fan-assisted, for example) means.

Dynamic insulation emerged as an idea in the 1960s, though its roots can be traced back to research into the transportation of air and moisture through buildings and building materials in mid-nineteenth-century Germany.

In 1965 David Pattie ran a series of research projects at Ontario Agricultural College in Canada which set out to quantify the contribution of dynamic insulation to ventilation, indoor air quality and the consumption of energy for space heating. He went on to design and construct his own dynamically insulated house.

Around the same time, Trygve Graee at the University of Agriculture at As, Norway, started to develop dynamic insulation in ceilings. This followed study of traditional steadings in which he identified that air was being drawn in under the eaves by stack effect, passing through the hay loft and exchanging heat with the stored material, then passing through the floor where the animals were kept, down into the midden and out through a sealed pipe which is vented at high level (see diagram).

Dynamic insulation came into common use in 1968 in Scandinavia in animal houses for pigs and poultry. The idea has been developed during the 1990s in Norway and Sweden in a range of projects including a number of houses, school buildings and sports halls. Rykkinnhalen Sports Hall near Oslo is reporting energy savings of up to 50 per cent.

The effect of this airflow through the wall is to modify the U-value. The dynamic U-value, Udyn, is the U-value of the wall modified to take account of air velocity and insulation thickness. The effect of heat exchange is to reduce the effective U-value of the wall by an amount which corresponds to the heat gains during air-inflow. While the U-value decreases as air velocity increases, very low U-values can occur at quite low air velocities. In a sports hall the velocity of the bulk air flow through the permeable fabric can be in the range 5-10m3/m2 of fabric/h. A near-zero U-value, or infinite thermal resistance, can be achieved while creating ventilation air velocities in the occupied zone well below the 0.1m/s recognised as the threshold for discomfort.

A number of advantages are claimed for dynamic insulation.

Improved indoor air quality

Reducing the number of intake ducts where dirt can accumulate

Reducing the number of particles in the air through filtration because of the extensive intake surface and low velocity. (It is estimated that it will take approximately 300 years for clogging of pores to become a problem. At this point the insulation could be extracted and new insulation blown in)

Delivering large quantities of air at a very low velocity avoids uncomfortable air movements.

Improved energy efficiency

Reduced U-value

Reduced quantity of mechanical plant

Elimination of the heat stratification through controlled air movement.

Reduction in condensation risk

Humidity damage in construction is due largely to air leakage from inside to outside because of local or general overpressure within a building. Creating an underpressure internally will prevent airflow out through the fabric and will tend to dry out the construction. The constant air stream from the cold side to the warm side effectively ventilates the construction and the building. And because the air is always moving to the warm side where its potential to hold moisture increases, interstitial condensation cannot occur.

Promoting health

At first sight it might seem strange, if refreshing, to find a body such as the Scottish Sports Council (scc) promoting innovative construction. The use of dynamic insulation in the McLaren Centre grew out of the scc's desire for the sports facilities which encourage healthy lifestyles to be user and environmentally friendly - healthy buildings for healthy pursuits.

In 1992 the scc was involved with Gaia Architects in developing proposals for a sports and ecological centre in Glasgow. As part of the project it commissioned research into the use of dynamic insulation in sports halls*. This concluded that dynamic insulation in sports facilities was likely to:

reduce capital costs by reducing technological/mechanical systems

reduce energy in use and thereby operating costs

provide a good-quality indoor environment

be environmentally sound.

This led to its adoption in the development of the McLaren Centre, for which the ssc was a major player alongside Stirling Council and others.

McLaren Community Leisure Centre

The £3.1 million centre provides a range of facilities for the community of Callander, including a sports hall, swimming pool, indoor bowling hall, two squash courts, fitness suite, climbing wall, cafe, creche, meeting room and team changing rooms for outdoor activities.

The building is designed to sit tight into the slope of the site to reduce its visual impact and leave the playing fields clear of development. It is organised around a central circulation spine, which also acts as the social heart (cafe) and provides viewing areas. The careful use of natural daylight, combined with natural finishes such as timber cladding and linoleum, creates a warm, friendly, non-institutional atmosphere.

It was decided early on to restrict the dynamic insulation to the key activity spaces in the building -the sports hall, bowls hall and squash courts on the dry side and the swimming pool on the wet side.It was also decided to design the building so that as a fall-back position it could perform conventionally. Dynamic insulation is introduced at ceiling level for all of these spaces.

Roof voids above ceilings are slightly pressurised by fans pulling outside air into the loft space. Rooms below these ceilings are slightly depressurised by extract fans removing air from the room. These create a pressure differential which pulls the air through the insulation across the entirety of the ceilings.

The system is designed for an outside winter temperature of -5degreesC and 100 per cent rh. But it operates in different ways according to the requirements of the room it is serving. The critical point for the swimming pool, for example, is in summer with high outdoor temperatures (above indoor level) and high humidity.

Bowls hall

Unheated air is drawn into the loft at 7-8Pa. With mechanical extract from the hall, a pressure differential of 10Pa draws air through the ceiling at 3m/h, and achieves an air temperature (when -5degreesC outside) of 15-17degreesC by exchange in the fabric and in the hot air at the top of the room. This is topped up to 20degreesC by radiators with a ventilation rate of 0.8ach/h (air changes per hour).

Sports hall

Unheated air is drawn into the loft at 7-10Pa and drawn through the ceiling at a velocity of 10m/h by an underpressure in the hall of 3-5Pa, achieving 9-11degreesC. This is topped up by underfloor heating to 16degreesC with a ventilation rate of 1.2ach/h.

Swimming pool

Air is blown into the loft at 10Pa (with boost to 25Pa when required) and preheated to 10-15degreesC. As it is drawn through the ceiling at 10m/h it picks up a further 10-13degreesC to achieve a design temperature of 23-26degreesC. Underfloor heating and radiators top this up to 30degreesC. Humidistats on the pool hall extracts adjust the fresh-air volume required to keep the air at or below 60 per cent RH +/-5 per cent. A minimum of 2ach/h is available to control humidity in winter and 5ach/h for peak humidity in summer.

Extract air from the internal depressurisation is passed through an air- to-water heat pump to preheat the swimming pool water. After initial heating from cold by the boilers, 100 per cent of the pool heating throughout the year should be provided by the heat-recovery system. (In part this relies on energy input to preheat the air in the loft.)

System design and construction

For dynamic-insulation ceilings to work correctly, careful detailing and good workmanship on site are essential. The loft space had to be exceptionally well sealed. The services engineer, dubious whether this could be achieved, assumed that 50 per cent of the air pulled into the loft space would be lost back to the outside rather than transferred to the room below and so significantly oversized the fan equipment.

The whole system operates automatically with limited localised user control. System balance in the swimming pool is so critical that it is fully automated. In other spaces, such as the squash courts and sports hall, users can open windows. In winter, when the dynamic system is operating, open windows will reduce the pressure differential and the quantity of heat-transfer air pulled through the ceiling. In summer, the dynamic system is likely to be turned off and users will open windows to get natural ventilation where required.

The dynamically insulated ceilings aside, the building is conventionally constructed, but with environmentally benign materials with low embodied energy, minimum environmental damage over their lifecycle and involving no toxins during manufacture and use. These include Warmcell recycled- newspaper insulation, Heraklith magnesite/woodwool slab, untreated timber, linoleum and vapour-permeable low-solvent paints.

In the light of the government's initiative of promoting 'healthy living centres' and encouraging sustainable construction, the commitment of the Scottish Sports Council and of Gaia Architects is to be applauded. The project pilots an interesting and innovative construction technique with great potential to reduce capital and energy running costs and to promote healthier internal environments.

Too many innovative buildings base their reputations on optimistic claims by their architects and hearsay. I will start this process by giving my subjective view that the quality of the air in the dynamically insulated spaces was noticeably good! More reliably, McLaren is the subject of a two-year monitoring project funded by detr's Construction Directorate, the ssc and Gaia Research which will give valuable technical and user feedback to aid the further development of dynamic insulation. Except in small buildings, where stack ventilation may work, dynamic insulation clearly requires technology to drive and control it.

It is anticipated that monitoring will allow a reduction in size of the oversized fan equipment in McLaren, giving further capital cost savings.

That Gaia and mcm Architects have managed not only to be innovative, but to design a charming and humane building, so different from the standard leisure-centre aesthetic, is to be applauded. It is a building that enhances spiritual as well as physical health.

Jonathan Hines is a director of Architype

Pore Ventilation: Sports Halls. Research Report 43. Ventilation: Swimming Pools. Research Report 47. Scottish Sports Council , tel 0131 317 7200.

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