Design and Detailing for Airtightness

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Key Principles

1. Most UK construction is ‘leaky’ and wastes energy and money. Building airtight buildings can save energy and money, improve comfort and reduce the risk of damage to building fabric.

2. Airtight building will NOT mean ‘stuffy’ buildings. Good ventilation is vital for health and comfort - it is the UNPLANNED leakage of air that we are aiming to stem.

3. Legislation is slowly catching up with best practice in Scotland, the UK and elsewhere and we can expect a greater emphasis on airtightness in all types of construction in due course.

4. Good and Best Practice Targets have been set for many types of buildings and are easily achievable.

2.1 Infiltration, Ventilation and Airtightness

Air infiltration is the uncontrolled flow of air through gaps in the fabric of buildings. It is driven by wind pressure and temperature differences and as a result is variable, responding in particular to changes in the weather. Infiltration levels are strongly affected by both design decisions and construction quality.

Ventilation, on the other hand, is the intended and controlled ingress and egress of air through buildings, delivering fresh air, and exhausting stale air in combination with the designed heating system and humidity control, and the fabric of the building itself.

Whilst some unwanted air infiltration will at times aid comfort levels, it is not reliable and moreover brings with it a range of significant disadvantages such as high levels of heat loss, reduction in performance of the installed thermal insulation, poor comfort, poor controllability and risks to the longevity of the building fabric itself. It cannot be considered an acceptable alternative to designed ventilation. Infiltration needs to be reduced as much as possible if we are to create efficient, controllable, comfortable, healthy and durable buildings. This can be achieved by delivering airtight buildings.

Airtightness is a term used to describe the ‘leakiness’ of the building fabric. An airtight building will resist most unwanted air infiltration while satisfying its fresh air requirements through a controlled ventilation strategy. Most existing buildings, even those built recently, are far from being airtight and because of unwanted air infiltration generate huge costs to owners and occupants, in environmental, financial and health terms.

It is important to emphasise the distinction between infiltration and ventilation, because while the primary purpose of this document is to show how buildings can be designed and constructed to be airtight, it is equally important to stress that good levels of ventilation and a clear ventilation strategy will be required in every case. As the saying goes: ‘build tight, ventilate right.’

13 gaps in the external wall at services entry

14 leakage around ceiling roses, recessed spotlights and pullcord switches between a warm room and roof space or intermediate floor

15 gaps around boiler flues (in walls and roofs)

16 small gaps where water / heating pipes enter rooms from floors, walls and boxed in spaces

17 gaps around waste pipe penetrations eg behind toilets, baths and kitchen sinks

18 service entry points, even in concrete slabs within a larger diameter pipe

19 airbrick / air entry to open-flued fires required by the regulations admit air at all times, not just when the fire is on use.

20 large gaps where soil pipes / ventilation flues penetrate the roof

21 other roof penetrations eg overflow pipes

22 gaps between heated spaces and a cold loft where water pipes and cables pass between, often in airing cupboards

23 poorly sealed wall mounted extract fans, also ducted extract from cooker hoods, tumble driers etc allow air directly into and out of the room, but also into the cavity

24 chimneys and flues, if not sealed properly will allow leakage at all times

2.2 Why Build Airtight?

Legislation

At a rather prosaic level, the issue is important because it is now part of the Building Regulations in England and Wales concerning non-domestic new buildings over 1000 sqm in area, and is likely to affect a wider range of buildings soon. Whilst the initial targets set for airtightness of buildings are easy to achieve (see 2.5), it is equally likely that once in place, those targets will be ratcheted up to create ever more airtight and efficient buildings in Scotland and the rest of the UK, in line with many of our European neighbours.

Energy and Cost Saving

Typically, the largest heat losses in most buildings are related to levels of thermal insulation, followed by those related to infiltration, followed by those related to inefficient plant. Quite rightly therefore, most efforts to save energy and costs have until recently been directed at increasing thermal insulation levels. But as these levels have risen, so the relative contribution of infiltration has increased to the point where it can represent around half of all heat loss in a building. In highly insulated buildings, the percentage may be higher.

This is reflected in the fact that total space heating costs in an airtight building may be as much as 40% less than in a leaky one [5].

We are at the stage where it is likely that any further increase in thermal insulation levels would be ineffective until levels of airtightness in construction have improved considerably.

Figure of heat losses per P. Jennings, ‘Airtightness in Buildings’ Building for a Future Winter ‘00/’01

Space Heating System Reduction

Clearly there is potential to reduce the capacity of space heating systems sized to cope with current levels of heat loss if those levels can be reduced by a half or more. In addition, airtight buildings are more predictable in terms of environmental control and the capital cost savings of installing smaller heating plant may be augmented by reduced plantroom sizes in certain cases, and particularly by reduced running costs in the longer term.

As well as reducing the need for heating plant, airtight buildings offer much greater potential to respond positively to the local external climate through passive, or climate responsive design strategies such as natural ventilation, daylighting, the use of thermal mass and passive solar gain. Energy savings, capital and running costs, along with CO2 emissions can thus be further reduced.

Comfort and Control

As noted above, airtight buildings are not as affected by variations in external conditions. This makes them easier to control from an Engineer or Designer’s point of view, but it also makes them more comfortable from the point of view of the occupant.

In buildings with high levels of infiltration those occupants near draughty windows, for example, will suffer the cold, particularly on windy days, whereas those elsewhere may well suffer from too much heat locally as the system tries to raise the temperature overall. Those who try to achieve comfortable levels through the use of the provided ventilation controls will find these to be relatively ineffective, whereas in more airtight buildings greater levels of control and comfort generally are achievable and local control and variation by occupants can have a more direct effect. In one example of an existing superstore, the ambient temperature in the store was raised by 5oC after the store had been sealed [6].

Complaints by occupants in leaky buildings are common, and remedial measures are usually difficult and expensive.

Deterioration of Fabric

Leaky buildings allow cold air in through the construction causing discomfort, they also allow warm (and often moist) air out, causing heat loss. This warm and often moist air can find itself in colder parts of the outer construction where it can cool, and the moisture in the air can condense, leading to a build up of moisture. This in turn can lead to:

• decay of organic materials such as timber frames
• saturation of insulating materials thus reducing their insulative effect (which increases heat loss further)
• corrosion of metal components
• frost damage where moisture has accumulated on the cold side of the insulation.

2.3 Legislation

In England and Wales the relevant regulation on airtightness is contained within Approved Document L1 for dwellings and L2 for non-domestic buildings (2002). There is general encouragement to consider airtightness issues, with a target air permeability for all buildings of 10 m3/hr/m2 envelope area at 50 Pa. In L2, buildings with a floor area of greater than 1000 m2 are required to be tested if approved details are not used. Further tightening of the regulations are due in 2006 and 2010.

In the new Scottish Building Standards, the relevant section is 6.2.5 for both domestic and non-domestic buildings. In the domestic version, Designers are directed to Building Research Establishment (BRE) Report 262 – “Thermal insulation, avoiding risks” 2002 edition, and in the non-domestic version, to the BRE document BR 448: Airtightness in Commercial and Public Buildings but it is stated explicitly that “within the Building (Scotland) Regulations 2004 there is no requirement, mandatory or otherwise to test buildings”.

Proposals for changes to the Energy standards were issued to public consultation in March 2006, including guidance that air tightness testing would be required if the calculation of energy performance included air permeability rates lower than 10m3/m2h at 50 Pa.

2.4 Measurement

A range of units for measuring airtightness have been used in the past and this can complicate matters. However, one method only – “air permeability” - is the measure used in European Standards, the new editions of the various UK Building regulations and in CIBSE’s TM23 Testing methodology and has been used throughout this document. The Air Permeability is defined as the volume flow in cubic metres of air per hour per square metre of the total building surface area (including the floor) at 50 Pascals pressure differential, expressed in m3/hr/m2 @ 50 Pa.

The main difference between the air permeability and previous practice in the UK is the inclusion of the non-exposed ground floor in the calculation of the ‘total surface area’ of the building. The difference between the new measurements and older ones tend only to be marked therefore where there are large volumes and ground floor areas.

Of the range of measurements used previously, the “Average Air Leakage Rate (or Index)” is similar to the “Air Permeability” except that non-exposed floors are excluded from the measurement. Another common expression is the “Air Changes per Hour at 50 Pascals (ACH @ 50 Pa). This is a useful measurement in particular because, when divided by twenty, it gives an approximate value of the natural infiltration rate of the building at normal atmospheric pressure, which can then be used to help size heating and ventilating plant etc.

Yet another measurement is the “Equivalent Leakage Area” (ELA) at 50, 10 and/or 4 Pascals. This figure gives a representation of the sum of all of the individual cracks, gaps and openings as a single orifice and helps to visualise the scale of the leakage problem. The main problem of changing the measurement technique is the ability to compare data. See P Jennings, Airtightness in Buildings in ‘Building for a Future’ for a good account of the issues.

The standard pressure differential used is 50 Pascals. This is not in fact a very large pressure differential and corresponds to the pressure exerted by a column of water 5mm high. Compared to the fact that buildings can withstand wind induced pressures of at least 500 Pascals, this seems insignificant, but it is larger than wind induced pressure on a calm day, and by testing and quoting air leakage figures at 50 Pascals, inaccuracies are reduced and repeatability is improved. See Chapter 4 for more on this.

2.5 Targets

As noted above, the only ‘official’ guidance in the UK applies in England and Wales and relates to non-domestic buildings over 1000 sq.m in area. As can be seen from the table below, the target of 10 m3/hr/m2 at 50 Pa. is relatively easily achieved compared to the good and best practice noted in the 2000 document by CIBSE, TM23. This sets out the testing methodology which is the de-facto methodology now followed in the UK.

A number of airtightness experts believe the stated targets are inadequate when compared with the overwhelming need to address carbon emission reductions, and the potential to do so through airtightness measures. For example, the house illustrated to the right was built in 1992 for the same cost as nearby houses and improved upon the standards noted above by two thirds.

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Footnotes:

5. BRE, Airtighness in Commercial and Public Buildings 2002, p.3

6. Quoted by HRS Services in their Airtightness Information Pack, p.5 -
see www.air-tightness.co.uk or 0114 272 3004.

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