Air Tightness

Best Practice > Air Tightness

Under new building regulations, dwellings must be designed to minimum standards of air tightness. So how can they be achieved?


  • Infiltration – this is accidental ventilation caused by cracks and gaps in the building fabric.
  • Ventilation – this is the designed air changes needed to maintain good air quality in the building.
  • Air tightness – representation of the level of infiltration (i.e. not including elements of the designed ventilation)

Whole-house ventilation systems will work most successfully with an airtight fabric, so that ventilation is provided by the system rather than by infiltration. The new Building Regulations in England and Wales, and also in Scotland and in Northern Ireland and in the Republic of Ireland, require dwellings to be designed to minimum standards of air tightness and, in most cases, to undergo pressure testing on completion to confirm that design standards have been met. This will not apply to existing buildings being refurbished, nor to extensions, as the existing building would affect the result.

The normal minimum air tightness level for new dwellings is 10m³/(hm²) at 50 Pa (Pascals, or N/m²), with a normal minimum of 10m³/(hm²) at 50Pa. This means that, when the dwelling is pressurised, (via use of a door fan,) to a pressure of 50Pa, no more than 10m³ of air should leave the building each hour for every m² of envelope (walls, floor and roof). This would equate to around 0.5 air changes per hour (ach) at normal pressure. The current average value for new UK dwellings is about 14 m³/(hm²) at 50 Pa, equivalent to an average infiltration rate at normal pressure of about 0.7 ach. A target value for an energy efficient home would be around 0.1 ach at normal pressure.

With wet trades on internal surfaces, reasonably good levels of air tightness should be possible. High levels of air tightness can be achieved with timber frame with correct use of air permeability membranes.

Air tightness Testing

A high level of workmanship is necessary to achieve an airtight building – it is not enough simply to rely on well-draught-sealed windows and doors. With new build, if working to a low target air tightness level it is important to advise all tradesmen and sub-contractors that you may have coming on site, especially those whose operations are likely to influence air tightness, that this is an integral part of your design. But how will you know if you have achieved the target? The only way to tell for sure is to carry out a fan pressurisation test. This is a specialised operation involving expertise and the right kit. More companies are moving into this area as interest increases and in anticipation of legislative changes.

On a domestic scale one or two fans mounted in a door-sized frame are used to blow air into the house. The door fan is sealed in the door frame and all windows and specific ventilators (e.g. fans, trickle vents, etc) are sealed off. In this way only infiltration is measured, not designed ventilation. The fans are then allowed to run until a pressure of 50Pa is reached and a measurement is taken of the airflow through the fan required to maintain that pressure. Different readings may also be taken at different pressures.

The rate of airflow across the fan will indicate the level of leakiness – the greater the airflow required to maintain the pressure the more leaky the building. A smoke pen (available from scientific instrument suppliers) can also be used to pinpoint local areas of air leakage, for example around window frames, at skirtings, in order for remedial measures to be taken if necessary. Relatively small gaps can be sealed with expanding foam or even flexible sealants. Larger gaps may need something a little more substantial, such as insulation, mortar, etc.

It is a good idea to carry out the pressurisation test before installing fitted kitchens etc, in case remedial measures are needed. However if water appliances are installed ensure that water traps are filled or in some way blocked, otherwise a false reading will be given. Pressurisation tests can also be carried out on existing houses, though the presence of fitted units may make remedial measures impractical.

It is also possible to use smoke pens to identify leaks without pressurising the house, though they will work much better with a higher pressure difference between inside and out – a windy day might suffice!

Air movement paths:
Pay particular attention to the following areas:

  • Services/Trunking
  • Recess Lighting
  • Joists/skirtings
  • Windows/Doors/Openings
  • Chimneys
  • Loft Hatches
  • Dry Lining/Blocks
  • Suspended Floors

MVHR/Passive ventilation

Heat Recovery Ventilation:

While ventilation is essential and some heat loss inevitable, the amount of heat lost can be reduced by use of a heat recovery system. This can be done on a whole-house basis or in individual rooms.

Single Room Heat Recovery Ventilation:

Individual rooms can be fitted with heat recovery fans, which combine an extract fan with a fresh air supply fan. A heat exchanger allows some of the heat from the extracted air to be transferred to the incoming fresh air, thus pre-heating it. Most types have two settings, a low flow rate designed for constant use and a high flow 545rate for intermittent use, controlled on a humidistat if required. These units can be as effective as extract fans in removing humid and stale air, while also providing fresh air and recovering up to 60% of the heat otherwise lost. It should be noted, however, that concern has been voiced regarding the ability of these systems to operate effectively other than in still air. Claims have been made that any degree of wind movement renders the fan incapable of extracting. Look for independent rigorous testing of systems in typical wind speeds in UK and Ireland of 4-5m/s.

Whole House Mechanical Ventilation with Heat Recovery (MVHR):

In general the scale of domestic buildings means that it is not necessary to introduce mechanical ventilation except in specific areas as covered above, natural ventilation being effective in most cases. However, there may be circumstances in which you would wish to consider mechanical ventilation for your whole house. For example, if you had no permanent background ventilation in an existing house and particular air quality requirements for health reasons; or if aiming for a very airtight fabric in the case of a new building (a prerequisite for these systems in any case).

There are a number of such systems now on the market. Some systems use trickle vents as the supply air source, but then do not include heat recovery. Most share common features, an extract system and a fresh air supply system (each with fan and ducts) and a heat exchanger. Heat is recovered from waste air extracted from areas such as bathrooms and transferred to incoming fresh air supplied to other habitable rooms. The “tempered” air provided can reduce cold draughts that may result from other systems, improving comfort levels, while up to 80-90% of heat can be recovered from the waste air streams. A system with variable extract rate will be better able to cope with high levels of moisture production. If the kitchen is included, are re-circulating cooker hood should be installed and the filters regularly changed as grease will foul the heat exchanger heat transfer surfaces.

The advantages of this kind of system are clear – guaranteed sufficient ventilation (assuming correct design and sizing) with reduced heat loss and improved comfort conditions. Such systems may also help to combat dust mites and thus improve conditions for asthma sufferers (see below). The disadvantages include capital costs and running costs, and also maintenance liability.

Passive Stack Ventilation:

If you want designed-in ventilation but without the energy consumption and running costs, you should consider passive stack ventilation (PSV). “Stack” in this case refers to the “stack effect” – a fancy term for the well-known physical property of warm air to rise up. In fact it is not only the stack effect that is used here, but also the “chimney effect” – that of air passing over the top of the duct drawing in air at the bottom. Care should be taken to avoid down-draughts via correct termination at roof level. System manufacturers and suppliers will be able to advise on this and other matters regarding installation.

PSV, in conjunction with trickle vents, can provide ventilation in kitchens and bathrooms. With appropriate location of transfer grilles between these and other rooms having trickle vents, it is possible to provide ventilation to these other rooms too. Ceiling grilles are connected via ducts, as straight and vertical as possible, to terminals, usually located at the roof ridge, as this will provide the longest possible run for maximum stack effect. If bends cannot be avoided these should be as shallow as possible – ideally no more than 30° (45° as an absolute maximum) to the vertical.

The advantages of PSV are absence of energy loads, running costs and noise and also reduced maintenance requirement. Disadvantages include variable ventilation rate and the requirement for straight vertical duct runs, which are not always easily incorporated. Additional extract fans can be installed if required to cover periods of high moisture and humidistat-controlled dampers can be used at the ceiling grilles. More advice on PSV may be found in BRE Information Paper IP13/94

Low Energy Window System

In a recently developed window system, two single-glazed frames are placed close together to form a double-glazed unit as is common practice. However instead of being hermetically sealed, the gap is ventilated via openings at the bottom of the outer frame and at the top of the inner one. This allows air to enter the gap from outside, gain heat and rise up under solar action, entering the room via the upper vent. The vents are automatically controlled according to air quality inside the house and low-E glazing is used to reduce heat loss from the room. The efficiency of the system, which preliminary indications show can save 10-15% of heating costs, is better with a very airtight fabric.

Draught Lobbies

Think of the heat lost from your house when standing on the doorstep bidding farewell to guests! This could be avoided by use of a draught lobby, which is formed via two sets of doors at the main entrances. Many Victorian and Edwardian houses have two sets of doors on the front entrance – the Victorians knew a thing or two.
In order to be effective only one set of doors can be open at any one time – like an air-lock – the idea being that only a small volume of air is displaced each time the entrances are used. To be officially termed a draught lobby, the two sets of doors should be separated by a space at least 2m² in area – this being the area deemed sufficient to wheel a pram or buggy into the lobby without needing both sets of doors open at once. Draught lobbies do take up space and can also interfere with wheelchair access (unless made sufficiently large – in which case they take up more space). However, the lobby can also form a useful place to remove muddy boots and leave wet umbrellas to dry, for example.