The 'hole' truth about ventilation

As buildings have become progressively more airtight in order to improve energy efficiency, the need to dilute indoor air and provide the necessary number of air changes to keep occupants healthy has become a more exacting science.

Building services engineers have tended to focus on increasing the quantity of air in the space to dilute contaminants, reduce stuffiness and manage the relative humidity (RH). However, the presence of increased levels of outside air pollution has made this a trickier balancing act.

Designers must now take into account a dizzying array of factors when deciding what approach to take including: energy efficiency; IAQ; aesthetics; the presence and type of contaminants; the impact of potential noise pollution; weather effects; and security.

All this in tandem with a sealed building envelope means there is a lot more to designing ventilation intakes and the weather louvres that protect them than simply calculating aerodynamic performance and keeping out the rain.

Success

Avoiding unintentional openings in the building envelope is essential to reduce heat losses, but equally important, therefore, is the role of purpose provided openings (PPOs), which are critical to the success of any type of ventilation system. Whether mechanical, natural or mixed-mode; in the end every ventilation system relies on the PPOs to exhaust stale air to the outside of the building and introduce fresh air into the inside of the building.

With all the factors currently impacting on ventilation performance, there will have to bea lot more attention paid to the position and nature of these vital ‘holes’ and how they are protected and supported by louvres.

Our Building Regulations have traditionally focussed on progressive improvement of fabric insulation through better insulation and airtightness. Ultra-low infiltration rates are, therefore, crucial when it comes to meeting energy efficiency standards and in achieving BREEAM points.

However, if you ‘build tight you must ventilate right’ in order to avoid suffocating the occupants and increasing condensation and mould problems. Higher rates of asthma and other respiratory conditions have been reported in many modern housing developments, often as a result of the ventilation failing to adequately compensate for the heavily insulated structure – either through misapplication; poor design or inexpert operation.

Indoor air quality (IAQ) is increasingly vexing building designers and operators against this tightly engineered backdrop. If your structure is not ‘leaky’ you can better measure ventilation rates and design the system more accurately, but you must also correctly size and position PPOs in order to achieve acceptable IAQ. Positioning a PPO is not a random exercise.

To demonstrate acceptable air permeability, new buildings that are not dwellings must be subject to pressure testing unless <500m2 total useful floor area or a default value of 15m3/(h.m2)@50Pa is used to calculate the buildings emission rate (BER). The maximum permitted air permeability is 10m3/(h.m2)@50Pa.

Traditionally, designers have focused on air quantity (not quality) with prescribed ventilation rates such as one air change per hour or up to 10 litres per second per person. However, this approach does not take into account the type or effectiveness of the ventilation system. Designers have come to realise that the problem is not one of ventilation rates, but of pollution levels – ventilation is just one part of the solution and it is current practice to use CO2 levels as a general indicator of IAQ.

The current furore around the quality of outside air with the UK embroiled in a legal challenge from the European Union over our pollution rates and 10 British cities found to be exceeding World Health Organisation (WHO) limits for air toxicity puts our efforts to protect building occupants in a significant context.

Hazardous

The presence of odours, traffic fumes, particulates, pollen, products of combustion, which can be seriously (and invisibly) hazardous to health can be minimised by carefully siting air intakes; minimising face velocities; the use of appropriate filtration and the siting exhausts to avoid re-entry of contaminants. Simply increasing ventilation rates will not, necessarily, solve the problem and it may drive up energy costs.

The design of the building can, however, compromise a designer’s best efforts. For example, aesthetic considerations may dictate that PPOs are not positioned in the most advantageous place and openings are smaller than desired so creating higher air velocities that can lead to draughts and unbalanced ventilation across the building.

Terminations may also need to be hidden or disguised and the designer may need to consider particular specialised finishes to ensure louvres are in keeping with the appearance of the building.

Engineers must also take prevailing weather conditions into account to ensure protection from wind and rain and correctly specified louvres are crucial to this aspect of the system. The potential for noise pollution is also a key consideration that requires sensible location of intakes. Lowering air speeds inside ducts through careful design will also reduce ambient noise, but the team may consider whether sound attenuation is also required.

In order to achieve a successful ventilation strategy while also reducing energy usage, the engineering team should look to minimise cooling loads wherever possible. This requires siting air intakes away from objects or surfaces that may become hot and positioning them in shaded areas wherever possible.

They should maintain separation between intakes and exhausts; and enhance summertime ventilation rates to achieve as much ‘free cooling’ as possible – including the use of night-time cooling.

While some ventilation systems are specifically wind driven, all systems can be impacted by wind effects so the engineering team should consider the building’s orientation and, therefore, wind exposure and the potential for wind driven rain (or sand). They should also be aware of pressure differences between elevations.

Security is another issue that is bound to concern building users and obvious precautions like the size of openings and the position of low level intakes used with night cooling require particular attention.

Louvres are commonly used to treat PPOs and allow the passage of air into or from an air distribution system or part of a building whilst restricting the entry of rain. A wide range of designs and finishes are available to suit most applications, which means they can support an architect’s vision for how a building should look while also allowing a good level of flexibility for the ventilation designer.

Weather louvres are primarily designed to permit airflow and good aerodynamic performance can reduce energy consumption, but if insect screens are in place this can reduce airflow by up to 20%.

Contaminants

Care should be taken when designing plenums and duct transitions to ensure that the volume of intake air is evenly distributed over the whole of the inlet area and localised excessive face velocities are avoided to reduce the risk of entraining contaminants.

For best results; louvres should be sized to operate at a low and even air speed preferably at <1m/s as over-sizing risks pushing up the capital cost; compromising the aesthetics; and leading to greater heat losses. Under-sizing can, equally, lead to other disadvantages such as greater rain penetration; entrainment of pollutants; higher energy consumption; and noise/vibration.

However, specifiers should also ensure that products under consideration have been tested to BS EN 13030:2001 Ventilation for Buildings. Terminals. Performance testing of louvres subjected to simulated rain.

The standard test involves a 1m2 sample louvre subjected to simulated wind at 13m/s (30mph) and rainfall in the shape of water sprayed at a rate of 75l/hr.

Further specific guidance for good practice is detailed in BS EN 13779 (2007): Ventilation for non-residential buildings – performance requirements for ventilation and room-conditioning systems. BSRIA’s Weather Louvre Specification Guide (BG 36/2012) also gives comprehensive advice.