A wide range of factors need to be considered when specifying a louvre system to ensure improved air flow to HVAC systems while providing protection against rain ingress, says Simon Hunter
An estimated 45 per cent of the UK's carbon dioxide emissions come from buildings: principally through space heating and cooling, water heating, lighting and other building systems.

In an effort to reduce their environmental impact, public and private sector clients are seeking to commission ever-more sustainable buildings. The emphasis is on 'fabric first' solutions, incorporating high levels of natural light and ventilation, plus improved insulation, combined with more energy-efficient building systems
such as climate control.

Louvre systems have an important part to play in improving buildings' energy performance. Needed on most buildings, these systems allow air flow, both intake and exhaust, to HVAC and other building systems, while protecting these openings against rain ingress.

As well as allowing natural ventilation, an efficient louvre system can improve air flow to building systems. In the case of HVAC, this can sometimes mean a smaller and less powerful system is required for space heating and cooling, potentially reducing both initial outlay and running costs.

Crucial to specify correctly
However, to help improve a building's energy performance and get the best possible results from a louvre system, it is crucial that it is specified correctly.

System choice will depend upon the required balance of air flow with water ingress. There are three main types, offering varying levels of weather protection and air flow: screening, standard drainable and performance louvres.

Screening louvres typically have simple blade shapes, which allow good air flow but give limited rain defence. They are suitable for locations where water penetration will not cause significant problems and economy is the primary consideration, such as screening of rooftop plant or in a multi-storey car park.

Standard drainable blades give good air flow and some rain defence but do not perform so well with wind-driven rain. They are used when occasional ingress is not a major concern and where some drainage of cascading water is required.

Defence against wind and rain

Storm-resistant or performance louvres typically provide moderate to good air flow with excellent defence against wind-driven rain. These are used when high levels of ventilation and maximum protection from the rain is needed, for instance when sensitive equipment such as HVAC plant is behind the louvre. They have blades with complex profiles that force air entering the louvre to change direction rapidly and shed water droplets that then drain away.

Traditionally, louvres are specified based on free area (typically 50%), calculated by measuring the clear distance between the blades and multiplying it by the width of the louvre panel (or height, if the blades are arranged vertically).

Free area is dictated by the size of the louvre. Industry commonly uses a 1m by 1m louvre when quoting free area but, in reality, louvres are never precisely this size. In fact, the smaller the louvre, the lower the proportion of free area, as it is affected by the space taken up by the head, cill, jambs or mullions. Free area is also affected by other factors, such as additional structural support, or if bird screens or insect meshes are fitted.

Perhaps most importantly, free area alone does not take into account how the air flows through a louvre. All louvres restrict the passage of air and this resistance, the pressure drop, dictates how much air gets through the louvre and therefore how much can effectively be used. Too high a pressure drop and not enough air will be allowed through, which can cause a rapid temperature rise inside a building, in turn causing problems with plant such as generators and HVAC equipment.

In fact, louvre specification should consider a number of factors, including site location, prevailing weather conditions (in particular wind direction) and the location and exposure of the louvres (which will dictate air flow rate and the amount of potential wind-driven rain).

Balanced with required air flow

These should then be balanced with the required air flow, the maximum acceptable pressure drop, the degree acceptable water penetration and finally, the building's exterior design, which can dictate where louvres can be placed. Aesthetics should not compromise performance, however. For example, hiding louvres behind features or perforated panels can increase resistance to air flow.

Louvres come in a wide range of designs and colours to suit most buildings. Those with visible mullions, such as Construction Specialties RSH-5700 and RSV-5700 performance louvres, can be used as a design feature, to line up with curtain wall panels or windows. Visible mullions bring additional benefits, as they are designed to be part of the drainage system.

A louvre system using hidden mullions gives clean, architectural lines because the support system is behind the blades. These are normally used on buildings with multiple openings.

Additionally, most performance louvres can be shaped, however, it is important to realise that performance can be reduced, as the drainage characteristics of their complex blade profiles can be affected.

By considering all of these factors, louvres can help improve building energy efficiency. For example, a well-designed louvre will reduce the pressure drop, allowing a larger volume of air through and thus cutting the amount of fan power needed to deliver the required air flow. Conversely, poor specification can lead to rainwater ingress, not enough ventilation, wasted energy and poor performance, not only of the equipment being ventilated but of the whole building.

Louvres are an excellent way of helping building designers, engineers and architects meet ever-more demanding environmental legislation and client requirements for sustainable buildings, playing an important role in improving energy efficiency, lowering power consumption and reducing carbon emissions.

However, louvre design has developed in response to demand for better rain protection and more efficient ventilation. As a result, performance characteristics have also changed and traditional methods of louvre selection based purely on free area are no longer valid. Assessment of a wide range of contributing factors is needed to ensure that louvre systems provide a balance between ventilation, rain water protection, energy efficiency and aesthetics.

// The author is product manager of louvres at Construction Specialties //