Costs: Natural ventilation for offices

Naturally ventilated offices have 25-50% lower annual energy costs than air-conditioned offices. A recent British Council for Offices report on office sustainability, undertaken by multidisciplinary consultant Arup, suggests natural ventilation has a payback period of two-and-a-half years.

Getting the right system and strategy for an office requires careful design and management to take into account a multitude of factors. These include:

• External environment Temperature and rainfall regimes, cloud cover, wind direction and speed
• Office building issues Thermal mass, shape, height, layout, internal wall reflectance, usage patterns, internal energy input and required ventilation rates
• Window issues Area, orientation, glazing and shading.

Design guidance may be based on BS 5925, Code of Practice for Natural Ventilation. There is also a wealth of information on natural ventilation strategies, detailing and energy efficiency from bodies such as CIBSE and BRE.

Large deep-plan offices or where external traffic noise levels are unacceptably high or where security is a risk may preclude the use of openable windows. Stack ventilation may be an alternative in these cases.

Natural ventilation options

The simpler the window system, the lower the capital and whole-life costs – although at the expense of climatic control. More expensive options offer better control and management of internal conditions in response to the external weather and climate. There is no perfect solution; if there were, all offices would employ the same ventilation solution.

Manually operated windows

The opening arrangement is a balance between ventilation control, air flow and weather protection.

Windows with an upper fanlight and an outward opening casement offer the best all-round performance but at a higher relative cost. The upper fanlight ensures controllable ventilation on breezy days without disruptive draughts at workspace level as well as allowing secure, night-time cooling.

A top-hung outward opening casement offers a medium cost window option with reduced ventilation control.

Hinges are a small cost of the whole window assembly but critical in ensuring satisfactory long-term functionality as this is the bit that wears out. Careful specification of the hinge system pays dividends in the long run.

Specify stainless steel variable geometry friction hinges, preferably in austenitic stainless steel. Corrosion resistance is judged by EN 1670. Class four indicates suitability for polluted environments and offers greater corrosion resistance than class three, which is acceptable for wet or slightly polluted environments. Proof of performance by cyclical testing is a measure of resistance to wear and fatigue. Hinges may be tested to 30,000 or up to 50,000 operating cycles representing 25 to 50 years’ typical use. Low friction washers and sliders ensure smooth operation and minimise risk of metal-to-metal wear.

Trickle vents allow ventilation during winter without excessive heat loss. A range of options are available, from the basic with no control to types that allow manual control of air flow, through to automatically controlled systems by pressure or pollution sensors.

Powered window actuators

Natural ventilation works best when office users understand the system. Powered window actuators are a means of making manual control easier. The more complex the ventilation and control system, the higher the initial cost as well as servicing costs.

Window actuators are based around electric motors that operate chain, piston or spindle mechanisms contained in metal, plastic or concealed housings. Specification issues to consider include appearance, protrusion into the room, noise, range of movement, operating pressure, battery or mains power supply and energy use.

There are no specific standards by which to assess actuators. Initial installation is an important factor in future performance. A guide to long-term functionality may be assessed from cyclic bench tests: manufacturers test from 10,000 to 100,000 operations.

Users may control windows using fixed switches or remote control devices. Linking two or more opening lights to an actuator is a cost-effective strategy.

Automatic window controls are a more costly option combining a sensor with the actuator; this may be part of a building management system. The most effective sensor control systems operate the windows in response to internal and external temperature and rain sensors. More sophisticated systems may include sensors that adjust windows in response to carbon dioxide levels, wind speed and direction, solar gain, air quality and time.