Whole life costing: Natural ventilation in offices
Natural ventilation not only makes financial
sense but also offers a sustainable solution to
Peter Mayer of Building LifePlans examines the whole-life costs.
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 Arups, 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 of partitions etc, internal wall reflectance, usage patterns, internal energy input, ventilation rate required. and required ventilation rates.
• Window issues Area, orientation, glazing system, window 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 organisations 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 opening windows. Stack ventilation may be an alternative in these cases..
Natural ventilation options
The simpler the window system, the lower the capital costs 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.
Manual 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 fan light 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 long run. Specify stainless steel variable geometry friction hinges, preferably in austenitic stainless steel. Corrosion resistance is judged by EN 1670. Class 4 indicates suitability for polluted environments and offers greater corrosion resistance than Class 3 which is acceptable for wet or slightly polluted envionrments. 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 which 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 electrical motors that operate chain, pistons or spindle mechanisms contained in metal, plastics 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.
Net present value for 60 years£/m²
Manual operated windows
Top hung casement: Aluminium windows frame to BS 4873. Finish: anodised to BS 3987 minimum thickness 25 microns. Insulating glass units to EN 1279. Austenitic stainless steel hinges to BS EN 10088–2 grade 1.4301; UK type 304.
Aluminium window as above with upper fanlight and outward opening casement
Top hung casement, aluminium window frame as above with basic coated mild steel trickle vent providing 400mm² opening per m² of floor area.
Powered window actuators
Top hung casement, aluminium window frame as above operator remote control window control.
Top hung casement, aluminium window frame as above automatic window control based on temperature and rain sensors.
• The natural ventilation options are listed in order of increasing control and sophistication. The aluminium side hung casement window is the reference window to which the other options are added. Life expectancy for powered window actuators and controls is 15 years.
• Only aluminium windows are considered, as this analysis is about the whole life cost implication of alternative natural ventilation approached rather than a comparison of window frame options.
• Cost data relates to capital costs, replacement and maintenance costs only. Energy costs are not included as these will vary by building, location and use. The difference between the options in net present value gives an indication of the energy savings required to justify a solution on costs grounds alone.
• A whole life cost analysis including energy usage, fabric components and related functional requirements such as smoke control, should be carried out for specific buildings to assess the viability of natural ventilation options.