In the second of our series, David Weight of Currie & Brown looks at the differences in whole-life costs between a deep-plan, air-conditioned base office building and a shallow-plan scheme that is naturally ventilated

The SAS Institute at Upper Whittington is narrow-plan and naturally ventilated


With the increasing focus on sustainability issues and the need to reduce carbon emissions the whole-life costs of naturally ventilated versus air-conditioned buildings have become particularly pertinent.

Here we examine how lifetime costs are affected by comparing a new-build air-conditioned office building scheme with one that is naturally ventilated. Although this example considers offices, the change scenarios shown here should provide some guidance for many other building types.

Live Options: The whole-life costing computer model

The whole-life costings presented here are based upon Currie & Brown’s Live Options software, which is an integrated suite of geometric, engineering and energy calculation programs. Whether the development is an office, school, hospital or warehouse, the process starts in the same way, by using a basic but faculty-specific design template. This template is modified to incorporate, for example, changes in the size, shape, and operational performance.

The program integrates fabric and building services components and can show how costs change when a building element is changed – for example, increasing the proportion of glazing. It does this by providing a detailed breakdown of changes and shows the capital costs of physical items and the effect these have on building performance. Any changes to heating and cooling loads will be quantified in terms of the effect this has on, say, chillers and the electrical supply loads.

Changes to the energy profile of the facility are also calculated. The program uses this data to assess the cost effectiveness and overall viability of alternative solutions when considered against the facility’s projected economic life.

The model also takes account of repair and maintenance costs. This includes regular inspections, planned and unplanned maintenance and replacement costs for individual building components. These costs are automatically cash-flowed, discounted and summarised and net present costs are shown against each component, which assists in making choices and in value-engineering.

Shown below are the typical capital, energy and repair and maintenance costs for a typical deep-plan, eight-storey, air-conditioned office building. Overleaf are the increases or decreases in those costs when this building is remodelled into a naturally ventilated, shallow-plan alternative.

Changing the office from air-conditioned to naturally ventilated

Here, we take the 12,000 m2 base office building outlined on the previous pages, which is air-conditioned using fan coil units. This is a deep-plan design eight storeys high with the floor area measuring 30 × 50 m.

We want to consider an alternative form, which enables and relies upon natural ventilation. Service engineers Roger Preston Associates advises that even for open-plan use, the maximum acceptable span to achieve reasonable cross flow ventilation and comfort is 15 m.

Therefore, modelling required eight inputs:

  • The average depth on plan was changed from 30 m to 15 m, resulting in floorplates measuring 15 × 100 m.
  • The air-conditioning was taken out, reducing the ceiling void depth.
  • The clear height was increased from 2.7 m to 3 m to assist air circulation.
  • 50% of the glazed area of the facade was changed from sealed to opening windows.
  • Heat recovery was taken out as this cannot be used on naturally ventilated buildings.
  • The stand-by generator for the air-conditioning was taken out.
  • Preliminaries were increased because the naturally ventilated narrower building will need either more scaffolding or an extra tower crane because the building length is increased and there is a greater proportion of external envelope.
  • The average air change rate was increased from the recommended design rate of 12 l/person/sec, to 24 l/person/sec. This is because the ventilation rate is user-controlled, and people may have more windows open than necessary. This does affect energy consumption predictions but not capital costs.

Changes in capital costs

Taking the main elements in order as they appear on the graph above:

  • Substructure: Piling costs increase slightly as there are more columns and more external envelope to carry, although the building weight changes little (green). There is an increase in the external girth of the building, which increases the lengths of ground beams and walls between ground beams up to damp proof course level (blue). The modelled scenario here assumes the site is flat. If it slopes, a narrow-plan building could be cheaper than a deep-plan building if it was laid out parallel to the site contours, as this would save on excavation and retaining walls.
  • Structure: The frame costs increases, as there are more columns and the ratio of perimeter columns to internal columns increases (red).
  • Envelope: The main effect here is the increase in the external girth and area of vertical envelope, including walls (red), windows (yellow), and external shading (pink). The window costs were increased further by the increased cost of opening as opposed to fixed lights. With this scenario the height of the windows stay the same because although there is less space needed for air-conditioning in the ceiling void, this is countered by the increase in the clear span height needed for effective natural ventilation. However, in practice, the shallower building depth means smaller windows could be used for the same amount of daylight inside the building, saving on window and shading costs.
  • Internal division: Because the external wall length increases the internal wall length decreases. An external wall forms a room boundary on one side only whereas an internal wall forms a room boundary on two sides. Because of this any increase in the external wall length will cause a reduction in the internal wall length by half of the external wall length. This is indicated by red (masonry or similar internal walls) and green (for stud walls or similar).
  • Internal finishes: Areas of drylining to external walls has increased (red).
  • Mechanical: A small reduction is made for under-slab drainage as on narrow-plan buildings nearly all the drainage can be put on the building’s perimeter, whereas with deep-plan buildings, more downpipes are needed within it. There is a significant saving and sprinkler protection to the ceiling void (green), which was required on the base scheme as this is normally required where the ceiling void depth exceeds 800 mm. The heating load and boiler capacity have gone up (blue) in response to the increased area of envelope with its associated losses from fabric conduction and air infiltration. Also the length of perimeter trench heating has increased. However, these extras on heating have been outweighed by the savings on distribution pipework, which is much simplified, so there is an overall saving on the heating installation (blue). The yellow denotes the saving on air-conditioning. There is also a saving on deep-plan ventilation (pink). Controls are also much simplified, and there are some savings on commissioning (light blue).
  • Electrical and communications: The saving on pumps for chillers and fans saves on the electrical load (red). Just as sprinklers over the ceiling void were no longer required, the same applies to smoke alarms to the reduced void (yellow). However, the security installation has increased a little, because of the increased girth of the building (light blue).
  • Builders’ work is based on a percentage of the services content, and preliminaries are also based on a percentage (light blue). The effects on external works could be considered, but have been excluded as this is so site-specific.
  • Profits and overheads (light blue) are based on simple but consistent percentage allowances.

Changes in energy costs

Changes in energy costs are illustrated in the table. Prices for fuels are taken as 6.5p/kWh for electricity and 2p/kWh for gas, which is used for heating.

To explain the main changes:

  • Heat losses through the building envelope increase according to its extra girth and area.
  • Air infiltration losses through the envelope increase for the same reason.
  • Supply air is assumed to increase from a controlled average of 12 l/person/sec to user controlled rate of up to 24 l/person/sec. This figure depends upon the occupants. If there spare capacity in the heating system, there is a danger that people will open lots of windows unnecessarily as long as the heating system copes.
  • The extra window area means there is some solar gain. It is assumed that radiators are fitted with thermostatic valves so that this gain reduces the heat demand.
  • The need for mechanical cooling and fans are reduced dramatically, but it is assumed that some cooling is still required to some meeting rooms.
  • Pumps and controls are reduced.
  • Lighting use is reduced in response to improved daylight factors created by the narrower form. It is assumed there are daylight sensing controls on the lighting, or that occupants switch off the lights when daylight levels are adequate. The lights will contribute to the heating during the winter slightly, as there are less lights this contribution is also reduced.

Changes in net present costs of repair, maintenance and replacement

Changes in net present costs over 30 years of repair, maintenance and replacement are illustrated in the graph right. These include:

  • The maintenance of gutters, walls and windows all increase with the extra cartilage of the building.
  • The increase in envelope area and therefore fabric losses has increased the boiler capacity and associated maintenance and replacement costs.
  • Not surprisingly, the big saving is the air-conditioning, which includes chillers, pumps, and fan coil units.
  • Ventilation to deep-plan areas and areas like toilets is all much reduced, because there is much less deep plan area, and toilets can be sited on the perimeter more easily.
  • Lamps last longer because the improved daylight factors should lead to reduced usage. (Daylight sensing controls are assumed.)

Whole-life cost savings

In summary, over 30 years, the combined effect for a change from a deep-plan air-conditioned office to a shallow-plan naturally ventilated office are:

Capital cost changes -£1,891,000
Energy cost changes -£121,000
Maintenance and replacement changes -£1,543,000
Total change in net present cost -£3,555,000

If land costs are excluded, a shallower plan design, which is naturally ventilated, will normally be cheaper both to build and to operate than a deep-plan air-conditioned design. Of course this is dependent on a number of factors. A more compact, air-conditioned, deep-plan design may be better if the plot is very restricted and local conditions dictate against a naturally ventilated building because of outside noise, dust or pollution.

Note to readers

Although the model used by Currie & Brown is good at calculating quantities automatically, this does not replace the need for judgment. Rates may need to be adjusted for economies of scale. Construction time and preliminaries for the form of construction also need to be examined. There will always be matters which sit outside of the model – for example, enabling works such as diverting drains, or works in connection with phasing such as allowing for service diversions and/ or temporary services. The limitations of the model and as well as its strengths must be recognised.

About the author

  • David Weight is a chartered surveyor working with Currie & Brown. Email, telephone 01202-749020 or alternatively contact Liam Kirby at, telephone 020-7834 8400 or 07977 574136

    Thanks to engineer Roger Preston Associates