b) In a multi story office block with a flat roof a vapour barrier would be necessary to allow natural convection of warm air rising to escape through the roof and hence prevent interstitial condensation.
The vapour barrier is likely to be installed in the warm side of the roof (internal) so that condensation in the cold side (external) can escape. If a vapour barrier is not present continual build up of condensation would occur in the ceiling and ceiling space, this would dampen materials and if not controlled, lead to problems such as decay in timber or corrosion in metal fixings.
Flat roofs can cause a problem as there is a lack of airspace for the hot air to escape and the taller the building is, the more the flow of moist air increases as hot air rises because it is less dense than cold air. The flat roof also brings both internal and external temperatures closer together and this affects thermal transmittance. The vapour barrier is an impermeable membrane that helps the warm air in the building escape thus reducing the likeliness of any potential moisture problems.
Other factors, which affect thermal resistance in airspaces, are ventilation and surface emissivity; for example, a concrete deck roof generally has a higher vapour resistance in comparison with a metal deck roof therefore concrete copes better emitting water vapour.
Task 2
Give an outline description of how a modern 3-storey office block could be constructed as a “high thermal mass” building, and state the advantages of this method of construction for the temperature control and ventilation of the building.
A high thermal mass building (HTM) incorporates building materials which have good thermal storage properties because they absorb, store and release heat when necessary ensuring the internal temperature within the building is more constant throughout all seasons of the year. Heavy building materials such as concrete generally have higher thermal masses in comparison to lighter building materials e.g. timber. Thermal capacity of a wall is often enhanced by painting it a dark colour as it absorbs the heat. HTM delays heat flow through the building envelope therefore making the building warmer in the winter and cooler in the summer.
If constructing a 3-storey office with HTM, it may be an idea to use precast concrete for both the floor and roof; and concrete blocks to form the external wall. As mentioned in question (1a), louvres may also be incorporated into the design; these could be controlled by a building management system (BMS), a computer that automatically monitor and controls the building conditions automatically.
HTM is particularly useful for temperature control as it helps maintain a constant internal temperature all year round, less heating is therefore required making it more economical. A building with HTM uses night ventilation, which provides passive cooling. This is well explained by Nick Barnard of FaberMaunsell:
“Outside air is circulated through the building where it comes into contact with and cools the building fabric. The cooling that is stored in the building fabric is then available to offset heat gains the following day and keep temperatures within comfort levels.”
The higher the thermal mass of the building the more cooling it can store.
Barnard goes on to say that it is most suitable for offices and when using night ventilation in conjunction with HTM, you reduce the requirement for mechanical cooling, yet again making the building more economical and environmentally friendly
Task 3
a.) Cellular beams have an advantage, to the Structural Engineer, in having a higher strength to weight ratio than a standard UB section.
What benefit does their use give to the Services Engineer?
b.) A services shaft makes a convenient means of routing M & E and waste disposal services to each floor of a multi-storey building for the Services Engineer. What benefit can this be to the structural Engineer, particularly if integrated with the lift and stair wells?
a.) Ever since cellular beams were introduced in 1987, service engineers have benefited as they allow services such as lighting and electrical, to be passed through the circular openings in the beam. Universal beam (UB) sections do not have openings and are therefore less convenient for services engineers as they need to pay more consideration in how they are going to arrange the services around them. The diagram below illustrates both types of beam. A suspended ceiling may be used to hide the services, when a universal beam is used. This would be another expense involved in the project and in a multi-storey design can reduce the amount of storeys, due to the reduced floor to ceiling height.
b.) Structural engineers benefit when a service shaft is incorporated in the lift and stair well of a multi-storey building as it makes designing and construction less challenging as they do not have to consider the location where they are going to integrate the service shaft i.e. preventing the necessity of drilling holes in ceilings or floors etc. for services as the void provided by the lift or stair well, is ideal. A services shaft will also reduce any conflict between the designs of the structural and services engineer as they both know where the services are going to be located and conveyed from on each floor. Maintenance is also easier to carry out as the service shaft is in one specific place, and not all over the building.
Bibliography
Websites
- Explanation of the Earths seasonal variations and the reasons behind them.
- Article from the ‘Sustainable Energy Development Office of the Government of Western Australia’ about windows and shading.
- A brief explanation of louvres.
- ‘Canadian Building Digest’ commenting on vapour barriers in ceilings and roofs.
- This was a 6-page document about thermal mass and I found it extremely useful when answering Task 2.
- Bringing the Environment into the Workplace - this was yet another useful resource in answering Task 2, and highlighted concrete as a useful material when attempting to create a high thermal mass.
- FaberMaunsell produced this article, which I found useful when finding out how night ventilation occurs in conjunction with thermal mass.
- This website explained when cellular beams were first introduced and had some useful diagrams on.
Books
March & Roy, F & D. (1995) Flat Roofing – A Guide to Good Practise. London: Bryant Creative! - This book explained various information about flat roofs.
McMullan, R. (2002) Environmental Science in Building, Fifth Edition. Hampshire: Palgrave Macmillan - I used formulae and diagrams from here to help answer Task 4.