BUILDING DESIGN MODELLING


DISCOUNT ON CIBSE GUIDE A


CIBSE Guide A: Environmental Design is available online free to CIBSE members through the Knowledge Portal. It can also be purchased in hard copy (by both members and non-members) at a discount of 50% by using the code GVAMAR. A new edition will be published in the second half of 2012. Visit www.cibseknowledgeportal.co.uk


Additional features While developing the PDA, the development team at Arup supplemented the methodology with a number of additional algorithms that support features including the ability to specify a variable ventilation rate and the use of U- and G-values to specify glazing. Similarly, walls and floors can by defined


by their U- and Y-values, or alternatively by listing the materials that make up each layer of the construction element. Another useful feature is the ability to specify weather data for any location in the world.


Overheating risk As with all tools, users should be aware of any limitations they may have and, in respect of the admittance method, this largely relates to the assumed steady cyclic state that represents a repeating 24-hour period. The implication of this is that the PDA


results are based on a period of similar external conditions – that is, the room has experienced a repeating load profile during the preceding days. Actual weather patterns are, of course, generally more erratic. However, this limitation is not as significant as might be expected, particularly when using the PDA to predict peak summertime conditions during a warm spell, when external conditions are


generally repeated over several days. This makes the PDA particularly useful for investigating the risk of overheating in new and existing buildings subject to either normal or warm external conditions – the latter being representative of a hot summer or the more frequent summertime conditions we are likely to experience in response to future climate change.


Software validation To check that the PDA is sufficiently accurate, it has been subject to a series of CEN validation tests and has also been compared with results from the Arup in- house thermal modelling program (Oasys ROOM). In general, the exercise found that the


PDA gives results that are very consistent with those of the CEN tests4 ROOM5


and Oasys . This demonstrates that the


admittance procedure is capable of reproducing the results of more complex calculation methods when the same assumptions are made. However, a problem that did come to


light was the treatment of thin internal walls in the admittance method, which tends to break down when a small thickness is specified. This issue, along with other details of


the PDA development and validation, is the subject of a technical paper6


that


has been submitted to CIBSE ASHRAE Technical Symposium being held in April in London. Hopefully the the PDA tool – the download details are below – will make a helpful contribution to the understanding of building physics and passive design. CJ


l Tom De SaulleS is the building sustainability manager at the Concrete Centre. Version 1.0 of the PDA can be downloaded without charge at www.arup.com/ Publications/Passive_Design_Assistant.aspx


REFERENCES 1. Danter. E., Heat exchanges in a room and the definition of room temperature, IHVE Symposium, June 1973.


2. Loudon A. G., Summertime temperatures in buildings without air conditioning, BRS CP 47/68, 1968.


3. CIBSE Guide A (Environmental Design), Section 5 – Thermal response and plant sizing, 2006.


4. CEN European Committee for Standardisation. BS EN ISO 13791 Thermal performance of buildings – Calculation of internal temperatures of a room in summer without mechanical cooling – General criteria and validation procedures, 2004.


5. www.oasys-software.com The PDA tool is now available to download free. Visit www.arup.com/Publications/Passive_Design_Assistant.aspx


6. White A. et al., Passive Design Assistant – A tool to elucidate the principals of passive design, CIBSE ASHRAE Technical Symposium, April 2012 (paper awaiting approval).


62 CIBSE Journal March 2012 www.cibsejournal.com


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