DESIGN MASTERCLASS 8 HEAT LOSS
ENVELOPING ISSUE T
MASTERCLASS
Professor Doug King
This month’s article questions whether simple, steady-state heat loss calculations will continue to be of use in a world of high-performance buildings
he design of building envelopes presently falls into an unhappy void between the responsibilities of the architect and of the engineer. I
believe that, as guardians of building carbon performance, building services engineers should be responsible for the performance of the building envelope. However, to the architect the envelope is the means of sculpting the form of the building and generating its outward appearance. We must therefore be prepared to collaborate more closely and develop a clear understanding of envelope thermal performance issues in order to be able to communicate these with our architectural colleagues. As we approach the limit of efficiency
gains in equipment and systems, we need to achieve a step change in our understanding of building envelopes. The building physics that underlie thermal performance is well developed and we have sophisticated software to help us. Nevertheless, it is still essential for engineers to have a good understanding of
the principles of heat transfer and storage, so that they can validate or challenge the veracity of the results generated from software. When I started my career in building
services, the building envelope was entirely the architect’s responsibility and calculating heat losses was a simple business. Everybody assumed that construction was homogeneous and the Building Regulations had prescriptive standards for insulation. All the engineer had to do was choose the design temperatures and undertake steady state heat loss calculations using the U-values prescribed. This is far from the case in the present day. Insulation standards have increased dramatically and it is simply not the case that you can go on adding insulation to a wall or roof and things will keep getting better. Adding insulation to traditional constructions changes the temperature gradient and thus introduces the risk of interstitial condensation. So, as we increase the insulation and air tightness of our buildings, we must also attend to the transit of moisture vapour through the construction, or provide means to exclude it. From 1985 the Building Regulations
required us to account for repetitive thermal bridging (construction elements that spanned the insulation thickness, such as framing in the walls of timber housing). This meant the steady-state calculation had to be expanded to include the linear conduction of thermal bridges represented by the psi-value. As insulation requirements increased
The traditional U-value calculation models a simple, linear flow of energy from the internal air to the external air by conduction alone (accounting for the resistance of a stagnant air boundary layer). In fact, to represent the true picture, we ought to account for absorption and emission of energy by radiation, differential conduction into the internal face at high and low levels due to stratification, and for three dimensional flows within the construction
in subsequent revisions to the Building Regulations, it became necessary to insulate overstuds and rafters to reduce the conduction at thermal bridges, but this made calculations almost impossible without finite element analysis software to calculate the three-dimensional heat flows. The notional building used in Part L 2010 includes psi-values for all the common
38 CIBSE Journal August 2011
www.cibsejournal.com
Doug King
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