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15 December 2014

Insulation, Thermal Mass and Glazing: The Juggling Game

Over the past year I have been writing this blog as a means to, in a small way, stimulate discussion and critical thinking on the thermal performance of our build environment, especially the design and construction of the housing and small commercial sectors. While large commercial, industrial, educational, and public buildings are a large proportion of the construction dollar spent each year, they are relative few in comparison to the far greater number of small projects with thermal performance decisions being made by a multitude of clients and designers. Improvements in this area of the construction sector will have a disproportionate positive effect on the energy performance of the built environment.

Over successive blogs I have written individually of insulation, thermal mass and glazing as primary drivers of performance, but these are each strongly influenced by the physics we learnt at school – conduction, radiation, and convection. When designing, consideration of both the individual and combined effects of these environment factors is just as important as the selection and placing of the construction materials. For example, (and excluding shading, curtains etc.), a window on the north and west elevations of a room will reduce the need for supplementary heating, whereas the same window on a south wall will increase it. Radiation dominates in the first, and conduction and convection in the second.

I have written of a 'sweet-spot' where all these individual variables would come together to produce the 'perfect' thermal answer, but this is a fiction. While for a given site theoretically there could be a perfect design/materials answer, it would only be so for a moment. As soon as the sun goes behind a cloud the radiation factor diminishes. If there is little thermal mass the temperature difference between day and night will fluctuate significantly to the point where comfort might be compromised. This being said it does not mean that a 'practical sweet-spot' cannot be strived for, after due allowance is made for costs, aesthetics and durability etc. Just because a theoretical target is unattainable does not mean that any effort to move towards it is wasted; there is much to be gained for both the individual and the planet by doing the best we can.

Recently I analysed a house where an additional zone of insulation, constructed on the inner face of the structural exterior wall, was being considered. While this did produce an insulation gain, the client needed to consider the additional construction cost and time, and the reduced room dimensions, against this small benefit. There is also the 'embodied energy' aspect of the construction to be considered. The room had a high proportion of glazing so the extra insulation had less influence than if there were smaller windows, but with smaller windows there would be less solar gain for the insulation to contain. Add in the variable of thermal mass and the calculations change again. There is always the option to don a jersey as the last fine tuning of an individual’s ideal comfort level.

A holistic approach to the total design is necessary for good building; site selection and proposed use have the greatest influence.

As a different example, I analysed an adobe/rammed earth house where the exterior walls have an R-value below R0.5, (non-compliant with NZBC-E3 internal moisture); the windows are single glazed; and the roof was just above R2.9, a failure by common building standards. By the 'Modelling Method' this house was shown to have a Building Performance Index (BPI) of 1.23, well within the limit of 1.55, and a HERS rating of 5½ stars. Although the result was achieved through thermal mass, it was intelligent and knowledgeable design which was vital to the pass – a different plan and elevations could have failed.

These days the commercial world uses computer simulation software to quickly and objectively explore the multiplicity of business planning options and variables in a way that could not be done manually before. Through the analysis I do, the same objectivity can be applied to the many thermal performance variables as a check of the initial intuitive design decisions. Even so, once a reasonable level of thermal performance is achieved, in my opinion, liveability, aesthetics and buildability should still take precedence. After all, leaving the back door open after rushing in for dinner throws all design calculations to the wind.

In closing the year I would like to thank those who responded with comments and differing viewpoints as thermal design is not a simple 'textbook' exercise where standard construction details can be applied without thought. We can all learn from each other.

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