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21 June 2018

Can Thermal Performance Be Measured Objectively?

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In the past designers, builders, councils and homeowners have been happy to apply the minimum requirements of the NZ Building Code, clause H1 (Energy Efficiency) to the design of new homes, but there is a growing realisation that higher standards should be aimed for. This is especially so given that a house is around for many decades, during which it is expensive and difficult to make significant changes to the primary fabric of the construction.

Often the requirements to meet a building standard are treated as a maximum, rather than as the minimum that they are. The New Zealand Standard associated with NZBC-H1 (NZS 4218:2009 Thermal Insulation – Housing and Small Buildings) recognises this and states in its foreword that: "The construction R-values in this standard result in a low life cycle cost... Applying these construction R-values does not necessarily achieve good passive solar design. Better thermal performance can often be achieved by applying good passive solar design principles."

At the time of writing this, June 2018, the news media are reporting comment from economist Shamubeel Eaqub regarding the opportunity provided by KiwiBuild to construct a large number of dwellings with an emphasis on thermal performance rather than on minimising the initial cost. While this change of focus may slightly increase the purchase price, over the lifetime of the home the heating/cooling costs will be significantly reduced, as well as the comfort increasing. For the occupants it is the sum of the capital (mortgage payments) and regular energy costs which are of concern for the family’s monthly budget.

Councils cannot demand a higher standard than that prescribed by the NZBC, but often for subdivisions higher levels of thermal performance are put into place by council through the resource consent process. I recently came across a development where the dwellings to be built on the 39 sections were each to have minimum insulation R-values of R2.6 for walls, R4.6 for roofs, and R2.0 for the floors. While this is laudable, it is also very simplistic if it is trying to increase the passive solar performance of the building. However, at least it is a recognition of the need for improvement. The role of windows is crucial but difficult to regulate. The variables of dimensions, glass, frames, and especially orientation have a far greater influence on the thermal performance of the home than just adjusting the insulation R-values.

It is easy to pick on insulation as the means to improve thermal performance, but where available, the flow of heat, outwards and inwards, takes other less restrictive paths (e.g. windows), thereby by-passing the insulation. In my blogs of August 2014 ‘Is More Insulation the Best Solution?’, and September 2017 ‘There is More to Insulation Than Just the R-value’, I discuss this emphasis on insulation as the solution. The problem with focusing on insulation is that it ignores the importance of windows and their placement, as well as the role of thermal mass.

The same foreword to NZS 4218:2009 states that: “In the New Zealand climate, good passive solar design incorporating appropriate areas of glazing and thermal mass, combined with a well-insulated thermal envelope, can significantly reduce the energy requirements to heat and cool buildings." The problem with this inter-relationship, as I discuss in my blog of December 2014 ‘Insulation, Thermal Mass and Glazing: The Juggling Game’, is that the number of variables to be considered, even only taking the primary ones, is too complex to be analysed manually. This is why during the 2000s, the government developed the HERS (NZ Home Energy Rating Scheme) through EECA (Energy Efficiency & Conservation Authority). The scheme used thermal simulation software from CSIRO Australia, adapted to 18 climate zones throughout the country, as a means of evaluating the quality of NZ housing. The scheme was put into abeyance around 2010 primarily due to the complexity of its use and the public disinterest in star-ratings. Even so, the analysis is still valid and accepted by councils to show NZBC-H1 compliance by the modelling method.

Since that time, the Homestar rating scheme has been developed by the NZ Green Building Council as a broad measure of the sustainability of our housing stock, with a portion of the assessment considering the thermal performance of the dwelling. The scheme recognises that the simple NZBC compliance standards are not sufficient for good long-term thermal performance, and so requires that if more than 7 stars are to be gained (out of a possible 10 stars), then computer modelling is required for the thermal analysis credits. (N.B: Of course the final rating is dependent upon the total points gained from all seven assessment categories). This mandatory limit is appropriate because as performance improves, a building and site specific analysis is important.

Without using thermal simulation computer modelling, houses are too complex, with too many significant variables to have their thermal performance objectively assessed by manual or tick-box methods. Even so, these answers are not perfect because buildings exist in a dynamic environment where the ever-changing weather and comfort preferences of the occupants upset the performance of the building, unchanged from when constructed.

In addition to demonstrating NZBC-H1 compliance, since 2010 I have been using the HERS software analysis to assist architects, ADNZ designers, developers and homeowners to refine their designs through the exploration of plan, elevation, orientation and material options using the objective results from the analyses.

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