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20 September 2019

Assessing the Thermal Performance of Homes

photo by Sayan nath

Since early 2014, when I started these occasional blogs with “Does Passive Thermal Design Have To Mean Greater Cost,” I have been writing to explore the principles and factors which influence the passive thermal performance of our built environment. The problem with buildings is that there is a need to 'get it right' at the time of construction because 'upgrading' at a later date is challenging and expensive. But who decides, at any particular time, what is sufficient to ‘get it right’? For the past few decades, it has been the market — the NZ Building Code minimums are treated as maximums and the lowest initial cost is the mantra for many providers of our building stock.

The design, material selections and construction decisions being made this year will be with us for much more than half a century. Fifty years is no time at all in the life of our building stock; think of the recent keen market for pre-WW2 state houses, and the sought-after century-old homes in our inner cities. With society’s slow realisation of the need to prioritise the planet’s dwindling non-renewable energy resources and the contribution they make to climate change, we are beginning to acknowledge the importance of thinking much further ahead with our decision making and the incorporation of passive energy features into our building design and construction. The past decade of promotion of thermal insulation, which has now reached the stage of instructing landlords to upgrade their tenancies, is an example of this realisation. Once installed there is no continuing cost to maintain the greater thermal comfort. This is just the beginning of the possibilities.

In the early 1980s, the importance of the need to consider thermal performance and finite energy resources reached well beyond the Greenies to the point of being discussed in the building industry trade magazines, but it faded away with the expanding ‘market-forces’ counter-ideology sweeping the world. In the 2000s, the Government encouraged energy efficiencies and saving, and through EECA (Energy Efficiency & Conservation Authority) promoted research and the development of the Home Energy Rating Scheme, (HERS). Before HERS could get going, there was a change in political philosophy and the process went into abeyance. Soon afterwards the NZ branch of the not-for-profit World Green Building Council began to develop the more wide-ranging Homestar Rating scheme which is finally starting to gain traction with the general population. The need to save non-renewable energy is nothing new. When I was a youngster we were made very aware of energy shortages through power-cuts, and at the time DSIRO did a lot of research on solar water heating. Once the hydro-electricity network was completed memories faded and it was back to the old prolific ways. This time, hopefully, there will be a permanent shift.

Designing houses to maximise the passive thermal performance of a home is not a simple matter of just throwing in some more insulation — there is a complex inter-relationship of the primary factors of glazing, thermal mass and insulation interacting with solar radiation, diurnal ambient temperatures and the ever-changing wind. I discuss this in my April 2014 blog, “Insulation, Glazing and Thermal Mass: Is there a Simple Relationship?” Since receiving HERS training in 2009 I have been using the software to objectively assess and support the intuitive thermal design principles of experienced architects and designers so as to better inform their projects. This is equivalent to bringing in a structural engineer when a more sophisticated solution is desired. This normally does not involve anything complicated or expensive; often a simple adjustment to the windows can give significant improvement to passive thermal performance. Also, as a Homestar Assessor, I have used the thermal simulation software to calculate the heating and cooling deficits for the specific dwelling so that higher points can be gained for this credit.

Case Studies

Christchurch Triplex Development

As an illustration of what can be achieved without any special attention to looking for a ‘thermal solution’ to the project, my Homestar assessment of the middle unit of a triplex development for Generation Homes in Christchurch, designed by Emerge Architecture of Palmerston North, showed a heating plus cooling demand of only 35.8 kWhr/m² per annum after account is taken of the passive inputs. This is an excellent result for a building which did not have a focus on thermal performance. It was well designed and built to above Building Code standards while still meeting the demands of its market. If a few extra Homestar features had been included it would have easily achieved an 8 Homestar rating.

Stand-alone Auckland House

A standalone house, in Auckland, achieved a calculated heating plus cooling demand of a very low 14.1 kWhr/m² per annum without concentrating design attention only on passive thermal matters. Through his considerable experience, the architect was able to produce a very liveable and aesthetic house, well suited to its difficult site, by considering holistically all aspects of the design of which passive thermal performance was only one factor of many.

Wellington Home with a Homestar 8 Star Built Rating

Recently I undertook a Homestar assessment for a single storey home of normal construction in the Wellington region which gained an 8 Star Built-Rating. It was designed by Richard Wright of Aonui Architecture, an architect with many decades of experience in the passive thermal aspects of building design and construction. The house has two wings — one for living areas and the other for bedrooms, with a glazed sunroom and entry between. The living area primarily relies on passive solar gains to the exposed concrete floor slab (supplemented by a small woodburner) for its heating. The eastern bedrooms have carpet over a hydronically heated insulated slab. The water is heated by free energy from roof-top solar panels. In this case, the heating plus cooling demand is only 25.8 kWhr/m² per annum.

As an exercise, after completing the HERS analysis of the house for the EHC-1 Credit I ran the calculations again with carpet to the Living/Dining/Kitchen area, and then again adding carpet to the 50% glazed sun room. With carpet to the living wing, an additional 25% heating input would be required, and if the sun room were also carpeted then an additional 40% heating input would be necessary.

Given the long life ahead of the buildings we are constructing today, a robust objective analysis of their thermal performance is essential if we are not to perpetuate the mistakes of the past.

Through EcoRate Ltd – Architect I provide independent analysis and advice, to architects, designers, builders, manufacturers, and others in the construction industry, included those proposing to build a new home. As a Homestar Assessor, I’m able to assist with providing the thermal modelling requirements for the EHC-1 Thermal Comfort Credit.

For more information on passive solar design contact Keith at Ecorate on 021 890 251,, or visit our website:

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