TV and building product merchants continually tell us that our existing houses need more insulation, and over time the New Zealand Standards and the Building Code have increased the insulation requirements (and probably will continue to do so). Does it follow from this that 100% insulation will be the best? Obviously not, because a building with a 100% insulated exterior skin is often referred to as a 'cold room' or refrigerator.

Yes, thermal insulation will always work – but not always as you planned. Insulation only starts ‘working’ when the temperatures of the spaces on each side of the insulation are different. We all know that 'heat' flows from the hotter to the colder area until an equilibrium is reached, but what is important is the speed of this flow: the more efficient the insulation material, the slower the transfer of the heat (but it will still flow). 

With buildings in the real world there is a problem in that the temperatures on each side of the insulated exterior wall, floor, ceiling or roof are continually fluctuating – day cf., night; summer cf., winter; shaded cf., not shaded; etc., and so the direction and speed of flow is constantly changing.

Sometimes the heat within a building is moving from the interior and at other times into the interior, and yet the construction of the wall, floor, or roof cannot be changed; what was built at the beginning has to perform for all the continually changing situations. The exception to this is a window, where curtains can easily modify the thermal performance. Unfortunately this form of temperature control doesn’t always work, especially if you have paid a million dollars for the night-time view, or the curtains are too short.

We are all familiar with R-values as a measure of insulation performance of materials, and how they increase with greater thickness, but this figure is not the R-value of the completed wall into which it is fitted.  A normal wall has timber or steel framing, (lower R-value materials), which connects the inner and outer faces of the wall through the insulation.  ‘Heat’ is lazy and so flows through the frame instead of struggling through the insulation. This lesser actual performance is labelled as the ‘construction R-value’ of the building element and is the figure to achieve for NZBC-H1 compliance.

While a specific material will have particular R-values for given thicknesses, this does not mean that different insulation materials of the same thickness have the same R-value. Designers and architects tell me that their client wishes to change the wall insulation type after I have completed my thermal analysis and get annoyed when I tell them that the thermal performance will drop (the usual direction), because the substitute materials requires a greater thickness to achieve the stated R-value. (No, it is not a solution to squash the insulation to fit it into the wall framing.)

When considering the construction detailing for insulation, be it for warmth or cooling, the critical factor is the thermal bridging. Zooming in on the areas of greatest leakage gives the maximum benefit – subject of course to buildability, cost, aesthetics, etc. I often give the example of a boat: no matter how thick and strong the hull is, if it has a 10mm hole below the waterline, it will sink. Conversely much attention can be given to seemingly major heat leakage points, e.g. it is vital to have external insulation to concrete slab edges, (is it?), while ignoring the massively deep timber thermal bridges over the windows which have floor-joist-depth edge beams running parallel 200mm above.

Regarding insulation, a perennial question is the dilemma of which side, (or both), of a concrete block external wall do you fix the insulation for maximum performance.

Through Ecorate Ltd, using my experience as an architect, I provide thermal simulation analysis as a design tool, which gives an objective measure of the thermal performance of the proposed building, rather than relying on guesswork. Starting with the base results it is then an efficient process to quickly explore the effects of different materials, construction methods, proportions of thermal bridging in relation to structure and aesthetic requirements, and so on.