Last week’s column elicited a few groans that the story of passive house design was just too complex: great idea, poor implementation, good science, bad myths, NatHERS good, NatHERS bad. Let’s KISS* goodbye to all that with a little formula: Five O’s that spell out how to do passive houses, simply.
The ‘esky’ is a good place to start: stop the heat getting in or out. You can’t maintain thermal comfort if heat leaks uncontrollably. We all know the virtues of ‘bulk insulation’, well covered in the textbooks and examples from the United States and Europe where the main worry is winter heat loss.
But that’s not enough in Australia, where our warm temperate to hot climates necessitate keeping out the radiant summer heat; where cooling is increasingly more energy demanding than heating, increasingly with climate change. This is achieved with ‘outsulation’: layers of bulk and reflective insulation at the outside edge of the envelope.
Bulk insulation, batts or blankets, within the walls and ceiling, is widely known and regulated by an ‘R or resistance number’. By contrast, reflective ‘outsulation’, usually made of large rolls of bonded aluminium foil, has no corresponding R value. Thus, it is discounted in regulations, more difficult to specify and is under-measured in simulations.
In most Australian cities ‘outsulation’ is more important than the more easily specified bulk insulation, especially in the roof, but it gets a bad wrap. Its shiny reflectivity makes it hard to install; and early on builders had to supply sunglasses, before a surface coating was developed that reduced the reflectivity for humans but not heat.
‘Outsulation’ needs to go in during construction, it’s hard to retrofit. Labor’s very worthy roof insulation program (30 percent of houses at that time had none) became known as the ‘Pink Batts’ fiasco / farrago, because it was so much easier to install batts than foil, which was also needed. The tragic deaths were often a result of untrained young workers stapling reflective insulation through live wires.
If ‘outsulation’ is difficult to retrofit in roofs, it’s impossible to do so in walls, where it is just as necessary. And equally the reflective wrap can improve moisture resistance, but it must be vapour permeable (with tiny holes) to prevent condensation.
‘Outsulation’ is also valuable in insulating floors, at the edge of ground slabs, (but not under unless the ground is frozen), and under exposed suspended concrete floors. But termites can make successful detailing, and certification of ‘outsulation’ extremely costly, if not impossible.
Finally, and critically, in making our ‘Esky House’, we need to make houses airtight. Air, and heat, leaks through myriad gaps: around windows and doors, through vents and exhausts, in poor junctions and connections; they may have 10 to 12 air changes per hour instead of the minimum necessary of 0.5 air change / hour.
Sometimes called the ‘thermal wound in the house skin’, the glazed openings of windows and doors are a key passive design issue, and two aspects need to be addressed: how are they made, and where are they placed?
Australia has a long tradition of lousy quality in windows and doors, driven by price above all else: single glazing with no coatings, flimsy aluminium or cheap timber frames. For so long the regulators were loath to overhaul this low standard, so vernacular construction sets such a low benchmark that the sophisticated systems needed in passive design are too rare and too costly.
Poorly made glazing leaks heat three ways: conduction, convection and radiation. Conduction heat loss is most often seen in the condensation on the glass and frames as warm inside air meets the cold glass and frames. Short of stopping breathing (and showering and making cups of tea) in winter we need much better products.
Poor convection performance is hard to gauge as the R value used for all other products in a house is replaced by a Uw value for windows. You couldn’t do a better job of confusing architects, the very people you want to influence, and who want simplicity when it comes to science.
Winter heat loss happens as the interior ‘radiates’ its heat to the black night sky (the absorber). Double and triple glazing and high-level coatings do little to stop it. What’s needed are blinds and curtains, that cut the ‘line-of-sight’, preferably in good pelmets. Old fashions trump modern technology. On the other hand, the issues of radiant heat gain are best covered under ‘orientation’.
Thankfully glazing manufacture is changing: double glazing is much more common, with ‘low e’ and other selective or reflective coatings, in much better frames. Thermally broken aluminium frames and sashes, which prevent heat conduction and winter condensation, have been available for a while. Hybrids of timber/aluminium or timber/PVC are available with better mechanisms (the ones that you loved in that European hotel you last stayed in).
So much for materials, how about size and placement? Architects are enthralled to huge windows and doors as markers of prestige, but the houses are way overglazed, even the best glazing performs at only a tenth of wall insulation. A good rule of thumb is to have habitable rooms with a glazing ratio (glass area/floor area) between 10 percent (BCA minimum) and 25 percent.
More than 25 percent is lazy design. This ‘rule’ promotes design creativity in placement in response to view, sun and breeze, rather than the fully glazed wall. One wonders how long pale imitations of the Farnsworth or the Johnson New Canaan house can garner the RAIA awards. We can be a lot cleverer and make better thermal comfort.
The next issue, radiant heat transfer, happens in two ways: desirable heat in during winter (think north-east and north) and undesirable heat in summer (think north-west and west). Glazing is often placed poorly in relation to orientation for thermal performance, either for other issues such as views, or indiscriminately in relation to the street or some arbitrary external composition. A little cleverness or compromise is called for: thinking of the insides when making the outsides.
Crucially in orientation considerations is shade. Most architects learn how to design for good sunshade, but don’t practice it. Generations of designers learnt Sunshine and Shade in Australasia, by R.O. Phillips for the CSIRO, but narrow eaves are the universal default. Instead, think of extensive wide overhangs, adjustable sunshades or pergolas as good passive design.
Having achieved a highly insulated ‘Esky House’, we can increase the energy efficiency by gaining free solar energy into the house in winter; and by having night-time cooling in summer, both of which require thermal mass, or heavyweight materials that can store warmth, or coolth (an absence of warmth, and listed in the OED).
Our vernacular methods are the wrong way round when we consider thermal mass: a brick veneer house has the mass on the outside, where it performs very poorly even counterproductively. We should build our houses inside-out: brickwork (or concrete) on the inside, then ‘outsulation’, then a waterproof cladding.
The desirability of thermal mass is different from summer to winter. In winter, the thermal mass works best on the floor where the solar sun strike can warm the slab. And the heat rises gently from the floor. Additional mass of heavyweight materials in the walls can help absorb extra heat and store the warmth to prevent overheating. Recently rigid and flexible insulation products have been developed as interstitial insulation for brick cavity construction, another form of ‘outsulation’.
Summer is the inverse: the thermal mass is needed above, the convection law of physics dictates that the heat will rise, and the overhead mass will absorb it during the day. The cooler evening breeze passing across it will lower the temperature by exhausting the warm air to the outside, returning coolth to the mass. Called ‘diurnal’ cooling or night flushing (the latter also being two of the three things the Porter warns Macduff off, if you recall your Shakespeare)**.
Having built the ultimate maxi-Esky, and stuffed it full of mass, we can operate it for minimum additional energy use. Opening blinds or curtains on the NE to NW glazing can bring in the winter heat, and trap it at night, even if it seems a tad too 1966, with the housewife at home to adjust the house settings. Consider an update: electronic controls on servo motors controlling shutters or blinds, seamlessly responding to outdoor sun and indoor temperature.
Likewise, we can use controls to open highlight windows for summer evening coolth, perhaps aided by low power fans to ensure airflow: the energy used is a small portion of the net cooling effect achieved. And the best occupant gain from operation may not be free energy, but a closer relation to the external weather patterns and seasonal changes.
In summary, the entire passive design strategy is simple: a house with a well-insulated, airtight interior for minimal heat exchange between the inside and outside, with well distributed thermal mass on all surfaces, offers better warmth in winter and cooling in the summer and reduces the energy demand for better thermal comfort.
But my concern from last few weeks remains: our main problem lies with the vast bulk of existing Australian housing which needs to be retrofitted with the first two O’s: better ‘Outsulation’ and better ‘Openings’, before we can seriously address the other elements of passive design to achieve lower energy costs for better thermal comfort.
*Keep it Simple Stupid
****Macduff asks the Porter, “What three things does drink especially provoke?” The Porter replies, “nose painting, sleep, and urine”—the first of which is usually taken to mean the red flush that comes across a drinker’s face.
Tone Wheeler is principal architect at Environa Studio, Adjunct Professor at UNSW and is President of the Australian Architecture Association. Please note that I do not read Instagram, Facebook, Twitter or Linked In. My sanity is preserved by replying only to comments addressed to [email protected]. The views expressed here are solely those of the author and are not held or endorsed by A+D, the AAA or UNSW.