We’re building a lot of solar ovens

One of the issues we urgently need to talk about is the failure of some newer buildings to perform well in extreme weather, despite what looks like a high NABERS or NatHERS rating. Ratings focus on annual or seasonal performance, and decisions made at the final design, delivery and then operational stage often result in poor outcomes.

If you build a room with lots of glass that is highly insulated, or with mostly shared walls, which faces anywhere between east and west – including north – in many Australian climate zones, you have a potential solar oven problem. This problem exists even if it has advanced “low emissivity and tinted double glazing” and some fixed shading.

The laws of physics are tricky.

Many new “high thermal performance” houses, apartments and commercial buildings with big windows face challenges, and not just in summer. Hot, sunny weather in spring and especially autumn, when the sun is relatively low in the sky, can cause overheating.

Annual averages are not a good benchmark

Fixed eaves also don’t work very well in autumn and spring, especially if heat is reflected or re-radiated from surfaces near the windows and conducted through concrete slabs from balconies or walkways into the building. Even walls exposed to sun, which are typically not as well insulated as ceilings and may be painted dark colours, can add to overheating.

Unfortunately, our present rating systems, NatHERS for residential but also NABERS and the National Construction Code for commercial buildings, focus on annual and seasonal energy flows, not extreme situations.

This is not just a problem for discomfort, health, glare and overheating, it also drives a need for larger capacity (and costly) heating and cooling systems and higher capacity, more expensive, electricity supply infrastructure that has low utilisation. This adds to fixed electricity charges that can’t be avoided by investment in renewable energy.

The physics of baking humans

Why does this happen? A well-insulated room, or one with walls, ceiling and floor shared with neighbouring conditioned spaces doesn’t require much heat input to reach an uncomfortably hot temperature in warm weather, though it can be beneficial in cold weather. Just one square metre of window exposed to sun can deliver hundreds of watts of heat to a room, through direct solar radiation and other sources of heat not necessarily considered by building modelling.

Wide eaves still allow solar radiation to heat the ground or balconies near the walls. In autumn, they allow sun to reach the lower sections of full-height windows. Thermal bridging means significant amounts of heat can flow in via thermal bridges from balconies, paved areas and walls connected to concrete slab floors and through window frames. Even reasonably well insulated walls heat up and warm the room.

Limited shading just above north windows fails to deal with heat inputs during mornings and afternoons when sun bypasses the shading device.

Our rating and modelling approaches tend to understate the problems

NatHERS assumes someone will be home to pull down an internal blind if the solar radiation on a window exceeds a few hundred watts. For homes without internal blinds or someone at home to use them, overheating issues are seriously understated, not just for clear glazing but also for those with energy saving window films.

For commercial buildings, modellers may assume blinds are used. If cooling air is blown over windows, the indoor insulating air film when standard window insulation properties are assumed doesn’t apply. This indoor film provides around two-thirds of the insulation value of a single glazed window. Disrupting it by blowing cooling air over the window dramatically changes the heat flow. Failure to use blinds as modelled adds to the problem.

The energy consequences are significant

My recent article in AIRAH’s Ecolibrium magazine showed that peak electricity demand for some buildings was up to four times that of other buildings with the same annual electricity consumption per square metre.

Several “green” apartment buildings are also facing overheating issues. For example, I have been hearing from occupants of some 9 Star NatHERs apartments I helped with that they are overheating in summer and autumn because the advice I gave to include appropriate shading was not adopted by the architect.

Another apartment building is getting resident complaints in summer because it was designed without mechanical cooling as the architect believed thermal comfort could rely on passive thermal performance and crossflow ventilation. However, people are not ventilating spaces as predicted due to noise from nearby rail and roads.

We desperately need to deal with the “solar oven” problem, especially as our climate continues its trajectory of longer, hotter warm seasons.  That means for a start perhaps rethinking the design aesthetic of wide expanses of glazing, reliance on predicted behaviour to justify performance assumptions, and considering the impact of heat gain in spring and autumn, not just the height of summer.