During the recent heat waves here in Perth I’ve received a lot of enquiries asking how my own Passive House is performing.
Perth, located in Western Australia, experiences a Mediterranean climate with hot, dry summers, which typically span from December to February. During this time, temperatures range from 20 to 40 degrees Celsius (68 to 104 degrees Fahrenheit), although temperatures over 40 degrees are becoming more common.
Perth Maximum temperature diagram - Meteoblue.
Our Leanhaus is a single storey timber framed home designed and built to Passive House standards.
It’s airtight and we have a Heat Recovery Ventilation (HRV) system that runs 24/7, supplying fresh, filtered air to all living rooms. It can replace all the air in the home in three hours and keeps the relative humidity stable at around 50%.
The insulation is R5.0 in the walls and R6.0 in the roof. This may sound like a lot, but heat will eventually pass through all wall types to accumulate inside. This, however, is where timber framed homes show their benefits. Compared to mass homes (like brick), timber framed homes quickly lose this accumulated heat during cooler overnight temperatures, while brick homes will continue to radiate this stored heat back into the house.
So, how has our house been performing?
Overall, the house has been working very well. Only one room receives direct air conditioning, and with some basic management we’ve been able to keep all the spaces under 26 degrees.
Our Netatmo Weatherstation data for a hot week of December 2023.
Integrated roller shutters on many of our windows helped keep out direct sun.
What is our summer comfort strategy
Firstly, when the external temperature rises above 25 degrees, we make sure to shut the house down completely. All windows and doors are sealed shut to keep the heat out as much as possible.
Secondly, we make sure all the windows are well shaded. Our roller shutters go down before the sun starts hitting the glass on the east and west facing windows.
And, thirdly, we run the air conditioner during the day as soon as the solar panels are generating electricity. Our 220sqm home has a very small 6kw Inverter AC system, so roughly 27w/m2 of cooling capacity. Doing this allows the AC to maintain a constant internal temperature without much effort. In fact, I estimate that the AC is rarely working above 50% capacity.
Diagram of room-to-room air tranfer fan.
Moving cool air around
Last winter we retrofitted two room-to-room ventilation fans to help distribute the cool air in larger volumes than the HRV system can do by itself. This has helped both the guest unit and activity room get 300 litres/minute of additional cooled air redirected from our living space. This has worked well and makes it possible for these rooms to stay within 2 degrees of the AC set temperature. The room-to-room ventilation fans are very cost effective and easy to install in the ceiling zone, so I’ve even started to specify these on new homes that only have one AC unit.
If it is going to be a really hot night, we will leave the AC on just to maintain the internal temperature. So far this summer, we’ve only had to do this about twice a month. In general, though, the ceiling fans in the bedrooms are all we need to create that cooling effect at night. Special mention to the Big Ass Fans Haiku L fans which are whisper quiet and move the air well without being disruptive.
Monitor the weather and make adjustments
Perth, being coastal, benefits from a cooling sea breeze on all but the very hottest summer days. The temperature can change quite quickly from, say, 37 degrees at 2pm to 25 degrees at 6pm. When this happens, we turn the AC off and open the windows on both the windward and leeward sides of the house to create natural cross-ventilation.
Comfort at night
With no AC in the bedrooms, we’re generally sleeping at around 26 degrees with the fan on. This is comfortable for us, and we typically sleep under a sheet or sometimes a light blanket. If the temperatures are expected to drop overnight, we’ll open the bedroom windows a crack to let the cool air naturally ventilate and cool the room.
Energy use
Over summer we average about 12kw/h of mains energy a day (less than $5/day).
We do have a 10kw solar PV that easily runs the pool pump, pool heater, hot water system and AC, plus the washing, cooling and drying needs of 7 people. A significant portion of our mains energy usage is at night when the house is most active and there is no solar generation.
What would we do differently?
Two of our bathroom windows, one facing east and one west, don’t have roller shutters. I excluded these to save $2400, but it would be nice to be able to stop all heat to these rooms in summer.
We also saved about $4500 by not providing AC to our guest apartment. As we rent this unit on Airbnb I’m sometimes concerned about the comfort of our guests, but even after two summers we haven’t had any complaints. We made this decision to save money on the build, and make sure guests couldn’t abuse the AC when it’s very rarely needed.
Summary
High levels of insulation and airtightness make our homes very efficient and easy to keep cool and comfortable. It is, however, important for occupants to understand the strategies required to keep the house operating within a comfortable range.