Outdoor and indoor temperature graph
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Testing outdoor versus indoor temperature

Living in this house during summer feels more comfortable than any of our previous houses.  But, is that just a placebo effect? Are we simply thinking it should be cool by design, so that’s the way we feel?  As one of my Engineering bosses said, “If you can’t measure something, it doesn’t exist.” I need facts and figures to support this feeling of comfort. Luckily, I have a nifty bit of technology to track outdoor and indoor temperature.

Good to have data on indoor temperature

Click on THIS LINK to check our temperatures in real time.  Fingers crossed the technology works and you can see the graph.

Wireless tag monitors temperatures.

My Wireless Tag system includes temperature sensors and a small base station connected to the broadband WiFi router.

The sensors (or tags) measure temperature every 30 minutes. I could program the time interval to be shorter or longer, but this seems alright for now.

Wireless Tag temperature sensor
Wireless Tag temperature sensor

Each sensor talks to the base station which then uploads the data into “the cloud”.  The fun part is reviewing these numbers via my laptop or the phone app.

Wireless Tag sensor base station
Wireless Tag base station

Five temperature sensors in place.

Two sensors monitor outdoor temperature.  The first tag sits on top of a decorative timber slat on the northern wall, under the hopper window. This location experiences the biggest temperature fluctuations. That’s not surprising. Only the eave protects the tag from the elements.

Wireless Tag measuring outdoor temperature on north wall

My other outdoor temperature tag monitors the southern side of the house. Look carefully to see it hanging on the Timbercrete wall near the front entrance (above Jack’s indoor/outdoor temperature sensor).  Temperatures here are less variable due to the secluded location and moderating effect of the blocks’ thermal mass.

Wireless Tag measuring outdoor temperature on the southern side

I watch the indoor temperature at three locations, starting with the kitchen/living area.  The kitchen sensor is attached to the inside of the northern wall. It’s on the other side of the wall to the outdoor sensor so I can easily check the insulating properties of reverse brick veneer construction.

Wireless Tag measuring indoor temperature in the kitchen

The other two sensors record temperatures in the main and second bedrooms.  Monitoring both bedrooms highlights any temperature differences due to the rooms’ construction and location.  Our main bedroom is deep within the structure, surrounded by other rooms. The second bedroom sits on the south-eastern corner so morning sun hits one wall.

Wireless Tag measuring indoor temperature in the main bedroom
Wireless tag on main bedroom wall

Comfortable indoor temperature using passive solar design.

Right, so what’s the theory I’m testing by plastering little tags all over the place?

Passive solar design uses the Sun to heat a home in winter and then “turns off” the world’s best energy source in summer.  Get the design right and there is minimal need for mechanical heating or cooling to maintain a comfortable indoor temperature.

In winter, north-facing double-glazed windows allow sunlight to stream into the house, warming the internal thermal mass.  Good insulation prevents the energy leaking out. At night, the thermal mass releases stored energy to keep the indoor temperature within a comfortable range.

During summer, eaves shade the north facing windows so they let light into the house, but direct sunlight can’t reach the thermal mass. Having light, but not direct sunlight, passing through the double glazed windows keeps the rooms open and inviting. We don’t live in the dark with heavy curtains blocking direct sunlight.

Insulation prevents heat flowing through the walls and ceiling during the hottest parts of a summer’s day. The cool thermal mass moderates indoor temperature.  When the temperature drops at night, opening all the windows allows natural circulation to flush out warm air with cooler, fresh air. I like to think of this as clearing the decks to “reset” the house for the next hot day.

Testing the theory during summer.

I selected two batches of data to give an idea of how Summer affects indoor temperature.

For testing purposes, we did not use the air conditioner.  Our house has an 8.2 Star NatHERS rating so, theoretically, we need air conditioning only for extended periods of very hot or cold weather.  I hoped this test run would tell us exactly what “very hot” weather triggered the need for our heat pump.

In scientific parlance, we have an experiment testing an hypothesis.  In Summer, mechanical cooling is only required to maintain a comfortable indoor temperature after an extended period of heat wave conditions.

A couple of really hot days pushed up the indoor temperature.

Outdoor and indoor temperature graph
Series of hot days with a heat wave in the middle

This time period started with days peaking in the 35 to 40 degree range. Fortunately, at night, the temperature dropped below 20 so natural circulation did the trick to keep indoors between 20 and 25. This felt comfortable with the coolness of the concrete floor underfoot.

Then the really hot weather arrived.  Melbourne experienced two days of 40+ temperatures, separated by a night when the mercury couldn’t be bothered falling below 30.  Whew! Our overnight indoor temperature of 26 degrees was lower than the nighttime minimum, so we kept the windows closed. Next day, the kitchen temperature inched up to 29 before the cool change arrived.  Ventilating the house that night brought immediate relief. Cool air flowed through the lower windows as warm air vented from the clerestory windows.

To be honest, I considered using the air conditioner during the second afternoon of the heat wave.  However, we weren’t really desperate for mechanical cooling. Dressing appropriately and remaining calm avoided us feeling uncomfortable.

This experience showed we can ride through two 40+ degree days but are likely to need some mechanical cooling if the heat wave lasts longer.

Cold snap in the middle of hot days.

Indoor temperature variations with a cold snap in summer
Series of hot days with a cold snap in the middle

After a few warm days, a cold front arrived.  Hey, this is Melbourne after all. Nighttime temperatures plummeted to as low as 10 degrees while the daytime maximum topped out around the 20 mark.

With such a low outdoor temperature, we protected our indoor temperature by keeping windows closed at night.  Thermal mass maintained a toasty 18 to 20 degrees inside despite the cold outside.

As soon as the colder weather moved on, we reverted to overnight ventilation to vent warm air and bring in fresh, cooler air.

Passive solar design plus ventilation smooths indoor temperature.

I suggest the data supports our hypothesis that a passive solar home remains comfortable with minimal mechanical cooling during summer. Since moving in last November, the cooling system has only been used for a couple of hours one hot afternoon when we had a group of visitors. Extra bodies plus external doors opening/closing pushed us out of the passive design comfort zone.

Temperature graphs for a hot day compared to a cooler day are a perfect illustration of how well the house performs.  Good design plus natural air circulation ensures the indoor temperature remains relatively constant despite significant differences outside.

Outdoor and indoor temperature averages over two days
Indoor temperature remains stable despite fluctuations outdoors

Data collection continues with the transition from summer to winter being the next test. With climate change well underway, we experience unusually hot days at the start of Autumn. Will this be an issue with indoor temperature? As the Sun travels lower in the sky, sunlight hits more and more of the thermal mass in our kitchen and living area floor. That’s a good thing moving into cooler weather, but what if daytime temperatures remain stubbornly high?

I’ll keep you posted.

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