January "Coolth" in a House without Air-Conditioning
I have now 15 years of January average temperature data for my house at Manilla, North-west Slopes, NSW. These graphs show how the house temperature relates to the outdoor (or ambient) maximum, mean, and minimum temperatures.
This post can also be found, with a larger sized graph, here:
The house is not too hot and not too cold
In January (the hottest month) the rooms* in this solar-passive house do not heat up much during the day, nor do they cool down much at night. Since the indoor temperature always rises and falls just one or two degrees from the mean, only the mean is shown. Green lines on the graphs, which are drawn to pass through the middle of each cloud of data points, show by how much (on the average) the indoor temperatures have differed from the outdoor maximum, mean, and minimum temperatures. On the middle graph the green line shows that the rooms have been 0.5 degrees cooler than the mean temperature outdoors. The left graph shows that the rooms have been 8.2 degrees cooler than the daily maximum outdoor temperatures. The right graph shows that the rooms have been 7.3 degrees warmer than the daily minimum overnight temperatures.
The design of the house aimed to protect those living there from excessive summer heat. It may seem that reducing the mean temperature by only half a degree is a failure. Not so! The January mean temperature at this site (26.1 degrees) is near the middle of the adaptive comfort zone for this month, and so is the indoor mean temperature (25.6 degrees). The house succeeds in keeping the indoor temperature comfortable in the heat of the day, when that outdoors is an uncomfortable 34 degrees. The high thermal mass that achieves this has the unfortunate result that the minimum indoor temperature overnight (not shown) is some five degrees warmer than the outdoor minimum. However, on average, it is still a comfortable 23.5 degrees. (Curiously, no-one knows the best room temperature for sleep.)
Which of the outdoor temperatures affect the room temperature?(dashed black lines)
On each graph there is a dashed black line and an equation (from Excel charts) showing how the room temperature relates to the outdoor temperature (Max, Mean, or Min) that is plotted along the bottom.
On these January graphs, the points line up from bottom left to top right very much better than on the July graphs posted earlier. The R-squared values are all much higher, ranging from 0.62 for maxima to 0.97 for minima. To explain, I can only suggest that even this house has limited ability to reject the extreme solar radiation of this climate in summer. The night-time relation (on the right) is the closest by far: each degree that the outdoor daily minimum falls produces a 0.87 degree fall of indoor temperature. To say simply that the January temperature of the house is 7.3 degrees higher than the outdoor daily minimum temperature is is correct.
Why is it so?
In summer the house has many defences against high ambient temperature and solar radiation in daytime. It is well insulated and well sealed. There are air-locks at the main doors. No windows receive summer sun, and automated curtains are kept closed against stray radiation during the day. Morning and evening shade is given by trees, shrubs, vines, water tanks, awnings, porches and attached garages. An effective thermal mass of over 100 tonnes carries coolness from spring into summer. Yet, daytime conditions still have a big effect.
By night, warm air in the house is "purged". External doors at ground level are opened at bed-time (and closed on getting up). Air is vented through clear-story windows that are kept open, and the venting is assisted by fans that run from midnight to 5 am. It is this night purge that links the house temperature strongly to the outdoor minimum temperature. Two factors prevent the house from getting as cool as the pre-dawn outdoor minimum:
1. The air outdoors is near minimum temperature only for a short time;
2. The mass of the house takes a long time to cool down. The cooling effect of the air flow is not lost, though, because it ensures that the daily and summer season heating of the floor slab is reduced.
Warm and cool January months
At Manilla, January 2013 and January 2014 had (on average) the hottest days in the 15 years, and January 2012 and January 2008 the coldest. Inside the house, January 2013 was the hottest, but January 2012 was only the second-coldest, and the other two months were not extreme.
As the third graph shows, the January month with the warmest nights (2013) was also the warmest indoors, the one with the coolest nights (2000) was also the coolest indoors, and the others kept almost the same sequence in between.
Speaking of climate change, hotter summer days might cause more heat-stroke, but would not over-heat well designed houses. Hotter summer nights would have the opposite effect.
*Room temperature is mean daily room temperature, measured at head height on an internal stud wall. The daily temperature range at that point in July is only 3.2 degrees.
Outdoor (ambient) temperatures are measured in a Gill-type screen.
Posted Monday 21 Apr 2014 @ 10:32:36 am from IP #