Jan 5, 2018: Keeping Your Cool

Thank you very much to all of you who sent such nice thoughts to Cintia and I for our marriage and for the New Year. We’ve had a nice break and a bit of time at the beach. Now we’re ready to get back into the swing of things for 2018.

This coming Sunday is forecast to be scorching hot so I thought we’d start the year with some thoughts on how to keep your house cool during the summer. A good place to start is this article from Domain.com.au

(Source: Domain)

(Source: Domain)

The article lists nine ‘Tips and Tricks to Keep Your House Cool the Natural Way’. I’ll let you read it for yourself but I’d like to add the following comments and qualifications to what the author wrote.

Item 1: Create Shade

The more you can shade the paved areas around your house the cooler you’ll be. Heavy materials like paving, concrete, bitumen and stone will soak up heat from the sun during the day and stay warm long into the night. This is true of brick walls and tile roofs too (which is why I don’t use them). The more you can keep these areas cool during the day, the more you’ll stay comfortable through the night. Personally I prefer to use shade cloths, awnings and blinds rather than relying on deciduous trees or vines for shading for two reasons. It gives me more control over when I let heat into the house and when I keep it out. Also, in winter, even though the vines or trees might have lost their leaves, the bare branches can still provide a significant amount of shade that reduces the amount of solar heat gain when you need it most. If you live in a really hot climate this is not really a problem and deciduous trees can be an excellent option because they can shade the roof and walls of the house too (especially on the west). A light-coloured Colorbond roof also really helps to reflect a lot of the heat from the sun and won’t get nearly as hot as a dark-coloured roof. If your house has a dark-coloured roof you will find it much harder to keep it cool inside.

Item 2: Improve Your Eaves

Having the right amount of roof overhang on the north side of your home is a vital part of good Passive Solar Design however, it is important to understand that correct eave overhang sizing is only effective at controlling summer sun on the north side of the house. It can help a bit on the east and west but won’t stop the low morning and afternoon sun from making things pretty toasty. That is why a good Passive Solar Home will be oriented with the long side of the house facing north. This provides maximum shading in summer, maximum solar gain in winter and minimum exposure to the hot, low afternoon sun from the west.

Item 3: Let it in/Shut it out

The article correctly says… ‘On hot summer days, get into the habit of shutting up during the day – all doors, blinds, curtains – and then opening everything up in the evening to vent the house with evening breezes.’ This is exactly how we operate the Greeny Flat in summer with the help of our indoor and outdoor thermometer. As soon as the outdoor temperature drops below the indoor temperature we know it’s time to open up and start cooling her down. BUT there is one very important exception to this rule. It will only work if your house is reasonably well insulated (and air-sealed) and has the right shading on the windows, otherwise, when you close everything up during the heat of the day, the house could easily get hotter than the outdoors. If you have poor insulation or big windows with no shading the heat from outside will build up inside until it becomes unbearably hot and stuffy.

Item 4: Create Flow

Good cross-flow ventilation is very important for cooling the house down once you do open it up (or if you live near the coast you might keep it open during the day to catch the cooling sea breezes). This can be tricky in existing houses, especially in bedrooms that only have one window. The difficult thing with bedrooms is that, if there are more than one or two people in the house, you are likely to keep the bedroom doors closed throughout the night. This can block any sort of cross-ventilation if the bedroom only has one window. Ideally each bedroom should have two windows located as far from each other as possible and near opposite corners. This allows for good cross-ventilation, even if the door is closed. The following diagram shows how we planned for cross-ventilation in the Greeny Flat.

Natural cross ventilation in summer.

Note how the bedrooms have two windows near opposite corners of the rooms.

Item 5: Go for glazing

Here the article talks about the benefit of double-glazing for reducing heat gain. When it says ‘check the rating’ it’s referring to the U-value and the Solar Heat Gain Coefficient (SHGC). The lower the U-value, the better the window will be at insulating the house from summer heat. And the lower the SHGC, the better the windows will be at reflecting heat away from the house.

What the article doesn’t mention is Low-E coatings on the glass. Low-E coatings help reduce summer heat gain by reducing the U-value and lowering the SHGC. This is great for east, west and south-facing windows and for poorly shaded north-facing windows (or all windows if you live in a really hot climate). HOWEVER, if you live in a cool-to-cold climate and you have the right amount of roof overhang then you don’t want Low-E coatings on your north-facing windows because they will limit the amount of heat gain you get from the low-angled sun in winter. (Read through our Passive Solar Design guidelines for more information).

Regarding window frame materials, other factors to consider are:

  • With timber windows there are ongoing maintenance requirements plus the possibility of the frames warping or shrinking which can create air leaks.
  • With PVC windows there are longevity and durability concerns in my opinion (fibreglass is a much more durable option).
  • As the article says, you can get ‘Thermally Broken’ Aluminium frames but they are likely to be very expensive and watch out for companies that simply coat the inside of the frame with PVC and call it ‘Thermally Broken’… it’s not.

For all those reasons we decided to go with standard aluminium frames but double-glazed windows for the Greeny Flat. We knew we were compromising on the energy performance but we really wanted the low-maintenance, longevity and cost-effectiveness that aluminium provides. And we’re very happy with the result.

Item 6: Treat Windows Well

For window treatments at the Greeny Flat we chose double-cell cellular blinds from Kresta. These are not full block-out blinds as suggested in the article but we like the filtered light that comes through them, especially since we generally keep the blinds closed during the day in summer and the house would be very dark if we had block-out blinds.

Insulating cellular blinds with top-down-bottom-up feature.

Our insulating cellular blinds with top-down-bottom-up feature are very good for controlling sun, light, views and privacy.

The article suggests external blinds and these can be a very effective way of keeping heat out in summer but they tend to be very expensive and they don’t generally do much for keeping heat in in the winter time.

Item 7: Think Thermal Mass

The article is correct that Thermal Mass can be very effective at helping to keep a home cool in summer. HOWEVER it fails to explain some VERY important points about how to make this work.

The first is that, in order to be effective at helping to maintain a comfortable temperature inside a house both summer and winter, Thermal Mass MUST BE INSIDE THE INSULATION LAYER! This is why brick veneer walls and tiles roofs are SUCH a bad idea. They put Thermal Mass in the wrong place where it will soak up heat all day long and then hold that heat and radiate it in towards the interior for most of the night. This is why we used light-coloured corrugated metal for the roof and wall cladding on the Greeny Flat. The light colour helps to reflect the heat and the light-weight (opposite of Thermal Mass) means that, as soon as the sun stops shining on it, it cools down. I would go so far as to say that a brick veneer house with a dark tile roof (like almost all new homes I see in greenfield subdivisions these days) are about the worst thing you could do in our hot, sunny climate.

Instead the Thermal Mass should be inside the building. For us it takes the form of our concrete floor slab but it doesn’t just work magically by itself. We have to operate the house correctly to take advantage of it. In summer this means that we have to open the house up at night in order to let the floor slab cool down. Then it can help to maintain a cool temperature during the day. If we didn’t do that the house would just continue to get a bit warmer every day until it was unbearable. So it is very important that Thermal Mass is a) in the right place and b) operated effectively otherwise it can’t do its job correctly.

Item 8: Be Fan Savvy

Using ceiling fans is not exactly a ‘natural’ method of keeping cool but it certainly uses MUCH less energy than running an air-conditioner. Just bear in mind that a ceiling fan doesn’t actually lower the air temperature, it just makes you feel cooler by moving air across your skin which helps your sweat to evaporate more effectively.

Item 9: Insulate Well

I would have listed insulation as item 3, right after orienting your house correctly and providing the right amount of roof overhang but, obviously, good insulation is essential for keeping out summer heat as well as keeping in winter heat. I looked at a house which had no insulation in the ceiling with my infrared camera just a few days ago and the whole ceiling was glowing like a radiator and sitting at about 42degC. In fact, an uninsulated ceiling is exactly like a huge radiator beaming heat into your house all day long.

An uninsulated ceiling glowing like a radiator.

An uninsulated ceiling glowing like a radiator.

Other Items

The article contains a pretty good list and here are a few other things to consider:

  1. Solar panels on your roof can actually help keep the building cool by shading the roof.
  2. A solar power system also gives you the option to run a high-efficiency reverse cycle air-conditioning system during the day (when it will be using power directly from the solar system) to pre-cool the house (or pre-cool it in winter). That way the house is already cool when you come home and you don’t have to run the a/c at peak times (i.e. in the evening) when it will cost you a lot more and put extra strain on the grid.
  3. Also think about when and where you run electrical or gas appliances. Lights, fridges, computers, TV’s, microwaves, stoves, and cooktops all give off heat into your house. On really hot summer days we try to do all our cooking outdoors and run other appliances as little as possible in order to help keep the house cool.
  4. We also limit our use of kitchen or bathroom exhaust fans which blow air out of the house and, in the process, suck hot outside air in through leaks in the building fabric. Remember too that a dryer (assuming it is properly ducted to the outdoors) also acts like an exhaust fan blowing air out of the house and therefore sucking hot air in.
  5. You can use evaporation to help cool the area around your house in the evening. If you wait until the sun goes down and then water the grass and plants around your home (it also helps to spray a bit on any paved areas that might have been in the sun) the natural effect of that water evaporating will cool the air around your house but obviously you need to be conscious about not wasting water. We only use this technique on the very hottest days and we’re very careful about how much water we use.

Thanks for reading. Stay cool. More next week.

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