Cool House

Thoughts on keeping a house cool (and conversely keeping it warm).

Summary [added May 08]: A house is a complex thermo dynamic system, so there is no right answer. You will need to apply a different strategy depending on: the aspect of your house, the materials it is made of, whether it has party walls, where the wind comes from, what sort of wind it is, how much shade the house gets, etc etc etc.

Rule: you can’t use an external energy source (fan, air conditioner etc) to keep cool.

We used to live in a brick townhouse in Newtown. Our bedroom had french doors which opened onto a very narrow and rather pitched balcony. We also had small children so our assessment of the risk was that those doors needed to be kept closed at all times… or at least at all times that the children might be able to access them. Unfortunately those doors were also the only connection the room had to the outside world. It had no windows. The townhouse also had a tin roof. It could get very hot in summer. Indeed the whole house could get very hot in summer. So the question was – was it better to keep the windows in the house closed or open.

Making Tea

Take a cup at room temperature and pour hot water into it. Pour the water into the sink then feel the temperature of the cup. If you’ve done this right the cup should now be hot. The water has transferred its heat to the cup. Conversely, if you now put cold water in the cup and pour it out it will cool down. Heat flows from hot bodies to cold bodies. In this case the water is a body.

Cooling the House Down

When the windows are open people feel cool from the wind blowing through the windows and blowing over them. Having a cool wind blowing through the house will also blow the hot air out of the house. To take the tea analogy above, the cup is the house and the air is the water.

Drying your Hands

Hot air can feel cool because it evaporates water from your skin and that evaporation takes energy with it. This can be seen in any washroom which has a hot air hand dryer. When you first place your hands under the dryer the air feels cool – or at least not hot. As the dryer evaporates the air from your hands there is less water to evaporate and therefore the air from the dryer begins to feel warmer and warmer. When your hands are dry the air feels hot. However the temperature of the air has stayed the same (ie hot) the whole time (test it with a dry part of your arm).

Opening the Window

As I mentioned above, opening the window lets in a blast of air which feels very cool. What a relief! Except that the wind in summer is typically a hot wind. So leaving the windows open is actually heating up the house. It feels cool for the same reason the air from the hand dryer feels cool – you have water (sweat) on your body which is evaporating. Meanwhile the house (which doesn’t have sweat to evaporate) is being heated by the hot air which can now get in. If this occurs throughout the day, heat will be stored in the materials of the house itself (eg the walls, the floor and the ceiling as well as all the furniture etc inside).

Note: for all the air that enters your house an equal volume of air must simultaneously leave your house (… unless your house is sealed and the air is forced in under pressure). If it is leaving by the same window the air is entering from then the flow of air will be greatly reduced. If possible open at least two windows in such a manner as to achieve airflow (eg on opposing walls or the top and bottom of a sash window). Better yet put some obstacles impeding the direct flow of the air to create turbulence, mixing the fresh air with the air in the house and therefore cooling it more (no point the cool air flowing in a current through your house and right out another other window).  If you’re interested you can verify this when you are driving by noting the airflow through the car with one window down and then with two windows half down (ie nominally one window open).  Neither window should have a wind shield on it though…

Becalmed

Then, night falls and often the wind stops blowing. Then, when you’re trying to go to sleep, the house is releasing all the heat that has been stored in it through the day. With no wind to evaporate the sweat the nights can be unbearable. Frankly if you can’t sleep at night it doesn’t matter what things are like in the day.

Shutting Up

So is the answer in closing the windows? Maybe. All other things being equal, it would be better to monitor the air temperature and compare it to the internal temperature. If the air outside is cooler, then open the windows to cool the house. This is not necessarily a complete answer because the internal temperature may be high enough to be unbearable anyway. It might be worth it to raise the internal temperature in order to take advantage of whatever breeze there is. Also, if you’re confident that the wind will blow all day and all night, then you may have nothing to lose by letting warm air into the house because as long as it keeps coming you will be able to sleep.

Opening up windows at night is especially important in trying to reduce the internal temperature of the house – as it’s the only time that the air is cooler than the house and therefore the only time the house can be cooled. Over summer a house will typically not be able to rid itself at night of as much heat as it gained through the day, so it will become progressively hotter. This will depend on its structure and the materials from which it is built.

In the Dark

Another aspect of shutting up is the external heat energy which enters the house. The discussion above was mainly about the transfer of heat as a result of contact between two things. Heat can also be transfered by radiation (eg radiant heaters). The most obvious form of radiation is sunlight, both direct and reflected. In each case the sunlight enters the house through a window gets absorbed by whatever it hits and re-emitted by that thing. This wouldn’t be so bad, except for a certain quality of the glass in windows – it is typically transparent to visible light, but opaque to heat. So when the light is absorbed and re-emitted, it is often re-emitted as heat and cannot get out through the window (this is the reason cars get very hot in the sun). It therefore stays inside the house, heating it up more. For this reason either external blinds or awnings, or internal blinds or curtains on every external window will play an important role in keeping the temperature of the house down (internal curtains and blinds which are light coloured mostly reflect the light rather than re-emitting it as heat so the energy can travel out through the glass in the window). However, they will also play an important role in making the inside of your house look like a tomb. By the way, if you turn on a light you are heating up the room (by roughly the wattage of the bulb).

On Reflection

We tried some reflective insulation in the roof space at one stage. It was total rubbish. Maybe I didn’t lay it properly. Maybe I should have tweaked when the guy who I bought it from had a house like an oven. In any event we chose reflective insulation because it seemed a logical way of keeping out heat. The insulation comes in flat batts which are pulled up right into a batt with three reflective layers. The idea is that they stop the transfer of radiant heat from the layer of air above the batts (which are lain across the joists) and the ceiling below. Anyway, we couldn’t detect any improvement in temperature in the house. We also tried wool fibre (small fibres of wool are pumped into the voids between the joists). The wool fibre creates pockets of air which retard the transfer of heat. I can’t say we found that much use either. We also tried an exhaust system in the roof (one of those whirly bird things that lets hot air out of the roof). Unfortunately the roof space was shared with the two townhouses beside us so it was always unrealistic that the venting system that we bought was going to cope with that volume of space. In any event we didn’t notice any improvement.

The other option, which we didn’t try, was solid batts. The point of solid batts is that they are a large insulating mass which will absorb a lot of heat. Hopefully they will be able to absorb heat all day and then let off the heat overnight. With these insulation systems if the roof space gets hot enough the batts act like coals and warm the house through the evening.

I think that the house we had was just a rubbish house for keeping cool. I suspect that the shared roofspace and the shared party wall meant our house took in a lot of heat. We had a free wall, but it was on the south east side, so was heated for a good part of the day. We had a townhouse development across the road to the south, which blunted the force of the cool southerlies (although sometimes they would produce a substantial temperature drop (eg 6 C)).