|our secondhand house|
- conduction - the transfer of heat from substances in direct contact with each other
- convection - the movement of hot gases or liquids due to density and radiation - the transfer of heat energy by electromagnetic wave motion (this is the hardest one to explain, the best example is heat from the sun, which reaches us without contact with any substance or through gas or liquid movement)
How can we use these principles to keep a house cool or warm?
Depending on your climate, one of these aspects may be more of a priority than another. For example, in a very cold climate, the priority is heating the house. In a hot dry desert climate, the priority is keeping the house cool during the day. If you have a weird climate like mine, in which we have frost in winter and over 35degC in summer, you just have to pick your priority. In our case, we have lots of wood to burn to heat the house, so setting up the house to be cool is more important.
The main way to use conduction to your advantage is through insulation in roofs, walls and even underfloor. This prevents heat being transferred out of the house when the house is warmer than the atmosphere, and from being transferred into the house on a hot day. The only problem is that if the house does heat up during the day, it will slow heat loss overnight.
Convection can help to cool the house in the form of drafts, which are of course unwelcome when you want the house to be warm. In summer, this means opening lots of windows and creating cross-breezes through the house. The main recommendation is to open windows low on the shady side of the house and high on the sunny side, air will rise as it heats and exit through the top window, while fresh air is sucked into the house through the low window. In winter you need to prevent drafts and cold convection currents. Drafts around doors and windows can be blocked using cute "draft snakes" (a clever one here too). And the convection current around a window, which sucks warm air over the cold window pan, can be blocked using pelmets and floor length curtains. Heat from wood stoves and heaters is transferred by convection (unless you are standing in front of the fire, and then its radiation).
Anything that is warmer than its surroundings will radiate heat. In summer, we need to block the radiant heat coming directly from the sun, and the heat reflected from other surfaces. The main way to do this is to block windows using curtains or shutters/blinds on the outside, the latter is more effective as it stops the heat before it enters the house. It is also possible to reflect or absorb radiant heat from the roof of the house. In winter, the best way to take advantage of radiant heat is to let the sun shine in your windows. These strategies work best if the house is also orientated correctly. Most of the windows should face north (in the southern hemisphere). In summer, the sun is high in the sky and will not shine directly into north-facing windows, but in winter when the sun is lower, it will shine into north-facing windows. You can look up the solar path at your latitude and shade your north-facing windows at the appropriate angle to provide summer shade and winter sun.
What about solar mass?
Solar mass is a common passive solar technique, and is great if you have a climate in which you get reasonable day time temperatures in which to heat a slab floor or internal wall and then a cool or cold night which would benefit from the gradual release of that heat. In our climate, that would be ok in winter, but in our humid summer, true thermal mass would only keep the house hot all day and all night. Also in really cold climates, if the mass never heats up, it just becomes a massive heat sink that makes it hard to heat your house. Make sure you read about thermal mass and how it would perform in your climate before you include it in your design.
More information here and here.
How do we apply this concept in our house(s)?
Our existing house has not been orientated correctly. The veranda is on the south, so it is cold in winter. We at least have no windows on the western side (hot afternoon sun) and a car port on the east shades us from morning sun. The house is well-sealed, and heats up quickly in winter. Unfortunately it also heats up quickly in summer! There are not enough windows to create airflow, allow we have installed ceiling fans in every room to assist with this. The house is insulated in room and wall cavities (looks like the minimum requirement though) and has a zinc alum roof, which reflects radiant heat.
The "new" second-hand house at Cheslyn Rise (see yesterday's post) will be orientated with the veranda facing north and the master bedroom east (lucky we are early risers). The enclosed veranda (storage area) is on the south. The house has lots of windows to allow cross-breezes. We will insulate the roof and the walls where possible. I suspect that it is going to be drafty in winter, but we have plenty of wood to burn to keep us warm and I will be knitting some door snakes and sewing curtains. I'd like to get some outdoor window shades when we see where the radiant heat is a problem.