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> What is convection and how it effects your heating costs

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article imageLast updated 10:13 am, Tuesday 19th February 2013

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Convection - the heat stealer..

Basically put convection can be one of the main ways a house will loose heat during Winter. If not properly managed convection can add considerably to your heating bills. We explain what convection is and how to combat it in the home in practical ways.

What is convection?

Convection is defined as the transfer of energy from one point to another by the movement of a medium, such as air or water. Heating most substances causes them to become lighter, and, in a gravity field such as we experience here on Earth, the substance will rise, because it is lighter than the surrounding air. So a wood stove or oil filled heater on one side of the room will cause air to become warm, which causes it to rise, which pulls cool air into the area of the heat source. This is how a heater on the other side of the room can make the whole room warm.

Similarly you can have the opposite cooling effect occur when a cold substance interacts with warmer air and cools it and that air then 'drops' drawing in more warm air to be in contact with the cold substance and the cycle continues..  This is the effect that can 'pull' heat out of a room...

Windows and convection

A window is basically a structure designed to let light pass but keep the room separate from the environment on the other side of the window. Principally this is to keep things like wind, rain and dust out; but another function a window provides is to block the transfer of heat or cold. Of course depending on the window construction its ability to block the transfer of energy (which is what heat is or cold the lack of) will varying greatly and this is where convection comes into play. The ability, or not, of a substance to carry heat is known as its thermal conductivity.

During Winter you will find the inside of the window is warmer than the outside on average, so the window will act as a medium for the transfer of heat from the inside of the property to the outside. Now this on its own is not that much of a problem but if you add in the effect of convection, you get a 'heat conveyor' - basically the window 'sucks' heat from the air inside that is closest to the window, that colder air then drops to be replaced by new warmer air, and so the process repeats until the inside air is the same temperature as the outside...  You can feel this simply by placing your hand on the sill at the bottom of a window, it will have a downward draft over it and feel cold...  That is due to your heat going out through the window..

So what can you do to stop this? read on..

Ways to stop convection

Basically stopping convection and heat loss revolves around breaking the link between heat transfer through the window and its interaction with the larger air mass in the room and property. In effect you are making the windows more energy efficient by not loosing so much heat to the outside environment; there are several ways of doing this, explained below.

1. Insulating the glass in the window

This involves improving the insulation qualities of the glass itself, either by:
  • a form of thermal window film put over the glass (i.e. a thin plastic film),
  • modifying the thermal characteristics of the glass itself (i.e. dopping), or
  • putting in two panes of glass with a vacuum or high isolating gas trapped between them (double glazing)
The first technique is probably the cheapest and can be done to pre-existing windows - although needs to be done carefully or the end result can be very 'variable'. The second technique could be done to an existing window by replacing the glass, but probably not financially effective. The third is definitely something only done with new windows.

2. Insulating the window frame

In this technique the window frame itself is designed to reduce heat transmission; so done either by:
  • using materials with high insulation qualities (i.e. wood as opposed to metal), or
  • putting a form of 'thermal break' into the frame structure itself to allow the inside and outside frame elements to be at different temperatures.
Both these techniques require new windows. The first looks more natural (if you want that look), but has the downside you will need to paint and maintain the frames. The second technique allows you to use metal frames..

3. Stopping the draughts through the window

An ill fitting window, no matter its insulating qualities, will loose you heat. So it is most important that the frame surround is air tight. Also for any panes that open there needs to be excluder type mechanisms incorporated into the frame.

4. Isolate the window from the room

This is probably the cheapest and one of the most effective ways of minimizing heat loss. Basically you put something in front of the window in the room to exclude it from interacting with the air in the larger room. The simplest way to do this is to use a set of curtains covering the whole window all the way to floor. Why all the way to floor? Think about it, if you did not do this all you have done is put a block in front of the window, but nothing to the top and bottom, so the convection cycle is still free to occur. Where as if the curtain goes all the way to the floor, the cycle is broken. If you really want to ensure the cycle has no way of occurring, also put in a pelmet on the top of the curtains.

Another way of achieving the same goal is to use something called 'secondary double glazing' - this is just a clear plastic sheet put over the whole frame on the inside of the window, making a still section of air trapped against the frame.. Now in principal this seems great, but it has several down sides. First, unless you invest in a magnetic based secondary glazing (i.e. solid plastic sheet with magnetic latches) you won't be able to open the window without breaking the plastic..  Secondly, you will trap humid air against the glass, so in very cold conditions water will accumulate in the gap and can cause mold to develop over time - which can cause rot in wooden frames... So be very careful.

What would we recommend?

If you are doing a new build or reno, basically think carefully about the windows you have in your main living spaces (i.e. lounge, kitchen and bedrooms) this is where you need to invest in being 'convection smart'. Also you need to be aware of how much it really costs you to heat the space (i.e. if the lounge has a log fire in it, you might not need to deal with convection there as much as say an unheated kitchen).

In order of ease/cost I would look at:
  1. curtains and pelmets,
  2. isolating glass,
  3. thermal break or wooden frames,
  4. double glazing.
If you have an existing property I would recommend:
  1. curtains and pelmets,
  2. draught proofing,
  3. secondary glazing

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Related Tags: glass, insulation, convection, double glazing, secondary glazing, winter, energy efficiency

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