Draught-excluding snake. Image source: Together.com.

Together.com, the climate change campaigning organisation, has launched a new product for its annual campaign. It is promoting the traditional door ‘snake’ in its autumn publicity drive. Filled with insulating material, these colourful sausages stop heat leaving a warm room in winter through the gap at the bottom of the door. As part of the campaign, B&Q is selling snake-making kits for £1 on 20 October. National Trust properties are holding snake-making workshops over the half-term period.

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I like door snakes. When I was young my sister and I would carefully push the snake against the door every evening to keep the living room at a bearable temperature when we were watching the television. It kept the room at least three or four degrees hotter than our freezing front hall. But gradually snakes have fallen out of use as homeowners heat all their rooms and don’t worry so much about wasting heat.

When the PR company rang me to tell me about the new ‘snake’ campaign, I was intrigued. All forms of insulation make sense, but how much energy did they think the snake would save? Was it worth going along to B&Q to buy the kit and make yourself a snake for the main room? How much would you save? ‘We don’t know,’ came the slightly embarrassed reply.

Well, I don’t know either. But we can use a bit of guesswork to come up with an estimate. I’m sure that readers with a better grasp of the physics of home heating will come up with better figures, but here is my guess. Please tell me where I have gone wrong.

Let’s say that a snake stops air flowing from the living-room into the colder hall. The air flows at 1 metre per second (just over two miles an hour) into the hall through a gap 60cm long and 1cm high. This is 6,000 cubic centimetres or 0.006 cubic metres per second. Let’s say that the snake will stop this completely.
The air leaving the warm living-room before the snake is pushed against the door is being heated by radiators to 15 degrees above the outdoor temperature. So, for example, the room is 20 degrees when it is a brisk 5 outside.
The air leaving the warm room is replaced by a similar volume of air coming in through the draughty windows. So the effect of the air leak under the door is to require the central heating system to heat 0.06 cubic metres more air all the time during the heating season. The snake stops this.
There are 1,000 hours in the heating season each year. (Approximately five hours a day for six months a year.)
If I have done my maths right, this amount of heating will require an extra 100 or so kilowatt hours of central heating a year. Your boiler isn’t 100% efficient so this means you need 120 kilowatt hours of extra gas. This adds about 0.5% to the typical heating bill. Since one kilowatt hour of gas is about 3.5p pence at today’s prices, each snake will save something over £4 a year. In an old and draughty house, it will be more.
What is the climate change benefit? About 20kg of CO2 a year. More than the typical cost of the supermarket plastic bags that you no longer use, of course.
So go to the shops and buy yourself some kits for making snakes this half-term.