Lifting the curtain on loss of heat from your house

Last week's column contained two sentences that have bearing on this week's column: "Up to 30 per cent of heat loss from a home is through glass doors and windows. Nearly as much as the heat lost through the ceiling"; and "Thermal curtains are sweet as, if they're fitted properly".

The sad truth is that most of the curtains I've seen in Wanganui are not fitted for high thermal performance, ie insulation. Recently, Verti, Dani and I spent the night at an accommodation outside of Wanganui, which I'll use to illustrate my point.

The unit in which we stayed employed three distinctly different approaches to curtaining: the good, the bad and the ugly. I'll start with the bad because it's the most common thing I see in homes where the occupants are not even aware that their curtains aren't doing the job and that they're losing heat unnecessarily.

When it comes to holding heat inside a home, having a standard arm-hung curtain rail without a pelmet or floor-length curtains is almost like having no curtains at all. Even the best thermal curtains are practically useless if air can flow uninhibited behind them from top to bottom. This is because of physics (hooray science!), and the inverse of a principle we learned in school: hot air rises.

Luckily, it doesn't take a physicist to figure out that if hot air rises, then cold air sinks. Now that we have the science sorted, on to a tale of loss, betrayal and, ultimately, sorrow.

The loss comes from warm, indoor air coming into contact with a cold pane of glass. Heat is conducted through the glass to the outdoors, leaving the indoor air colder than the air directly below it. This cold air sinks, creating negative pressure - or a vacuum - between the curtain and the window, which pulls warm air from the ceiling down and against the cold glass. This air cools and sinks, causing another vacuum that pulls more warm air down from the ceiling.

Nek minnit, a convection current "in reverse" is flowing through the room, cooling it down. Naturally, a sense of betrayal consumes the occupants of the room when they discover the thermal curtains they've purchased might just be useless. Shortly thereafter, that sense of betrayal turns to sorrow when they realise how long they've been needlessly losing heat and that they should have paid attention during physics class at school.

The two common ways to prevent the above from occurring were well-known to our grandparents: pelmets and/or floor-length curtains. Contrary to what most people believe, pelmets are not just a beautiful accessory to a well-appointed home - they're an energy-saving device.

However, thanks to advances in curtain-hanging technology, of which our grandparents never dreamed, a third possibility now exists for preventing the dreaded reverse convection current. Although I'm unaware of the official name of this technology, I have come up with my own: Screw-it.

What could possibly be a more elegant way to describe screwing a curtain rail into the wall directly above a window? I characterise this as the good because it does restrict the flow of air behind the curtain from top to bottom, though not as well as a pelmet or floor-length curtains. But hey, there's nothing wrong with bronze when only three medals are being awarded.

As for the ugly, to quote my Himalayan eco-engineer friend, Sonam Wangchuk: "Warm is always beautiful."

If warm is beautiful, cold must be ugly, and lace curtains do nothing to prevent heat loss through windows, even if they're fixed hard against the frame.

Dr Nelson Lebo is an eco-design and education consultant. He and his wife, Dani, recently renovated an abandoned villa in Castlecliff into a healthy, energy-efficient home with abundant organic vege gardens.

Visit www. ecothriftydoup.blogspot.com, email theecoschool@gmail.com, or phone 344 5013.