But Palmerston North City Council eco design adviser Nelson Lebo said most New Zealander’s did not know how to fit them properly.

At their worst, curtains were worse than no window treatment at all.

Lebo, who last month advised New Zealanders to “turn the fan around” to draw cool air into a hot house, has turned his attention to teaching people how to trap heat in.

The problem with ill-fitting curtains was that hot air rose to the top of the curtain and through the gap to the cold glass.

Cold heavy air then sank down between the glass and the curtain to the floor, escaping in a cold draught.

Part of the solution was to have a pelmet interrupting the cycle at the top, and curtains that sat snug down on the floor, not a few centimetres higher, to work as a dam at the bottom.

The rule applied to all windows, not just floor-to-ceiling ranch-sliders.

“Almost all of the so-called floor length curtains I see do not touch the floor. Most curtain makers do not install them that way.”

Lebo said the third ingredient was layering. Separate linings, and even a layer of net curtains, worked better than thermal drapes.

“We need to dress our windows like we dress ourselves in winter, snug-fitting, with layers.

“We do not spray rubber onto a T-shirt and go for a walk in the middle of winter.”

Insulation advice applied to windows in kitchens, bathrooms and toilets as well as the rest of the house.

“Roman blinds are fantastic because they fix directly to the wall.”

New cellular blinds were even better, fitting snugly within the window frame.

Lebo said people thought of curtains as decoration, not insulation, and he did not argue that they served many purposes — looks, privacy, and closing out the light.

But they could also be practical.

He said curtains that worked need not cost a lot of money.

There was a curtain bank in Palmerston North for community service card holders, and there were cheap ways of creating a makeshift pelmet.

Mid-length curtains could be tucked up on the window sill to stop the gap at the bottom.

The result he wanted was for people to enjoy warmer, healthier homes.

Power savings were likely to follow, but that was a bonus.

Quick tips:

• Fit a pelmet over the curtain rail
• Make sure curtains sit on the floor or window sill
• Use layers
• Cover all windows in the house.

Curtain Clean comments.

Comments from our experience.

In general terms he is correct. The warm air circulates down behind the curtains, is cooled on the window panes (which is where the crying windows come from see here and here ), and the colder air falls to the floor level.

The closer the curtains go to the floor the less air movement but having the curtains touch the floor creates other problems.

• They wear out on the floor where they touch, leaving holes which of the curtain material shrinks when washed will make it unsightly to let the hems down to cover the shrinkage.
• The bottoms of the curtains get very dirty encouraging mould to grow and remember mould grows in warm places so it not only gets on the curtains but will also grow on the walls.
• Cats and dogs love sitting in the warm and contribute to making the curtains mouldy and dirty.

Fitting a pelmet is a good idea although mostly out of fashion these days. You can cover that space by placing a piece of timber on the curtain brackets or even a piece of stiff cardboard wrapped in lining fabric.Use a bit of double sided tape to keep it in place. Just helps slow the air circulation. Its best removed in the warm weather though as the warmth with accelerate the rate the curtains go mouldy.

Using layers is the way to go.

• Thermal curtains are a misnomer and offer little or no thermal heat exchange protection despite what all the “experts” say. Curtains with linings will always be better and you can choose a poly cotton or better.
• Block-out is fine but in the summer will soon go mouldy as they stay warm for a long time and mould likes the warm humid conditions surrounding the curtains.
• Best combination is ordinary poly cotton lined curtains with a separate rail with block-out only on it. or well fitted nets inside the window frame.

The consumers institute tested thermal heat loss through windows using various curtain types. The best result was from using nets on the windows You read their test here

The trick is to use nets that are well ruffled. Twice the length of the window being covered. Lots of people don’t like them but they work and are what was used to keep rooms warm in the past.

Here is the consumer comment;

“Surprisingly, old-fashioned net curtains were also effective at reducing heat loss. We think this is because the netting, which was installed close to the panes, interferes with downward air movement over the window. Net curtains are worth having – especially because they’re relatively cheap.”