It's curtains for Kiwi's cold houses

I will admit that in the first four weeks of this column I did not offer very many examples of low-hanging fruit with short payback periods as implied in the first column.

I will also admit that I love Neil Diamond and I love pelmets.

While I spent the last three columns carefully explaining the components of passive solar design and how they work together, I am finally at a point where I can bring two of those components together in an eco-thrifty context and provide some examples of low-hanging fruit that promise short payback periods.

Besides that last sentence being extraordinarily long, it implies that an eco-thrifty approach to renovation requires one to take a big picture view and retain attention to details.

I'll use the example of thermal curtains, pelmets and window quilts to illustrate this point.

As previously described, north-facing windows in New Zealand are net energy gainers in the winter. While double-glazing is probably best for all windows in a home (and, if building new, they should be installed), they are expensive to have made to replace existing single-glazed ones.

As an alternative to double-glazing, we wanted to look at how the performance of single-glazed windows could be maximised with low investment of money and time.

What we came up with is a low-budget combination of familiar Kiwi practices and potentially unfamiliar ones from North America.

I'll start with thermal curtains because they are probably the most familiar to everyone. There is not much to say except - wait for it - sometimes even the best thermal curtains won't hold in very much heat if not installed properly. Let the physics lesson begin.

A free-hanging thermal curtain that does not touch the floor can be almost as useless at heat retention as no curtain at all.

Here is how it works: 1) indoor air between the curtain and window cools and sinks to the floor; 2) this creates negative pressure between the curtain and window that "vacuums" warm air from the ceiling and places it against the cold glass; 3) this air cools and sinks drawing more warm air from the ceiling to replace it; 4) and the cycle repeats.

What forms is akin to a convection current through the room powered by the cold outside air against the window. The good news is that the cycle can be interrupted in two ways. The bad news (for some) is that one involves a pelmet. Fashion aside, I love pelmets because they are so practical. A properly fitted pelmet breaks the convection current by blocking the flow of warm air down from the ceiling.

Alternatively - or better yet, additionally - floor-length curtains achieve roughly the same by slowing the free flow of cooled air out across the floor. In other words, heads: you win / tails: you win, Both: Double win. And for a fraction of the cost of replacing all the windows in your home.

But there is a catch. A human being is required to open and close the curtains according to the level of sunlight and the difference between indoor and outdoor temperatures. I highly recommend purchasing an indoor/outdoor thermometer to help with the energy management of your home. We got ours for $20 at local hardware store and I reckon it has paid itself back within the first year. Oops, out of words and never got to window quilts. Until next week.

Nelson Lebo is co-founder of the ECO School with his wife, Dani. theecoschool@gmail.com - 022 635 0868 - 06 344 5013. They have extensively renovated an old villa at Castlecliff with green principles and sustainability in mind.