One of the surest tests of a period house, in a practical as well as aesthetic sense, is its windows. Much of the ambience of a building depends on its light and in a Northern European country such as Ireland, light, not shade, is crucial.

In the course of the past 30 years or so, a belated awareness of Ireland's rich architectural heritage has emerged just in time to preserve the last surviving examples of a range of Georgian houses and buildings, from the modest to the grand. Central to the value of these houses is the presence of the original sash timber windows with, in some cases, authentic period crown or spun glass with its beautiful colour and watery patterns.

In many cases, these wonderful windows are in need of repair, indeed often extensive restoration, but they are seldom beyond rescue.

Many Georgian windows had their glazing bars removed during the 19th century and single panes of glass were fitted to allow more light. But the fact is, many of these sashes have lasted for close on, in some cases, more than 200 years. What further testimony could be needed as to the quality of the original workmanship and design?

In contrast to the proven longevity of our historic timber sash windows is the brief life expectancy of uPVC replacements. Aside from being eyesores, destroying the line of a street facade and dismissing chapters of social history, they seriously reduce the market value, never mind visual appeal of a period house and often introduce dampness, by preventing air circulating and so contributing to creating conditions in which rot flourishes.

Put bluntly, uPVC, which neither contracts or expands with the temperature, is a rigid material which simply can not compete with either the beauty or efficiency of well treated timber.

Architectural historian and conservationist Nessa Roche has approached the subject of Ireland's historic windows and the old crown glass with expertise, practicality, common sense and slight traces of controlled exasperation. Though conscious of the beauty of the simplicity of classical design, and that windows are the main external decoration of Irish houses, she has not resorted to romantic lamentation. Her tone is unemotional but direct. Above all, her arguments are based on hard fact.

Who knows, she asks, whether the firm which offers a guarantee with its uPVC or aluminium double-glazed windows will still be around in 20 years to fulfil the promise? "Aluminium and uPVC are expensive, and research shows that these so-called maintenance-free windows will commonly need replacement after about 30 years. Their hinges, catches and gaskets will not last even that long, and major repairs are impossible when damage is done."

Elsewhere she questions the myth of double glazing as a cost-effective method of energy saving. "You are unlikely to recoup the investment cost in the form of lower heating bills for 60 years." And as she points out uPVC, in common with aluminium, has a 30-year life span.

There is also an environmental aspect; as many chemicals are involved in the production of uPVC, discarded windows can not be safely recycled. When dumped into landfill sites, uPVC releases contaminating chemicals into the ground water. In the event of fire, uPVC fumes are lethal.

No one could dispute that Dr Roche has written a campaigning book. Indeed, it is a persuasive manifesto arguing for the urgent preservation of an important legacy. Just as the editors of the Atlas of the Irish Rural Landscape ordered planners and developers to rethink their policy, Dr Roche is alerting architects, builders and home owners and house buyers to the implications of careless replacement versus responsible preservation and restoration.

Included are an excellent glossary and advice on maintaining windows. She does not underestimate the power of paint and putty.

However, there is a great deal more to be learnt from this deceptively brief, if impressively informative, study. In many ways, a relaxed social history, it is also a beautiful, well-designed book, graced with Hugh MacConville's photographs which in keeping with the tone of the book, reveals the rich diversity of windows in an everyday context of life as lived rather than in moody isolation.

You never lose the sense that many of these windows, however beautiful, were primarily created to allow light. Admittedly, there are some very small ones, but this illustrates that window size was, for a time, affected by politics. The Window Tax became law in England in 1695. Imposed on Ireland in 1799, it was not repealed until 1851. A Glass Tax was introduced in Ireland in 1825, and endured, despite protests, until 1845.

Irish history, she writes, "can be tracked through the style and shape of windows, which were determined by the political and economic conditions of the day".

Dr Roche has tended to concentrate on more ordinary dwellings, including traditional shop fronts, but also outlines the development of the Palladian style as adapted in Ireland. It was not until the middle of the 17th century that windows stopped being haphazardly placed and became central to design.

Simplicity and consistency, particularly in the ordering of the windows, are keynotes of Irish Neo-Palladianism or Georgian style. All decoration was concentrated on the interior, windows and a decorative doorway were the only external statements.

In contrast to the cohesiveness of Georgian design is the apparent readiness in the Victorian period to accept almost any architectural style.

At about this time, the gothic revival was also beginning to challenge the classical. Changing techniques in glass-making also had an impact on windows. Dr Roche is particularly good on the development of glass-making from high-risk craft to urban business by the 18th century, and eventually to an industry based on a number of processes, all of which would be overshadowed by the arrival in the 1950s of float glass.

Conversational clarity, information and balanced opinion are as alluring as they are rare to find. The Legacy of Light offers each of these. It should also make us open our eyes to the subtle art and lasting value of the simple sash window.

The Legacy of Light by Nessa Roche is published by Wordwell. It costs £19.95.