Preservationists Don’t Do Windows
March 17, 2009
As one who has spent his career trying to convince others of the value of historic buildings, I am amused by the new alliance between the green energy folks and the National Trust for Historic Preservation. The Trust has its purpose in this democracy–mainly to remind civic minded people that “heritage” matters–but it often sounds as if it is preaching to the choir rather than leading a new movement. The Trust is always behind the curve, proclaiming yesterday’s news to people who love the past enough to tolerate a little deja vu.
The most recent issue of Historic Preservation magazine, the Trust’s main mouthpiece, featured another “green” issue, close on the heals of the first one issued last year. Not much has changed in a year as far as preservation technology is concerned, but one or two things emerged that caught my attention. Articles on the adaptive re-use of historic buildings started to make sense as models of energy efficiency, not because the buildings were better insulated or more advanced than new structures, but because they maintained their “old” materials and technologies. It turns out that when considered holistically, older structures often make sense as examples of environmental conservation even if they haven’t been modernized.
To an architect who lived through the first energy crisis in the 1970s, and saw the preservation movement through its highs and lows, this revelation was not news. Many traditional buildings use passive heating and cooling, clever means of ventilation, and inherently sustainable materials. One bugaboo, however, has existed since I began restoring houses 30 years ago–the question of what to do about “leaky” single glazed windows. Conventional wisdom was to throw them away in favor of double glazed sashes that would seal the house and keep heat and cold inside in hostile climates.
Finally the tide has changed and preservationists have started to look at windows as pieces of the treasured fabric of older buildings that don’t need to be sacrificed to the altar of sustainability. It turns out, according to recent research, that a well-made wooden double-hung or casement, equipped with tight-fitting wood storm sash, can perform almost as well as a double-glazed unit in terms of thermal resistance and infiltration. Moreover, the cost of replacing beautifully-crafted wooden sash continues to rise, increasing their potential “embodied energy.” There is really no compelling reason to remove character-defining wood windows from any historic structure, if storm/screen units can be installed outside.
Though this may not sound revolutionary, it frees preservationists from one of the most vexing problems in building conservation. If one wants to be green, don’t install windows made of green wood. Why? The seasoned, old-growth wood in most historic windows will continue to perform better than either new wood units or comparable synthetic or metal sashes (metal conducts heat faster than wood). It turns out that when it comes to windows, they really don’t and can’t make ’em like the used to.
From now on my answer to clients who tell me their energy bills will be intolerable if they don’t replace their “leaky” windows will be: it’s not easy being green; add some foam to the roof deck.