New Ideas about Old Windows (I)
Is it time to replace the original wooden windows on your older home? According to many window manufacturing and sales companies, the answer is yes.
In fact, if we are to adopt the recommendations of one window manufacturer whose windows are commonly available to Canadians through Home Depot, we would replace all windows older than 15 years with more energy efficient models.
But not all older windows are created equal. Historic wooden windows were constructed with old growth softwoods and hardwoods that are more durable and long lasting than modern window materials such as vinyl.1 In contrast to their modern counterparts, historic wooden windows can be more readily and cheaply repaired to improve their energy efficiency. On the other hand, homeowners with wooden windows often have to expend a bit more energy and time maintaining their windows. Finding qualified contractors who are experienced with wooden window repair or storm window construction can also require more effort on the part of the homeowner.
Deciding whether or not to replace historic wooden windows is clearly more complicated than a mere assessment of the window’s age. This article is the first in a series that reviews the commonly cited reasons for wooden window replacement and how they measure up to the current research.
To replace or not to replace? Energy Efficiency and Environmental Considerations
Few historic wooden windows can outperform top of the line, thermally improved, non-metal framed, triple-glazed windows with low-solar-gain, low-e glass and argon/krypton gas. But when in good repair, old wooden windows equipped with storm windows are comparable to new windows in terms of energy efficiency.2,3 Problems such as draftiness and heat loss emerge when wooden windows are not properly maintained, or when storm windows are not installed. Repair or restoration of such windows is then required for them to perform at their optimal rate of efficiency.
Restoring older windows is a two step approach involving: 1) reducing air infiltration by draft stopping and repair, and 2) minimizing heat loss. Homeowners can carry out their own draft stopping and repair (see resources below for sample DIY websites and upcoming available courses), or they may prefer to hire a local contractor to carry out the work. Minimizing heat loss requires relatively simple, inexpensive techniques such as installation of storm windows, shutters, curtains or roller blinds. In a recent laboratory study on heat loss reduction measures, researchers reported that storm windows reduced window heat loss by 63%, followed by wooden shutters (51%), roller blinds (22%),and curtains (14%).4 Using storm windows or shutters in combination with insulated blinds or curtains further decreased the U-value (a measure of heat loss) of historic windows to levels comparable to modern vinyl windows.
Resources for homeowners that provide guidance on energy conservation measures commonly compare products based on their energy efficiency while the product is in use. Energy Star is one such resource that provides a ranking system for household products based on their end use energy consumption. These are useful resources in new home construction for which the installation of new windows is the only practical option. But for energy-conscious owners of historic homes, lifecycle energy use is a much more meaningful measure of a window’s actual performance. Lifecycle energy use includes that which is expended during its manufacture, installation, operation, maintenance, disposal and replacement.
It is this full accounting of energy use that led Carl Elefante of Quinn Evans Architects to coin the popular phrase “the greenest building is the one that is already built”. Historic buildings and their component parts such as windows, already embody a vast amount of forgone energy and resources. Energy efficiency gains associated with new windows are heavily offset by energy losses associated with the disposal of the older windows and manufacture, shipping and installation of replacement windows.1,5
Window lifespan also affects the net environmental impact and energy usage of a window. Historic wooden windows are highly durable due to their fabrication from old growth timber and the simplicity of their design. Vinyl window frames degrade and discolour when exposed to ultraviolet light, and typically have a life expectancy of only about 25 years6, after which time the window usually requires wholesale replacement. Aluminum is more durable than vinyl but has a very high embodied energy due to the significant energy inputs required for its production and its high thermal conductivity results in greater energy consumption during product use compared with vinyl and wood. Insulating glass seals on new windows tend to degrade more quickly than the window frame and manufacturer warranties typically do not extend past the 20 year mark.7 In contrast, historic building preservation experts argue that properly maintained historic windows can last 100-200 years or more.2
There are, of course, cases where wooden windows have degraded beyond repair and replacement is the only option. Installation of new wood frame windows are a good option from an energy efficiency standpoint since wood has low lifecycle energy inputs when compared with other products such vinyl and aluminum.6 Wooden window frames clad in aluminum are estimated to embody slightly more energy than unclad wood, but are still significantly lower in lifecycle energy consumption than frames constructed entirely of vinyl or aluminum.6 Although relatively new on the market, wood/polymer composite frames also show considerable potential for lower embodied energy compared with vinyl and aluminum windows since they are resistant to decay, stable in UV light and can be manufactured using byproducts of the construction industry such as sawdust. Homeowners seeking additional guidance regarding the energy efficiency of windows, including information about window seals and glazing options, are encouraged to investigate third-party research, such as that provided by the Efficient Windows Collaborative (EWC) (http://www.efficientwindows.org/).
Watch for Part 2 of this series: To replace or not replace? Cost, Maintenance and Ease of Operation
Further reading and free online resources:
- Sims, C. (2007). Maintenance and Repair of Historic Wood Windows. Heritage Magazine, Fall/Winter, 2007, p. 50-62.
- Old House Web, Repairing Wood Windows
- National Trust for Historic Preservation, Weatherization Guide for Older and Historic Buildings
- Cluver, J. H. and Randall, B. (2010). Saving Energy in Historic Buildings: Balancing Efficiency and Value. APT Bulletin 41(1): p. 5-12.
Window restoration workshops (available periodically):
1 Sedovic, W. and Gotthelf, J. H. (2005). What replacement windows can’t replace: The real cost of removing historic windows. APT Bulletin, 36(4): 25-29.
2 Sims, C. (2006). Repair or replace. Windows in historic buildings: Arriving at a sustainable solution. Heritage, Summer, p. 40-49. Website: http://ohp.parks.ca.gov/pages/1054/files/heritage%20canada.pdf.
3 Klems, J. H. (2002). Measured winter performance of storm windows. University of California: Lawrence Berkeley National Laboratory. Website: http://escholarship.org/uc/item/05p5881m
4 Baker, P., Curtis, R., Kennedy, C., and Wood, C. (2010). Thermal performance of traditional windows and low-cost energy-saving retrofits. APT Bulletin, 41(1): 29-34.
5 Jackson, M. (2005). Embodied energy and historic preservation: A needed reassessment. APT Bulletin, 36:4, 2005.
6 Asif, M., Davidson, A., and Muneer, T. (2002). Life cycle of window materials— a comparative assessment. School of Engineering, Napier University, United Kingdom. Website: http://www.cibse.org/pdfs/Masif.pdf
7 Efficient Windows Collaborative, Alliance to Save Energy, Washington DC, Website: http://www.efficientwindows.org/city_all.cfm?id=202