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Open Sash Window Retrofit; adding glass to old windows

May 22nd, 2012 · No Comments

My, limited, opinion on this process is that although commendable for the intent to save old growth historic windows and bring them into the next century, there are a few potential drawbacks to increasing the thickness and weight of original sash.

My first concern is that by adding a new veneer of wood and a glass panel to the exterior of original sash you are increasing the weight of each sash. This will have to be addressed with an increase in the size of the sash weights making them less functional. Often the pockets provided are made just large enough to remove the weights for cord and chain maintenance.

In addition to the weight concern, adding a layer of less durable new growth wood to a surface that will potentially be exposed to moisture may invite rot at the interface of old and new. This creates an ongoing maintenance concern. And although everyone should take care of their windows we know from experience that you can ignore the condition of your old windows for years and they’ll still bounce back to new with a minimum of attention and repair.

Also, I like the idea of insulating the weight pockets. We have experimented along those same lines. The challenge of foaming in place PVC channels for the weights to ride in is again accessibility. Sometimes there is lathe and plaster behind the window casings and that prevents or makes difficult the access needed to install the parts for this treatment. We take the path recommended by Efficiency Vermont and make sure that the window is thoroughly caulked on the interior side to prevent air infiltration. If access is relatively easy a 1 inch layer of foam board insulation can be added to the exterior side of the window pocket and foamed in place. This will provide a thermal break of about R-7 and help with creating a dead air space while not impacting the function of the sash weights as designed.

While I whole heartedly support making the sustainable decision in keeping your old windows I think that adopting the more conservative approach outlined in the Secretary of the Interiors Standards, of weather-stripping your old windows and adding a properly fitted storm window, either interior, exterior, or traditional wood style, is just as efficient. This is also reversible in the event that the next owners want to upgrade or change the storm windows impact on the fenestration of the house.

As stewards of our old and historic homes I think we need to take care of them by “the gentlest means necessary” so that the next generation isn’t trying to reverse what we thought was the best practice at the time.

Ron Wanamaker

Tags: Wanamaker Restoration

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