Retrofit Insulated Glass Unit
Author: Bob Yapp
Preservation Brief #3 Improving Energy Efficiency in Historic Buildings, National Park Service, 2012.
The Secretary of the Interior’s Standards for the Treatment of Historic Properties, National Park Service, 1995.
Contributors: John Leeke
Title of Treatment: Retrofit Insulated Glass Unit (IGU)
Class of Treatment: [ ] Identify, [ ] Maintain, [ ] Stabilize, [ ] Repair, [x] Upgrade, [ ] Exception
Type of Treatment: [ ] Traditional, [x] Contemporary, [ ] Conservation
The sash is removed from the frame and deglazed. The original glass pane is discarded. The glazing rabbet is routed out to make it deep enough for the thickness of the IGU and the glazing stick. Then the sash is glazed with the IGU and reinstalled in the frame.
In 2012 the National Park Service updated their "Preservation Brief #3 Improving Energy Efficiency in Historic Buildings". This is an excellent document with good, solid information. While the Window Preservation Standards Collaborative is grateful for the cautionary comments about not replacing windows and promoting the use of storm windows, the same document gives advice that could be detrimental to the preservation and structural integrity of historic wood window sashes.
Preservation Brief #3 states, "Where the sashes are sound, but improved thermal performance without the use of a storm window is desired, some windows may be retrofitted with insulated glass. If the existing sash is of sufficient thickness, it may be routed to accept insulated, clear low-e glass without extensive loss of historic material or historic character."
The following constitutes a gathering of best practice opinions from WPSC collaborators:
With great respect to our colleagues at the National Park Service, the WPSC contends that insulated glass unit retrofits into existing wood sashes is not best practice regardless of sash thickness. Over the last 30 years, these retrofits have weakened the structural integrity of wooden sashes. Standard residential windows, 1" to 1-3/8" thick, cannot accommodate typical 1/2" to 3/4" thick IGUs. This is no less true for commercial wood sashes, which are typically 1-3/4" to 2-1/4" thick.
The problem begins with the loss of the original glass pane, which is removed from the window and is usually discarded and replace with the new glass IGU. The Secretary of the Interior’s Standards for Preservation states, “The replacement of intact or repairable historic materials...that characterize a property will be avoided.” “Distinctive materials, features, finishes and construction techniques or examples of craftsmanship that characterize a property will be preserved.” It is clear that retrofitting a sash with an IGU is not good preservation.
The problem continues when the sashes are deeply routed to accept the IGU. This step causes permanent and irreversible damage to the mortise and tenon joints which hold the sash together. These efforts to increase the depth of the glazing bed penetrate into the most vulnerable areas of the mortise and tenon joints, weakening them permanently. The added weight of the IGU then puts stress on these structural joints creating the potential for complete failure of the entire window sash. It is also important to note that these changes to the sashes are not reversible.
In a typical 1-3/4" commercial sash, the deepening of the glazing bed for an IGU and wood glazing strips puts the depth of these beds at 1" to 1-1/4". This is well over half the thickness of the sash and structurally damaging. This practice is at odds with thousands of years of basic woodworking knowledge, and hundreds of years of practical development of the double-hung window system.
In almost all the cases, traditional putty glazing is not used in favor of wood glazing strips. These wood strips can allow moisture to penetrate the stiles and rails of the sashes accounting for more deterioration of the original sashes.
To date, the WPSC can find no comprehensive studies conducted on the results of this practice as it relates to the durability of the wooden sashes. Over the next two years the WPSC will conduct research into IGU retro-fits. If better energy performance is the goal for wood windows, the WPSC provides standards for several effective energy saving upgrades suitable for all residential and commercial applications.
Quality of Results
Inadequate Work: Routing of wood sash for UGU retrofit results in the loss of the original glass panes and diminishes the strength of the sash joints and increases the weight that the joints must sustain. Until comprehensive research is conducted on this treatment, the WPSC does not recommend retro-fitting historic wood window sashes with insulated glass units.
Controlling the movement of air and heat through windows.
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