To Replace, or not to Replace

Online Image_This Old House Journal_by Jonathan Poore_date1982

Image Credit: The Old-House Journal, 1982, p. 19, Jonathan Poore’s Drawing “Anatomy of a Double-Hung Window”

Fortunately, that is NOT the question. But it is a topic we discuss a lot, and not because it’s a fresh debate! More so for its ambiguity, and sometimes research studies on window glazing systems [1] can be too academic to dilute for today’s lightning-fast culture [2].

In simple terms, no reasonable person is going to argue the fact that with more material (two or more panes of glass) between you and the exterior you’ll have better energy efficiency [3]. It’s basic physics. The ambiguity though is due to misunderstanding the terms ‘energy efficiency’ and ‘sustainability.’ They’re not synonyms!

Most preservationists are pro-IGU (insulated glazing units) when the multiple pane window system is installed for new construction. Double pane, triple pane, we’re ready to embrace them …in fifty years [4]. The controversy is with their installation in historic buildings, and about legitimate observations regarding IGU quality. They deteriorate at a rapid and wasteful rate, and when installed adjacent to original long-lasting building fabric, there is no denying there’s an unsustainable problem with them.

This is what historic preservationists are protesting. When it comes to original windows we, the VOH team, consistently champion for repair and restoration for several reasons:

Image Credit: APT Bulletin: Journal of Preservation Technology, 36:4, 2005, by Walter Sedovic and Jill H. Gotthelf, "What Replacement Windows Can't Replace: The Real Cost of Removing Historic Windows."

Image Credit: APT Bulletin: Journal of Preservation Technology, 36:4, 2005, by Walter Sedovic and Jill H. Gotthelf, “What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows.”

  1. From a preservationist’s perspective, windows are considered one of the most prominent and important character-defining features [5] of a building. IGU with two panes of glass cause a distracting double reflection, an effect that’s magnified when films are added. The IGU spacers that separate the glazing are not only visible but also their thickness obscures the original sticking [6] and proportions of the sash muntins [7]. And finally, the most troublesome issue, the original sash design usually cannot accommodate the extra material and weight. Historic double-hung windows operate with carefully sized sash-weights hidden in recessed pockets to counterbalance the windows for easy operation. When the sash is heavier, so must be the sash-weight. Increasing the sash-weight usually means it will not fit in the existing pocket.
  2. We often work with the Texas Historical Commission and have found the THC’s position to be that IGU have an appropriate but limited use within historic applications.
  • If original window sashes remain, then they are restored including the single pane feature.  If the craftsmanship is well executed, this resolves air leakage issues.  If additional insulating value is necessary, then consider near-transparent thermal window films, and lastly look to interior storm windows.
  • If the original sashes no longer exist and energy savings is urgently needed that cannot otherwise be accomplished by the above methods, then insulated glass is evaluated.  A major consideration is the construction of the windows themselves.  If the windows have distinctive glass (stained, beveled, prismatic/ribbed, etc.), if there are a number of panes, or if the sash components are thin, then IGU is generally unacceptable or unworkable; however, if the sash is a single large pane of glass, it may work.
  • The need for IGU must be significant. For example if an analysis indicates a new HVAC [8] system would have to be so large to accommodate the single pane feature that it negatively impacts the restored appearance of a building more so than theIGU, then that would be an appropriate application. The Harris County Courthouse project demonstrated this. Their original double hung windows have a single clear pane on the bottom with an upper sash decorated with prismatic panes and metal cames. The energy code requirements were met withIGU and thermal film installed on the lower sashes only, thereby leaving the historic detailing of the upper sash intact. This reduced the physical size of the HVAC system, better preserving the architectural character of the space. In this case, THC approved the use of IGU within a historic context.

    Image Credit: Harris County Courthouse, Houston, Texas; Vaughn Construction website

    The restored original windows of the 1891 Colorado County Courthouse, Columbus, Texas, VOH Architects

    The restored original windows of the 1891 Colorado County Courthouse, Columbus, Texas, VOH Architects

  1. Traditional window assemblies, when properly maintained, can last the life of a building. Their construction is straightforward and can be repaired by local craftspeople; no factory-based manufacturing process is required. The wood used in original window frames and sash is typically tight-grained, old-growth, extremely sturdy, rot-resistant wood [9]. When IGU windows deteriorate, which happens quickly for an array of reasons (extreme weather and temperatures, building movement, fragile materials, use/abuse by people, etc.) they have no repairable qualities and have to be replaced. Once IGU are compromised, they don’t have a second life like historic window systems that so often appear in architectural and construction salvage shops.
  1. Window manufacturing corporations have, with high-paid lobbyists and an incredibly successful marketing campaign promoting energy efficiency, created the perfect feed-back loop to their pocketbook. With IGU products they take more material and more consumer money to support their effort to lay waste to the infinite stock of pre-existing windows all in the name of Energy Efficiency. However, in spite of their best efforts even U.S. government studies continue to support window repair over replacement.[10]
  1. Modern window assemblies use synthetic and fabricated components, encapsulating an interior frame of one type of material with cladding of another type. The components are designed for replacement only and are not recyclable. Most IGUs will end up in the landfill within 20 to 30 years.  Nothing is maintenance-free, and everything – buildings included – moves and shifts. Large temperature swings magnify the different expansion-contraction rates of dissimilar materials. Once joints open even slightly the decline begins. The seals on IGU typically deteriorate between 10-20 years, and the argon or krypton gas leaks out, fogging up the window and reducing energy performance, ultimately requiring replacement. The synthetic materials and new growth wood used have a shorter life span and requires more maintenance due to the lower quality of the materials.[11]
  1. When factoring in the collection of raw materials, manufacturing, and shipping from remote locations, comprehensive analysis shows a simple wood sash and glass pane requires less energy consumption to manufacture with longer life spans (if high quality materials are used). IGUs have higher maintenance costs and consequently higher life cycle costs because they have plastic parts and custom-fabricated replacement window glass.[12]
  1. If a pane of IGU glass breaks, it has to be specially ordered to fit the window, whereas if a single pane of glass breaks, it can be replaced within hours. This is where window restoration provides for the economic sustainability of the community, something IGU windows do not do.[13]

Comal County Courthouse Restoration – Window “Repairs”

I’ll wrap up with an example of how a project, missing its original windows, was slated for IGU and suddenly reconsidered the single pane feature. When the c. 1960 aluminum windows were removed in our most recent courthouse project, the historic 1898 frames were uncovered, still intact in very good condition. This allowed the reuse of the frames in situ, and reclassified the window work as “repairs” rather than “replacement.” If the windows and frames were replaced completely then the project would have been held to a higher – subsequently more expensive – energy efficiency standard that would have negative life cycle benefits in the long run [14].

The historic window sashes are 1 3/8-inch thick. There was not enough space for a thicker sash in the original frame to accommodate inherently thicker IGU. The design team, including the owner, found the simple design and traditional material of wood sash and frames more desirable as they are easily maintained by local craftspeople; therefore, the single glazed windows were more economically sustainable by continuing to support community employment for any repairs or restoration window work in the future.

Additional measures were taken to enhance the original design [15]:

  • The biggest energy loss actually occurs around the windows byway of loose sash and unsealed framing. This project repaired, sealed and tightened up the original frames, the new sash are seated snugly, and weatherstripping was added throughout;
  • The installation of a performance-enhancing (low-E) window film, applied to the interior side of the glass, filters out damaging UV light, reflects solar radiation and reduces heat transfer [16]. The film has a 15-year life span that was factored into the long-range maintenance program;
  • And, finally, the installation method for the window shades increased energy efficiency as well. THC would have preferred the window shades hung to match the historic photos, but when installed with inside mounts, the heat loss was reduced by about 25% (a shallow air space of less than 1-1/2″ is insulating, whereas a deep air space permitting air circulation diminishes that effectiveness).
P.S. To really geek out about IGU spacers and argon/krypton gas…
Image of IGU spacer and the double reflection

Image of IGU spacer and the double reflection

When retrofitting historic windows with IGU, the contractor has two spacer sizes to choose from, low profile and regular. For the abutting IGU, with two dark colored spacers and clear sealant in between, the muntins need to be at least 3/8-inch thick, plus a whisper more for hiding ability. Regular sized spacers are used for much larger heavier IGU. Originally, when IGU first came out, spacers were made of metal, a material chosen for its quality and longevity, but it was later determined to compromise the IGU energy performance due to its natural ability to conduct heat. Spacers are now produced with less-conductive matter similar to structural foam with a desiccant interior fill. The desiccant is added only to assist with the drying phase immediately after manufacturing. The sealant selection is another consideration when switching to IGU. For example DAP 33 not compatible with any IGU spacers. Together they deteriorate prematurely. It is important to learn from the IGU manufacturer which spacer was used and from whom it was fabricated. Compatible sealants for the IGU spacer will be a unique list provided by its maker. IGU manufacturers are aware of this problem and are prepared to provide their spacer sources and provide a list of compatible sealants.

And regarding the different uses of Argon and Krypton gas:

On the advice of IGU reps, our window contractors shy away from using any gas, argon or otherwise, in their retrofitted IGUs because general consensus is that it’s an unnecessary expense for such a marginal increase in performance and it has a very limited life expectancy. The addition of argon or krypton gas adds about R-1 value to the IGU window system; it is more effective than regular air at reflecting heat. Argon is nontoxic, clear, odorless and cheaper to produce. It is the preferred choice if the IGU air space is at least ½-inch thick, whereas krypton gas, which has the same characteristics but is more expensive to develop, is used if the air space is thinner than ½-inch. While a 3/8-inch air space is the ideal gap dimension for using krypton, it is used for even smaller spacing, which is often the case when trying to retrofit IGU within constraints associated with pre-existing window openings, especially historic ones.

Oh, and footnotes for a little light reading…
[1] Definition: ‘glaze’ – to install glass in windows; ‘glazing’ – glass
[2] Capital Development Board, Lincoln Hall Windows Research Report, A Case Study of Options for Treatment for Windows at Lincoln Hall University of Illinois, Urbana Champaign, Revised 2009, by Bailey Edward Architecture
[3] National Park Service, Technical Preservation Services, Preservation Brief 3 “Improving Energy Efficiency in Historic Buildings.
[4] National Park Service U.S. Department of the Interior, National Register of Historic Places, Frequently Asked Questions “How old does a property have to be to qualify for [historic] listing?” ;
[5] U.S. Department of the Interior National Park Service, The Secretary of the Interior’s Standards for Rehabilitation and Guidelines for Rehabilitating Historic Buildings, p. 25
[6] Definition: ‘sticking’ – the shaping of molding; it is also referred to as the stile-and-rail profile.
[7] Definition: ‘muntin’ – the secondary framing members (vertical and horizontal) that hold panes within a window or door. “Mullions” – (often misused as interchangeable with’ muntin’) identifies only vertical members.
[8] Acronym: “HVAC” – heating, ventilation and air conditioning.
[9] National Trust for Historic Preservation, ‘Windows,’
[10] U.S. Department of Energy, Energy Efficiency & Renewable Energy Building Technologies Program, “Measure Guideline: Wood Window Repair, Rehabilitation, and Replacement,” by P. Baker, P.E., Building Science Corporation, December 2012
[11] ‘The Greenest Building’ transcript, Film by Wagging Tale Productions, Inc., Jane Turville, 2011.
[12] APT Bulletin: Journal of Preservation Technology / 36:4, 2005, “What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows,” by Walter Sedovic and Jill H. Gotthelf
[13] Economic Benefits of Preservation Session, “Sustainability and Historic Preservation, 2007, by Donovan Rypkema
[14] Texas Historical Commission Comal County Courthouse Restoration Completion Report, 2013, by Volz O’Connell Hutson, Inc.
[15] The Center for Resource Conservation, Study ‘The Effects of Energy Efficiency Treatments on Historic Windows, 2011
[16] WAAC [Western Association for Art Conservation] Newsletter (ISSN 1052-006), Volume 30, Number 2, May 2008, “UV and Visible Light Filtering Window Films.”