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IDL - Bozeman eNEWS - April 2013 - Vol. 6 No. 4

Entries in windows (3)


Simple Steps to Improving the Energy Efficiency of Historic Buildings

Within the realm of energy efficient design, building professionals deal not only with new construction, but also with existing and historic buildings. Whether it is a retrofit or a preservation undertaking, these projects have great potential to extend the life of a building and encourage sustainable attitudes towards existing buildings in society. Yet, working with historic buildings can be a challenging process. Historic buildings are defined by key features in their construction and architectural style as well as in their historical significance; thus, when those features need repairing, replacing, or retrofitting, the methods and manner in which those features are handled are critical to the historic integrity of the building.

These issues often arise with windows and other elements that significantly affect the energy efficiency of the existing building. While the initial thought may be to alter or replace these elements, the building should first be evaluated for its current energy saving potential. Many historic buildings were designed to be inherently energy saving due to their lack of reliance on mechanical systems. Window shading, natural ventilation and circulation, light reflecting surfaces, local materials, thick insulated walls, and operable windows and shutters are often part of the original fabric of the building and could thus be utilized for their continued energy saving characteristics.

After identifying these features, it is critical to determine what type of work will be completed due to the restrictions and suggestions that are unique to each treatment category. It is important to note that a qualified historic preservation professional such as a Historic Preservation Officer should be consulted early in the design process to ensure adequate interpretation and execution of the treatment standards based on the agreed treatment category. Guidelines and standards vary to some degree between the different organizations and institutions, but the most widely accepted and trusted information comes from the National Park Service. They oversee the National Register of Historic Places Program and its various branches, which include the Secretary of the Interior’s Standards for the Treatment of Historic Properties. These standards outline the different types of work and the subsequent methods of treatment that could be done to a historic building. The standards divide the work into the four categories of Preservation, Rehabilitation, Restoration, and Reconstruction. According to their website…

“The Standards for the first treatment, Preservation, require retention of the greatest amount of historic fabric, along with the building's historic form, features, and detailing as they have evolved over time. The Rehabilitation Standards acknowledge the need to alter or add to a historic building to meet continuing or new uses while retaining the building's historic character. The Restoration Standards allow for the depiction of a building at a particular time in its history by preserving materials from the period of significance and removing materials from other periods. The Reconstruction Standards establish a limited framework for re-creating a vanished or non-surviving building with new materials, primarily for interpretive purposes.”*      

* www.nps.gov/history/hps/tps/standguide/overview/using_standguide.htm


While some treatment methods are unique to a particular category, there are a few guidelines that consistently apply to all four categories:

  1. The most important and emphasized standard is that it is always preferable to repair damaged elements as opposed to replacing or retrofitting. The introduction of new materials and styles can do irreversible damage to the historic integrity of the building, especially when dealing with windows. For example, wood windows that were installed prior to the 1950’s have a greater chance of containing very valuable old growth wood that, if removed, are essentially impossible to replace and will compromise the value of the building’s historic significance.
  2. Any new materials or elements must match the appearance of the original element, and if possible should match the original material. A few examples would include not changing the glazing color in a repaired window, replacing wood windows with wood, not vinyl windows, etc.
  3. All changes to the historic building must meet the specified code requirements based on either the existing building code (Preservation, Restoration, Rehabilitation) or new building code guidelines (Rehabilitation, Reconstruction.)



Upon taking a closer look at the four categories, each category has outlined a set of steps for design professionals to take towards completing a project without compromising the building’s integrity. When working within a Preservation project, the goal is to preserve as much of the existing building as it is in its current state.

The Steps for Preservation:

Identify, Retain & Preserve, Stabilize, Protect & Maintain, Repair, and Limited Replacement in Kind.


Restoration projects are similar to Preservation projects in that they attempt to retain a large amount of the historic fabric of the original design. However, Restoration allows for the removal of elements that did not exist during the period to which the project is being restored.

The Steps for Restoration:

Identify, Retain & Preserve, Protect & Maintain, Repair, Replace, Removal of Existing features from Other Historic Periods, and Recreation of Missing Features from the Restoration Period.


Rehabilitation Projects are commonly known as adaptive re-use projects where a building is reworked to better accommodate a current use while still retaining its historic features. It is important to note that with Rehabilitation Projects, new additions to the building cannot alter the appearance of the existing features of the building such as adding a mezzanine whose floor plate crosses through window planes, or lowering the ceiling below the top of the windows. However, windows can be added to party walls and non-dominant facades if they are compatible with the building design while not matching windows on character defining elevations.

The Steps for Rehabilitation:

Identify, Retain & Preserve, Protect & Maintain, Repair, Replace, Design for the Replacement of Missing Historic Features, Alterations/Additions for the New Use.


Reconstruction Projects constitute a reconstruction of a demolished or missing historic building and/or feature based upon reliable documented evidence.

The Steps for Reconstruction (if any features remain):
Identify, Protect & Preserve.

To learn more about the restrictions and suggestions identified above, visit the Secretary of the Interior’s Standards for the Treatment of Historic Properties’ website.


With these four categories outlined, there are several ways in which all of the project types can be updated to improve their energy efficiency:

  1. Additional Insulation may be added in out-of-site areas such as attics, unheated cellars, crawlspaces, etc. It is important that added insulation avoid high moisture content areas to prevent damage of historical elements and features, and it must not affect the interior or exterior appearance of the building.
  2. Original windows, louvered blinds, and exterior shading devices should be maintained to utilize their inherent energy conserving properties.
  3. Interior shades, blinds and, if appropriate, awnings may be added to improve efficiency.
  4. It is acceptable to recaulk windows and replace and/or add weather stripping to improve the thermal efficiency of the windows so long as it does not detract from the overall historic appearance.
  5. If a non-operable window is beyond repair and must be replaced, operable windows may be used to capitalize on energy conserving potential so long as the appearance of the window matches that of the original.
  6. Non-damaging exterior storm windows and shutters or interior storm windows “with air-tight gaskets, ventilating holes, and/or removable clips to ensure proper maintenance and to avoid condensation damage to historic windows” can be added to improve thermal efficiency. In many cases, they can be ordered with Low–E glass or laminated glass to further improve the performance of the windows.
  7. In Rehabilitation projects, additions such as skylights can be added to non-character defining elevations to help improve natural daylighting.

So how do you go about finding windows that meet all of the criteria in these categories? Window manufacturers have begun developing windows that meet the stipulations outlined in these guidelines to make is easier for designers to find quality windows that will work within these projects. Jeld–Wen and Kolbe Windows & Doors have created lines of historically designed windows, and Dynamic Architectural Windows and Doors are a custom order company that produces thermally broken steel windows to name a few of the options available for designers today.

So while working with historic buildings can be difficult, there is great energy saving potential to be found in many historic properties. As mentioned earlier, historic buildings are often inherently designed to be energy saving, and the guidelines for Preservation, Restoration, Rehabilitation, and Reconstruction allow some flexibility to accommodate additions and changes that can improve the energy efficiency of the building. To learn more information, visit the National Park Service’s website or contact your local Historic Preservation Officer to help you determine the best treatment method for your next historic project.


Refining the Southern Window

Breaking down a southern window into components helps to design efficient windows.  These components can be arranged in multiple ways, to design a specific design detailed to the project. 

1.  The Daylight Window and the View Window

The Daylight window’s primary function is to provide a maximum amount of daylight deep into the space from the perimeter.  This window is located above the View Window, or generally above 7’-8’.  Glazing specifications are recommended to be a window glass with a high visible transmission value (Tvis 70% or higher), to allow the most light through the window.  Also, a low solar heat gain coefficient (SHGC .38 or less).

The View window’s primary function is to provide a view to the exterior from the interior.  This window is generally located within the view frame of the occupants, 3.5’ sill height and 6’-7’ head height.  This gives the occupants a view to relax their eyes, reducing eyestrain, while also giving a connection to the outdoor environment.  The glazing specifications for this window are generally tinted to a transmission value of 50%.  This reduction in light transmission reduces the contrast between the brighter window and the darker interior wall surfaces.

2.  Shade the View Window to Block Direct Sun, Eliminate Glare, and Reduce Solar Gain.

An exterior overhang that is about as deep as the window is high (1:1 ratio) will shade a south facing window during the summer months, which will dramatically reduce solar heat gain into the interior. 

In building types such as schools and offices, where the occupant hours are limited to the mornings through early afternoons (until 3 pm), this strategy can be used on the West Elevations.  By the time the sun comes around to the west side, with the sun altitude low enough, the building will be unoccupied.  Heat gain incurred after this time can be night flushed and the space will be cool by the next morning.

3.  Use an Interior Light Shelf to Provide Diffuse Daylight

Interior light shelves provide three major benefits, they block direct sun from penetrating the space through the upper daylight window, they reduce light levels at the perimeter, and they reflect diffuse daylight onto the ceiling plane.  All these benefits together combine to give the space a more diffuse, uniform lighting level.  Illuminating the surfaces such as the walls and ceiling plane makes the space look larger and brighter than it is actually.

A bright white or translucent, matte finish surface is recommended to provide the best light diffusing quality.  This material specification will have less contrast with the bright exterior also.

4.  Provide Adjustable Louver Blinds at the Upper Daylight Window

Interior louver blinds in the upper daylight window can provide direct sun control without significantly reducing the amount of diffuse daylight into the space.  They can be adjusted seasonally or when low sun angles penetrate the space.  They can be specified as mechanical units or manual.

5.  Use a Roll-Down Shade to Control Direct Sun in the View Window

An interior roll-down shade can control direct sun in the view window without significantly reducing the view to the exterior.  It is recommended to use a dark colored surface on the interior of the shade fabric.  This allows for a comfortable view through the fabric, while allowing room darkening capabilities.  Many newer fabrics provide a dark interior surface while maintaining a light exterior surface.  The bright light exterior surface helps reduce heat gain into the space. 

For more information and to read the full article with diagrams, click here.


Resource: Tips for Daylighting with Windows

Looking for a quick way to size windows and shading devices? Wondering what all those letters stand for on glazing data sheets? Need a quick consult on sensors and controls? Then Tips for Daylighting with Windows is for you!

Lawrence Berkeley National Laboratory compiled this excellent resource to help with basic daylighting needs. This pdf covers a range of topics including: daylight feasibility, envelope and room design, selecting a glazing type, sizing shading devices, coordinating with electrical and mechanical systems, and using sensors and controls.

The guide is simple, with key points, vocabulary explanations (and a glossary), as well as easy-to-understand diagrams. At the end of every section is a page called “If you have ...” which goes through the steps a designer can take if they have “no time,” “a little time,” or “more time.”

Despite its California-oriented data, this guide can be a valuable tool to any architect looking to daylight a building. Get it here!