Continued from Building Energy Benchmarking and Window/Curtainwall Retrofit Part 1

I am excited about a little-known approach for window or curtainwall retrofit that can create dramatic improvements in building performance at a far lower cost with no down-time, showing energy paybacks in as little as only five years!

Interior mounted supplemental window system

Secondary window, also called a commercial interior storm window
A secondary window, also called a commercial interior storm window can be packed with features that address the building’s unique needs for insulation, solar control, noise control and security

The solution, sometimes called a commercial interior storm window is to add an interior mounted layer of glazing to your existing window. This sounds simple, but there can be a lot to it. This custom interior window can be packed with features that will benefit your building in all sorts of ways.

For instance you can add a single glass, low-e glass, or insulated glass for far better thermal performance. You can add spectrally selective low-e coatings to “tune” the way energy enters and leaves through the windows.

Between glass blinds work extremely well with supplemental windows because the blinds stay cleaner and receive less wear from handling. You can even use dimmable glass for the ultimate in solar control.

The new secondary windows fit tightly, creating dramatic improvements in air infiltration, which in turn reduces street noise.

You can add laminated glass to an interior window for safety, UV resistance and sound attenuation. You can even make these commercial interior storm windows blast and hurricane resistant! Government, military and bank buildings can benefit greatly with this added security.

When well designed interior mounted secondary windows are added to drafty, too bright, uncomfortable work spaces, occupants report surprising improvements in the work environment. Worker productivity and property value climb rapidly as a result.

Historic window renovation

Historic building and older curtainwall buildings are candidates for an interior secondary window solution
Historic building and older curtainwall buildings are candidates for an interior secondary window solution

Interior secondary windows are the darling of the historic rehabilitation crowd. Design professionals and building owners with historically significant buildings are now free to carefully restore the windows without awkwardly butchering them with clumsy weatherstrips, heavy insulated glass or ugly exterior storm windows.

Curtainwall retrofit

Office buildings with single-glazed glass curtainwalls can be brought into the age of building science and energy efficiency with a well designed interior glazing solution that manages air infiltration, thermal conductivity, solar heat gain and acoustical performance in one swift economical swoop. The interior glazing solution, far beyond what you might think of as an interior storm window, can be designed to blend perfectly with the existing curtainwall so that the interior aesthetics are preserved and even improved.

Easy, non-intrusive installation of secondary glazing

A real beauty of the interior window is its ease of installation and lack of intrusiveness during installation. Interior windows can be installed in an hour or less with far less mess and disruption than a full window replacement. It is not uncommon that a worker might enter his office after the interior windows were installed, and not even notice that construction had just occurred.

In addition to all of these benefits, when benchmarking time comes along, your property will rise to the top of the list of desirable, energy efficient properties, and your energy cost savings will be the gift that keeps on giving year after year.

Window/curtainwall retrofit scenarios

Let’s consider a 15 story, 100,000 sf office building in Chicago with 40% window to wall ratio.

To this we will add a commercial interior storm window that is also glazed, and creates an air space of about 2 inches between the new and old glass.

Types of interior storm windows
Our three window retrofit scenarios

Here are our three scenarios. None have blinds or shades in this simulation:

#1. Original window: Existing 1/4″ clear glass (U=1.03, SC=.95, VT=.90)

#2. Add Type 1 supplemental window designed for security glazed with 5/16″ laminated low-e glass (U=.37, SC=.52, VT=.90)

#3. Add Type 2 supplemental window designed for energy glazed with 1″ insulated glass with low-e coating and argon fill (U=.14, SC=.43, VT=.90)

In an authoritative Energy White Paper, these scenarios were extensively analyzed using WINDOW 6.3 and eQuest simulation software. Here are the results:

#1. Original window: Annual energy cost $228,523

#2. Type 1 interior window added (Designed for security): Annual energy savings $57,191 (25%)

#3. Type 2 interior window added (Designed for energy): Annual energy savings $66,207 (29%)

The report points out that these 25%-29% savings are for energy alone. Far greater savings can be realized when coupled with replacement heating and cooling equipment which can now be downsized due to the lower demand. By reducing peak electrical demand, there can also be a significant effect on the overall $/kwh pricing. The benefits keep on coming!

Is there any reason to delay exploring this exciting new option? When benchmarking comes to your building, be ready!

Related articles and resources

U.S. Department of Energy Commercial Reference Building Models of the National Building Stock

Advanced Energy Retrofit Guide – Practical Ways to Improve Energy Performance

New York, NY Benchmarking Data Definitions

New York, NY Benchmarking Data Disclosure – the first in the US to disclose actual building performance

Need help planning for benchmarking with a window or curtainwall retrofit?

Mark Meshulam, Chicago Window Expert
Mark Meshulam, Chicago Window Expert will help you successfully benchmark your building with the best window & curtainwall retrofit solutions


6 thoughts on “Energy Benchmarking and Window/Curtainwall Retrofit Part 2”

  1. Mark,

    Thanks for the post. I have a question for you: In Scenario #3 with insulated secondary glass, how do you manage the very likely condensation on the interior surface of the original single pane (in this scenario, surface #2)? The newly created cavity should be significantly cooler that the indoor air temp and I’d expect to see moisture, especially in Chicago.

    I look forward to your thoughts.

    Thank you,

  2. Hi Brandon,
    Good question. These concerns always come up when non-hermetically sealed glazing layers are used.
    If the new interior glazing layer is very tight, then in winter the RH of the outside air will trend towaard equalization with the airspace behind the outside lite.
    If you don’t see condensation on the exterior face of the outer original lite, you should not see condensation on its interior side. The key is equalizing the airspace with the exterior. Sometimes this is accomplished with weeps. The time you might see condensation on the outer lite would be a rapid drop in temperature, where the higher RH of the warmer air is in the airspace when the cold snap comes. Depending upon the severity of the cold snap, you might even see a bit of frost in there. The key is to wait out the cold snap. Such cold snaps only tend to occur only once or twice a season.

  3. I am interested doing a similar installation for my home. After making sure that I get correct RH, I am still disappointed with the performance of my windows when it is really cold.

    My existing windows are double pane / low-e, but the thickness of my walls prevent the heat to reach the lower part of my windows so they condense at -20C and I am not happy with the situation.

    Would adding another layer of clear glass, well sealed on the interior part of the window assembly be a good idea?

    I am concerned about risks of condensation on the interior face of the double pane glazing assembly, behind the storm window.

    I can’t really do more on the other fronts (my house has a infiltration rate of 0.57 CPH, blower test, highly insulated walls and ceiling, constantly running HRV unit) so I am trying to improve the windows performance.

  4. Maybe it is time to look at getting a heat source at the bottom of the window. Google “window sill heater”. Let me know how it goes.

  5. I’m in California. There is excessive noise outside my bedroom window in the very early morning. I am desperate to find the best solution for soundproofing. I have looked at removing the window (have a patio door, so don’t need the window.) Also, blowing in more insulation into the exterior wall. I have also wondered about a storm window, and sound blankets, or maybe just removing my neighbor from the neighborhood so I can get some sleep. My window is 4 x 5 vinyl, double pane. House has wood siding. Would adding “inside window” be a solution.?

    Thank you

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