Sustainable Window Design

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What is It?


Windows and doors comprise a large part of the exterior building envelope and have a major impact on the human activities inside the building and on energy use. Sustainable window design maximizes the benefits of daylighting while minimizing energy use. It also considers the need for raw materials, life-cycle and maintenance costs and transportation. Windows and doors are available in a variety of material configurations, with the most common frame types being either wood, aluminum, vinyl, wood with aluminum or vinyl cladding, fiberglass or composite. The efficiency of windows varies greatly, primarily based on the type of frame construction and the glass type.

  • Also known as: energy efficient window, high performance windows

Why is it Important?


Sustainable Window Design:

  • Connects the building and its occupants to the sun, the wind and the world outside.
  • Supports a net zero energy design with passive strategies for lighting, heating and cooling, and ventilation.
  • Allows people to control their own environments and be healthier and more productive as a result.

When to Use It?


Use Sustainable Window Design:

  • In northern climates, there should be more windows on the south side than on any other elevation
    • Maximizes winter heat gain
    • Minimizes summer cooling loads since radiation is high in winter and low in summer.
  • Minimize east and west windows because they are difficult and expensive to control daylighting, glare, and summer heat gain.
  • May have restrictions in historic districts and places where nearby buildings or trees block light.

Green Garage Use of Sustainable Window Design


Window selection and placement is a critical step in the design strategy for the Green Garage. Windows and doors have an enormous impact on the heating, cooling, daylighting, and security of the building. Together with the rest of the building envelope, they form the boundary to the neighborhood community.

Sustainability Goals
  • Provide 80% of the interior lighting with natural daylight.
  • Provide adequate natural ventilation to minimize the need for mechanical ventilation whenever possible.
  • Provide a sense of security and well-being within the living building, while providing an attractive and inviting addition to the community.
  • U Factor (Thermal) < 0.30
  • Window -to-Wall Ratio (WWR) < 27%
  • Solar Heat Gain Coefficient (SHGC): E = .30; W = .30; N = .30 ; S = .55; Skylight = .55
  • Visual Light Transmittance (VLT or VT); E = .50; W = .50; N = .50; S(up) = .70; S (view)= .50; Skylight = .70


Strategy and Conceptual Design

* U Factor (Thermal): The U factor (or U value) is a measure of how well heat is transferred through an assembly such as a window. The lower the U factor, the less heat is transferred. An old, poorly insulated and poorly fitted single pane window can have a U factor of 1.20 or more. A super insulated wall assembly could have a very low U factor of around 0.02. The best insulated, most expensive triple-pane windows can have a U factor of 0.09, and many commonly available and affordable double-pane windows have a U factor of 0.35. Our strategy includes balancing the cost of a low U-value window with the expected energy savings and life-cycle costs.

We did life-cycle costs for U factors of 0.35, 0.27 and 0.10. Because we have a super insulated building and a relatively low WWR, we decided that a window with around a 0.27 U-value is the best fit for our needs.


* Window to Wall Ratio (WWR): The WWR is a ratio of window (glazing) area to wall area. This relationship can also be expressed as a window to wall percent (or glazing percent). Too high a WWR can result in too much light in the building, creating glare on computer screens and fading upholstery, artwork, printed materials and carpets. It can also contribute to the building being too cold in the winter from the heat lost through windows, and too hot in the summer from all the sunlight and heat coming in.

Based on many published strategies for daylighting a building, we found that a whole building WWR = 0.20 to 0.30 ratio of window area to wall area is preferred. An overall WWR < 0.20 does not provide enough daylight, WWR > 0.30 allows too much heat loss in winter and too much heat gain in summer. Our overall WWR comes out to about 0.24, falling into the preferred range to achieve both thermal and daylighting performance.

WWR per exposure

South side: In cold climates, it's recommended that you maximize the WWR on the south side to allow sufficient daylight in with the maximum heat gain in the winter. There is less heat gain from south windows in the summer than there is from east and west windows because of the sun's angle. Our south side is shaded by the building next door, which will further reduce the amount of light and heat entering our building from that side.

North side: It's recommended that you minimize the WWR on the north side because of high winter heat loss through windows on that side. Our north side faces another building, which will further reduce the amount of reflected light coming from that side. The amount of reflected light also depends on the surfaces the light is reflected from and how far the surfaces are from the window. The building adjacent to our north side is very close and has a low reflectance value material (red brick). For these reasons, we chose to reduce the WWR on this side of the building to around 0.16, which will still allow some daylighting from the windows without causing too much heat loss through them.

East and West sides: Because of the angle of the sun, the east and west windows can interfere with the activities inside by introducing too much direct light into a space, which can cause glare in addition to contributing to heat gain issues in the summer. Our east side is shaded by the building next door, so this will not be a problem most times of the year. Our west side faces Second Ave. and is considered the front of our building. Because we are in an historic district, this facade needs to be restored to its original configuration, which was much more glass than the other walls. The WWR for this elevation is around 0.40, 10% higher than the recommended overall WWR, and 20% higher than what's recommended for this elevation in this climate. We have compensated for this by choosing a low solar heat gain coefficient and perhaps adding some internal shades to control the glare and heat gain associated with this high WWR.

  • The Sun Map below follows the sun's path around our building.

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* Solar Heat Gain Coefficient (SHGC): The Solar Heat Gain Coefficient is a measure of how much of the sun's heat energy passes through a window assembly. It is measured on a theoretical scale from 0 to 1, with 0 meaning that none of the heat energy that hits the window passes through (like a theoretical solid wall), and 1 meaning that all of it does (like a hole in the wall). Part of choosing a high-performance window is choosing the right SHGC so that daylighting and winter solar gain goals can be achieved, but also so the impact to energy use for cooling the building in the summer is minimized. For our East, West, and North walls, we chose an SHGC of 0.30 to minimize heat gain in the summer. For our South wall, we chose an SHGC of 0.55 to allow for maximum heat gain in the winter.

*Low-Emissivity (Low-e) Glass: Low-e glass (aka selective low-e glazing) has a special coating to make it an excellent way to let in light but reduce solar gain.

*Visible Light Transmittance (VLT), aka Visible Tranmittance (VT): Visible transmittance is a measure of how much light passes through a window assembly. It is measured on a scale of 0 to 1, with zero meaning that none of the visible light is transmitted, (like a solid wall), and one meaning that all of it is, (like a hole in the wall). Typical window values range from 0.3 to 0.8, since glass alone will reduce VT. We chose a VT value of 0.50 for most of our windows to maximize the daylighting opportunities (which in turn reduces energy needed to run electric lights) and minimize glare.

*Light to Solar Gain (LSG): Light to solar gain is a ratio comparing a window's light transmittal efficiency in relation to it's solar gain. The higher the number, the better the window is at blocking solar gain while allowing visible light to pass through. For typical values, Poor < 1.00 ... Excellent > 1.55. Our LSG is VT 0.50/SHGC 0.30 = 1.67 for most windows, which is excellent.

*Other Design Criteria:

    • Glazing the wall areas below desk height (0-30 in. above the floor) offers little or no benefits for daylighting an office or view for the occupants, so we chose not to do this.
    • Use neutral colored glass, avoid tinted glass (e.g. green or bronze)Why??cuts daylighting??
    • Because we have thick, super insulated walls, we can angle the vertical walls on the interior of the window opening to allow more light in. This is a more pleasing visual transition from the bright window to the darker wall. This also allows us to use less glazing area while achieving the same level of daylighting.




Window Design Evaluation Matrix

Through the window design process for the Green Garage, we evaluated different methods of window security, materials, window operation, and how the design might impact daylighting, energy use, and the historic appearance of the building. Our evaluation is summarized below:

(o) = neutral, (+) = positive, (-) = negative


Design Daylight Thermal Ventilation Security Historic Cost Materials Life-cycle and Maintenance
* A. Aluminum/wood
  • Casement-swing out
  • Security screen inside
o
o
o
+
o
?
o
+
* C. Aluminum/wood
  • Casement-swing out
  • Shutters inside
+
+
o
+
o
-
o
o
* F. Aluminum/wood
  • Casement-swing out
  • Security glass
o
o
o
+
o
-
o
+
* B. Aluminum/wood
  • Casement- swing in
  • Metal design security panel-outside
o
o
-
+
o
-
o
-
* D. Steel/metal old style small panes
  • Casement-swing out
o
-
o
-
+
-
o
-
* E. Aluminum/wood
  • Casement-either?
  • Sliding shutters
o
+
o
+
-
-
o
-

Supporting science

People are healthier and more productive when they have access to fresh air and daylight.


Glossary

VLT - Visual Light Transmission

SHGC - solar heat gain coefficient

WWR - Window Wall Ratio

EA - Effective Aperture

Proposed Materials / Suppliers

Development Story

The Sustainable Window Design - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our sustainable window design.

Related Internal Links

Resources


Open points

  • Why can't Joe open the Window types page in google docs?
  • How much of this info should go to the Daylighting page?
  • What should I say about Sola Tubes here?
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Peggy edited this page :)