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This article discusses design details for site-built double-glazed windows, including which window pane should be sealed most carefully, venting the space between window panes, the amount of space that should separate window panes, and comparing the cost of factory-built windows with site-built windows. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Our photograph (left) shows an interesting sloped window installation on the roof of a barn silo that had been converted to living space. Conventional wood-frame double-hung windows were set into the sloped silo roof - this was not a successful installation and the windows rapidly rotted, leaked, and disintegrated.

Readers who are building their own windows or who are installing factory-built windows should also see  SLOPED GLAZING DETAILS and VERTICAL GLAZING DETAILS. For suggestions about how to diagnose and repair leaks in existing skylights, see SKYLIGHT LEAK DIAGNOSIS & REPAIR.

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Site-Built Double Pane Windows

The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.

• Q&A on Do-It-Yourself-Windows - PDF form, use your browser's back button to return to this page

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article, (see links just above) from Solar Age Magazine and written by Steven Bliss.

Topic: ways to construct site-built double-pane windows.

Question:

I have heard of various ways to construct site-built, double-pane windows. One way is to seal the inner pane airtight, then to fit the outer window pane somewhat loose. Other ways are similar, involving small holes to let the inner air space "breathe". What do you recommend? Also, what maximum spacing do you recommend between panes of fixed glass in a double-glazed window? - David Lile, Santa Cruz, CA

Answer:

Since it is virtually impossible to achieve a hermetic seal in site-built windows (except with exotic hot melt window construction systems used sometimes in commercial window retrofits), we agree with your suggestions.

Inner window pane: Keep the inside window pane as airtight as possible to keep moist household air out of the window unit. In sealing window glass it is essential that proper allowances are made for contraction and expansion of materials over the expected temperature range -see THERMAL EXPANSION of MATERIALS.

Outer window pane: provide a moisture escape route to the outdoors once moisture does enter the space between the two window panes. A few 3/8-inch weep holes, drilled through the sill and stuffed with fiberglass or screening (to keep bugs out) is one detail we've seen. This seems preferable to a "loose" outer window pane.

Watch Out for Leaks at Drain Holes Drilled in Wood Window Frames

DJF Note: but beware: drilling through wood window frame components, if they are not properly protected from the weather and from leaks, can lead to serious window frame rot.

Recommended Space Between Window Panes in Double-Glazed Window Construction

As for the recommended spacing distance between panes of glass in a multi-glazed window, beyond 3/4-inch there is no gain in thermal performance. A one-inch double glass window unit is rated at about R-2 versus about R-1.8 for a 5/8-inch spaced window glass unit. [Specific research citations needed here.]

Window glass panes too far apart?

DJF Note: other studies have shown that if the space between window panes becomes too great, even if the window is a factory-sealed unit, thermal convection can cause air movement inside the window, increasing building heat loss during cold weather. For an explanation of convection currents and how they can cause even sealed building cavities to act as heat loss conductors. As we detail at Convective Loops & Thermal Bypass Leaks, stack effects in buildings chill the interior walls - increasing conduction (heat) losses through them. Sealing in the home's interior will reduce infiltration, but it won't stop partitions and plumbing or electrical chases that are open to the attic from filling with cold air. A window whose glass panes are set too far apart can create these same effects, turning a window into a heat pump, sending indoor heat outdoors.

Window glass panes too close together?

DJF Note: other studies have shown that if the space between window panes is too small, even if the window is a factory-sealed unit, radiation losses from the warm inner window surface across the air space between the panes and onto the cold outer window pane will significantly reduce the window's R-value. Stick with the common window pane spacings used by window manufacturers, typically 3/16" to 3/4" between glass panes. [Specific research citations needed here.]

Window Cost Comparison: Site-Built versus Factory Made Double-Pane Windows

Incidentally, in standard window sizes, factory-sealed double-glazed window units cost about the same as two individual lites of glass you would buy to build your own site-built double-glazed windows. So unless you are retrofitting windows of odd sizes, or require extra thick windows, factory-sealed window units may be your best bet.

Our photo (left) shows a skylight that has suffered recurrent leaks at the glazing frame itself.

This unit, patched with tar and roof cement is on a New York college campus and installed in a slate roof where skylight repairs, properly performed, will reduce the frequent need to access this high and fragile slate roof.

For advice on finding and curing the cause of leaks at existing skylights and other sloped glazing, see SKYLIGHT LEAK DIAGNOSIS & REPAIR.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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  SKYLIGHT LEAK DIAGNOSIS & REPAIR
    Diagnose & Repair Skylight Leaks
    Rooftop Skylight Leaks
    Water Testing Skylights
    Repairing Skylight Leaks
    Avoiding or Preventing Skylight Leaks
  SITE BUILT DOUBLE GLAZED WINDOWS
  SLOPED GLAZING DETAILS
    Seals, sealants & leaks
    Sloped Glazing Design
    Temperature effects on windows
    Sealants & Gaskets
    Sealant Joint Depth & Width
    Sealed Window Joint Failures
    Avoid Joint Depth Sealant Failures
    Avoid Adhesion Window Sealant Failures
    Using Window Glazing Tapes
    Using Window Glazing Gaskets
    Using Window Setting Blocks
    Choosing Skylight Glass
    Skylight Glass Breakage
    Window Glass Seal Failures
    Stopless Window Glazing
  VERTICAL GLAZING DETAILS

• Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years. ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
  
• Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
  Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in BUILDINGS, and other topics.
• Engineering Toolbox website, provides a more extensive table of coefficients of linear expansion at http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html
• Western Washington University Thermal Expansion is described and defined in a clear article that also gives both linear and volumetric coefficients of thermal expansion at 20 degC. for a variety of materials at http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Thermal/ThermExpan.html
• General Electric Corporation, Silicone Products Division, Waterford NY 12188, 518-237-3330 also see www.ge.com/ [Beware of online web-links to the GE Silicone Products Division - the company's contact information for silicone products will redirect to a not-helpful website on silicone products hosted by supplier Momentive.
• Tremco, 10701 Shaker Blvd., Cleveland, OH
• Identification of Silicone Oil/PETN Interaction, PDF information from General Electric Corporation, Silicone Products Division, Product Information. Pamphlet RHB-4B. c A General Electric polydimethylsiloxane, document from the U.S. Department of Energy at http://www.osti.gov
• Idea Development, Inc., PO Box 44, Antrim NH 03440 603-588-6544. Updated contact: www.bigideagroup.net, Manchester NH, 03101 603-641-5955 - Bigideagroup is a network of inventors assisting in marketing products.

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