Sloped windows on barn silo (C) Daniel FriedmanSite-Built Double Glazed Window Construction Details

  • Do-it-yourself window construction details
    • Home made double-glazed window sealing suggestions
    • Double pane window venting details for site built windows
    • How much space should be provided between double-glazed window panes?
    • Cost comparison of site built versus factory made insulated glass windows
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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.

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Site-Built Double Pane 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.

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

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


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


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.

Also see our discussion of storm window weep holes and why they are critical: STORM WINDOW WEEP HOLES

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.

Leaky steel framed skylight Vassar College (C) Daniel Friedman

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.

Original article

The link to the original Q&A article in PDF form immediately below was preceded (above) by an expanded/updated online version of this article.

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