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Windows & Skylights: vertical glazing details: this article discusses the proper installation details for vertical glazing: fixed vertical window glass on buildings.
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The problems inherent in vertical glazing (discussed here ) are multiplied in sloped glazing - readers should also see SLOPED GLAZING DETAILS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Fixed Vertical Glazing: less demanding than sloped glass, it still takes careful detailing" as we explain here.
This article explains the details of handling and installing fixed vertical window glass or "vertical glazing" to avoid leaks, cracks, and other window problems on buildings. The author, Steven Bliss, points out that while fixed vertical glazing is less demanding than sloped glass (see SLOPED GLAZING DETAILS), it still takes careful detailing to avoid future problems with the window installation.
Mr. Bliss points out that many carpenters are accustomed to installing pre-hung windows and doors, but are not experienced or trained in the details of successful installation of fixed double glass windows.
The text below paraphrases, quotes-from, updates, and comments an original article original article (see links just above)"Fixed Vertical Glazing" by Steven Bliss.
Our photograph (left) shows two triangular double-glazed vertical fixed glass windows installed by Daniel Friedman in the gable-end wall of this workshop. These vertical fixed glass windows were made by a local window glazing company to dimensions chosen to fit in the framed window opening leaving the clearances discussed below.
What are Insulated Glass Units & How do They Work to Save Energy?
An insulated glass window unit is made from two or more panes of glass bonded to an aluminum spacer. The spacer itself is filled with a desiccant - a material that absorbs moisture from the air to dry out the space between the panes.
The glass panes are sealed to the dessicant-filled spacer using polysulfide, polyisobutylene, silicone, urethane or other sealants. Premium sealed glass units may use two sealants: an inner seal of butyl as that material provides both a strong mountant and an effective moisture seal, and an outer seal of silicone.
How Condensation and "Fogging" Occurs in Sealed Glass Windows
In understanding the problem of condensation that occurs between double paned windows, keep in mind that the role of the desiccant is to absorb any excess moisture in the original air or gas in the sealed space between the panes at the time of window assembly.
If improper assembly or improper window installation causes the window glass seal to leak, allowing new moist air to enter the glass space, the desiccant will eventually be overwhelmed and foggy condensation can appear on the interior of the glass. Our photograph (left) shows a vertical fixed glass bay window that whose seals failed, leading to condensation and fogging between the window panes.
As the sealed glass window is exposed to varying temperatures and to air of varying moisture levels, warm moist air can be drawn into the space between the glass panes, followed by moisture condensation on the interior glass surfaces.
As the window is exposed to cycles of temperature changes, the amount of moisture between the panes accumulates until ultimately the leaky previously-sealed glass widow becomes fogged and even opaque.
Common sealed glass failure modes are
And see SLOPED GLAZING DETAILS for a discussion of how thermal expansion and contraction can lead to damage and leaks in various window glazing systems.
How to Handle Insulated Glass During Window Installation
A double-glazed window unit built using 3/16" thick glass weighs over five pounds per square foot. Such window units should be handled with extra care and plenty of hands - at least two workers for vertical glass.
Any nicks or scratches in the glass will weaken it significantly by compromising the tensile strength of the glass surface. Tempered glass is a little easier to handle as it is less fragile and because its edges are sanded smooth.
If you are installing lots of glass, buy the suction cups that professional glaziers use to handle glass units and to place them accurately into their final position in the window frame.
How to Construct the Frame for Vertical Fixed Glass Installations
Take the time to frame accurately, particularly if the framing is also the finish, as is often the case in a sunspace. Check the opening for squareness by comparing diagonal corner-to-corner measurements or use a full-size template. Check that the window opening is in plane by sighting, or design a system with stops that can be adjusted after the glass is set. Forcing the glass into an out-of-plane opening guarantees a short life - glass failure mode No. 2 that we described above.
There are many ways to set up the openings. Unless maximum glass area is required, I like to have a separate frame that can be shimmed or adjusted. The finish opening should be 1/2-inch larger than the glass unit in each dimension, thus leaving 1/4" around all sides of the glass. The rabbet (or stops) should be 3/4" thick to allow a 1/2" bite against the glass plus the 1/4" clearance. The goal is to have the glass floating in the opening with a space on all sides and edges. This float space is what prevents damage to the glass by forces exerted by building weight or flexing.
The frame can be built out of rabbeted 2-by stock or 1-by stock with separate stops. A 2-by frame can be its own finish (interior and exterior) if left projecting past the drywall on the interior and siding on the exterior. In either case the bottom of the jamb should be level where the glass bears and should be beveled to shed water on the exterior. In a humid space such as a greenhouse, the interior sill should also be sloped to shed condensing water.
The Glazing System - Installation Details for Fixed Vertical Window Glazing
Place the glazing tape carefully, not stretching it, and fitting it tightly at the corners. Set the window glass glazing tape flush with the lip of the rabbet or stop so that it seals well to the glass. Leave the paper facing on until the glass has been positioned.
The glass should sit on two 4-inch long hard neoprene glass-setting blocks obtained from the glass distributor. Set these window glass blocks a quarter of the way in from either end. If necessary, one block can be shimmed (from below) with a non-compressible material such as metal flashing. The glass setting blocks should be a little wider than the thickness of the glass to ensure continuous support of both lights (panes) of glass. The neoprene setting blocks are shown in the center of the illustration at left.
Once the glass has been fit and centered and its position marked, remove it carefully and peel back the paper facing by holding one finger on the paper as you go. Then set the glass in position. The glass unit must not touch the framing - that could transmit structural loads to the glass (insulated glass failure type #2 above). Compressible foam or rubber spacers can be used to help center the glass in the opening.
Next place the glazing tape on the glass and install the finish stops or battens.
The stops should be set to compress the window glazing tapes by 35 to 50 percent. For example, if the glass is 1-inch thick, and the glazing tapes are 1/4-inch thick each, the overall rabbet should be 1 1/4 inches deep - compressing the glazing tape by 50 percent. I (DJF) like to also cut the exterior stops with a slight bevel cut to assure that wind-blown rain running down the glass drains readily off of the stop, prolonging its life.
Install the finish stops with the greatest of care, protecting the glass from tools, nails, and screws with ample pieces of fiberboard, hardboard, or any non-abrasive material. Point nails away from the glass and watch out for knots or grain in the stop wood that might deflect a nail towards the glass. It's a heart breaker to see a large triple-glazed tempered window shatter into thousands of tiny fragments due to a wayward nail. I speak here from experience.
A secondary seal of silicone caulking should be installed on the exterior, sealing from glass to stop. If neatness is a consideration, use masking tape and tool the bead to the tape. Remove the tape after the caulk has skinned over. Remove excess silicone from the glass with a razor. Paint the wood first or use a pigmented or paintable caulk.
Glazing is best done when the temperature is above 40 deg. F. outside. Below that condensation may form on the glass, preventing a good seal to the butyl. If you must, dry the glass with a solvent-dampened rag just before installing the butyl tape. The glass should be cleaned with solvent (such as alcohol) anyway where it seals to tape or caulks.
One final consideration for a high-quality glazing job is the need for weep openings. The purpose of weeping a glazing system is to allow any water that penetrates the system to escape. If it remains trapped it will tend to undermine the sealants and could diffuse into the sealed glass unit causing fogging (failure type #4 above).
You can allow for weeping by drilling small holes (1/4" diameter) at a downward angle from beneath the glass to the outside through the bottom of the stop. Be very careful to drill low enough to completely miss the edge of the glass or you'll break the window. The weep opening should drain across the sloped sill and away from the building.
Readers should also see SLOPED GLAZING DETAILS where we describe the use of window glazing tapes, dry gaskets, caulk, and neoprene setting blocks in more detail.
Links to the original article in PDF form immediately below are preceded (above) by an expanded/updated online version of this article.
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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