Leaky steel framed skylight Vassar College (C) Daniel FriedmanSloped Glazing Details for buildings
     


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This article discusses how to install sloped window glazing on buildings: how to install sloped glazing to avoid leaks, rot, mold in buildings. Careful and informed design can bring success to these popular but problematic building details.

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Sloped window glazing, skylights, seals, sealants and leaks

Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

For suggestions about how to diagnose and repair leaks in existing skylights, see SKYLIGHT LEAK DIAGNOSIS & REPAIR.

"Sloped Glazing: careful and informed design can bring success tothese popular but problematic building details"

Sloped windows on barn silo (C) Daniel FriedmanThe text below paraphrases, quotes-from, updates, and comments an original article original article (see links just above)"Sloped Glazing" by 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.

As Mr. Bliss points out in the original article (links above), a glazing system must perform many functions. It has to shed and drain water, support and cushion the glass to avoid mechanical pressure points, and seal against air and water leakage. It should be attractive and economical.

Yet many subtle and pervasive forces are working against you: thermal and structural movement, high UV radiation, wind and weather. A pretty redwood cross batten is no match for sliding sheets of ice.

The problems inherent in vertical glazing (see VERTICAL GLAZING DETAILS) are multiplied in sloped glazing: higher levels of UV, water damming at the lower edges of sloped window frames or between the roof and the upper section of window and skylight frames, and structural loading.

Professionals in the large scale curtain wall industries, as well as residential contractors, have encountered problems with skylights and other forms of sloped window glazing. Fortunately, many new products and systems have been developed to beat the elements as well as to reduce the chances of installation errors leading to leaky skylights and related structural damage or even leak-related mold contamination in buildings.

Sloped Glazing - skylight system design

The three types of glazing systems are wet, dry, and combination wet/dry. Wet glazing systems are gunned-in-place sealants. Dry glazing systems use pre-formed gasket materials. The choice among these window glass glazing methods (really window glass sealing methods) depends largely on labor and material costs and the appearance desired.

Generally, dry window glazing systems require less labor time and less demanding workmanship, but more expensive materials. For residential work, combination systems are often the best choice. The combination of using window glazing tape or gaskets along with a final silicone sealant described at VERTICAL GLAZING DETAILS is an example of a combination wet/dry window glazing system.

Temperature effects on window glazing and sealing

In sealing window glass it is essential that proper allowances are made for contraction and expansion of materials over the expected temperature range. This is particularly important when using materials with high coefficient of expansion such as metals and plastics. (See THERMAL EXPANSION of MATERIALS.) To keep thermal movement to a minimum, use shorter lengths of material, joining them with expansion joints.

Readers can see from these building material coefficients of thermal expansion (also called coefficient of linear temperature expansion) that assembling a building component that uses multiple materials requires methods that allow for these differences in the degree of expansion as temperatures change. Failing to permit movement of abutting or connected building materials whose rate of thermal expansion varies significantly will lead to separation, cracks, leaks, or damage in many instances.

Guide to Choosing & Using Sealants & Gaskets for Skylights & Other Sloped Glazing

The critical issues with window glazing sealants are joint size and shape, compatibility among the building materials used, and the type and preparation of the surfaces to be bonded and sealed.

The Role of Joint Depth & Joint Width in Window Sealing

With a window product having a + / - 25 percent movement capability, such as silicones, a 1/4-inch wide joint can safely handle 1/16" movement in either direction. If the sealant is installed with materials at one end of the temperature range (very hot or very cold) then all of the subsequent movement will be in one direction (cooling and shrinking, or warming and expanding), requiring a wider joint that can tolerate more total movement.

Up to a point, the wider the joint the better it is able to tolerate movement caused by temperature changes in the materials. Joint depth also affects the performance of the sealant. Deeper joints create greater stresses at the bond line. The recommended joint depth for silicone is from 1/8-inch to 3/8-inch.

Where Sealed Window Joints Fail

Leaky steel framed skylight Vassar College (C) Daniel FriedmanA window joint sealant can fail either in adhesion to the substrate or in cohesion within itself. That is, the sealant may separate from the surface of the material being sealed, or the sealant may tear within itself. Either failure leaves an opening, a leak, and either leads to eventual failure at the window seal joint.

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.

Where an old but leaky skylight is already installed in a location where complete replacement would be very costly, careful inspection, surface cleaning, and sealing is an appealing alternative.

How to Avoid Joint Depth Sealant Failures at Windows

Bond-breaking tapes or rods (backer rods) should be used under the sealant to prevent this sealant joint breakage due to joint depth, and also to control the shape and depth of the joint. Tooling of the sealant's surface should be done immediately after the sealant has been applied. Masking tape applied to the glass and removed while the sealant is still set will provide a neat attractive line.

How to Avoid Adhesion Failures at Window Sealants & Caulks

To achieve good adhesion between the sealant and the surfaces it is sealing against leaks, clean the surfaces well with an appropriate solvent and, if necessary, with the correct primer. When cleaning each surface use a clean portion of the rag and pour solvent onto the rag (don't hold the rag against the solvent bottle or can mouth and tip the bottle or can). This will avoid contaminating the solvent itself. Using contaminated solvent to "clean" a surface will simply redeposit debris onto that surface, possibly leading to a caulk or sealant failure later.

Wipe the cleaned surfaces dry (also using a clean rag) so that contaminants are not re deposited when the solvent evaporates.

Apply the sealant to the cleaned surfaces quickly, before the surfaces have a chance to oxidize or collect fresh dust and dirt.

Tips for Using Window Glazing Tapes

When using glazing tapes - such as butyl or the newer PVC foam tapes - below the glass, choose a product with sufficient hardness that it will not over-compress under the weight of the glass at the lower edge. Butyl glazing tape has been used successfully for many years in vertical applications, but it has no structural properties. It will not absorb movement, nor will it return to its original shape if it is deformed. If over compressed, butyl glazing tape may run. Pre-shimmed butyl glazing tape has a hard core designed to withstand the compression of glass weight and loads in sloped applications.

Tips for Using Dry Window or Glass Glazing Gaskets

In dry systems, dense neoprene and EPDM (50 to 70 Durometer) have proven themselves. The material should comply with ASTM standards C-864 and ASTM C-509. The proper compression must be achieved. As we discussed at VERTICAL GLAZING DETAILS, for a water-tight fit, this is usually between 25 and 40 percent of the original gasket thickness.

Since both glazing tapes and window gaskets rely on a fairly narrow range of compression to be effective, a secondary wet seal is often a good idea, such as using a bead of silicone caulk on the outside of the window, at the joint between the glass and the wood stop.

Tips for Using Window Setting Blocks During Glass Installation

For setting blocks, which support the weight of the glass at the bottom of the window or skylight unit, dense neoprene or silicone rubber (80 to 90 Durometer) should be used. Two blocks should be used per glass unit, set in from the corners about one quarter of the length of the glass bottom.

The window setting blocks must support both lites (both the outermost and innermost glass panes of a double-glazed or triple-glazed window pane) well. Otherwise the glass lite that is not well supported may slide down past its mate(s), breaking the window seal and causing leaks and condensation.

In wet window sealing systems (using sealant caulks), the setting blocks should ideally bond to the sealant gummed around them. The sealant should cover the top exposed edge of the setting blocks to avoid leaks in that location.

The use of neoprene (setting blocks) in contact with silicone sealants (caulks) may cause discoloration of the silicone over time, and can lead to a sealant bond failure.

Finally, use common sense. Do not use sealants when there is moisture or frost on the surfaces. Find out the temperature range for application of the sealant you've chosen (read the package instructions). If you don't find the answer in reading the instructions that come with your sealant, gasket, or tape, consult with the manufacturer about the selection and use of cleaners, primers, proper substrates, and compatibility problems.

Glass coatings, plastic glazings, and oily woods (e.g. redwood) can present sealant problems and lead to leaks and window failures. Prepare a sealant test sample. If the fully-cured sealant peels easily off of the substrate the bond may be poor and the joint is more likely to fail. For more information about the interaction between silicone sealants and window setting blocks, see Identification of Silicone Oil/PETN Interaction.

Advice for Choosing Glass for Sloped Skylights & Window Glazing

Other than cost and thermal performance, the major issues in the choice of glass are breakage (glass safety in skylights, doors, and in windows located on stair landings), and the integrity of the hermetic seal between lites of multple-pane double-glazed or triple-glazed windows.

Window Glass Breakage

Most window manufacturers will not guarantee their units for overhead use unless the inboard lite (pane of glass) is laminated like auto safety glass. These units are quite expensive.

Tempered glass is least prone to mechanical and thermal breakage, followed by heat-strengthened glass, then annealed glass. Tempered glass tends to break into small (rectangular) fragments when broken. These particles, while dangerous, are less so than large pointed glass shards produced when other types of glass fracture.

Plastic films applied to the inboard lite are considered by some to increase safety. Surface scratches and edge nicks severely weaken any glass - inspect for these before you install any window, especially windows in locations where safety is an extra concern.

Hermetic Double-pane or Triple-pane Window Glass Seal Failures

Sealed glass failure at skylight (C) Daniel Friedman

Our closeup photo of the down-roof corner of a leaky skylight (left) shows what is probably a double failure. Leaks at the insulated-glass frame permitted water to enter the window structure where the freeze-thaw climate at this New York home continued to damage the window by forcing apart and losing the seal of the insulated glass itself.

That's why you can see condensation stains inside of the insulated glass unit as well as (a bit too late) someone's try at stopping leaks at this skylight using silicone caulk.

In sloped applications, hermetic window seals (between panes of glass in a multi-glazed window) are only guaranteed on a case by case basis. While a variety of single and double-sealed, even triple-sealed glass units are available, the best choice for sloped glazing such as skylights has a polyisobutylene inner seal for moisture protection and a silicone outer seal for strength.

Polysulfide window glass seals are not recommended where high UV exposure is anticipated.

Single-sealed units are likely to fail over time with the increased live and dead loads in sloped window installations.

Stopless Window Glazing

Also called Structural Silicone Glazing, stopless window glazing is an attractive and conceptually simple window system that has gained in popularity in large-scale curtain wall construction since the mid 1980's. Stopless window glazing uses no mechanical stops or battens.The window is flush with the building surface, creating a smooth, clean wall.

In structural window glazing (stopless glazing) a high-modulus (elasticity) silicone is used to bond the glass units directly to a metal frame. This system must be designed by an architect or engineer, and it requires meticulous workmanship for a successful installation. Details are available from the Silicone Products Division of G.E. and from Tremco.

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 in PDF form

- links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.

  • SLOPED GLAZING ARTICLE - PDF form, use your browser's back button to return to this page
  • SLOPED GLAZING ARTICLE page 2 - from Solar Age Magazine- use your browser's back button to return to this page
  • FAQs below discusses field reports of problems & solutions for this topic

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