The Multi-Light Insulating Glass Debate
- To Seal or Not To Seal -
The seal in all Insulating Glass used in wooden windows and doors will ultimately fail requiring replacement of the Insulating Glass. The decision whether or not to use Sealed Insulating Glass must be carefully considered. How your windows and doors are fabricated should be carefully reviewed before trying to determine the value and practicality
of energy efficiency, structural performance, aesthetics and convenience
versus maintenance.
Background:
Basically, there are two ways of glazing wooden multi-light sash and doors. While the concept of Sealed Insulating Glass has reportedly been around for a century or more, layers of unsealed single glazing has been the primary fabrication technique used to enhance energy efficiency until 20 to 30 years ago. Today, Sealed Insulating Glass (IG) is not only the most widely used glazing method, but is often specified by building codes requiring energy efficiency and structural performance in conjunction with landmark design conformance.
But, again, the seal in all IG will fail requiring periodic replacement. Warranties against seal failure range from 1 year to 10 years. And, recently, some 20 year warranties have become available depending upon the type, size and shape of the IG unit. But IG is warranted for a specific time frame because it can not be expected to perform for periods of time exceeding that specified by its warranty.
The question is, do the benefits of IG out-weigh or off-set the inevitable maintenance cost of IG replacement over the life of a window or door? To answer that question, we have to understand the elements of Window and Door Fabrication, Energy Efficiency, Structural Performance, Aesthetics, Convenience and Maintenance.
Fabrication - Unsealed & Sealed:
For all practical considerations, windows and doors are either single, double or triple glazed. Single glazing assumes that there is one layer of glass separating the interior from the exterior, double glazing assumes that there are two layers of glass, and so on. Some recent glazing methods include layers of transparent materials other than glass. The layering of glass or other transparent materials creates an air space that increases the insulating properties of the window or door. The air space is either Unsealed, in which air is permitted to circulate in and out of the air space, or Sealed, to prevent air from migrating in and out of the air space.
Unsealed:
Layers of unsealed single glazing typically take the form of a traditional single glazed sash or door set in a common jamb with a second single glazed sash or door or a loosely fit glass panel let into a single glazed sash.
Sealed:
There are, currently, two methods of fabricating multi-light sash and doors with IG. They are True Divided Light (TDL) IG and Simulated Divided Light (SDL) IG.
TDL fabrication includes traditional full sash thickness muntins and muntin bars separating individual panes of IG. Each IG pane is (or should be) installed from the interior face of the window or door and held in place with interior wooden glazing stops. The IG is bedded in a silicone seal against the exterior rabbet preventing moisture from penetrating the exterior surface of the window or door. The interior glazing stops are not (or should not be) sealed to the interior. Instead, the glazing stops should be applied to the interior glazing rabbet and allowed to breath thereby allowing any moisture that does reach the glazing rabbet to weep or evaporate. It is important that this weeping be allowed to insure that moisture does not accumulate in the glazing rabbet thereby voiding the IG warranty. The use of interior glazing stops also prevents moisture from accumulating in and deteriorating the wood joinery and provides an early warning in the unlikely advent that the exterior silicone seal between the glass and wood has failed. Early detection allows simple maintenance of the exterior seal before any leakage can cause serious damage.
SDL fabrication includes fabrication with one full-light piece of IG bedded in a full-light frame much the same way TDL IG is installed. However, the SDL IG is usually fabricated with internal spacer bars sandwiched inside the sealed air space along with applied moldings simulating the muntin grille laminated to the outside surface of the glass and aligned over the internal spacer bars to simulate the appearance of traditional TDL sash and doors.
ENERGY EFFICIENCY:
IG, in it's simplest double glazed configuration, is at least twice as energy efficient as single glazing. Storm sash and storm panels included with single glazing close that gap. But even the simplest IG configuration is more energy efficient than single glazed units with storm panels or storm sash.
IG with Low E coatings and/or various gas inserts are often 2.5 times more energy efficient than traditional single glazing with storm sash or storm panels. And some IG, i.e. Southwall Technologies Heat Mirror, can be as much a 5 times more efficient than single glazing. However, Heat Mirror is rarely used in small pane multi-light sash because it is 2 to 3 times thicker than conventional IG and more expensive per square foot given minimum square foot pricing structures. Furthermore, currently there are drawbacks to gas filled IG units that diminish energy efficiency and longevity requiring that its use be carefully considered as well.
The cost benefit ratio of energy efficiency depends upon the size and insulating characteristics of the building in which the sash or doors are installed, sash or door quantity, size/glass area, location and solar orientation.
STRUCTURAL PERFORMANCE:
Simply put, Sealed IG units resist pressure and impact better than Unsealed units. The degree of increased performance depends upon the type of glass being used. But in all cases, the Sealed airspace provides a cushioning effect. For example, a 10 square foot single piece of double strength glass in an unsealed application can withstand nearly 40 psf. That same piece of glass can withstand more than 60 psf in a sealed application.
AESTHETICS:
Years ago, small true divided lights in windows and doors were the product of expediency rather that aesthetics. In colonial days, large pieces of glass were not readily available. Small panes of glass were more easily replaced and one might presume that the risk of breakage from an errant bullet, arrow or other missile of the day would have a lesser impact when breaking a small pane in a window or door as opposed to breaking one large pane of glass.
Fenestration, or the arrangement, proportioning, and design of windows and doors in a building, is an important architectural design consideration.
IG allows the appearance of traditional TDL sash and doors from both the interior and the exterior while Storm Sash or Storm Panels obscure that appearance. Of course, TDL single glazed storm sash are also available. But the appearance of two TDL sash or doors juxtaposed in a common jamb is often considered less than attractive, especially when there are many small panes of glass in each sash.
CONVENIENCE:
Insulating glass provides energy efficiency eliminating the need for Storm Windows & Doors or Storm Panels. Typically, storm windows and doors are separate sash installed in a common jamb. Storm panels are usually large panes of glass installed in a rabbet let into a sash or door covering either side of the true divided light muntins. In either case, a Storm Window, Door or Panel is not sealed in place. Air is allowed to circulate from either the exterior or interior into the air space between the Storm Sash and the Primary Sash or Door. In fact, in many applications, air is encouraged to circulate to allow the periodic and inevitable condensation that forms within the airspace to evaporate and escape. Dirt particles, smoke, cooking greases and the like, migrate with the circulating air and accumulate on the glass surfaces requiring regular cleaning. Insects can plug weep holes and also migrate into the air space.
When applied in windows and doors that open and close, Storm Sash or Storm Doors present a second sash or door that must also be opened and closed. Storm Panels are more convenient in windows but present a risk in egress doors. Because the Storm Panel is typically larger in a door, the risk of breakage from vibration or impact with the loosely fit panel is increased. The use of Storm Panels in doors is not usually recommended.
MAINTENANCE:
Unsealed units require regular maintenance, albeit not intensive or costly for the most part on a constant year to year basis. But yearly maintenance is recommended and failure to do so will result in serious long term damage.
All glass surfaces should be cleaned regularly. And there are typically twice as many glass surfaces to clean in an unsealed unit as there are in comparable sealed units.
Glazing stops, glazing putty and points should be inspected yearly and will require varying degrees of maintenance over the years. Because of increased condensation, glazing stops, glazing putty, and, to some extent, glazing points, must be repainted, resealed or replaced on a regular basis. Failure to do so will put the sash or door frame, bars and muntins in jeopardy of wood joinery deterioration. Moisture can seep into the mortise and tenon through the glazing rabbet and rot the sash from the inside out.
Once moisture penetrates the wood joinery, it migrates and tries to escape through the surface of the sash or door typically evidenced by peeling paint. Once the moisture content of the wood increases, repainting is less effective. And with traditional glazing putty and point applications, the glazing rabbet can not be pre-painted because the glazing compound will not adhere to a painted surface.
The glass in layers of single glazed unsealed sash and doors, given the improvements of today's silicone sealants, should be installed from the interior, sealed to the exterior of the glazing rabbet, and held in place with wooden glazing stops. Note that single glazed sash & doors have traditionally been glazed from the exterior side. The glazing stops should not be sealed against the glass in order to allow the inevitable condensation to weep. This fabrication should be used for both the primary sash or door and the storm sash or door.
Storm panels can be installed to either the exterior or interior sides of a properly fabricated sash. Again, Storm Panels are not appropriate for most door applications.
None-the-less, condensation is an unsealed units worst enemy. Not only does condensation impede the view, unless it is minimal or removed during times of concentrated occurrence, it will discolor and deteriorate painted surfaces and rot the wood joinery.
Sealed units must be replaced from time to time. It's as simple as that. And replacement is a more intensive and costly process compared to the costs of regularly scheduled maintenance required by an Unsealed unit in a given year.
But handling and storage are not an issue for Sealed units. There are fewer glass surfaces to clean in a Sealed unit. Condensation is virtually eliminated by Sealed units, thereby eliminating the need for yearly maintenance. And all sash and door surfaces can be pre-painted with virtually no repainting or resealing required for more years than is extended by the typical IG warranty.
The cost to replace all of the IG in a TDL (True Divided Light) multi-light sash or door can cost as much as 20% of the original cost of the entire unit for sash or doors with many panes. On the other hand, if the TDL IG unit is fabricated properly, the unit can be converted to a traditional Unsealed format for approximately the same cost as the IG replacement.
The cost to replace the IG in an SDL (Simulated Divided Light) unit can, and usually will, cost as much as 65% of the original cost of the unit regardless of the number of panes. An SDL unit can not be converted to a traditional Unsealed format. In fact, the IG in an SDL unit can not be replaced. The entire sash must be replaced because the interior spacer is permanently installed within the sealed airspace and the applied moldings are permanently laminated to the exposed faces of the glass.
CONCLUSION:
Again, the question is, do the benefits of IG out-weigh or off-set the inevitable maintenance costs of IG replacement over the life of a window or door? The answer, at least today, is that it depends.
Maintenance, Energy Efficiency, Structural Performance, Convenience and even Aesthetics are all value and practicality considerations.
What are the costs of removing Storm Sash and Doors for cleaning year to year or seasonal replacing with screen sash and doors? What are the costs associated with the increased potential for breakage that may result from handling and storage? What are the potential costs of diminished Structural Performance? What are the costs of design conformance, building code conformance and your own personal preferences? What are the costs of decreased Energy Efficiency? And does Convenience have a price?
Furthermore, as IG technology improves, how much does that improvement offset the inevitable replacement costs. Today, as a rule of thumb, the wider the muntin profile, the more sealant can be applied to the edges of the IG and the longer the IG will perform before seal failure. But whether your muntin profile widths are the minimum 7/8" or you have Simulated Divided Lights with perhaps half again as much perimeter sealant, the warranty tells the tale. On average, you can expect 10 years of performance from a typical IG unit before the seal fails requiring replacement.
At Woodstone, we will fabricate both Sealed True Divided Light IG units and layered Unsealed units. However, Woodstone has chosen to not provide Sealed Simulated Divided Light IG units.
In the final analysis, we expect all of our windows and doors to remain useful for generations as long as they are properly maintained. Clearly, however, the many benefits of True Divided Light Insulating Glass are undeniable and we will continue to focus on using the best IG materials and fabrication techniques available to increase each units durability and cost effectiveness.
Please refer to Woodstone's Tradition and Technology specification manual or visit our Web Site at http://www.woodstone.com/index.html for illustrations of various glazing methods.