Factors influencing glazing performances
Selecting the proper glazing system for particular requirement must take into account a number of important factors :
Life cycle costing is being used more and more too determine the total cost over a specified period of time. For example, what may seem to be an economical sealant in terms of initial cost may eventually become more expensive if it requires frequent (and costly) maintenance and/or replacement.
In high rise construction, this situation can become particularly acute. The cost of frequent maintenance or replacement is more than the initial cost of installing a higher performance glazing system.
Conversely, the selection of an expensive glazing system may not be necessary on a single-storey structure or buildings that afford protection from the elements. In the final analysis, a careful determination should be made between the glazing system’s initial cost and its anticipated service life before final selection is made.
Any glazing system, regardless of its initial cost, must be properly installed if it is to achieve its maximum performance capability. The most expensive system, if not properly installed, will perform no better than an inexpensive system.
Similarly, the use of an expensive system does not justify poor workmanship if the system is expected to properly perform. For example, sealants, regardless of cost, will not bond properly to dirt, damp, or frosty surfaces. Two-component compounds must be uniformly mixed before applying to achieve proper cure. Surfaces to be caulked must be clean, dry, and free from contaminating substances to ensure a good bond. Glazing tapes must be neatly butted (not overlapped) at corners. Setting blocks, spacer’s shims and edge blocks should be installed properly when and where they are required. Window washing should not be done until sealant systems have been allowed to set properly. It is important to remember that the finest workmanship can never compensate for a poorly designed flimsy sash, improperly specified sealant, adverse weather conditions, or damaged by trades not directly engaged in the glazing operation.
Sash and Rabbet Dimensions:
The design and dimensions of the sash can dramatically affect the ability of the glazing system to properly perform. In the case of gunnable sealants, the volume of sealant that can be installed is very important. A large volume of sealant is better able to absorb and compensate for stresses and strains and lose bond, as compared to a smaller volume of sealant. Liberal sash dimensioning-specifically ample face clearances- is recommended to allow glazing to withstand these dynamic forces in shear, tension and compression.
Type and Size of Glass:
The larger the lite of the glass or plastic, the greater the stresses placed on the glazing system. This is due to excessive movement of glass in the in the sash caused by deflection, and differences in the coefficient of expansion between the glass and the metal.
With advent of high-performance glass such as reflective, heat absorbing, insulating and laminating, additional strains are placed on the glazing system. Heat absorbing glass; for example, transmit higher temperatures to the glazing sealant than does clear glass. This tends to dramatically shorten the service life of oil-base type sealant. Elastomeric sealants, such as MONO and DYmeric are necessary to ensure long life expectancy.
When specifying larger lites of glass, particular attention should be directed to sash design and dimensioning, sturdiness of sash, proper anchorage of framing members and the use of a high performance glazing system.
Type of Sash:
All types of sash-whether aluminium, steel, or wood- are subject to dimensional changes caused by fluctuations in temperature and atmospheric conditions. This imposes a burden on the glazing system to accommodate stresses and tension, yet maintain a positive bond to sash surfaces as well as to the glass.
Operating sash… projected, pivot, sliding and others… create further demands on the glazing sealant due to racking, discordant, shock and vibration-as compared to the non-operated sash.
Additional considerations include positioning of the fixed and removable stops in relation to interior or exterior exposure. In many instances when properly located, glazing combination can be used to permit interior glazing, thereby eliminating costly exterior scaffolds on multi-story buildings. Occasionally, the use of “light-gauge“, Snap-On glazing stops may prevent the effective use of glazing tapes because the stop is sufficiently sturdy to insure compression of the glazing tape.
Glazing sealants are subject to physical change caused by variations in both temperature and humidity. Cold weather conditions tend to stiffen most sealants, making them more difficult to apply. When glazing at temperatures as below 4 degrees, sash may be damp or frosty preventing the sealant from attaining a satisfactory bond. Normal curing of sealant is often retarted during cold weather, and the resultant “tacky“ surface is subject to “dirt“ pick up.
Generally, direct exposure to certain UV wave lengths, as well as prolonged heat, accelerate the aging processes of most glazing material.
Moisture collecting pockets in the sash tend to affect the bond of most sealants. This is particularly true during freeze-thaw cycles, or in areas of high humidity or frequent precipitation. Systems employing vents (rain screen) will generally solve this problem.