TECHNICAL
PREFORMED tape sealants, more commonly referred to as butyl tapes or glazing tapes, offer an economical approach that is neat and easy to install in both high-and low-performance glazing applications - and are the most popular sealant for many applications. Since correct specification and installation are the keys to successful performance, one of the most important steps in any tape-glazed project is to ensure that the correct tape has been selected.
Three kinds of tapes are available for glazing applications: shimmed, un-shimmed and hybrid. Hybrid tapes differ from standard shimmed and un-shimmed tapes in that they are highly elastic. Hybrid tapes make use of cross-linked rubber technology and are expressly designed for use without the aid of a rubber shim in compression glazing systems.
Compression glazing systems require the use of shimmed or hybrid tape to maintain the designed face clearance while under continuous pressure from a gasket installed at the opposite side of the glass. Tapes in a compression system are designed to be compressed to as little as one-third of their original thickness.
The gasket used to provide compression on the glass must be designed for use with the specific tape being used and in the specific glazing system. If the components of a compression glazing system are not designed to function together, performance may be less than desired or glass breakage may occur.
Non-compression glazing systems typically make use of un-shimmed tapes. In these systems, the tape is initially compressed during installation of the glass. This compression can be done by hand or mechanically in factory applications. The tape must completely wet out on the glass and framing surfaces.
In non-compression systems, continuous pressure is not maintained on the glass. On the side of the glass opposite the tape, the glass is held in position by a specially designed removable stop or by a spacer sized to the designed face clearance.
Tapes intended for non-compression glazing systems are easily deformed; therefore, their use is limited to smaller openings subject to less demanding performance conditions. Because tapes have different performance characteristics, manufacturers vary in the limitations they impose on their individual products.
Wind loading and thermally-induced movement are two key influences on the performance of a window. Therefore, the size of the glass panes is the most common factor used to determine when a glazing application needs to be upgraded to a better-performing system. In a non-compression glazing system, it may be necessary to upgrade from an un-shimmed tape to a shimmed tape when the glass pane exceeds 0.85m2 . Demands on performance, such as coastal wind exposure, are also reasons to consider upgrading a system.
Choosing the right tape size is as important as choosing the right tape. Glass, gaskets and framing members have specific dimensions and manufacturing tolerances.
Preformed tape sealants are supplied in a variety of thicknesses and shim diameters and each is intended for a specific designed face clearance. Just like glass, gaskets and framing, tapes are subject to manufacturing tolerances. Poor performance or even glass breakage can result from failure to select the correct type and size of tape.
There are several basic rules of thumb to determine the correct tape size:
For non-compression systems, the tape thickness should be the same as the designed face clearance (3.2mm thick tape for a 3.2mm face clearance).
For compression glazing systems using a shimmed tape, the shim in the tape should be the same as the designed face clearance (a tape with 3.2mm diameter shim for a 3.2mm face clearance).
For compression glazing systems using a hybrid tape, the tape thickness should be greater than the designed face clearance (a 3.2mm thick tape for a 2mm face clearance).
Proper treatment of the joinery is critical to the performance of any glazing application, and the first step in a successful installation is to prepare and seal the joinery in the framing system. Butt, mitre and overlapping joinery conditions need to be sealed. Proper procedures include cleaning and, if necessary, priming the joinery at all corners of the framing. A variety of sealants may be used to seal the joinery, but neutral curing silicone sealants perform especially well in joinery conditions.
The following general guidelines for tape glazing apply regardless of the type of tape or system being glazed:
Tape should be applied in four separate lengths; do not use a single continuous length that wraps around the corners. When tape is wrapped around corners it wrinkles, which can prevent a complete seal. This can also create a pressure point at the corners, possibly increasing the potential for glass breakage;
Tape should be cut to the proper length for each side of the opening. It must not be stretched or placed together to fit.
Tapes should never overlap at the corners. While this may seem to offer the best seal, it actually creates a greater risk of leakage as well as a pressure point that can cause glass breakage;
All tape applications require some use of a gunnable sealant. The minimum sealant requirement is to seal around the butt-joined corners of the tape. Cap beads, toe beads, heel beads and air seals may also be desirable;
Release paper or film should be left on the tape until just prior to setting the glass
Tape should be applied only to the openings for which glazing can be completed in the same day;
In some systems, tape is placed on both sides of the glass or panel. These lower-performing systems should be limited to residential or other less-severe applications;
Lower-performing systems that do not use a continuous toe bead should be limited to residential or other less-severe applications.
Joint surfaces should be inspected before application. Any conditions detrimental to achieving a positive, weather-tight seal should be corrected prior to commencing work.
All openings, joints or channels to be sealed must be clean and dry. They should be free from dust, oil, grease, loose mortar or any other foreign matter; and If the tape will come in contact with surfaces with protective coatings, a suitable solvent should be used to remove the coatings and any oily deposits.
As with all glazing systems the correct selection of materials to be used and the correct installation will ensure the maximum service life of the complete system. Following the above guidelines should ensure that any tape glazing system will perform satisfactorily for the full extent of its intended service life.