Glass
In our work as cladding consultants advising on the specification and control on site of all cladding and roofing materials the product which always demands our greatest attention in glass and glazing.
Architects are increasingly using glass as a means of their architectural expression. At the same time there is a need to improve standards for thermal insulation and energy savings demanded by BREAM requirements. Typically our first fork is to advise the project team on the formulation of the glass required to meet these requirements. It is not unusual for the architect's specification to require a thermal value of 1.0 w/m2/k for the centre pane of glass, with a solar gain (g value) of 0.35 - 0.42 and a light transmission of 60-70%. For lower g values the % light transmission with also decrease approximately a ratio of 2 to 1. These values can be met by coating the glass which will also affect its appearance. Glass suppliers will now provide superior coatings which have a more neutral colour. These are normally supplied with the coating on face 2 of the glass for fabricators to cut to size and make up as insulated units. Argon gas is commonly used to fill the cavity. Normal cavity width is 16mm but this can be increased to provide additional acoustic performance. Schuco system for example can accept double glazed units up to 64mm.
At Coventry University where a light transmission of 73% and g value of 0.42 was required, Arup Associates adopted to use spanish glass by Avino Douglas, but in UK it would unusual to specify glass from St. Gobain, Pilkington, Interpane, Cavy glass and Guardian all of which will offer high performance coatings.
The type and thickness of the glass is affected in requirements for wind loading, impact resistance and size of the unit. Glass producers and fabricators now have equipment to fabricate and handle larger and larger units up to 3 m x 3.5 m.
For larger units toughened glass, which is 4 times stronger than annealed glass, can be used but this should always be heat soaked to reduce the risk of spontaneous explosion due to nickel sulphide. The DIN standard 18516 used to require 8 hours heat soaking but this has now been reduced to 2 hours in euro standard BSEN14179. Even so although glass manufacturers claim that improved methods of production have reduced contamination by nickel sulphide inclusions a small risk still remains. For this reason we would never advise clients to agree to the use of single pane toughened glass on the outside skin of high rise/medium rise building particularly over public areas. The alternative is to use heat strengthened glass which may have to be thicker due to its reduced strength or laminated glass which may also have additional risk of thermal breakage which is another key consideration in the design of glazing particularly if internal blinds are used or if the client requires fitting of opaque filmed glass.
This fitting will be on side 3 of a double glazed unit as this process is normally carried out by the fabricator using a digital printer to architects design.
Acoustic performance can be improved by the use of an acoustic laminate in the inner pane of a double glazed unit. The overall acoustic performance of the glazing system will also be affected by any acoustic loss through the aluminium framing.
In large projects where the system of glazing is specific to that project is may be a requirement to carry out a full scale weather test as at Singapore Theatre on the Bay but in most projects the time and cost of building such a mockup is not possible and the design team will specify tried and tested systems such as Shuceo, Kawneer, Reynolds and others. However we should be aware that any bespoke changes to these systems may affect their system warranties. Even small scale mockups for the purpose of showing buildability and general appearance of detail are difficult to provide within tight programmes of design. Design and build contractors are reluctant to identify the precise specification of glass related to a particular supplier in order to give their subcontractors greater maneuverability in pricing.
Thus sourcing of samples or mockups to show clients and planning authorities is not an easy matter as the mock up can often be late due to glass delivery times.
Our role is often to assist the design team to come to an early discussion about performance requirements and have available a palette of different glass formulations to meet these requirements.
At one time as Alan Brookes Associates we were keen to promote the advantages of suspended glazing but the gradual understanding of the complexity of the fittings required for double and triple glazing and the small number of subcontractors with the skills to engineer the combination of lightweight structures linked with suspended glazing has shown that on most projects in UK we are working with ‘stick’ curtain walling. Most of our attention is now given to interface details between curtain walling and the adjacent construction. All too often there is a complete lack of understanding of the need for an epdm edge sent to overlap on to the adjacent framing and the complexity of forming the corner joints. We constantly have to remind the subcontractors to maintain a thermal break across the intersection between glazing and solid back up walls or kingspan panels in SFS framing. The purpose and position of vapour barriers and breather membranes always gives cause for concern.
On site we have to frequently monitor the method of storage and condition of the edge of the glazing. Our standard specification requires glass edges to be generally undamaged. Shells and chips not more than 2mm deep and extending not more than 5mm across the surface are acceptable if ground out. It is important particularly when using toughened glass which suffers from roller wave for the glass to be installed in the correct orientation. For this reason we ask for the kitemark (which should include a mark showing heat soaking) to be in the same position on the glass which is the bottom right hand corner when viewed from the outside. Unfortunately this is not always achieved by some subcontractors. We have even had cases where the glass has been installed completely the wrong way round i.e. inner face facing outwards. We now have the means of carrying out careful checks on the glass supplied using equipment to check the type and thickness of the glass. Other common issues included in our site reports are gaps at the ends of EPDM or silicone gaskets and spacer bars projecting through the silicone sealant.
Thus although glass has been available since the time of the Egyptians there are many obstacles to its successful use in modern buildings. The standard NBS specification in our view is not sufficient to prevent problems. Early specification of glass types and agreement on position to EPDM seals and tolerances is essential in our view.
