The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
The use of double skin composite or insulated panels for roof and wall cladding requires the same or similar detailing for flashings, penetrations and design considerations as those for single skin roof and wall cladding described in this Code of Practice. Reference should be made to the relevant section when designing insulated panel systems, as this section only describes specific differences.
Because insulated roof and wall panels are specialised proprietary systems, few specific details are discussed. However, the principles of water shedding, fastening, and maintenance described in this COP are all applicable.
Composite or insulated panels are factory made laminated products, using different core materials permanently bonded by adhesive or foaming to act as a single structural element.
Insulated or sandwich panels have metal facings on both sides; the space between them filled with an insulating core which is permanently bonded to both surfaces.
Three types of sheeting are used on insulated panels.
The manufacturing process for bonded panels consists of roll forming the flat or profiled sheeting, followed by the adhesion of the insulation core to both surfaces or skins.
There are three methods to do this :
- Continuous metal panel production by bonding panels of insulation to metal skins.
- Individual panel production.
- Continuous metal panel production by foaming.
Site assembled, or built-up systems are also known as composite panels and are of two main types.
- Two profiled sheets have rigid insulation boards adhered to their troughs, without metal spacers.
- The sheeting is mechanically fixed on both sides to a structural girt. The girt can form a thermal bridge unless spaced away from the structure. This type of built-up system commonly uses fibre insulation.
Bonded composite panels develop their strength from the sandwich of skins and insulation, and are made with a tongue and groove side lap detail that incorporates concealed fasteners.
Flat continuously produced panels suffer minor undulations in the metal skins that arise from built-in tensions in the metal coil and introduced during panel manufacture. Panning can be minimised by using an embossed or matt finish or forming minor ribs or swages on the flat face of the panel.
The metal facing or skin is commonly made from grade G300 steel of 0.40 – 0.63 BMT thickness, with pre-painted organic finish over a metallic coating of ZM 275.Aluminium facings are used in very humid conditions or a severe marine environment and can be supplied with a mill surface, embossed surface, or they can be pre-painted.
The core can be made from different types of material all with different insulating values, fire ratings, and strengths. The most common are EPS Expanded Polystyrene, PIR Polyisocyanurate, and PPS Phenolic/Polystyrene.
Expanded polystyrene is used for flat factory bonded panels and can be shaped to the profile of the top skin.
The insulation thickness of a profiled roof panel varies from 30 mm – 300 mm. To achieve the same insulating value as a flat panel, the profiled roof panel needs to be thicker.
Dense rigid mineral-fibre insulation may be selected for applications where fire resistance or acoustic insulation properties are considered to be most important.
Built up or composite panels insulated with extruded closed cell polystyrene or fibre insulation material may need to be of a different thickness to achieve the same insulation value.
The number and strength of the fasteners under wind suction loads can limit the maximum purlin spacing. If roof-lights are required, the maximum purlin spacing will be limited by the strength of the roof-light sheeting. Polycarbonate or G.R.P. barrel vault roof-lighting may avoid this restriction.
Insulated panels, unlike single skin profiles, can support normal foot traffic without damage, because the foam core provides continuous support to the external sheeting to resist deformation and indentation.
All persons walking on the cladding should wear footwear suitable to comply with the safety requirements in 12.1 Safety, and also to avoid marking or scratching the surface coatings.
Composite panels are supported on purlins or girts, which should be accurately erected to a maximum tolerance of 3 mm and L/600; due to their inherent stiffness, insulated panels do not have the flexibility to follow uneven structures.
All transverse laps should be fixed and sealed to prevent the passage of air, water or water vapour.
If composite panels are expected to provide restraint to the purlin or girt flanges, through fixing with oversized holes is required which allows panels to slide under thermal movement, as clips do not provide sufficient restraint. Where fixings are widely spaced panels may not effectively restrain the purlin or girt flange.
Composite panels should not be used in lieu of sag bars as their function is to hold the purlins or girts in their correct location while the panels are erected.
Where larger holes are required trimmers should be in place before the erection of the panels.
A method of limiting the thermal bow is to make stress relief cuts in the panels as follows.
- When a panel is restrained at three or more points, a cut completely severing the cold skin may be required at the intermediate point.
- When a panel is attached along its edge, a partial stress relief cut may be required.
The through fasteners or fixing clips are cold bridges, but it has been shown that these are unlikely to increase the U-value by more than 1 –2 %.
A joint may be required when the roof panel is longer than 15 m. It can be a sealed lap joint with provision for expansion, or a stepped or waterfall detail. See 22.214.171.124 Step Apron.
Most panels have a fire resistance when used as a non-loading panel, and the cores are made from insulating foam incorporating fire retardant materials. Fire regulations aim at reducing the risk of death or injury to occupants, the public and the fire service, and it is achieved by the selection of materials which behave in a predictable manner.
Steel and aluminium liners achieve classifications for combustibility, ignitability, and surface spread of flame; for fire resistant wall construction, steel-skinned composite panels must be used because the melting point of aluminium is too low.
Polystyrene cores are not easily ignited behind the metal skins but can melt and flow out of the panel. Such cores must not be used for internal partitions or ceilings, where there is a high fire risk. Polystyrene cored panels must be isolated and protected from radiation from hot flues.
Once a fire has started within the foam core, fire services are unable to trace or extinguish it and the building should be regarded as unsafe.
Because nylon bolts may jeopardise the integrity of the building during a fire, other mechanical connections should be used if the building is required to have a fire rating or is considered a likely fire risk.
N.B. Fire ratings are available for non-load bearing applications.
When composite panels are used as cold store insulation a complete and continuous vapour barrier is essential to prevent inward moisture vapour pressure. Any discontinuity will result in a build-up of ice which can destroy the panel.
Acoustic absorption depends on the nature of the lining. Flat metal linings absorb very little acoustic energy, and it may be necessary to install additional acoustic lining systems.
Profiled cladding side laps require stitching at the rib at 500 mm centres with a strip sealant of approximately 9 mm x 3 mm or similar. See 7.8.2 Secondary Fasteners.
The through fixings may also be pan fixed or located on a mini-rib or swage within the trough, but purpose-designed fasteners are required to maintain the weather seal between the metal skin and the washer. Pan fasteners should not be over-tightened as this causes shallow dents around the fastener head and washer. The washer should have a minimum diameter of 25 mm to provide good pull-over strength.
- At end-laps, the lining and insulation is butt jointed over the purlin, and a 150 mm overlap is formed in the external weather skin only using two lines of sealant. The sealant should be silicone or preformed strips and positioned at the top and bottom of the lap. To provide a secure seal with flat or wide pan profiles, additional sealed rivets or stitching screws are required through the top skins only. This detail is only suitable where the roof pitch is more than 10° and where the maximum length is less than 1 5 m.
- Where the pitch is below 10° or the length is more than 15 m, a stepped or waterfall joint is required. See 126.96.36.199 Step Apron.
The bottom skins of composite panels have an integral side lap with a re-entrant sealing space which acts as a vapour control, but in high-risk applications such as food processing buildings, textile mills, and indoor swimming pools an additional sealer strip is required at the lining. Concealed fix systems may be used on very low pitches to conceal the fasteners from the weather and keep it out of sight.
Flashings detailing is similar to that used with single skin roof and wall cladding or built-up systems, but there are minor differences that may influence design decisions and special requirements that should be addressed.
The panels at the ridge should be sealed and the lining closed with a metal trim mounted on the ridge purlins. Any gap between the ends of the composite panels should be insulated to eliminate cold spots or cold bridging. They can be sealed using in-situ injected foam or mineral fibre. In high humidity applications the liner trim should be sealed to the panels, and at end-laps or gaps, foam should be injected to provide a vapour tight seal.
Eaves panels should have the ends turned down to direct water to drip into the gutter, and to have a metal flashing to cover the exposed end of the insulation and metal liner.