Warm roofs are where insulation is in direct contact with the roof cladding. It can be divided into three main categories:
- insulated panels,
- composite or site assembled systems, and
- roofs with insulation sprayed on after installation.
Warm roofs are where insulation is in direct contact with the roof cladding. It can be divided into three main categories:
Insulated or sandwich panels are factory-made laminated products using different core materials permanently bonded by adhesive or foam to metal skins to act as a single structural element.
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:
The structure and composition of an insulated panel are chiefly determined by its end use.
Three types of profiles are used on insulated panels. This is largely determined by the spanning requirements and ease of maintenance.
Flat continuously produced panels suffer minor undulations in the metal skins that arise from built-in tensions in the metal coil and are introduced during panel manufacture. Panning can be minimised by using a matt finish or forming minor ribs or swages on the flat face of the panel.
The facings or skins of composite panels are typically metallic coated, pre-painted steel or aluminium, and are either profiled or flat on either or both sides of the panel. The internal skin is also known as the liner skin or sheeting.
The metal facing is commonly made from grade G300 steel of 0.40 – 0.63 BMT thickness, with a pre-painted organic finish over a metallic coating. Cool room panels have a galvanised Z275 metallic coating, while interior/exterior structural insulated panels have an aluminium/zinc coating of AZ150 or AZ200. Paint coatings are specifically developed to assist in bonding the insulation to the panel
Aluminium facings are used in very humid conditions or severe marine environments and can be supplied plain or pre-painted.
The core can be made from different types of material with different insulating values, fire ratings, and strengths. The most common are EPS (Expanded Polystyrene), PPS (Phenolic/Polystyrene), PIR (Polyisocyanurate), and mineral fibre.
EPS foam is mainly used for applications where high fire resistance is not a requirement.
PPS may be used where greater fire resistance is required.
PIR foam is increasingly specified because of its fire-resistant properties and better insulation efficiency.
Mineral FibreMineral-fibre insulation may be selected for applications where fire resistance and/or acoustic insulation properties are of prime importance.
The insulation thickness of a profiled roof panel varies from 30 mm to 300 mm. To achieve the same insulating value as a flat panel, a profiled roof panel needs to be thicker. The through fasteners or fixing clips are thermal bridges, but it has been shown that these are unlikely to decrease the R-value by more than 2%.
Different cores will also have different values for thermal insulation and noise attenuation, as will the details of panel joints and interfaces with other materials.
The nett insulation value of a panel must be calculated and stated by the manufacturer.
Since we say it won't increase by more than...we can stay with the top value.
R-values are the reciprocal of U-values. We will stick with R-value because that is the most commonly used value for roofs and walls.
Insulated panels are integral units in which the insulation layer together with the two metal skins act as a beam to resist wind and point loads. The bonded insulation core material contributes to the panel's strength by effectively increasing the web depth of the profile and resisting buckling; the depth of the core largely determines the panel's resistance to deflection. Panel stiffness is also affected by skin thickness and profile shape.
Insulated panels are excellent at supporting normal foot traffic without damage because the foam core provides continuous support to the external sheeting. The number and strength of the fasteners under wind suction loads can limit the maximum purlin spacing.
Where roof lights are required, the maximum purlin spacing will be limited by the strength of the roof light sheeting; it can be extended by using mid-span supports. Polycarbonate or G.R.P. barrel vault roof lighting may be used for greater spans or proprietary systems may be supplied by the manufacturer.
Insulated panels do not have inherently good acoustic insulation properties. Sound can be lessened by using sealed joints, but where higher levels of acoustic attenuation or absorption are required, it may be necessary to install additional acoustic lining systems.
Metal facings are effectively impervious to penetration by vapour, and panel cores have a closed cell structure which does not permit significant transmission or absorption of vapour. However, to prevent the possibility of interstitial condensation, it is necessary to fasten and seal all laps and gaps, side-lap joints, transverse laps, and joints and ridges that are exposed to the internal environment.
When insulated 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.
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 in the side lap joint.
Thermal bowing can occur when the two skins are at significantly different temperatures such as north-facing walls, e.g., when a cool room roof panel is in direct sunshine. The effect is accentuated when the external surface is a dark colour and is more severe for aluminium facings.
The use of insulated panels for roof and wall cladding requires the same or similar detailing for Structure, Durability, External Moisture, and other design considerations as those for single-skin roof and wall cladding.
In most applications insulated panels can be installed by workers operating off the already installed sheet, negating the need for safely mesh. Many panels are installed using underslung safety nets. Other than that, all safety precautions should be followed as for any other cladding material.
Due to their inherent stiffness, insulated panels do not have the flexibility to follow uneven structures. Insulated panels are supported on purlins or girts, which should be accurately erected to a maximum tolerance of 3 mm and a deflection limit of l/600.
Insulated panels can accommodate penetration openings of 350 mm diameter or 300 mm square without the need for additional structural supports or trimmers. Where larger holes are required, trimmers should be in place before the erection of the panels.
Purpose-designed high thread-type fasteners are required to spread upwards loads and maintain the weather seal between the metal skin and the washer. Through fixings for roof panels may be rib fixed or located on a mini-rib within the trough, and should be set snugly to achieve a weatherproof seal without distorting the rib profile. Care must be taken not to over-drive the fasteners as this can cause damage to the G300 outer skin of the panel which can result in water ingress issues
Insulated roof panels with trapezoidal ribs are through-fixed with a load spreading washer on the rib and require lap fixing and sealing at the side laps to the manufacturer’s recommendations.
The maximum practical length of panels for transport and handling efficiency is restricted to approximately 25 m. Where a transverse joint is required, there are two options:
Butt End Lap
The lining and insulation are butt-jointed over the purlin, and a 150 mm overlap of the top sheet is formed in the external weather skin only, using three or more lines of sealant and fasteners. The sealant should be butyl tape, silicone or MS sealant, or self-adhesive closed cell tape according to the specifications of the manufacturer and should be 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.
The challenge with this detail is that there are four layers of material to consider at the side/end junction. While successful in many instances, problems developing later are very difficult to remedy.
A waterfall step can be achieved by putting a step in the rafter or by increasing purlin cleat heights,
The first challenge of this detail is that the cold skin of the upper sheet cannot rest on the upper surface of the lower sheet, so a thermal break must be created. The second challenge is that any gap between the upper and lower sheets must be adequately insulated to ensure that the thermal efficiency of the system is not compromised.
Flashings detailing is similar to that used with single-skin roof and wall cladding. The main exception is that internal joints should be designed and sealed so that water vapour cannot impregnate the system.
The panels at the ridge and other edges of the roof should be sealed and the lining closed with a metal trim mounted on the ridge purlins as detailed by the manufacturer. 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. At end-laps or gaps, foam should be injected to provide a vapour-tight seal.
Where required by the manufacturer, eaves panels should have the ends turned down to prevent capillary action on the underside of the sheet. A metal flashing may also be installed to cover the exposed end of the insulation and metal liner when panel ends are visually exposed.
Site-assembled or built-up warm roof systems are also known as composite systems. They are not a true warm roof, as the ribs are typically not in contact with the insulation, but they have many of the same attributes as warm roofs.
The advantages of composite systems are that they offer longer runs without the need for roof laps or waterfall junctions, and when the time comes for the replacement of the outer weather-proofing roof sheet, the system allows for it to be done without disturbance to the operations within the building.
Acoustic boards can be laid within the system to increase sound absorption either from within the building or from outside. Installation of the under-liner sheets can make a building largely watertight allowing other trades to continue below while the roof is completed.
There are three main types.
Composite systems typically incorporate a sealed vapour control layer under the insulating layer, and an absorbent roofing underlay under the top skin. They may be designed as closed systems, or ventilation may be introduced between the insulation and the top surface. The insulation material may be expanded foam, mineral wool, or fibreglass blanket. Top surfaces may be profiled metal or membrane.
Post and rail systems allow thicker insulation to be used, increasing thermal and fire performance.
Composite systems are normally laid layer by layer to form the desired system. Typically, they comprise a metal base layer to provide support for the insulation, which is overlaid by a vapour check layer, insulation, underlay, and the top roofing sheet.
Post and rail systems are often used for over-laying existing roofs in good, safe condition. Another form of overlay uses custom cut blocks as pan infills, over which is laid a continuous insulating layer. These forms of roof repair avoid the need for removal and replacement of an existing roof, allowing the building tenants’ operations to continue uninterrupted.
When installing composite systems, it is important that the vapour control layer is correctly installed and sealed to manufacturers requirements, and insulation is laid correctly without gaps. Bulk insulation must be able to loft up to its design thickness to achieve the design R-value.
Warm roofs can also be constructed by applying spray foam after installation.
Spray foam is typically polyurethane closed cell foam applied in thicknesses according to the insulation requirement. It can also be applied to walls, ceilings, and under floors. Spray foam may be applied to any roofing profile. It has a greater insulation value per millimetre than other insulation materials, so is particularly suitable for flat roofs and skillion roofs where there is limited air space.
Spray foam is non-absorbent (less than 1%) and has strong contact with all adjacent building elements. This makes it an effective vapour check, so internal moisture issues must be considered when using this or any other non-permeable system.
When sprayed foam is applied directly to the underside of profiled metal sheet roofing on a lined building, the ceiling space must be adequately ventilated by other means. (See 10 Internal Moisture.)
Installation with underlay.
Polyurethane Spray foam may typically be applied directly to any type of purpose-made bituminous or synthetic building wrap, including foil-backed, excluding polyethylene sheet which is not a suitable substrate as there is inadequate adhesion.
The typical procedure for application is:
1. Check the building to confirm that the wrap is not loose or floppy, is properly attached to the studs, and is not sagging between purlins.
2. Spray a “flash-coat” of Polyurethane Spray Foam over the building wrap or cladding contacting the timber or metal framing. Allow to set — it will take 5-10 minutes depending on the ambient conditions. The "Flash-coat" will tension the building wrap, give it some rigidity, and assist with the curing and adhesion of subsequent layers.
3. Spray the first pass of Polyurethane Spray Foam onto the flash-coat. Do not exceed 25-30 mm with this pass. This will render the building wrap/foam composite rigid and able to take the balance of the foam with no distortion.
4. Spray the second pass of Polyurethane Spray Foam to the desired thickness to achieve the required R-value (or up to a maximum of 100mm per pass).