Insulation should maintain its full design depth throughout a roof assembly – using spacer systems is best practice.
This article is part of a series on metal roofing and cladding systems that promote sustainability.
Detailed installation methodologies for insulated roof spaces are often over-looked at the design stage. As a result, the performance of the insulation may be negatively affected during the on-site installation process, and it may not achieve the prescribed thermal resistance (R-values) as stipulated in the SANS 10400XA standards.
It is of key importance that insulation maintains its full design depth throughout the roof assembly in order for it to achieve its optimum R-value. Compression of insulation compromises the R-value and the thermal performance of the material. It also increases thermal bridging along purlin lines.
Challenges with different insulation types
Factors such as cost, aesthetics, thermal performance and combustibility all play a major role in the selection of an appropriate insulation material for a project. Currently, rigid insulation boards and flexible insulation blankets are the two most popular types of insulation used in commercial roof assemblies in South Africa.
Rigid insulation boards generally cope better under compression, but as their specified depths increase, so do the associated costs, as well as risks associated with long fasteners and system instability.
Flexible blanket insulation installed over purlin will typically be subjected to considerable compression along the purlin lines if the outer weather sheet is not elevated. When installing this type of insulation, suitable spacer systems should be employed to prevent or reduce compression.
There are various spacer systems available in South Africa that can broadly be categorised under continuous spacers and mechanical spacers.
Continuous spacer systems
Continuous spacers or packers are available in a number of forms such as timber, XPS or steel. They are secured directly to the purlins in continuous lengths on top of the insulation blanket, which is draped over purlin.
These packers will cause compression at the purlin while allowing the blanket to regain some loft between purlins by elevating the weather sheet. It is necessary to compensate for this loss in overall R-value caused by the compression, by increasing the depth of the specified insulation blanket and packer accordingly (see table).
Mechanical spacer systems
Mechanical spacer systems form the backbone of site-assembled roofing systems where warranted thermal performance is required. They eliminate compression of the insulation blanket and take the guesswork out of roof assembly performance.
Lightweight structural steel bars, which act as purlins, are mechanically locked into each other to create the lengths required. The bars are supported by brackets, which are available in a variety of heights to accommodate varying blanket thicknesses. The support brackets create a defined cavity for the insulation material and keep fasteners within safe working lengths. Specific loading requirements are achieved by varying the support bracket centres.
Performance of blanket insulation installed with different spacer systems.
*It is imperative that fastener limitations should be considered when increasing the depths of certain types of packers. Professional advice should be sought from a roof sheeting manufacturer and fastener supplier, especially when using packers in conjunction with concealed fix outer weather sheets.
Safintra South Africa
Tel: 011 323 6300
Website: www.safintra.co.za and www.safintra.com
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Branches in South Africa: Johannesburg, Durban, Cape Town, Nelspruit, Polokwane, Port Elizabeth and Bloemfontein.
Further operations: Namibia, Zambia, Mozambique, Malawi, Tanzania and East Africa.
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