Recticel Insulation > Articles
Published on 20 January 2016 09:24

Specifying a Flat Roof with PIR Insulation

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Key Learning outcomes
  • Choosing PIR (polyisocyanurate) insulation boards for different waterproofing.
  • Requirements/products characteristics of flat roof PIR insulation.
  • Fixing methods for different flat roof build ups.
  • Condensation risk in flat roofs.
  • Understanding vapour control layers
  • The dangers of hybrid roof constructions.
Introduction

A warm flat roof represents one of the simplest forms of construction – and therefore one of the best ways of achieving the fabric first principle. A build-up of structural deck, vapour control layer, PIR (polyisocyanurate) insulation and waterproofing offers reliable performance for the lifetime of the waterproofing membrane.

Unfortunately, on site constraints, particularly in existing buildings, or confusion over what the layers in that build-up do, can lead to compromises in a roof’s design and construction. These compromises can store up problems that may not be seen for several decades, but which can lead to worse-than-expected levels of thermal performance or a shortening of the roof’s active life.

Nor is it uncommon for a disconnect to occur between the designer/specifier and the roofing contractor who has “built roofs like this for years”. As an insulation manufacturer, Recticel Insulation is often asked to comment on the work of a contractor that has not met an architect’s design or does not accord with current accepted flat roofing practice.

By offering this CPD Article, we aim to demystify some common issues surrounding flat roofs, drawing on our specialism in producing flat roof insulation and the experience of our sister company, Gradient Insulation.

1.0 Specification Basics

Unless specifying a system, where the waterproofing manufacturer offers the insulation with their vapour control layer and roof finish, the choice of insulation board is dictated by the preferred waterproofing. Rigid PIR foam boards for flat roofs typically fall into two categories: foil faced and tissue faced boards.

Insulation board types
Foil faced boards are mainly used for mechanically fixed single ply membranes. They do not rely on adhesion of the waterproofing to the insulation board, and the foil facing helps the insulation to achieve the best possible thermal performance.

(Sometimes foil faced boards are used with liquid waterproofing systems, but these are tested systems with manufacturer approval).

Tissue faced boards can be divided into a further two categories. A mineral-coated glass tissue or fleece facing is usually offered for adhered single ply membranes and traditional techniques such as pour and roll, built up felt and mastic asphalt.

Bitumen fleece facings are most commonly offered for torch-on felt applications. There can be overlap with the applications of a tissue faced board, however, so you should always check with your preferred manufacturer.

Unless specifying a system, where the waterproofing manufacturer offers the insulation with their vapour control layer and roof finish, the choice of insulation board is dictated by the preferred waterproofing. Rigid PIR foam boards for flat roofs typically fall into two categories: foil faced and tissue faced boards.

Insulation board types
Foil faced boards are mainly used for mechanically fixed single ply membranes. They do not rely on adhesion of the waterproofing to the insulation board, and the foil facing helps the insulation to achieve the best possible thermal performance.

(Sometimes foil faced boards are used with liquid waterproofing systems, but these are tested systems with manufacturer approval).

Tissue faced boards can be divided into a further two categories. A mineral-coated glass tissue or fleece facing is usually offered for adhered single ply membranes and traditional techniques such as pour and roll, built up felt and mastic asphalt.

Bitumen fleece facings are most commonly offered for torch-on felt applications. There can be overlap with the applications of a tissue faced board, however, so you should always check with your preferred manufacturer.

Other insulation properties
Tissue faced boards are primarily used for flat roofing. Foil faced boards, by contrast, are used throughout other construction elements and often feature similar performance characteristics. It is not uncommon, for example, for them to be offered in similar dimensions and with an identical thermal performance for applications like floors and walls.

So what differentiates a foil faced board for flat roofing from other foil faced boards?

For a start, it will likely have an Agrément certificate – provided by the British Board of Agrément (BBA) or similar certifying body – to demonstrate its fitness for purpose as a flat roofing product. This will include a specific suite of tests related to flat roofing such as wind uplift, stability, and the fire performance of the roof build up.

The other principle performance criterion to look for is a compressive strength of at least 150kPa. This compares to typical performance declarations of 140kPa or 120kPa for PIR boards in other applications such as pitched roofs or cavity walls. The increased compressive strength offers greater robustness when contractors are working on the insulation to install the waterproofing over, and also gives enough loadbearing to take the weight of some roof plant.
Illustration of a typical warm flat roof construction, featuring tissue faced PIR insulation boards on a steel structural deck.
Illustration of a typical warm flat roof construction, featuring tissue faced PIR insulation boards on a steel structural deck.
 
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Enterprise Way
Meir Park
Stoke-on-Trent
Staffordshire
ST3 7UN
Mandy Baggaley
Tel +44 (0)1782 590470
technicalservices@recticel.com
The information contained in the CPD article web pages is not intended and accordingly shall not be relied upon either as a substitute for professional advice or judgement or to provide legal or other advice with respect to any particular circumstance. RIBA Enterprises accepts no responsibility for loss occasioned to any person acting or refraining from action as a result of the information contained.
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