Selecting roof insulation cannot be an isolated decision because the choice generally needs to be made in coordination with the membrane type and application method. Managers need to consider selecting insulation that addresses: membrane compatibility; energy-code — R-value — requirements; insurance and code requirements regarding wind and fire; and compressive strength for traffic and hail resistance.
Not all insulation types are compatible with all roof types. For instance, expanded and extruded polystyrenes should not be in direct contact with thermoplastic polyvinyl chloride, single-ply membranes. Also, modified-bitumen membranes and conventional built-up membranes are not recommended for direct application over foam-plastic insulation boards. Modified bitumen and built-up roof-covering manufacturers recommend cover-board insulation, and it is required when using foam-plastic insulation.
Managers also must consider code issues. State and local building codes have requirements for thermal resistances of insulation. Washington, D.C., and 47 states have adopted the International Building Code (IBC), and a chapter in the IBC references the International Energy Conservation Code (IECC). In turn, the IECC has adopted the ASHRAE 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings, as a reference standard.
Before selecting insulation, managers need to understand the impact insulation thickness can have on installation. For example, because of their composition, wood fiber and perlite require greater insulation thicknesses, compared with polyisocyanurate or composites. Installers might need to raise perimeter wood blocking, roof curbs, and rooftop-equipment elevations as roof-insulation thickness increases.
Managers also must consider insurance requirements when selecting roof insulation. FM Global Affiliate — or an insurance company that requires the use of FM Global-tested and -approved roof assemblies — insures many properties. Approvals for tested assemblies are specific regarding the types, thicknesses, configurations, attachments, and acceptable manufacturers of roof insulation.
Finally, concentrated roof traffic can be as hard on a roof system as any other factor. Managers should closely evaluate compressive strength based on anticipated traffic loads.
Alvin Nunnikhoven is a registered roof consultant (RRC) and senior consultant with Benchmark Inc.
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