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9.4.2.1 Calculating Roof Performance for "Over-the-Purlin" Installations Traditional methods of calculating assembly R-val-ues will not result in accurate estimates of thermal performance for "over-the-purlin" installations. The insulation is not installed uniformly, nor at the thickness required to perform at the rated R-value, and it is penetrated with multiple metal fasteners. The Thermal Insulation Manufacturers Association (TIMA) has developed an empirical formula based on testing of insulation meeting the TIMA 202 specification. Insulation not designed to be laminated (such as filler insulation) will show performance up to 15% less than indicated by the formula below. Assuming the insulation meets the TIMA specification, the U-factor of the completed assembly can be estimated as:

Rf = Sum of Inside and

Outside Air Film R-Values, hr - ft2°F Btu t = Pre-installed insulation thickness (for TIMA 202 type insulation), inches

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