In the following document we have attempted to put together some tools to make your job easier in the selection and use of the proper roof insulation for your specific project. We know it is very difficult with new roof designs, truss options and insulation levels, to make a decision on what to use without creating a problem. It gets very hard for the Builder and Architect to make decisions based on some manufacturers hearsay. The following is a guide to help determine Fact from Fiction.
Based on our almost 24 years of combined construction experience, we have attempted to separate these two for you as follows:
Fiction: It is better to cut a foil capsheet back 3 - 6" on either end of the sub-purlins to make sure you do not have condensation problems in a wood panelized deck system.
Fact: You probably would never experience any condensation problems in this installation. However, the foil would be a waste of money. Horizontal movement of air, by convection, would remove whatever thermal performance that would have been there with an uncut assembly. A 1/2" cutback at either end would not significantly effect the performance of the foil and would still vent the cavity.
Note: The 1/2" cutback would not work in a bar truss purlin type assembly because it could cause the bars to sweat by forcing hot air out the ends and onto the cold steel purlin surfaces. This would not be a problem with a capsheet and fiberglass combination.
Fiction: The bar truss type purlin is less costly and more efficient than using a solid wood purlin in combination with 2 x 4 or 2 x 6 sub-purlins.
Fact: It may cost less and help the electricians and plumbers but it sure doesn’t help the cosmetics of the roof insulation. Take a good look at the exposed insulation. It is almost impossible to make this installation cosmetically appealing for the owner or occupant. Architects should consider a “TJI" or “Gangnail" truss for use as purlins instead of a “TJL" or bar truss system if they want a ceiling, in an exposed condition, to look half way decent.
Fiction: It is O.K. to paint over an insulation facing without changing the performance of the product.
Fact: While painting over an insulation facing may not change its insulating R-Value (with the exception of a foil product), it does change its flammability characteristics and could make it more flammable.
Painting, or altering, any facing voids the manufacturers warranty. Painting over the surrounding wood purlins and sub-purlins, in a panel deck system, may also trap pre-existing moisture and create problems at a later date such as dripping and dryrot. Painting over a reflective foil surface will also reduce the overall system R-Value by approximately R-4 for cooling and R1 for heating by the elimination of the reflective air film. (see ASHRAE handbook)
Fiction: A reflective foil can be used to satisfy the requirement in a roof for a “One Hour" fire rated assembly.
Fact: A local building official may allow that but the facts do not. The UBC (Uniform Building Code) is very specific in its material requirements for a 1 hour assembly and those requirements don’t allow the use of an aluminum capsheet or foil facing.