FACT vs. FICTION

III

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:  You can achieve the full R-value benefit of a multi-layer foil system, in a panel roof deck with “TJL" (bar trusses) purlins, as long as you install a single layer foil capsheet beneath it (Fig. 1).

Fact:  Unless you cut the casheet through, at the webbing of the bar truss and overlap the pieces, you will not receive the full benefit of the R-rating for this type of system.  This type of installation may cause “sweating" of the bars because of the movement of hot convective air against the colder steel webbing.  The best method is to fill the joist cavity and the ends, around the bars (webbing) with unfaced fiberglass covered by an aluminum capsheet of some type, such as R3035HD.  This type of assembly gives you the best of both the mass insulation and the foil products (Fig. 2).

Fiction:  You can achieve  a better looking insulation application by utilizing a white facing when the installation is exposed, as in a retail store.

Fact:  It would not only be cosmetically more desirable but the white facing would reduce your lighting requirements also.  The only negative is you would lose a minimum of R-4 in cooling values and an R-1 in heating values when you use a white facing instead of an aluminum one.  These values are always added when you have a foil facing an interior air space.  This is called a “reflective film".  If your concerned about total R-value being as high as you can get it, in the cavity your working with, spec. a higher density unfaced batt with a white facing such as an R-21 which will fit in a 2 x 6 cavity.  It’s only 5.5" thick and would allow you a nice flat look while maximizing R-values.