Article #1491 - COMcheck Web-Based Training Questions - Roofs
This article contains questions asked during the August 18, 2005 live broadcast of the COMcheck Web-Based Training
session. The original language of the questions and answers has been modified to make them as generic as possible while retaining the applicable specific information.
I design pre-engineered metal buildings. They typically have 3" draped insulation at the roof panel. If we have interior built out space, we will generally put 6" (R19) insulation above the ceiling. Do we get any credit for this insulation?
If the interior built out space is not sealed according to the energy code, you cannot get credit for the R19 insulation above the ceiling. In which I am assuming you are describing a drop down ceiling that is not completely sealed with gyp board that would not be considered part of the exterior building envelope components required to meet the energy code. The only value of insulation that would be accounted for is the 3" draped insulation at the roof panel.
Can lay-in ceiling tile be considered sealed and part of the envelope?
No, ceiling tile is specifically not allowed as a ceiling barrier between conditioned and unconditioned space.
For roofs, how is the area calculated?
The ceiling area should be measured on the slope of the finished interior surface.
If we have tapered roof insulation do we use an average of the insulation thickness?
Tapered roof insulation in a typical attic configuration is already accounted for in COMcheck.
For an all-wood joist/rafter/truss, the base assembly consists of a roof truss with a 2x4 bottom chord. The ceiling is attached directly to the bottom chord of the truss, and the attic space above is ventilated. Insulation is located directly on top of the ceiling, first filling the cavities between the wood, then continuously covering wood and cavity insulation. No credit is given for roofing materials, because they are above the ventilated space. The heat flow path through the wood members is calculated to be the same depth as the insulation. The assembly includes R-0.17 for the exterior air film, R-0.56 for 0.625-in. gypsum board, and R-0.61 for interior air film with heat flow up. U-factors are calculated for standard framing, where insulation is tapered around the perimeter with resultant decreases in thermal resistance. The table shows the balance of assembly R-value calculation details. Area weighting factors for the parallel paths are 85% full-depth insulation, 5% half-depth insulation, and 10% framing.
Balance of Assembly R-values for All-Wood Joist Roof
| Description | R Value at Insulation | R Value at Joists |
|---|---|---|
| Outside Air Film | 0.17 | 0.17 |
| Wood Joists/Cavity | 0 | 4.38 |
| 5/8-in. Gypsum Board | 0.56 | 0.56 |
| Inside Air Film | 0.61 | 0.61 |
| Total Path R-value | 1.34 | 5.72 |
Total Assembly R-value = 1.0 / (0.85/1.34 + 0.05/1.34 + 0.10/5.72) = 1.45
Why is unknown ceiling insulation R-value amount already factored into the program?
The program automatically includes the insulation value of the standard construction items that are part of the roof (rafters, sheathing, deck, etc.) so that only added insulation (batts, blow, and insulated sheathing) will need to be input into the program.
How is roof insulation treated over unconditioned space? Should that square footage be included in the roof area?
No, only envelope components surrounding conditioned space should be included.