ASHRAE Journal - August 2009 - 65

Shingles Roofing Paper R-40 Rigid Insulation (6 in. of R-6.5/in. Rigid Insulation) in Two or Three layers With Horizontal and Vertical Joints Staggered Nail Base for Shingles (Plywood or OSB) Screwed Through Rigid Insulation to Wood Decking or Timber Rafters Air Barrier (Fully Adhered Peel and Stick Membrane) Air Barrier Membrane Gypsum Sheathing Fluted Steel Deck Wood Decking Timber Rafter or Exposed Joist Roof Membrane Fiberboard Hygric Buffer Rigid Insulation Figure 1 (left): Slope an unvented compact roof. This should be easy. Figure 2 (right): Sloping unvented compact roof. It is important to have an air barrier directly on the top of the structural roof deck and to have multiple layers of rigid insulation with the joints offset horizontally and vertically. It is not a good idea to have only one layer of rigid insulation due to convection. Photo 3 (left): SIP roofs. In Juneau, Alaska, rainfall is measured in feet not inches. Photo 4 (center): Mushrooms are growing out of the roof. The Wolfgang Puck effect. Photo 5 (right): Moisture damage. Rot is located at panel joints and ridge. The upper and lower horizontal SIP splines have air gaps along their edges creating parallel “micro-ducts” (Photo 6). The upper and lower pairs of microducts are connected at the panel joints (Figure 4). If the panel joints are not airtight at their interior surfaces, interior moisture-laden air can migrate into the interstitial joint cavities leading to the pattern of damage observed. Note that the damage is all along the upper part of the roof—at the ridge—and at the upper portions of the panel joints near the ridge. The damage at the upper portions of the roof assembly is due to thermal buoyancy and moisture buoyancy. The thermal buoyancy should be obvious. We sometimes refer to it as the stack effect. However, moisture buoyancy August 2009 is not always obvious. Yes, moisture-laden air is less dense than dry air and tends to rise. It is one of those counterintuitive, but true things, which amaze people in bars late at night. How to avoid the problem of SIP ridge rot? Make the panel joints airtight, especially over structural supports such as beams and purlins, and provide a mechanism for moisture removal such as a vented over-roof (Figure 5). It’s easy once you know that it is important. Well, we now know that it is important, and SIPs can rock on in cold climates. Let’s have some more fun and look at what is becoming an increasingly common roof assembly failure with another type of roofing panel. The intent is not to pick on panels. Let’s go to Lexington, Ky. Ridge Center of Panels Not Affected Eave Figure 3: SIP ridge rot. Distinctive pattern of moisture damage at roof ridge and at upper panel joint locations. Why, Lexington? It’s famous for bourbon, of course, and because the example we are going to discuss happens to be in Lexington. ASHRAE Journal 65

ASHRAE Journal - August 2009

Table of Contents for the Digital Edition of ASHRAE Journal - August 2009