COLUMN BUILDING SCIENCES FIGURE 2 Shingle ridging in structural insulated panels. Upper Surface Has a Greater Moisture Content Than Lower Surface When Upper Surface is Cold and Lower Surface Is Warm When Upper Surface Expands Relative to Lower Surface, Panels Can Bow; at a Minimum, Upper Joints Close-or Deform by Crushing-as Upper Surface Expands Distance Between Nails Closes as Moisture Content Initial Distance Between Nails Of Upper Surface Increases Gap Closes Initial Gap Between Panels Shingle Stressed Between Nail Leading to Telegraphing Of Joint Localized Increase in Moisture Content Can Occur Due to Air Leakage Through Joint or Via Vapor Diffusion Through Joint Where Vapor Diffusion Source is Wood Timber at High Initial Moisture Content Wood Timber atd Timber at High InitialHigh Initial Moisture ContentMoisture Content goes up it expands. Did I mention that the roof sheathing can't dry upward? I think I did. Interesting things happen when that occurs. The typical gaps in the sheathing joints the framer leaves close-and the shingles that span the joint tend to " ridge " upward. We have similar issues with structural insulated panels (SIPs) (Figure 2). It gets even better. Let's install black shingles. The sun eventually comes out. The black shingles get hot. Real hot. The moisture in the roof sheathing is driven downward. Not slowly, but quickly. The roof sheathing contracts. Not slowly, but quickly. The closed-cell spray foam does not like " quickly. " As mentioned before it can't handle the " quickly, " and the " stress " of the movement is taken at the " bond " between the spray foam and roof sheathing. The bond fails. Just like the thermal shock described earlier. It fails quickly. Very quickly. And once again it lets everyone know that it has failed quickly. It shouts out the failure. Yup. You hear the " pop. " It wants to be the center of attention-and lets you know-again. So why did we not have " popping " with SIPs? The SIP panel edges are not " restrained " as " strenuously " as the closed-cell spray foam is to the sides of rafters. Nails are not in the same league as the adhesive strength of closed-cell spray foam; the nail connections allow the SIPs And we build outside. Yup. Sometimes it rains. Sometimes the roof sheathing gets wet during construction-and does not dry " totally " and " completely " before the roofing is installed. Guess what? The moisture content of the roof sheathing goes up. And when the moisture content of the roof sheathing 40 ASHRAE JOURNAL ashrae.o rg J U LY 2023 to move. As such, the SIP foam cores do not de-bond from the top sheathing surfaces of the SIP panels. In closed-cell spray foam applications sometimes the " side " bond to the rafter fails-you see a crack at that location. Most of the time it is dismissed as " gee, the foam was applied too thickly, " and the exothermichttp://ashrae.org