## 3 Thermal Balance and Time Constant of a Solid Slab Containing Phase Change Materials (PCM)
The nominal latent heat storage density of the PCM packed in the plastic film pouches was around 156 Wh/m\({}^{2}\) of the roof area. As shown in Figure 16, PCM was macro packaged between two layers of heavy-duty plastic foil forming arrays of PCM cells. The fiberglass insulation had a nominal thermal resistance of 1.1 m\({}^{2}\)-K/W. Air cavities between the PCM cells and the air gap above the fiberglass insulation provided above-sheathing ventilation to help reduce the attic-generated cooling loads during the summer. In general, in roof assemblies where over-the-deck ventilation, radiant barrier, or thermal insulation are used, the attic space below is exposed to significantly lower thermal stresses as compared with typical North American roofs, where shingles are installed on top of a thin layer of underlayment which  
Figure 7.3 Components of the experimental roof containing roof-installed PV and advance thermal disconnect system with organic PCM heat storage. (**a**) Location of the PCM heat sink directly on top of the roofing deck material and under the fiberglass insulation with air channels above; (**b**) Top of the fiberglass insulation with radiant barrier facing up; (**c**) A view of the open roof’s ventilation cavity with visible PCM pouches and top of the fiberglass insulation; (**d**) Installed metal roof with PV laminates (**left**) and open roof ventilation cavity before installation of the metal pane (**right**). is directly placed on the roof deck surface. This experiment proved that a dynamic thermal disconnect system utilized in the construction of the PV-PCM roof worked exceptionally well. However, note that the PV-PCM roof was topped with highly absorbing PV laminate. From a thermal performance perspective, the PV laminate was obviously a disadvantage. This resulted in similar peak roof surface temperatures as in the case of conventional shingle roofs [44]. Therefore, any potential benefits of the previously discussed cool roofs using solar reflective coatings were eliminated. The overall thermal performance of the PV-PCM roof data depended on the physical characteristics and effectiveness of the thermal disconnect system installed below the roof surface. This experiment further showed that lightweight sloped roof structures utilizing an over-the-deck thermal disconnect could generate a significant time shift in the peak thermal load generated by the roof. Figures 17 and 18 present comparisons of heat fluxes measured on the bottom surfaces of the roof decks for a PV-PCM roof and a conventional shingle roof, during both the summer and winter. As shown in Figure 17, the test roof with PV laminate and thermal disconnect over the deck, reduced the peak time thermal load by over 90% during the summer. Furthermore, a time shift of four hours was recorded. Both of these features are very useful from the perspective of the whole building thermal response control and management of the cooling energy demand. Figure 7.4 Comparisons of summer-time heat fluxes measured on the bottom surfaces of the roof decks for a PV-PCM roof and a conventional shingle roof during the summer and winter seasons. Figure 7.5 Comparisons of winter-time heat fluxes measured on the bottom surfaces of the roof decks for the PV-PCM roof and conventional shingle roof during the summer and winter seasons. As shown in Figure 18, heat absorbed by PCM during the winter resulted in a 66% reduction of the overnight heating load (heat losses) and about 60% reduction of the maximum night heating load. In addition, the maximum peak heat was shifted by about two hours. Two-Phase Control of Roof-Generated Thermal Loads: The Impact of Peak Temperature Reduction on the Roof Deck  
Sloped roofs with insulated attic spaces have the potential to be one of the most energy efficient building components. This is accomplished by integrating attic floor insulation, attic air space, attic ventilation, and thermal disconnects installed at the roof deck level and use of cool roof materials.