PCMs have been examined as thermal mass components in buildings since the 1940s, when the first-ever residential house utilizing PCM heat storage was built in Massachusetts [27]. A key goal in low-energy building research is to find ways to manage differences in time between energy generation and energy consumption (i.e., building loads). According to the International Energy Agency (IEA) Annex 24/42, energy conservation through solar energy utilization, conventional energy, and advanced PCM thermal storage, is essential for energy efficiency. PCMs are emerging with the potential to dramatically reduce energy consumption for comfort conditioning and peak utility demand in buildings. The magnitude of the impact of PCMs on the cooling season electric power demand of high-performance building envelopes was recently demonstrated in Albuquerque, NM, USA. Fraunhofer CSE constructed two well-insulated wood framed test huts (shown in Figure 1) that were operated during the summer and fall of 2011. In addition to fiberglass insulation, one of these near-identical huts contained layers of PCM integrated into the attic and wall cavity insulation as follows:  
1. between two layers of insulation on the ceiling, i.e., on top of R-3.7 (m\({}^{2}\)-K/W) fiberglass insulation and under another R-2.3 (m\({}^{2}\)-K/W) layer of fiberglass insulation;
2. inside the wall cavities, i.e., within R-2.3 (m\({}^{2}\)-K/W) fiberglass insulation at about 3 cm from the internal surface of the cavities in between two layers of fiberglass baths of custom thickness. The amount of thermal insulation in the huts was the same. The test hut containing PCM also had reflective (cool) roof shingles and a layer of microchannel board installed on top of the roof deck and both test huts had very similar air-leakage rates, as confirmed with blower door testing. The whole-hut results presented in Figure 2 demonstrate that envelope-related cooling loads are similar during the early morning hours for the test huts. However, during and a few hours after peak solar irradiance, in the test hut with PCM (Hut #2) the heat flow across the envelope was dramatically reduced as compared with the other hut. As a result, the hut's load peak was shifted three hours in the afternoon and the total whole-hut cooling load was reduced by 63%. This shows the great potential for using dynamic, thermally enhanced envelopes containing PCMs, combined with thermal break (microchannel board) and reflective surface (cool roof shingles) to control building energy dynamics and peak-energy demand. Figure 1: Instrumented huts tested in Albuquerque, NM, USA, in 2011. Figure 2: Measured reduction of envelope-generated cooling loads in the insulated test huts located in Albuquerque, NM, USA. Meaningful predictions of the heat flow crossing the attic floor require accurate field measurements because of the multiplicity of variables affecting the thermal performance of the roof and attic. Several generations of novel roof designs, developed by the authors, have demonstrated as much as 90% reduction in roof-generated peak-hour cooling loads, significant peak-load time shifting, and reductions in overall cooling loads as compared with traditional roof-attic systems using the same level of thermal insulation [28]. In many historical PCM studies, the major interest invariably tilted toward the building operators' or owners' interest in energy savings associated load reductions. However, most recently, these interests have shifted to include building thermal response characteristics, energy consumption dynamics, peak-energy load management, and analyses of the dynamic thermal response of building structures. Specifically, a major focus of this paper is on time shifting of peak thermal loads and decrease in their maximum value.