Author: Marshall Sweet
With the support of the U.S. Department of Energy (DOE) Building America program, Southface partnered with the Housing Authority of Savannah to specify and construct a test home in the hot-humid climate of Savannah, Georgia.The home was designed and built to meet the requirements of green building certification programs (EarthCraft House, ENERGY STAR® v3, and DOE Challenge Home), while keeping the total cost in the realm of an affordable housing development. Southface’s long-term research interests in this project are to analyze the performance of two unique features of this home over the course of a year and expand the knowledge of actual field performance of these emerging technologies.
The first design implementation of interest is Huber’s Zip System® R-Sheathing in the vertical exterior walls. Huber’s Zip System® R-Sheathing panel features continuous foam insulation, engineered oriented strand board (OSB), water resistant barrier wrapping, a continuous air barrier, and an acrylic tape to form a permanent seal between panels. The Zip System® provides a new, convenient approach to sealing and insulating the building envelope with a single product installation instead of the traditional method of installing rigid foam, OSB, and house wrap in three phases. A major advantage of the extra insulation is to provide insulation between the exterior sheathing and the interior studs. The performance of the Zip-R sheathing will be compared to the performance of Huber’s traditional Zip system (same product minus rigid foam insulation) installed on the neighboring EarthCraft certified home. The structure of the two homes are of the same shape and size, however the neighboring home has a slightly different floor plan.
Throughout the first two weeks of 2014, when the Polar Vortex brought frigid temperatures to Georgia, the average wall stud and cavity temperatures during the evening were warmer in the Zip-R sheathed home (above). This result was expected since the rigid foam adds an additional R-3.6 (resistance to heat transfer) helping to keep the heat inside the home. More interesting were the temperature swings of the wall studs on the southwest facing walls during the daytime. The solar gains from being in direct sunlight heat the studs in the traditional Zip wall to significantly greater temperatures than the studs behind the Zip-R system. On January 8, the daily stud temperatures in the Zip wall increased 30.9°F (55.7°F to 86.6°F) while the studs in the Zip-R wall only gained 10.3°F. While these solar gains help keep the heat inside during the winter, they are counterproductive during the summer when the inside of the house is being cooled. Computational models of the homes were created in BEopt (the National Renewable Energy Laboratory’s building energy simulation program) and simulated for an entire year to determine the extra insulation’s effect on heating and cooling loads. The BEopt results predict a heating savings of 70 kilowatt-hour per year (kWh/yr), meaning that the extra insulation keeping the house warmer in the evening outweigh the benefits from solar gain during the daytime.
However, BEopt predicts annual cooling savings during summer months of only six kWh/yr for the Zip-R system. This contradicts what was just discussed about the stud temperatures during the winter. The stud temperatures during summer evenings should theoretically be warmer in the traditional Zip walls because they follow the outdoor temperature more closely due to less insulation (assuming the outdoor temperature is greater than indoors). The solar gains in the studs during the summer will also add heat to the interior of the home that is now in cooling mode. With the extra insulation of the Zip-R system now helping keep the house cool during both the daytime and evening (instead of just during the evening) it seems obvious that the cooling savings would be much greater than the heating savings, especially in a predominantly cooling season location like Savannah, Ga. Our goal is to use our data to enhance building simulation software results, like BEopt, so that energy savings of continuous foam insulation, like Zip-R, wall systems can be more accurately determined.
The second research interest of this project is a 60 gallon A.O. Smith Voltex® heat pump water heater (HPWH) (above) installed in the indirectly conditioned spray-foamed encapsulated attic. This project overlaps with a concurrent project in Lafayette, Ga., where two LEED certified affordable housing duplexes have HPWHs installed in the mechanical closet of each unit (four in total). Since the volume of air in the mechanical closets is not sufficient to provide the amount of heat required by a HPWH, those units need to be ducted to the encapsulated attic to draw and exhaust air. When adding ducts to the exhaust of the working fan in a HPWH, the static pressure is increased. This added pressure on the exhaust side reduces air flow across the heat pumps’ coils and potentially decreases its efficiency. However, the ability to duct a HPWH makes them more feasible for use in a wider array of buildings. Another point of interest is how hot-water-draw patterns affect HPWH performance. Long hot water draws cause the water temperature inside the tank to decrease as the tank’s volume is replenished with cold water. If the heat pump is unable to heat the incoming water fast enough, then an electric back-up heater is summoned for assistance. The use of the electric back-up heater decreases the HPWH’s energy performance, so the draw pattern can have a significant impact. Monitoring five HPWH installations will hopefully result in five significantly different draw patterns.
HPWHs have undergone extensive testing in a laboratory setting to quantify their individual performances, however there is still a large void of data from real-world installations. These two projects will fill that void with both ducted and ductless installation types, as well as insight to how different hot-water-draw patterns affect HPWH performance.