# Performance of a Mid-Size Net-Zero Energy Solar House
## 5 Energy Modeling and Discussion
### Modeling Results
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Recommended HVAC Sizing**} \\ \hline \multicolumn{4}{c}{**Birmingham**} & \multicolumn{2}{c}{**Denver**} \\ \hline ACH & Heating ton, (kW) & Cooling ton, (kW) & Heating ton, (kW) & Cooling ton, (kW) \\ \hline
0 & 0.73 (2.54) & 1.06 (3.71) & 1.00 (3.50) & 1.02 (3.57) \\ \hline
0.3 & 0.98 (3.41) & 1.15 (4.03) & 1.30 (4.56) & 1.09 (3.81) \\ \hline
0.6 & 1.22 (4  
For cooling, as seen in Figure 4, Birmingham requires more electricity for most months. The results presented in Figures 3 and 4 are in line with what was expected according to the weather condition of the two cities. The total electricity use for cooling in Denver and Birmingham is 2568 and 4155 kWh, respectively. In Birmingham, cooling constitutes 34% of total electricity use while that fraction is 20% in Denver. In contrast to heating, improving the building performance with respect to cooling load has a larger impact in the total electricity cost of the building in Birmingham. Based on the results of the energy modeling performed on the as-built condition of the house, a 2-ton ductlest heat pump system is sufficient to deliver heating and cooling needs of the building. The study revealed that, as expected, cooling is the dominant design factor in Birmingham while in Denver, the dominant design factor is heating. The annual cooling electricity in Birmingham is 62% more than that in Denver. Heating electricity required in Birmingham is 57% less than that in Denver. When both electricity use for cooling and heating is considered, the two cities require nearly the same energy in kWh per year. Selecting insulation material for the house is a multi-faceted task. The ultimate goal is to keep the inside conditions at a comfortable range by limiting heat transfer through the house's envelope, and also to minimize infiltration of unconditioned outside air to the house through gaps, cracks, and openings. In order to achieve these goals, conscientious decisions must be made. Health concerns are the most important factors, and most commercially available insulating materials have deficiencies in that area. Construction-phase issues includes skin- and eye-irritant chemical fumes, microfiber inhalation, and skin abrasion. Post-construction issues include flammability, off-gassing, end-of-life environmental issues, and also the same issues as those during construction-phase whenever repair and replace work is required. On the other hand, there is the issue of achieving the highest R-value that is possible in a compact space, and so hard choices have to be made. Usually, environmentally benign materials do not provide high R-values or lack other required features such as fireproofing. The exterior vertical walls of the house are built in a staggered double 2\" \(\times\) 4\" stud manner with 2\" \(\times\) 6\" plates on the top and bottom of the wall frame. This provides a cavity space for insulation equal to 5.5 in (14 cm). This cavity is filled with 1 in (2.5 cm) of closed-cell foam sprayed on the ZIP system(r) wall sheathing. A 2-in (5 cm) layer of fiberglass batt insulation is installed over the spray foam. An additional 3\" (7.6 cm) layer of fiberglass paper-faced batt insulation is in place. Then, in order to provide a rigid backing to prevent gypsum boards from cracking due to transportation-induced vibration, plywood sheets are installed to close the wall cavity. Gypsum board is installed on the plywood sheets. On the outside of the exterior sheathing, a 1-in (2.5 cm) slab of rigid foam insulation provides an additional R-5 insulation to the wall. Table 5 shows the thickness of each individual layer of the vertical exterior walls. Since the house is built in several modules for transportation, there are seams where the modules come in contact. All seams are sealed with weather-resistant tape. Monthly PV electricity generation by the installed system was simulated using the PVWatts calculator [19].