The Passive House Source Energy target equates to site energy usage multiplied by a utilization factor of 3.16 for US electricity; for a non-residential building this target is 38 kBTU/sf/yr. For a 2500 sf building, this translates to 8,782 kWh/yr in site energy (building energy) use. The relatively high occupant load for this project, combined with the restriction on combustion, makes this a difficult target to achieve.
LBC requires net-positive energy, but allows achievement of this target with unlimited renewables, whereas Passive House limits the amount of photovoltaics (PV) which is “deductible” from the project’s building energy use to reach the target. Incorporating battery backup energy storage increases the amount a project can deduct for PV.
This project has undergone a feasibility study by the Passive House Institute US. The design team provided design drawings, a 3-D model, design and mechanical narratives, occupant schedules and usages to the staff at PHIUS. PHIUS then used that information to create a WUFI energy model for the project, which contained two design cases – one as proposed by the design team, and a second modified as required to meet Passive House requirements. The model verified for the design team that the source energy target was the most challenging aspect of Passive House for this particular project, but that the target was achievable through incorporation of an 8kW PV array and minor modifications to envelope components and mechanical system efficiencies. The PV necessary to achieve the LBC Net-Positive Energy target is greater than the 8kW array needed for Passive House, so the project benefits from the combination of the two standards, in that the LBC Energy Petal also helps the project achieve Passive House.
For this project to achieve Passive House, based on the Passive House Case in the WUFI model provided by PHIUS, the ERV efficiency level needs to be 85%, and the envelope design needs to provide insulation levels of R-83 in the roof, R-61 in the above grade walls, R-50 in the below grade walls, and R-48 in the slab, with U .12 windows. The model is extremely useful as a tool to analyze trade- offs and decisions. For example, lowering the efficiency of the ERV results in a failure to meet the heating load limit, which would result in a need to increase insulation levels to compensate. As the insulation levels described are already high, which equates to thick envelope components, it would be preferable to find an 85% efficient ERV which meets the other LBC requirements. In another case, reducing the coefficient of performance (COP) of the heat pump results in a failure to meet the source energy target with the 10kW PV array as modeled; however, since the designed PV array is larger, the project may be able to absorb a slight reduction in heat pump efficiency. At the same time LBC does not allow a gas-fired water heater, and an electric water heater results in a failure of the source energy target. The PV could potentially cover the electric water heater energy use, depending on other trade-offs taken, otherwise a combination heat pump/water heater would need to be incorporated. As a goal of the project is to minimize the energy use to reduce the amount of PV required, the design team will need to make these choices carefully.
Our next post will briefly discuss the considerations for including composting toilets in a Passive House design, and provide a conclusion to our Passive House series.
Written by Christina Aßmann and Nicole Schuster, Ashley McGraw Architects