The Hollis Montessori School holds the distinction of being the first Certified Passive House Elementary School in the U.S.
Keeping in line with multiple project goals, such as education, Montessori principles, healthy indoor air, durability, cost effectiveness, and exceptional energy performance, a solution was designed with a high-performance building enclosure, high-efficiency systems, orientation for passive solar gain and maximum daylight, and adherence to the Passive House standard.
The enclosure includes a double stud wall system that provides R-41 dense pack cellulose continuous insulation, an R-111 roof assembly that combines dense-packed cellulose in the roof trusses and continuous rigid insulation outbound of the roof sheathing, R-54 under the concrete floor slab, and building details to mitigate all thermal bridging. High performance U-0.15 triple pane windows/doors offer a SHGC-glass of 0.50 to capture the sun's energy. The exterior sheathing was taped and sealed to serve as the primary air barrier, yielding a final infiltration rate of only 0.25ACH50.
An air source heat pump system with one head per classroom was installed to provide space conditioning. The ZehnderComfoair 550 HRVs provide high-efficiency ventilation and heat recovery. Since the building's very limited need for hot water comes in demand spikes, electric tankless water heaters were installed at each point of use.
In addition to capturing passive solar gain, the south-facing windows readily offer natural daylight throughout the classrooms. Vacancy sensors and daylight sensors help to further reduce lighting use and minimize electrical loads.
Preliminary monitoring results, even in the high heating environment of southern New Hampshire, estimate annual consumption of approximately 28,000 kWh for the all-electric building's energy requirements. Since a photovoltaic system of only 25 kW would offset all consumption, yielding net zero energy, the recently-installed 56.8 kW system makes the building exceptionally net positive, providing additional clean energy for use elsewhere on the site.