As the first Certified Passive House Elementary School in the US, the Hollis Montessori School exemplifies the future of low carbon, zero energy possible institutional buildings.

Goals.  The project successfully balances multiple goals, including education, Montessori principles (such as independence, freedom, and respect), and the institutional goals of healthy indoor air, durability/longevity, cost effectiveness, and exceptional energy performance.

Solution.  The solution for the school was a high performance building enclosure, very high efficiency systems, design to maximize daylighting, orientation for passive solar gain, and adherence to the Passive House Standard.

Enclosure.  The super-insulated air tight building 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.

Systems.  An air source heat pump system with one head per classroom provides space conditioning, offering heating, cooling, and dehumidification. Zehnder Comfoair 550 HRVs complete the package with highly efficient ventilation and heat recovery. The building's very limited need for hot water comes in demand spikes, justifying the electric tankless water heaters at each point of use. 

Lighting.  In addition to capturing passive solar gain, the south facing windows readily offer natural daylighting throughout the classrooms. Vacancy sensors and daylight sensors help to further reduce lighting use and minimize electrical loads.

Renewables. A roof mounted 55kW photovoltaic system is planned to offset all of the building's energy consumption; with abundant additional clean energy for use elsewhere on site and in the neighborhood.

Teaching.  One classroom includes a ventilation supply fabric duct that inflates when active, offering a teaching opportunity about indoor fresh air. Conduit and the heating/cooling systems are selectively exposed to openly display the building's functionality. Energy monitoring systems are kid accessible to provide insight regarding operational energy usage.

Result.  Preliminary monitoring results, even in the high heating environment of southern New Hampshire, project annual consumption of approximately 28,000kWh for all the building's energy requirements. Only a 25 kW photovoltaic system would offset all consumption, yielding net zero energy, so with the planned 55kW system will make the building exceptionally net positive, generating more than twice the energy consumed annually.



- PaSsive House Consulting

- Mechanical Design


Certified Passive House


High Performance Buildings (cover)
School Planning and Management
Mitsubishi Case Study


EUI: 10.8kBtu/sf/yr (before solar)
Air Leakage: 0.26ACH50
Better Than Code: 85%
Heat Demand  2.93 kBTU/(ft2yr)
Primary Energy  20.2 kBTU/(ft2yr)
Cooling Demand  0.10 kBTU/(ft2yr)
Heat Load 3.32 kBTU/(ft2)
Cooling Load  1.25 kBTU/(ft2)


Double stud walls
Truss roof
Slab-on grade foundation
U Value 0.15
Dense packed cellulose,
EPS and polyisocyanurate
R54 Slab
R41 Above Grade Walls
R111 Roof


Ductless and ducted air source heat pump (8.9HSPF, 15 SEER)
HRV (95% sensible, 0.53 W/cfm)
Hot Water
Electric On-demand


Windy Hill Associates
ZeroEnergy Design
Passive House Consulting
Mechanical Design
Eric Roth Photography