Hollis Montessori School: Hollis, NH
Cold Climate, Big Savings
At the Hollis Montessori School in southern New Hampshire, the school environment is a teaching tool. So, to create the best possible environment for students, it made sense to build the first Passive House certified elementary school in the U.S. It made financial sense, too. For a 10% increase in construction costs to reach certification status, the payback was only three years.
The campus is comprised of three energy-efficient buildings, the crowning jewel being the Passive House certified New Classroom Building: 11,000 ft2 with four classrooms for primary and elementary students and teachers. The school is located in a cold climate (6,950 heating degree days, 3;°F outdoor heating design temperature) on a repurposed 9.5 acre apple orchard. The site is fairly level with excellent solar access. A large portion of the orchard remains, and it is maintained by students as a business teaching model. Well water, septic, and stormwater are all managed on site. The electric utility provides all of the energy consumed; no fossil fuels are consumed on site.
This efficient building has a measured energy use intensity (EUI) of 10.8 kBtu/ft2•yr, compared to the ASHRAE Standard 90.1-2004 EUI of 70 kBtu/ft2•yr for a K–12 school. That’s an 85% reduction compared to other K-12 schools. The annual energy savings are on pace to pay off the roughly 10% additional construction cost within three years.
In the future, Hollis Montessori School plans to offset all of its energy consumption with photovoltaic (PV) panels, thereby achieving net zero energy performance. A 27 kW PV system would offset 100% of the 35,000 kWh/yr annual energy consumption. In comparison, a K-12 building of the same size built to ASHRAE Standard 90.1-2004 would require 175 kW of PV panels to achieve net zero energy performance.
The architecture is simple in its form to help it integrate with the fabric of small-town New England. The form also facilitates continuous insulation, air, moisture, and weather barriers. It was designed along an east-west axis to provide a south-facing roof for future PV panels, to maximize daylighting, and to capture passive solar gain through the south-facing windows. The southern roof overhang is designed to balance solar gain in winter and shading in summer.
Lighting is controlled by daylight and occupancy sensors, using three T-8 tubes per fixture to provide three lighting levels as controlled by the daylight sensor. The occupancy sensors will shut off the fixture when the room is unoccupied. In addition to the four classrooms the building also features a conference room, staff offices, reception area, laundry, and print-room with dedicated exhaust air. Each classroom has its own kitchen.
Each of the four classrooms has its own mini-split system that pairs an outdoor heat pump unit with a wall-mounted indoor unit. The super-insulated envelope and triple-paned windows make point-source heating viable and allow for the elimination of a ducted distribution system.
The super-insulated building envelope is the primary energy-efficiency strategy. Heating energy can comprise 50% (or more) of the overall energy profile of a building in New England, per the 2003 CBECS data. The individual components of the building envelope system include high R-values with no thermal bridging, a continuous air barrier with blower door performance testing, and triple-paned windows. The super-insulated building envelope will reduce the heating demand 90% compared to a code-compliant building, improve thermal comfort by eliminating drafts and cold surfaces, reduce the required capital expense on heating equipment, and maintain a viable indoor temperature in the event of a power outage. (The temperature in the building during winter construction, without any heat, did not drop below 60°F.)
The insulation strategy includes 12 in. type IX EPS under the slab (R-54), double stud 12 in. thick wood frame walls filled with dense pack cellulose (R-41), and parallel chord wood roof trusses filled with 20 in. dense pack cellulose and 6 in. polyisocuanurate nail-base insulation above the roof deck to warm the sheathing and prevent condensation (R-111).
All building envelope connections are thermally broken: the slab is completely thermally isolated from the exterior, the double-stud walls have separate top and bottom plates to avoid heat transmission through the framing, and the roof assembly has continuous rigid insulation over the framing. Windows are triple-glazed tilt-turn or fixed units with an overall U-value of 0.15. Similarly, doors are commercial grade aluminum frame triple glazed with an overall U-value of 0.176. Doors are equipped with a “guillotine” drop-down sill gasket for additional air sealing. All glazing has an SHGC of 0.5, which increases the passive solar gain through the windows.
A continuous air barrier was meticulously designed, with particular care given to transitions between different assemblies. The wall and roof sheathing comprise the primary air barrier system; all seams were taped. The under-slab vapor barrier wraps over the top of the foundation wall and is taped to the wall sheathing, eliminating air leakage at the perimeter sill plate. The taped sheathing wraps around the eaves and rakes to maintain continuity between the wall and roof sheathing. Window and door frames were taped to the exterior sheathing and expanding foam tape fills the rough opening cavities of each window and door.
All penetrations, including those under-slab, were diligently sealed to the air barrier. Air sealing was measured by blower door testing multiple times throughout the project. The building achieved an airtightness of 0.26 air changes per hour (ach) at 50 Pa, 50% better than the Passive House requirement (0.6 ach at 50 Pa) and almost 90% better than the building code requirement (3.0 ach at 50 Pa).
The framing crew and other subcontractors deserve enormous credit for the measured air-tightness. Before construction started, the project goals were clarified, the construction details reviewed, and the team was instructed on best practices to achieve the stringent requirements. Despite the fact that no one on the construction team had prior experience with the Passive House standard, they took an immense interest in the building’s performance, which fostered a sense of accountability and pride. On-the-job training for a thermally excellent building became a normal part of construction administration and everybody benefited from the experience. At the end of the process they all had a much greater respect and understanding for what was initially mysterious.
Heating and Cooling
Given the exceptionally well insulated and air-sealed building envelope, the heating and cooling systems are smaller and simpler than they would be ordinarily. Mini-split air-source heat pumps (ASHPs), without any backup heat source, provide 100% of the building’s heating and cooling needs. This might seem a strange choice in a cold climate; however, they are incredibly well-suited for the Hollis Montessori school. The installed systems are rated down to –13°F, well below the design temperature.
The relatively small heat output of a mini-split ASHP pairs well with the greatly reduced peak heat demand of the Passive House certified school. The school requires less than 4 Btu/h•ft2 at peak conditions, about 90% less than a code complaint school of the same size. Each of the four classrooms has its own mini-split system which pairs an outdoor heat pump unit with a wall-mounted indoor unit.
The super-insulated envelope and triple-paned windows make point-source heating viable and allow for the elimination of a ducted distribution system. The R-10 glazing maintains a warm interior surface temperature and eliminates the need to supply heat at the windows. The entry, circulation areas, and offices are served by a single ducted indoor mini-split unit combined with its corresponding outdoor unit. The ducted unit was selected, in this case, to provide a heat supply in each of several separate rooms.
A ground-source heat pump (GSHP) was not selected due to its high installation cost for drilling, piping, and distribution systems. Given the extremely small heating requirement, the increase in efficiency from an ASHP to a GSHP (COP 2.5 and COP 3.5, respectively) could not offset the higher first costs.
Heat recovery ventilators (HRVs) (84% sensible efficiency, 0.4 W/cfm) distribute fresh air to the classrooms and offices and exhaust stale air from the kitchenettes and bathrooms. Each HRV is controlled by a CO2 sensor as part of a demand-controlled ventilation system; the interior CO2 concentration acts as a proxy for occupancy. When the room is occupied, the CO2 sensor energizes the HRV into high speed. Once the CO2 concentration has fallen below its defined threshold of 700 ppm, the HRV reverts to its lowest speed. One classroom includes an exposed fabric duct that visibly inflates when the HRV is energized. This provides a learning opportunity; the students can see the air being distributed and understand how the system changes with fluctuations in occupancy.
Another learning opportunity comes from the building-wide energy monitoring system. Students use measurements to understand how much energy is being used per classroom, how their behavior affects consumption, and even compete against the other classrooms to minimize consumption.
The building is currently not used during summer vacation. As such, the mechanical systems are dormant except the small amount of dehumidification necessary to keep the hardwood finishes from warping, meaning that almost no summertime air conditioning is required.
Mechanical equipment is not hidden, rather it is displayed so students can learn how energy is used. The compressors for the air-source heat pumps are on racks to keep them above the snow. Mechanical rooms are not necessary for this type of equipment.
Having a Passive House certified school, makes a big impact on the budget. It only takes $5,000 to $6,000 annually to operate a 11,000 ft2 building with 112 people (includes heating, cooling, lighting, hot water and plug loads), which is a large enough savings that it has an effect on the whole organization. Another impact is the ease of running the building. Other than cleaning the filters a few times a year, not much adjustment is needed. And, finally, the large amounts of daylighting and comfortable environment contributes to a sense of wellness for the occupants. •