NET ZERO ENERGY HOUSE 02
Elliot J. Glassman AIA, NCARB, LEED AP BD+C
Introduction Defining Net Zero Renewable Energy
Net Zero House 01 (https://issuu.com/ejg6782/docs/ net_zero_house) explored one possible way of creating a compact yet flexible net zero energy house. The project explored the tension between the unsustainable aspects inherent in single family dwelling and the contradictory opportunities that the typology has to improve the environmental performance. The design utilized the relatively low energy use intensity of residential structures and the solar access provided by suburban zoning to create a home that can be run with photovoltaic panels. Design techniques to provide an open and flexible floor plan allowed the reduction in mechanically conditioned area, while abundant site area allowed for the use of a horizontal ground source heat pump loop to generate heating and cooling efficiently.
Building on that success, Net Zero House 02 looks at another high performance design concept for a single family home. The house has many of the same materials and mechanical systems as Net Zero House 01. It too seeks to provide an efficient floor plan but provide a continuous and flexible open space.
Here however, the entire house is wrapped around a passively conditioned wintergarden where the thermal conditions are less controlled than the house. The wintergarden acts both a thermal buffer for the spaces within the house and as an occasional extension of the living space. The wintergarden is naturally ventilated in the summer and shoulder months, and is sealed up to retain passive solar gains in the winter, acting as a warm blanket for the house.
A Net Zero Energy Building (NZEB) is one which produces as much energy as it uses on an annual basis.
Most Net Zero Energy Buildings are still connected to the electric grid, allowing for the grid electricity to be used when renewable energy generation cannot meet the building’s energy load.
When the on-site energy generation exceeds the building energy requirements, the surplus energy is exported back to the utility grid. The excess energy production offsets later periods of excess demand, resulting in a net energy consumption of zero.
On-site energy generation using renewable sources is better for the environment than using electricity for the grid since so much of the energy produced for a typical grid is lost in the transmission over long distances. Renewable energy generation does not pollute the air or water like conventional electricity generation does.
On-site energy generation for residential buildings is typically provided by photovoltaic (PV) panels, most commonly referred to as solar panels. The efficiency at which the PV panels convert solar radiation to usable electricity is constantly increasing with new technological breakthroughs. Simultaneously, the costs of the equipment and installation is decreasing as this form of renewable energy becomes more commonplace.
Combined with tax incentives and increasing energy costs, solar power is becoming a very affordable investment. Right now, the cost of solar produced energy is comparable or a little less per kWh than energy produced from the grid.
The more the energy usage of a house is reduced, the fewer panels are needed to provide for the energy demand, reducing costs for the PV array.
The entry foyer acts as a central hub, offering access to the garage, wintergarden, upstairs, and the living area. Natural light enters from celestory windows and sliding glass doors.
The wintergarden is the focal point of the house, with all the occupied spaces wrapping around it on two sides. It is an unconditioned room that acts as a thermal buffer for the adjacent spaces. It relies on passive methods such as solar heat gains and natural ventilation to moderate its thermal conditions compared to the outdoor temperature. It acts as an extension of the indoor spaces.
The main living area is completely open and continuous space, allowing a great deal of flexibility in its arraignment. The thin floor plate allows daylight to permeate throughout the space. The operable glass doors on either side of the room facilitates cross ventilation; the fresh outdoor air enters though openings on the north facade, exits out through the wintergarden side, and is then exhausted through the louvers out the top of the space.
The den can be used as a guest bedroom. It can also be converted to a first floor master bedroom as the owners age, allowing them to stay in the home for a number of additional years.
The upstairs corridor overlooks the winter garden. A built-in bookshelf below the windows offers storage space as well as a place to sit and read. The operable windows along the corridor facilitate natural ventilation and connection with the wintergarden below.
Three bedrooms line the corridor with north facing windows. Built in units offer a daylit working space and storage space.
The master suite contains a parlor/office area, a sleeping area, balcony, walk in closet, and master bath.
The wardrobe contains shelves and racks for clothing storage. A translucent window allows in abundant daylight while preserving privacy.
The washer and dryer are on the upper floor so that laundry does not have to be brought up and down stairs.
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The south facing overhang protects the wintergarden from solar heat gain. The roof of the wintergarden prevents direct sun from reaching the windows of the house.
The glass louvers at the bottom of the wintergarden facade allow fresh air to enter into the space.
Openings on both sides of the living space allow cross ventilation of the narrow floor plate.
The windows on the upper floor corridor are operable, also allowing ventilation.
The louvers at the top of the winter garden are open, allowing hot air to escape and creating a negative pressure for cooler air to be drawn in from the exterior.
Roof 20' - 0"
Low angle sun penetrates into the wintergarden, collecting solar radiation and warming up the air inside.
The louvers at the top and bottom of the space are closed to retain the warm air like a blanket around the house.
The wintergarden is warmer than the outdoors, meaning that less heat from the house will be lost to the outdoors through the envelope.
The high mass floor absorbs solar radiation during the day and radiates it out after the sun has set so the blanket effect continues longer.
ServicesThe following images show some of the components of the heating and cooling system in greater detail.
Generally, residential air conditioning systems have an outdoor unit which rejects heat to the outdoor air. However, the ambient temperatures are highest when cooling is most needed, therefore heat rejection becomes a more energy intensive process. Additionally, the compressor and fan in the outdoor units create noise. A separate furnace is required for heating.
In contrast, the proposed design uses a ground source heat pump in the mechanical room for both heating and cooling. Instead of rejecting heat to the air, it exchanges temperature with the ground, which has a more stable temperature, thus improving efficiency. A trench is dug in the yard and thermal exchange coils are placed before the trench is backfilled.
The thermal exchange and pump allows either hot and cold water to be generated. The hot water circulates through tubing underneath the finished floor, creating a radiant surface that creates more even heating. Cold water is circulated through either a radiant ceiling, a chilled beam, or a valance to provide cooling to the spaces in the summer.
Construction Stud framing
Insulation only between studs, creating thermal bridging which reduces insulation value
Gypsum wallboard
Plywood sheathing Solid and continuous insulation core
The construction is designed to minimize complexity and be erected quickly on the site.
The envelope is key to energy efficiency. In typical single family home construction, it is the convention to use wood studs to frame the bearing and non bearing walls. On external walls, insulation is placed between the studs, causing thermal bridges through the studs themselves. In addition, the insulation can sag within the stud cavity or air infiltration can occur between the insulation and the face of the stud.
In contrast, the proposed home will use structural insulated panels (SIPs) for the exterior walls. The SIPs are load bearing panels composed of thick continuous insulation sandwiched between sheets of structural plywood. This eliminates the thermal bridging found in the stud walls and virtually eliminates infiltration. The use of SIPs means the structure can be built very quickly since the panels can be prefabricated and brought to the site. There are less trades involved and less coordination, saving on labor costs. The prefabrication also reduces material waste at the site.
The structural bearing walls are arranged in parallel rows running west to east. The floors and roof span between them. Lateral support is provided by the west to east walls in the core and by the walls between the bedrooms on the second floor.
The south facades use a curtain wall of high performance glazing with solid wood framing, giving those facades a lighter, more transparent feel.