FAQs about a net zero energy home

FAQ #1.     What does net zero energy mean?

Net zero energy means that the home generates all its own heat and electricity on an annual basis.

An EnerGuide for Houses (EGH*) rating of 100 means the house is net zero energy.

An EGH* rating of 100 can be achieved by designing a house that produces as much energy as it consumes on an annual basis.

FAQ #2.    How is net zero energy achieved?

To achieve an EGH* rating of 100 (i.e., net zero energy) the house must use as little energy as possible to operate plus produce sufficient energy to meet those operating requirements.

To minimize the energy required to operate the house, the design begins with an air tight, highly insulated envelope structure that incorporates passive solar heating and is equipped with low energy appliances, lighting and equipment. The household water and home heating needs are supplied by an active solar heating system with a backup electrical heater. The electrical needs of the house are provided by a grid-connected solar electric power system.

FAQ #3.    How can the Riverdale NetZero home achieve net zero energy?

The Riverdale NetZero home can achieve its net zero energy goals by being hugely energy efficient in its building envelope construction (walls, ceiling, windows, doors, air tightness and efficient ventilation) and electrical appliances and lights and motors, and then by supplying this heat and electricity entirely using a solar heating system and a solar electric power system.

All solar heat produced is either used by the home directly at the time of its production, stored for later use, or wasted because it can’t be used or stored.

All solar electricity produced is either used by the home directly at the time of its production, or supplied to the electrical grid. Any supply to the grid offsets the electricity that was purchased at another time in the year. This yields a net zero annual energy purchase. The objective then of designing a net zero energy home is to minimize the amount of electricity imported from the grid.

FAQ #4.    Why no gas line?

The decision was based on economics. When you are needing only very small amounts of heat, it can be more cost effective to use electricity directly than natural gas.

It has been estimated that less than $80 per year of natural gas would be required to heat the Riverdale NetZero home. However, there is a flat service charge of $320 per year on every natural gas line. This means that natural gas would effectively cost approximately $400 per year (which makes the apparent price of natural gas over $70 per GJ). Whereas if we just used electricity this same amount of heat would cost only $160 per year (at a price of $30/GJ). We don’t count the additional $200 cost of the electricity service charges because we are already paying them.

FAQ #5.    Why are we able to heat the house at -32°C using four toasters?

This home is extremely well insulated. We have used R‑56 cellufibre insulation in the walls, R‑100 cellufibre insulation in the ceiling and R‑30 insulation in the basement floor. The average new house has R‑20 in the walls, R‑40 in the ceiling and doesn’t insulate the basement floor.

High performance windows (triple and quadruple glazed), air tight door, window and wall seals, plus a highly efficient ventilation system also reduce the energy required to heat the house.

The use of passive solar heating (through the large south facing windows) maximizes solar heat gain and stores the heat in the concrete and drywall used in the home.

The result of all the energy efficiency is that this home will use only 6500 W of heat at -32°C. This is the same amount of heat produced by 4 (4-slice) toasters.

FAQ #6.    Why no geothermal heat pump (GTHP)?

Standard geothermal heat pumps were evaluated during the design of the house. Because of the energy efficient design of the Riverdale NetZero home, the amount of energy required to heat the home is not sufficient to offset the cost of installing even a minimal system and the annual cost of operating the heat pump. It was more cost effective to simply add more PV. We are still considering at this time a very small and very simple geothermal heat pump connected in with the solar heating system. With the configuration we are considering, the heat pump would cause the solar heating system to be more efficient, which is very attractive, and we could get a wee bit of cooling from the ground in the summer if we needed it.

FAQ #7.    How can solar energy heat the Riverdale NetZero home at night?

The heat from the solar heating system is being stored in a 17,000 litre cistern in the basement. This is sufficient to store the heat from day to night on sunny days. We will still need some backup electric resistance heating in the middle of the winter.

FAQ #8.    What do we do with all our excess solar electricity?

Any surplus electricity generated by the house is exported to “the grid”. We will have a new meter that will separately measure both how much electricity we import from the grid to the house, and how much we export from the house to the grid.

FAQ #9.    What about ventilation?

The home incorporates fully ducted mechanical ventilation with a heat recovery ventilator (HRV) as the energy saving heart of it. Using what is essentially a cross-flow air-to-air heat exchanger, up to 80% of the energy from the outgoing exhaust air can be recaptured. Fresh air from the HRV is ducted to each room in quantities prescribed by building codes using oversized ducts to reduce fan power requirements. During the summer, the HRV can be turned off and windows can be opened to provide natural air circulation.

When the thermostat calls for heat, the fresh air system can also distribute heat to each room of the house at the same time, just like any furnace would do in a typical house. In this house, a coil of pipe in the air stream will deliver hot water from the storage tank to provide this heat. The technical term for this is a fan coil.

FAQ #10.   What about indoor air quality?

To ensure healthy air all year round, the Riverdale NetZero home has been built using materials with little to no volatile organic compound (VOC) emissions. Since building codes do not yet allow us to use lower ventilation rates in homes with low VOC levels as compared to normal VOC levels, we are required to have standard ventilation rates.  As a result, we should have a particularly fresh smelling environment.

FAQ #11.   What type of insulation is used?

The Riverdale NetZero home has insulation almost everywhere, from the floors to the ceilings, interior and exterior walls.

The basement floor uses 150 mm (6”) of Type 2 EPS Terra foam under the slab (R‑24).

The basement walls use a combination of 25 mm (1″) Type 2 EPS, 50 mm (2″) of poly‑isocyanurate board insulation and 230 mm (9″) of blown Wallbar™ insulation held in place with 2×4 framing 600 mm (24″) on centre  (R‑50).

The rim joint areas are insulated on both floors using blown cellufibre held in place with Fibreglass mesh (R‑50).

The exterior walls on the main and second floors are built as a double 2×4 wall and filled with blown Wallbar™ insulation to a thickness of 400 mm (16″) (R‑56).

The ceiling cavity is insulated with 685 mm (27″) of blown Wallbar™ insulation (R‑100).

FAQ #12.   What about shutters?

Window shutters have the potential of providing functions of privacy and heat loss reduction. Interior quilted shutters, used as manually operated curtains, can be used on bedroom, bathroom and smaller, easily accessible main floor windows. They are assumed to be closed for the night hours only, with a net additional insulating value of R‑4. Basement window shutters would likely not be operated unless the space was occupied daily.

Interior shutters are problematic in that they can cause extreme temperature changes in the windows they cover. As such, they can affect the window’s long term viability. In fact, if accidentally left closed, the direct sun shinning through a window on to a closed high R‑value shutter could break the sealed unit. Exterior shutters would be the best solution, but they are difficult to seal on all sides (and to operate).

FAQ #13.   What is thermal mass heat storage?

When solar radiation transmits through windows during the day, materials like concrete, rock, tile, drywall and water will slow the interior temperature rise by absorbing heat energy, and then gradually release the stored heat when the air temperature in the room begins to drop. In the wintertime, when the sun is low in the sky, the window surfaces are almost perpendicular to direct beam radiation, and because it is also reflecting off of snow cover, the greenhouse effect is maximized.

Thermal heat storage materials are more effective in a passive solar home if they are less than 4” thick when not exposed directly to the sun, but they can be quite thick when exposed directly to the sun.  In the Riverdale NetZero home, the main floor will utilize a concrete, masonry or rammed earth wall directly facing the sun, with concrete countertops and floor tiles elsewhere. The basement and second floor partition walls will be partially filled with small containers of water or dry wall scraps.

On a weight basis, water is still the best thermal mass storage material, having about 4 times the thermal capacity of most building materials. Phase change materials have the potential to store even more energy however this is called latent heat storage rather than mass heat storage.

FAQ #14.   What type of landscaping is to be used?

The landscape plan has minimal lawn. There is a priority given to native plants and species that are drought tolerant.

FAQ #15.   How is storm water managed?

The site is an in-fill site with no storm sewer connection. Water from the main roof will be collected and stored for landscape and garden watering. The site will be graded such that surplus rainwater and snow melt drains across the site toward the street and lane. Water collected by the weeping tile is directed to a sump from which it is pumped to a discharge outlet. The discharge outlet is located such that excess water is dispersed on the site and away from the building by gravity.

FAQ #16.   What water conservation features will be incorporated in the home?

Within the home, water conservation starts with equipment that delivers significantly reduced quantities of water through showerheads and faucets. As well, dual flush toilets will be installed and very low water consumption appliances (washing machine and dishwasher).

FAQ #17.   Why not SIPS?

SIPS, or Structural Insulated Panels, can be a good choice when space is limited because with the right choice of cavity insulation, they can typically achieve insulation values that are twice what a normal wall can provide. These more space-efficient insulations; however, are much more expensive than a wood framed cellufibre wall (and you don’t get the fantastic interior window sills). SIPS typically use a polyurethane core, which is a hydrocarbon, and are not as environmentally benign to produce as 2×4 wood studs and cellufibre. Our wood studs can be cut from even the smallest softwoods, and given the prognosis for much of our northern pine forests for the next few decades (i.e. the pine beetle deforestation taking place), we can think of no better place to store this carbon than in a duplex that will be around for at least 100 years. And cellufibre is a recycled product that also stores carbon, rather than going to a land fill.

Do SIPS reduce air leakage? We don’t think so. Air leakage is a joint sealing issue, and all panel type systems have many additional joints.

FAQ #18.    Why not ICFs?

ICFs, or Insulated Concrete Forms, are typically more expensive than an insulated standard 8” thick concrete wall, and do not provide the level of insulation we need. We need R‑50 or greater to achieve the optimum value from our envelope and solar collection systems, and ICFs typically top out at R‑20 to R‑25. ICFs have been advertised at ‘equivalent to R‑50’ values but this is not supported by engineering analysis. They do exhibit a thermal lag effect, but having the mass outside a layer of insulation means that we can’t really use this mass for effective passive thermal storage (moderating our interior air temperatures). When ICFs are used as foundation walls only, they still need a membrane on the outside and need drywall cover on the inside. And of course, they utilize hydrocarbon insulation material, rather than our choice, recycled newspaper.

Can an ICF wall be inspected? Not really. It would require removal of the insulation, so we are left to rely on good practice as our means of preventing leaking foundations.

Do ICF walls last longer than exposed concrete walls? The concrete in an ICF wall is typically sandwiched between two layers of Styrofoam, which places them at the freeze/ thaw interface during the winter months. Concrete behaves best either completely frozen or completely thawed, not in the transition zone where daily movement between frozen and unfrozen occurs. The jury will be out on this issue for decades to come.

FAQ #19   I hate dusting – is this house for me?

Yes! A number of design, system and operational factors combine to control particulates in the home. The airtight envelope design and construction reduces air borne dust particles from the exterior during the heating season.

The fan coil units will be installed with air filters, and depending on the frequency of use of the central vacuum system, they will still need to be changed.

All floor finishes will be hard surfaced and easily cleaned. No carpet will be used. The central vacuum system will be exhausted to the outside.