Net Zero?

Lily Pond House Solar Plant

It has been exactly a year since we moved to our Lily Pond House. So how did our passive and active solar design perform? How close did we get to our net zero goal where annual energy consumption is roughly equal to renewable energy generation?

Starting with the passive solar design with South facing wall of glass, every sunny day was a joy during winter. On those sunny days with freezing temperatures, we often had to open windows as the temperature reached about 78 degrees after the living room cement slab could no longer absorb the sun’s heat. If the cement slab was not covered with furniture and rugs, we could have stored more solar energy but life trumps energy efficiency. Radiant heating did not have to kick in about 3+ hours after sunset while the cement slab warmed the house releasing the stored heat.

One of the best decisions our HVAC contractor Jim Godbout Plumbing & Heating made at the suggestion of our solar contractor Revision Energy was to go with an all electric energy for the house. Daikin Altherma, air to water heat pump that provides low temperature hot water for radiant heating and radiators, proved to be an efficient workhouse. We didn’t have a single day laboring to heat the house thanks to Theodore and Theodore Architects high performance shell design with minimal thermal bridging. Having radiant heating in the living room and basement cement slab, and bathroom tiles also  helped the cause with heat retention.

For active solar, Revision Energy designed a ballasted system that does not require the installation to penetrate the roof, which is a plus. However, solar panels are then limited to 15 degree pitch because beyond that the wind load gets too high and the required ballast gets too heavy for roof support. Although the optimal angle for solar panels is about 40 degrees for our location, the total annual production is fairly insensitive to installation angle since we can bank excess production for 12 months by feeding the excess solar electricity  into the grid. This is because the lower installation angle is optimized for the summer months when the sun’s radiant energy is at its highest. The penalty for 15 degrees tilt is less than 8%. Low pitch installations also hold snow longer than steeper ones;however, the penalty for snow cover is about less than a 10% effect in southern Maine.

Revision Energy estimated our expected electricity consumption at 11 MWh per year:

Plug Loads: Design estimate was 450 kWh a month, or 5,400 kWh per year. Although it was on the low side for a 2,500 sq. ft. house, it was justified by all LED lighting and  energy efficient appliances.

Heat: Design estimate was 15,000 BTU/sq. ft. for the expected annual heat load. That translates into 38 MBTU/year for space heating, given the 2.5 Coefficient of Performance (COP) for Daikin Altherma heat pump  yields an expected electricity consumption of an additional 4,450 kWh per year.

Domestic Hot Water: Design estimate was 40 Gallons per Day (GPD) load with two occupants at 20 GPD each. That translates to an annual load of 10 MBTU/year. Given the 3.25 Energy Factor  performance of our Voltex Hybrid Electric Heat Pump  water heater, that translates to an additional electric load of 1,200 kWh per year.

11 MWh solar plant design called for 36 255 -Watt Canadian Solar photovoltaic panels. Unfortunately, our two roofs could only accommodate 27 panels. So we had to go with the more efficient 300 Watt LG NeON panels resulting in a 10 MWh per year power generation.

Best Month – May

Worst Month – January

Since roughly half of our total electric load is space heating, our electric consumption in winter is effectively supplied by our solar production in summer. Our best month was May with 50 kWh electricity production on sunny days while our worst month was January with 20 kWh electricity production on sunny days except when the panels were covered with snow, which was about 10 days. Since photovoltaic panels can capture scattered sunlight to create electricity, they generated electricity even on cloudy days.

Solar Electricity Production by Month

Revision Energy’s initial design employed Solectria PVI inverters, which convert the DC output of a photovoltaic (PV) solar panel into AC that can be fed into the electrical grid or used by house electrical network.  As the solar  crew was laying out the roof and moving panels around to accommodate the parapet, and roof obstacles (e.g. vent pipes), they felt there might be enough partial shading on this system so it would really benefit from a slightly more advanced inverter solution. At their suggestion, we went with a Solaredge system that combines a central string inverter in the basement with individual panel optimizers on the back of each individual panel to improve system performance, especially when part of the array is shaded. In contrast to a traditional inverter system where the partial shading of a panel typically results in the total loss of power from that panel in order to optimize the power of all panels in the same string, Solaredge recovers the partial power from the shaded panel. In addition, Solaredge provides module level monitoring and performance information, which is a pretty neat bonus.

Upper Roof Panels Performance

Solar system was designed to be a net generator in summer and net consumer in winter.  As the data shows, Lily Pond House Power Plant is a net generator roughly 5 months (May, June, July, August, September), a net consumer 5 months (November, December, January, February, March), and roughly balanced  in the transition months (April, October).

Generated vs. Consumed Electricity

So what is the bottom line? We consumed 11.5 MWh while the consumption estimate was 11MWh. So if we could generate 11 MWh per our original design, we would have a net zero home. We took a 10% hit due to roof constraints, and another 10% hit because 10 MWh solar design produced only 9 MWh, mostly due to parapet shading. At the end, we generated 9 MWh of solar electricity while we consumed 11.5MWh. For the 2.5 MWh shortfall, we paid $300, which we really can’t complain about given the environmental wellness, which encourages us to live in harmony with the Earth by taking action to protect it:

 

 

Topping out

Topping out Lily Pond House

Topping out is a builders’ rite traditionally held when the last joist is placed on the roof during construction. On our team, Wiebke Theodore makes sure that we celebrate these time honored traditions, and made sure that our framing crew Mike & Co. nailed a live tree branch to celebrate the occasion. In high performance home construction, it is distressing to see that US lags Europe, Australia, New Zealand, Japan, and Korea from high efficiency heat pumps to solar panels, and from Energy Recovery Ventilation (ERV) systems to radiant heating. So it was refreshing to witness US green building progress beyond the traditional dimensional lumber based framing. In our Lily Pond House, the engineered subfloor panel AdvanTech manufactured by the Maine company Huber proved its resistance to wet soaking during a Northeaster that lasted for about a week. AdvanTech subfloors consist of highly compressed wood chips with resins that prevent swelling during long-term exposure to the elements. It does work but I do think there has to be a better practical way to be invented to prevent new construction from such exposure. Based on Steven Theodore’s specifications, Spang Builders used Boise-Cascade VERSA-LAM laminated veneer beams and headers that eliminate twisting, shrinking and splitting, and deliver flatter, quieter floors along with BCI Joists that are wooden I-beams with flanges made from laminated veneer lumber sandwiching engineered sheathing. I-joists enable simpler floor/roof layouts while saving half of the wood fiber used in dimensional lumber. I can’t argue with Boise-Cascade’s I-joist marketing collateral: “Clean appearance pleases the structure’s owner which makes the builder look good:-)”

I-Joist

I-Joist close-up

I-Joist floor

AdvanTech flooring by Huber

Laminated engineered wood posts

Engineered laminated wood beam

In a high performance home, the house envelope’s insulation, its resistance to air leakage, and moisture control within the envelope are of paramount importance. If you want to want to get up to speed with high performance envelopes, Building Science Corporation’s list of high R-value wall assemblies is a good place to start. For Lily Pond House, our architect specified R-25 3.75″ closed cell foam to all exterior wood-framed walls with an interior 3/4″ R-5 foil faced polyiso rigid foam board that gave an effective R-30 insulation for the walls. For the roofs, we ended up with R-42 6.25″ thick closed cell foam to all rooflines above finished living space with 5.5″ R-23 Roxul stone wool batts that gave an effective R-65 roof insulation. For the underside of living space at unconditioned area, the we went with R-42 6.25″ thick closed cell foam. Finally, our insulation subcontractor applied sealant to vertical and horizontal seams between dimensional lumber of double top plate, bottom plate, multiple studs, around window headers, and wall corners to get ready for the blower test. Spang Builders foreman Chad Richardson made sure that all of the areas missed got sealed right by the insulation subcontractor, which shows the builder’s value in enforcing quality assurance. We also used Roxul mineral wool sound attenuation batts in all partitions and ceiling assemblies within the living space. It was interesting to see the impact of the insulation during installation. Closed cell spray foam improved the rigidity of the frame and attenuated outside noise while Roxul quieted inside the house quite a bit. Besides its strength, and high R-6 per inch insulation value, closed-cell foam provides a barrier against air or water vapor. In contrast to building materials that slap on the green label on marketing collateral, closed cell polyurethane foam is the choice of green building construction as full life-cycle analyses show that this insulation provides a substantial reduction in carbon footprint. We selected Roxul made from stone and recycled slag from steel manufacturing over fiberglass for the interior because of its fire retardant, water repellant, and sound proofing properties. The fascinating Discovery Channel video explains how Roxul is made.

Closed cell spray foam insulation

Roxul insulation for interior

Polyiso insulation board

While the insulation keeps the house warm or cool, in windy weather the envelope needs a breathable membrane to keep the wind and moisture out. In our house, the membrane is SIGA Majvest that forms a part of the vented rainscreen system along with the exterior tongue and groove siding with a strapping in between to provide the spacing to form the drainage plane. We enthuistically support our architects’ preference for local sourcing of construction materials so the original specs for the siding was Eastern white cedar. Unfortunately, we found out that the the Eastern white cedar in our original estimate was  knotty making it not desirable as we were not going for a rustic look. The clear grade Eastern white cedar proved to be too expensive. So we ended up going with Port Orford cedar from Oregon, which ended up costing 3 times as much as the original siding estimate. Oh well such is the fun of building a custom home. Port Orford cedar is used in building boats, and docks because of its resistance to the elements. We were happy to find out that Port Orford cedar plays a spiritual role in the lives of the Hupa indigenous people of Northwestern California as ornaments used in spiritual ceremonies and dances are made of the cedar wood. Finally, the windows and doors are one of the most important components of a high performance envelope, which we will cover in our next post.

 

Going for Contemporary

This blog chronicles our journey in building a contemporary energy efficient home in Maine. We have been spending our summers in our cottage for the last 27 years. In 2014, my wife Margaret and I decided to take the plunge by selling our Massachusetts home, and settling in Maine to experience life as it should be for the whole year.

Over the years, while our tastes varied with the styles of the times, our preference for the modern minimalist design stayed constant. We still adore our Scandinavian furniture that we bought right after getting married, and fondly remember our stays in Copenhagen, Rome, Prague shopping for contemporary home furnishings, and living with the modern design in Istanbul.

Scandinavian Design dining room furniture – circa 1979

Deck View of our Conantum Home

We lived 20 years in this Carl Koch and Donald Gillespie designed contemporary in Conantum – a neighborhood of Concord, MA with about 100 contemporary homes nestled next to Sudbury River and Walden Pond. Conantum was the idea of MIT economics professor, Rupert McLaurin, who envisioned affordable cost housing for young couples. Carl Koch, an architect and also a MIT professor, planned the houses. Joseph Kelley, a local contractor, was the builder. Carl Koch spent some time after graduation in Sweden where  blended  clean Scandinavian design into his work . Our Conantum home is an example of Koch’s mid-century architecture design with multi levels and walls of glass that invite the outdoors in. For an excellent overview of Conantum homes, see Bill Janovitz’ tour of Conantum.

Conantum houses were designed when energy was too cheap to matter. Our house tripled its original size with architect Donald Gillespie before we bought it. We spent our 20 years upgrading the heating system into a distributed one, insulating just about the whole house, installing energy efficient doors and windows house while remodeling without destroying the character of the house. No matter what we did, it never could reach the ACH (Air Changes per Hour) specification of a Pretty Good House. So we wanted a high performance contemporary design for our next house. So in the summer of 2013, our journey started with looking for an architect who could turn our needs and wants into a buildable vision.