Flywheel is current working on a new townhome development site in the Congress Heights neighborhood of Washington, DC. The 16 affordable homes will be built on a site awarded by District's Department of Housing & Community Development (DHCD) and will achieve net zero energy and Passivhouse performance. Stay tuned for more updates.
To achieve Passivhaus certification, the homes have to achieve a minimum air tightness level of 0.60 ACH 50, which is five times “tighter” than the 3.0 ACH 50 required of new Maryland code-built houses.
In these images, Robert Champ of Edge Energy operates a blower door, which pressurizes and depressurizes the house to test air-tightness and spot leaks. The initial blower door test, conducted after the modules were installed on site, was 0.36 ACH 50. This initial blower door test provides a crucial window to spot any glaring air leaks before they are covered in other building materials and become more difficult to access. The homes’ final blower door test, completed after construction was wrapped up, was 0.35 – below the Passivhaus requirement of 0.6.
After completing the initial blower door test, two inches of exterior insulation are added to the building’s exterior. In addition to improving the wall’s thermal performance, the foam is necessary to ensure the wall assembly avoids condensation and moisture problems within the wall cavity. Outside of the insulation, furring strips are applied, which allows air to circulate behind the siding and ensures bulk moisture is not trapped against the sheathing. With the furring strips installed, air can move freely from the bottom of the wall assembly to a vent under the parapet cap.
On the interior, an additional building envelope detail includes applying Intello-brand smart vapor retarder to the interior of the basement wall.
To improve the stormwater characteristics of the site, a “green” alley is installed using Ecoraster-brand geotextile panels. With an underlayment of compacted gravel and soil, the Ecoraster sheets snap together in a matrix. The open cells are then filled with additional topsoil and grass seed and are watered. The completed alley allows stormwater infiltration, captures pollutants, and will create additional green space for owners.
Passivhaus construction requires additional investments in the building envelope – triple pane windows and doors, and extra insulation. On this project, the insulation levels are on average two times the r-value required by code. Apart from utility savings achieved later by owners, there are offsetting construction cost savings as well. As a result of the superior insulation, one small ducted mini-split heat pump is enough to heat and cool each house. The footprint is so small it can be installed in ceiling of the first floor modular “box.”
Because of the homes’ air tightness, each house also includes an energy recovery ventilator (ERV) which continuously brings in fresh air. The system includes long flexible ducts which supply fresh air to bedrooms and living spaces, and return ducts which continuously remove air from bathrooms and the kitchen. Situated in the basement, the Zehnder-brand ERV unit is connected to the exterior with graphite polystyrene ducts – one supplying fresh air and the other exhausting indoor air. The ERV unit uses a heat exchanger to transfer 85% of the latent energy from exhausted indoor air to the incoming air. In the winter, warm indoor air is used to pre-heat the cold incoming outdoor air before it enters the house. The same effect operates in reverse in the winter.
To meet stormwater requirements and ensure the homes achieve net zero energy performance, a combined solar-green roof system is installed on the townhouse roofs. To guard against leaks in the roof itself, an Electronic Field Vector Mapping (EFVM) system is installed under the roof membrane. Consisting of a stainless steel wire mesh connected to an exposed copper wire, this allows the membrane to be tested for leaks by applying a charge to the mesh through the wire, and “sweeping” the roof with a copper bristle broom. Holes are detected when an electrical connection is made to the copper bristles through any holes in the membrane.
The green roof assembly on top of the roof deck integrates solar racking bases made by German-based company Optigreen. The Optigreen system allows the solar racking system to be ballasted by the green roof media, avoiding the need for penetrations in the roof membrane. Low-maintenance ‘sedum’ plants are selected for their hardiness, with shade-tolerant species placed under the PV panels. This green roof-solar combination offers the additional benefit of improving solar generation from the panels; the evapotranspiration of the green roof plants effectively cools the panels in hotter summer months, which studies suggest improve solar production by 8-12%.
With the foundations completed, the Beracah Homes modules are set on their foundations in two days by the Dover, Delaware-based Amish set crew, Ervin King & Sons.
Between modules, the set crew installs modular party walls. The walls are clipped into place and fire-stopped with mineral wool. Once set into place, seams between modules are taped and sealed to prevent air infiltration.
On Day 2, additional modules are set and the mansard roof is attached to the perimeter of the roof through four inches of high-density EPS foam.
The foundation walls for the new townhomes were poured offsite by Durham, North Carolina-based Ideal Precast. Trucked to the site, the walls were set into place by a crane in one day. Made of high-strength steel and concrete, the wall incorporates EPS insulation, which improves the basement’s thermal performance.
After these walls were set into place, a vapor and air barrier (Stego Wrap) and five inches of high-density insulation were installed under the slab and footers by Mount Rainier-based contractor Pete McAvoy. The foundation was completed with the addition of a slab and concrete block party walls.
At the same time as on-site work began at the project site, our project’s modular builder, Beracah Homes, began construction of the building modules. Because Beracah was able to transport 20 foot wide modules to the site, each home is made up of only two “boxes.”
In contrast to site-built construction, where contractors often cut corners and accept poor-quality workmanship, Beracah’s Homes are built by specialized crews in a factory-controlled environment, and must withstand transit and craning into place without damaging interior finished drywall.
At Beracah’s factory, modules are assembled on two lines, and boxes are rolled down an indoor rail track as they advance to each successive stage of construction: framing, mechanicals, drywall and window installation, paint, trim/finish carpentry and prep for transportation. Each box takes 11 days to complete, and the entire build of eight boxes was completed in three weeks.
Quality control is essential in Passive House construction, due to the construction detailing and superior air sealing that are required to meet the stringent standard. By building in a factory environment, Beracah is able to achieve these air sealing targets as all trades are part of an integrated team that is experienced working together.
With a company goal of achieving net zero energy performance on all our projects, Passive House is the way we get there. Achieving Passive House performance requires a tremendous commitment to quality construction -- something we'll write more about in future blog posts.
The Passive House standard is a proven method for the design and construction of energy-efficient, quiet, comfortable, healthy and durable buildings. It is the only internationally recognized performance-based energy standard in construction and is often considered the most rigorous voluntary standard in the industry today. Buildings that are Passive House-certified consume roughly 90% less heating and cooling energy and 80% less total energy when compared to typical building stock. Because of this dramatic reduction in the energy consumption of Passive House buildings, achieving Passive House is the most straight-forward way to achieve net zero energy performance.
The Passive House standard prescribes an energy “budget” per square foot (or Energy Use Intensity), and a maximum amount of air leakage. How to meet the rigorous requirements of Passive House is up to the project team. The flexibility of the Passive House standard enables it to be applied to any construction type, anywhere in the world.
Here's a great video that explains how Passive House works:
The fundamentals of Passive House design are as follows:
• Proper Insulation: Passive Houses dramatically lower energy needs by increasing the insulation levels in the building to ensure conditioned air is not lost to the outside. This can be cost effectively-achieved in new construction by increasing the thickness of wall and roof assemblies. For example, a code-built wall for a new home in Maryland is R-19, while a Passive House requirement is significantly higher. For the Perry Street Townhomes our insulation levels are more than twice code requirements.
• No Air Leakages: A building’s air tightness measures the amount of indoor air that “leaks” outside. Air leakage reduces efficiency and creates what’s commonly known as a “drafty” house. Air tightness requirements under Maryland’s current Energy Code requires homes to lose or “exchange” its outdoor air no more than 3.0 times per hour (3.0 ACH). In other words, under the current building code the air in the average new Maryland home leaks out every 20 minutes. Passive House requires air leakage to be less than 0.6 ACH, five times more stringent than code.
• No Thermal Bridging: Thermal bridges are elements within a building’s envelope (walls, ceiling, and floor) where energy is easily transferred between the inside and outside of the building. For example, in the average new home’s walls where the 2x6 studs touch both the exterior sheathing and the interior drywall, the 2x6s act as a conduit, radiating out heat in the winter and coolth in the summer. In many Passive House walls, including those built by Flywheel Development, 2x4 studs are staggered on a 2x8 or 2x10 base plate, creating a thicker wall and eliminating thermal bridging from studs. In the thermal image above, the Passive House clearly stands out from the other row houses as the only one that appears “blue” because it is not radiating any heat.
• High-Performance Windows: Passive House projects typically use high-performance, triple-paned windows that are airtight and have no thermal bridges.
• Orientation and Shading: Passive House design utilizes shading and solar orientation to optimize energy performance. In the Northern Hemisphere, large windows are designed to face south, taking advantage of solar heat gain in winter months to “passively” warm the home’s interior and lower heating demand. In the summer, trees (or shading devices) are used to dramatically lower solar heat gain into these same windows. On Perry Street this goal was met by retaining the oak tree in front of the houses and planting new trees.
• Energy Recovery Ventilation: Due to the air tightness of Passive Houses, these houses rely on Energy Recovery Ventilators (ERVs) to mechanically exchange indoor air for fresh outdoor air. These are the same air handlers used in commercial buildings, just on a smaller scale. In traditional homes, air simply leaks in through gaps within a home’s envelope. By contrast, in Passive Houses, the ERV recovers the heat (or coolth) of the indoor air when exchanging indoor and outdoor air. For example, in winter months, the ERV’s heat exchanger uses warm exhaust air to warm cold outdoor air before the air is brought into the building’s interior. The reverse is true in the summer, when the indoor air is cooler than the outdoor air.
Net zero energy is a performance target that describes buildings that produce as much energy as they use over a calendar year. Flywheel Development’s goal is to build all of our projects to meet this standard. For our Perry Street Townhome project this goal will mean homeowners will effectively eliminate electric and gas bills – although in practice our local electric utility charges a fee to homeowners to remain connected to the grid. As with other grid-tied homes, during the day, when solar PV on each roof produces a surplus of power, energy is sold back to the grid to be used by neighbors, while at night the homes draw power from the power grid like conventional homes.
Achieving net zero energy performance is challenging, and requires a commitment to building construction techniques that are far beyond code requirements. Flywheel has found that the best way to achieve net zero energy performance is by using the Passive House building standard, which allows us to reduce home energy use by 80% compared to traditional construction. The remaining energy needs to achieve net zero energy performance is met with onsite renewable energy in the form of solar PV.
Welcome to our new website. If you visited before you saw our old landing page -- and the map of greater Washington, DC. Well we're happy to report that we're on the map now, with a handful of proposals in the streetcar suburb of Mount Rainier, MD...