First US Passive House community almost complete
11 December 2012
By Imogen Schoots
11 December 2012 — The first Passive House community in the US will soon be due for completion in Philadelphia. Halted by the global financial crisis back in 2008, the project, known as The Stables, recommenced after the project was downsized.
Instead of the originally planned 70 condominiums, limited financing forced a scale back to 27, four-story modular homes in Northern Liberties, a suburb undergoing increasing gentrification.
A number of features are noteworthy
First there is Interdisciplinary team
The Stables is being designed, built and constructed by award winning architect/builders Onion Flats, an innovative firm comprising brothers Tim and Pat McDonald and architect Howard Steinberg.
In the past the partners would strap on their tool belts and do everything on building sites themselves for projects they designed and built. Now the scale of the projects that Onion Flats works on means that this is no longer possible, but the firm’s partners still maintain a hands-on role.
Steinberg explains the design/build approach of the firm is quite unusual in the area.
“Design and build in one firm as a way to deliver a project, in Philadelphia, is quite rare. Architects are inherently risk adverse, Steinberg says.
“Historically architects and builders battle each other. We wanted to break down that border. We want our student architects to be better builders, we do that by bringing them onto our job site.
“From an architectural view point you are mitigating risk by not just handing over the project to someone else for them to build. We can be onsite and controlling the detailing, things like water sealing – it’s dynamic and fun. Saying that, we are still learning, it’s a learning curve all round.”
Adapting Passive House
Steinberg, with some of his colleagues, was trained in Passive House standards in Dublin, Ireland in early 2011. There are a number of essential requirements when constructing a Passive House. One of these is to ensure that the home is air sealed, that no external air can enter other than via the mechanical ventilation system. all of the internal walls. However Steinberg and his colleagues could see the practical application of air sealing internal walls was difficult. There are many components or parts that come in to join at the wall such as all the joists. Steinberg said he and his colleagues thought, “Why not turn it around. On the outside you have these nice clean surfaces that are much easier to tape and therefore air seal”. So airsealing on The Stables homes is done externally on The Stables homes.
Another essential component of the Passive House standard is to eliminate all thermal bridges.
“A thermal bridge is where a material conducts temperature. In a stud wall, even though wood is not very conductive, you can still get thermal bridging through that stud from outside to inside the house,” Steinberg says.
“By putting the rigid insulation on top of that, it’s blocking the bridging.”
Each home of The Stables project has only four penetrations on the external wall. There are no wires running through any external wall. Such careful design prevents anything conducting temperature or interrupting the insulation.
Passive House design originated in Germany and Austria in the 1990s. Given the much higher humidity levels in Philadelphia compared to northern Europe, careful attention to detail of design and construction was required. Steinberg explains why: “Where cold air meets warm air is the dew point. You want to make sure this happens outside of your water and air barrier, so in this case outside of the zip panel. If it happens on the inside of the zip panel then that dew is going to collect inside the stud cavity and cause mould.”
So The Stables construction process includes methods to prevent humidity entering the buildings.
Other Passive House criteria
Insulation is an essential component of the Passive House. Required thicknesses depend on geographical location and house orientation. It is possible that different walls, whether facing west or not will require insulation of different thicknesses.
A specific requirement of the Passive House is a ventilation system known as energy recovery ventilation that operates as a heat exchanger. Heat from inside is extracted and then mixed with fresh air from outside. A constant fresh air supply is provided but with a lower energy demand than conventional mechanical systems. An additional heat pump, similar to an airconditioner, can de-humidify and also provide heating or cooling should it be needed.
Passive House standards don’t require the home to have a solar photo voltaic system. The Stables will be completed without solar PV systems, however, if purchasers do wish to eliminate all future energy bills, the required wiring has been already installed.
Green roofs were early in the project.
The kitchen window is above a balcony herb garden and the roofs of both the house and the car park will both have gardens.
In consideration of weight, a special engineered growing media is used instead of soil, which is much lighter. It has organic matter of about two per cent so it won’t break down over time.
The growing medium is also designed to prevent compaction while allowing for adequate oxygenation of the plants. Drought tolerant plants, predominately sedum, a succulent, are used so no supplementary irrigation system is required.
The roof of the car park, which is about 2.4 metres long, will also work aesthetically to be enjoyed as an outdoor living space but its main function is to help with storm water management. The whole development site has been designed to be 95 per cent pervious.
The Stables driveways will also be pervious, tying in with requirements set by the Philadelphia Water Department. The department requires that the level of storm water management must be at least 25 per cent better than prior to construction for a development project to be approved.
“The whole city has a combined sewerage and storm water system. It all goes down one pipe. Given the increased volume of water it is easy for water to back up, over flowing into rivers,” Steinberg says.
“It is obviously easier to change how developments are done than to spend millions on new infrastructure. In the past Philadelphia had basically been in non-compliance for years and years but now it is one of the most progressive water departments. They have done a great job.”
There are planning approval benefits to being green as well.
“Having a green roof means you get a green plan review so your file goes to the top. They are making it more desirable to do these things. The green roof industry across the US grew something like 115 per cent in 2011 and 30 per cent the year before.”
When considering buyer demand for such homes, Steinberg says, “Our perception is we just need to create it and put it in the market. We try to lead more by example than to give lectures and that kind of thing.
“My feeling is that take up is growing but there is still a perception of it costing more. Our whole mission is to strip this notion that it has to cost more. It is the same [components as regular construction] like studs and drywall, with careful attention to detail like air sealing and how things meet up.
“Sure we need to upgrade windows, which of course cost more but then the mechanical system is a third of the cost that you would need otherwise. You need to look at it as a whole, not a line item.”
Regarding sale price, the Stables homes are at a price point at the higher end of comparable sized homes in the area. Steinberg stresses that operating costs however will be considerably lower with an expected average of ninety percent lower energy demand.
The Stables is being built by Onion Flats’ sister company, BLOX Sustainable Building Systems. As the first Passive House community development in Philadelphia, it looks like the Onion Flats develop/design/build collective will continue its award winning streak for leadership and forging the way for sustainable building.