...creating a continuous WRB.
— We are excited to have started building the SURE HOUSE in a parking lot on the campus of Stevens Institute of Technology in Hoboken New Jersey. “SURE Construction” is a subset of our PopSci blog that we’ll use to chronicle our construction process. Check back often if you want to follow our progress and get a first hand view of how a sustainable and resilient house takes shape.
Water, Water Everywhere
SURE HOUSE is designed to survive a flood. But in terms of water, exposure to flooding will be the exception in the life of our building. The rule will be constant exposure to water as vapor in the air and liquid in the form of rain and melted snow.
In a building, a drainage plane is a vertical surface designed to prevent liquid water from penetrating inward while allowing it to flow down and away from the building by the force of gravity. For most of the walls in the SURE HOUSE, cedar shakes are the primary drainage plane.
A weather resistant barrier (WRB) is essentially a second drainage plane working in tandem with the exterior building surface (siding, shakes, plaster) to keep water out and direct it away. In SURE HOUSE we chose a structural sheathing product with integrated WRB that is designed to both seal out water and act as an air barrier. In our design the primary air barrier is attached to the interior of our framing, but this wall sheathing system is an important secondary air barrier in that it helps prevent migration of water vapor laden air into the building. The sheathing is attached to exterior framing…
…and then all joints are taped…
Weather Resistant Barrier
…creating a continuous WRB.
What’s Flat, Full of Holes, and Drains Water?
When it comes to liquid water, walls are easy because they are vertical and therefore just need to help trusty gravity do its work of moving water down and away. Roofs, especially flat roofs, are gravity challenged, so drainage planes are more a more complicated affair. We started with ¾” tongue and groove sheathing. This is thicker than most roof installation, chosen in our case to give our photovoltaic racking a stronger substrate to resist wind uplift.
We then taped the joints of our roof sheathing. This was both a temporary waterproofing maneuver and a decision to let the sheathing work double-duty as a secondary air barrier, similar to the ZIP sheathing in our walls, a WRB of sorts. If it were only that simple. The crazy matrix of blocks (PV racking, HVAC curb, conduit blocking) and pipes (ventilation supply/exhaust, plumbing stack, power supply, condenser line sets…) pictured here are the bones of a high performance house, each complicating the critical, seemingly simple but actually deceptively difficult job of convincing water to flow over and away, rather than through and into our house.
Vent Penetrations – Interior
For example, here is our energy recovery ventilator’s exhaust pipe exiting through the drop ceiling in the bathroom. We thought it was a good idea to insulate this penetration to prevent having a very conductive piece of metal pipe cutting through our building envelope. Seems at times, a hot summer day for example, the temperature differential between the pipe and the insulation could create a condensing surface that would bring liquid water into the roof cavity where it could become trapped and cause problems.
Vent Penetrations – Exterior
The pipe continues through the roof and was taped as part of our secondary air barrier. A metal pipe extension and cap will be installed later.
Flat Roofs Aren’t Really Flat
The main drainage plane for our roof starts with layers of tapered insulation, installed both to create a slope toward gutter drains and to add thermal resistance while alleviating thermal bridging from our roof framing.
Penetration and blocking installation that started as part of the roof sheathing was continued in tandem with the insulation process. Here, blocking for our solar panel racking has been fabricated to sit flush with the tapered insulation…
Solar Panel Wind Uplift
…with fasteners carefully specified to handle the wind uplift forces of the solar panels that will be mounted at these points. Rigid insulation is held in place with screws and large flat pan washers.
Preparation for Weatherproof Membrane
Here, the insulation, installed at a slope of ⅛”/foot, is prepared for the application of our vinyl roof membrane. Our generous sponsors Sika Sarnafil and Delta Contracting donated the materials, labor, and installation expertise.
Vinyl Roof Membrane
Last but not least, the membrane was installed…
Planning for Reassembly
…with the joints heat welded to create a seamless waterproof surface sloped to our gutter penetrations. We’ll have to cut the membrane in two places to separate our house into the three modules for shipping, then reweld it at the competition site in Irvine.
Spending a long, hot day working on a roof teaches you something. Roofs take a lot of abuse from sun, wind, water, and freeze-thaw cycles and are asked to be perfect….allowing absolutely no liquid water into a building for many years. With roofs, you really can’t mess around. We chose vinyl because it is very durable in a harsh environment and is easy to work with, taking penetrations well.