Passive solar design
The house is designed to use winter sun as its primary heater. Most of the windows face south, and the main living spaces are on the south side of the house. Bathrooms, storage, and staircase–parts of the house where less time is spent–are generally located on the north side. Solar heat provides 60% of the annual space heating needs, and heat from occupants and appliances inside the house provides another 15%, according to a simulation in the Passive House Planning Package (PHPP) software.
Outdoor living spaces are integrated on the south side of the house where they connect directly to the main rooms. The outdoor spaces will be comfortable for most of the year, shaded by trellises.
Designed for passive ventilation using skylights
Three electrically-operated skylights near the ridge of the roof are located to passively ventilate the house. Air moves freely through the loft balcony and stairwell, and the height difference of 20+ feet above the ground slab ensures a strong stack effect. The stack effect is the updraft created by the buoyancy of warm air released from a high opening.
2×8 studs at 24” spacing
Advanced Framing or Optimum Value Engineered (OVE) framing is a system that uses wood only where it is necessary structurally. American builders trying to conserve limited resources in the past centuries used a similar framing system. Today, OVE framing typically uses 2×6 studs (5-1/2” thick) at 24” spacing, with less superfluous wood around windows and doors, and at the top and bottom of walls.
Since wood conducts heat much faster than insulation, reducing wood in the walls not only saves forests, but also saves heating and cooling energy. In this house 2×8 studs (7-1/4”) are “balloon framed” over two stories, running from slab to roof, to reduce joints and connections at the intermediate floor. This also makes the house stronger against wind and earthquakes.
Framing with this system takes more detailed drawings and more coordination between builder, architect, and structural engineer. Most builder-architect-engineer teams have never framed this way, and the learning curve is steep.
A header is the structural member spanning over an opening in a wall. Headers are typically solid wood and occupy the entire thickness of the wall, creating a significant thermal bridge. Headers in this house are 3-1/2” thick engineered lumber, set to the inside of the 7-1/4” wall, with 3-3/4” of expanded polystyrene (EPS) insulating the header from outside temperatures. EPS (white and crumbly) is the eco-friendliest of the rigid, plastic foam insulations. White Cap Construction Supply, San Leandro.
Post & beam interior designed for remodeling
The exterior shell of the house is largely self-supporting, and the interior is post-and-beam construction. Most of the interior walls are non-load-bearing, so they do not need a lot of structural wood, and they can be rearranged in the future if the family’s needs change.
Unvented roof (full cavity insulation)
The entire shell of this house has full-cavity insulation. The attic is usable, conditioned space, and there is no outside air vented between the rafters. The California Building Code has required rafter venting to prevent damage from condensation in roof cavities constructed to a low level of quality. Condensation forms in roof cavities when a steady stream of humid interior air can enter the roof cavity through air leaks, and the top of the roof remains cold for long periods of time. This house is so well sealed against drafts that there is no supply of humid air, and the roof sheathing is insulated from above with 1” of rigid polyisocyanurate (“polyiso”) board. Theoretically, the rigid board is not even needed, since the mean monthly temperature never drops below 40ºF, which is the approximate dew point of interior air. The design team submitted an AMMR application to the City of Palo Alto to waive the prescriptive building code and allow the modern roof assembly. This was obtained. www.BuildingScience.com
A “White Roof” is not necessarily white, but is a light color so that it reflects more sunlight, keeping the house cool, and reflecting more light into space. A dark colored roof absorbs more light, and converts it to heat energy. For example, a roof that is actually black heats up by 90 degrees Fahrenheit in direct sunlight, while a true white roof heats up by 14.6 degrees. The light absorbed by a dark colored roof generates heat energy, and contributes directly to global warming.
The Palo Alto Planning Department was concerned about the aesthetic effect of glare on our neighborhood, so we are using a light grey, metallic color that is almost as reflective. Our roof’s reflectivity is 58%, while the white option we had available is 63.3%. Because our roof insulation is so thick, the gray roof will have minimal impacts on heating the house in the summer time, and should save a small amount of energy on heating in the winter. The only drawback of the grey roof is from a climate perspective: over a course of a year a little more heat is absorbed from sunlight, and will be released into the air outside the house.
Air Admittance Valves (AAVs)
An AAV is a durable, one-way air valve, the size of a vitamin bottle. It takes the place of a traditional plumbing vent through the roof. The purpose of both the traditional vent and the AAV is to admit relief air into the plumbing system when water is draining, in order to prevent a vacuum in the pipes that would suck water out of the P-traps under faucets. Typical houses have many vents breaking the integrity of the roof, acting as thermal bridges, and circulating outdoor air within the walls. This house has one such vent–the rest are AAVs. AAVs require less plumbing material and labor, and less roof work than conventional vents. When they are enclosed in a wall, AAVs require an access panel for inspection. Many building departments that are not educated about the technology do not allow AAVs. Oatey Sure-Vent, www.oatey.com
Radiant floor heating
A variable-speed pump circulates warm water through tubing in the ground floor slab for winter comfort. Because of the large surface area, radiant slabs can deliver heat using very lukewarm water (90ºF), compared to the hot temperatures required by other water-based heating equipment such as radiators and baseboards (160ºF). The lower temperature improves efficiency at the heat source, allows the use of future solar-hot-water for space heating, and allows the use of an electric water heater (the Daikin Altherma) that would otherwise be inefficient at higher delivery temperatures.
A conventional slab with tubing everywhere, running at full output, would be barely warm enough to feel, because so little heat is needed to maintain a warm room temperature. Because heat delivered is proportional to (water temperature) x (surface area), we had to greatly reduce the surface area of tubing in this house in order to keep the water temperature warm enough to feel.
Hot water tubing was placed where the noticeable warmth would be most comfortable and social
the dining room table, the kitchen, the bathrooms, and walking paths around the ground floor.
Daikin Altherma heat pump water heater
This electric water heater transfers heat from the outside air into a water storage tank using refrigerant in a vapor-compression cycle, like a chilled water-fountain or air-conditioner in reverse. The so-called “air-source heat pump” creates 3-4 times more heat from the same electricity as a standard electric water heater. The efficiency is comparable to a geothermal heat pump in this climate, but involves no boring or excavation.
The Altherma costs much more than a conventional water heater, but for a zero-net energy project, the Altherma costs less upfront to save electricity than the photovoltaic panels (PV) that would be necessary to generate that electricity. www.JTGMuir.com
Aging-in-place or extended family
The house has five bedrooms, including a ground floor suite with its own outside door that can accommodate an elderly relative, an au pair, elderly homeowners, or a young couple with a child. The bedroom and bathroom configuration strives to provide extended-family living, and flexible space for many future situations.
Meeting the Passive House standard
There are two components to the Passive House standard. One is the insulation (see Insulated Headers). The other is that the house should be airtight, so that heat is not transferred through gaps in the walls. Our walls were tested using an infrared camera, to show where cold air was seeping in. On the left is a picture taken to demonstrate the use of the camera to show heat. The picture on the right shows a plume of cold air, coming in around the edges of a beam, which (intentionally) punctures the building exterior membrane.