On April 19th, Lydia and I took a tour of the Midori House being built here in Santa Cruz. This house (which is currently under construction) utilizes the principles of the Passive House design to create an extremely energy efficient home. Passive Houses are designed to minimize heating and cooling needs by creating a virtually airtight structure and utilizing alternative energy sources to create a “Net Zero” structure. Passive structures all share the same core design elements, outlined here in a virtual tour of the Midori Passive House.
The ProjectChie and Kurt purchased the 1922 Craftsman bungalow with the intent of converting it into an extremely energy efficient home without having to actually tear the whole house down. As Chie put it, “It is analogous to taking a 90 year old person, keeping the skeleton, but putting in a new respiratory system, a new circulatory system, and giving them new skin and clothes.” With the help of Santa Cruz Green Builders, the homeowners plan to redesign the house into a Passive House while still maintaining the original craftsman architectural style of the neighborhood.
The Passive House Planning Package (PHPP)Our tour began in the front yard with Kurt, Chie, and Taylor (of Santa Cruz Green Builders) explaining the concepts of a Passive House. It all begins with the PHPP, a special software used in the development and planning of Passive Houses. The software maps climate data specific to the location of the property and incorporates natural components such as the distribution and intensity of sunlight throughout the year (where to put windows on which walls), the climate patterns throughout the year (which days require cooling or heating), and the surrounding air quality (to determine comfort and air quality specifications for the interior of the house).
Construction: A Super-Airtight Envelope
Airtightness is essential to the Passive House model. In order to test airtightness, a Blower Door Test is used to measure the structure’s ability to maintain pressurization. For Passive House specifications, at 50 Pascals of pressure the house should have less than .6 air changes (air leakage) per hour. In comparison, a typical house has 7-12 air changes per hour, while the original Midori House had 25. Superb airtightness was achieved through excessive calking of the walls, floor, ceiling, everything. Standard constructions usually do not put this much attention into calking, but for a Passive House this is essential. The windows jut out of the walls to allow insulation to be placed around them, and rigid foam insulation runs continuously along the entire bottom and ceiling of the house.
The Heat Recovery Ventilator: The “Central Respiratory System”You may wonder, if air cannot leak in and out of the house how does the air get circulated? This necessitates the Passive House’s second key element: The Heat (or Energy) Recovery Ventilator (HRV). This device, located at the heart of the Midori House, removes exhaust air and circulates fresh air to all the rooms of the house, transferring energy from the outgoing air to the incoming air. This system is 85-95% efficient at this direct heat transfer, and also utilizes solar thermal energy to make up additional heating needs. With this system, air quality in the home is maintained and heating/cooling costs are reduced.
Advanced WindowsWindows are crucial when constructing a Passive House. The Midori House windows are triple-pane with 6 different kinds of coatings to reflect and retain heat, managing the interaction of infrared and visible light. The specific coating is dependent on which wall the window is placed, determined by the sunlight patterns mapped in the PHPP. The southern facing wall has the highest solar heat gain; therefore, most of the windows are along this wall. Awnings are used to shade the windows in the summer months but allow in light from the low-sitting sun during the winter months.
Efficient Systems: PlumbingOne technique in creating an efficient structure is to assess the hot water demand for every plumbing feature in the home (every sink, toilet, shower, etc.), and instead of applying a one-size-fits-all plumbing system, install high efficiency low-flow pipes that minimize the time it takes to wait for hot water. The plumbing system in the Midori house relies on solar panels to heat up water (with a backup generator for cloudy days), and super-insulated pipes to minimize water waste. Another feature of the Midori House plumbing system is the use of greywater, a technique used to minimize potable municipal water waste. Water from the washing machine can be used for landscaping, while a rainwater catchment system in the backyard supplies cold water for the washing machine and toilets.
Efficient Systems: Fun with Numbers
The Passive House design stresses the concept of energy efficient systems in the home aside from plumbing. These are quantified in many ways, particularly the R-Factor or R-Value. The R-Factor is a measure of the heat retention, or resistance to heat transfer, of a substance. For example, wood, a very poor insulator, has an R-Factor of 1 (designated as R1), while the insulation used for the hot water pipes has a factor of R5. Rockwool (a rigid insulator) encapsulates the entire walls of the Midori house, reducing thermal bridging that occurs due to the poor insulation of wood. The walls, therefore, are R26, compared to R13 for standard walls. The home also boasts ceilings with R40, and floors with R25.
Shown below: heat loss before and after Passive House retrofit.
Materials Sourcing, Recycling, and RepurposingIt would seem counterintuitive to construct an energy efficient home using very un-green construction methods. Kurt, Chie, and the Santa Cruz Green Builders made it a point to recycle and repurpose as much of the waste material as possible. Wood removed from the site was recycled. The exterior siding is made from recycled cement fiber. The interior of the house is insulated with cellulose (recycled newspapers). The special windows were imported from Canada to reduce the carbon impact of transport.
Our tour ended in the backyard, where Kurt and Taylor made their final comments about green building practices. The key to the Passive House approach is biomimicry, particularly solar heat gain, efficiency, and energy storage. In the average American home, heating water is the biggest energy guzzler, following by air heating, appliances and lights. Occupant behavior is what ultimately makes the difference. You can have the greenest house in the world, but it will not matter if the occupant does not live a green lifestyle. Proponents of green building design wish to move society toward a “Net Zero” model, curbing our global impact on the environment.