One of the projects currently on Beausoleil’s drafting boards (i.e., on our computer screens) is a modest remodel of and addition to a typical 1950’s ranch house in San Carlos. Besides creation of a master suite, a guest suite and a better relationship to the back yard, the owners desired to go well beyond normal practice in remodeling the building envelope. They wanted to approach, and possibly meet, the rigorous requirements of the Passive House Institute and reduce the building’s energy use by as much as 90%.
‘Passive House’ (Passiv Haus in German) is an idea that was formalized in recent years in Germany, though the basic concepts have been around at least since the 1973 energy crisis started people thinking about curbing energy use in buildings. ‘Passive House’ is related to the broader concept of passive design in architecture, where the environmental comfort within buildings relies on local environmental forces, rather than utilizing complicated mechanical and electrical systems. In classic passive design, a building is designed to allow windows to admit solar heat in the winter, and to be shaded in the summer; occupants open windows for ventilation and cooling; heat is regulated through good insulation and air sealing; daily temperature cycling is smoothed by providing thermal mass within the building envelope. Space heating is minimized and people are expected to live with a bit of minor discomfort – putting on a sweater when the temperature swings a bit low, wearing shorts when its hot, getting up off the couch to adjust shades and windows – instead of cranking the thermostat.
The Passive House Institute standards require very heavy duty insulation (typically double to triple the insulation value most homes have), triple glazing on the windows, very tight air sealing, and (not so passive) ventilation using a heat recovery exchanger to retain the heat in air being exhausted. Very good detailing and supervision of construction is necessary to make these houses meet the standards, as it is very easy for air and heat to escape through a poorly conceived, or a poorly made, building shell. With excellent insulation and air control, most of the heat for the house can come from the building occupants and their electrical appliances. The need for air conditioning is eliminated. With good natural lighting and photovoltaic panels on the roof generating power, very little energy input from outside is required.
Getting back to the project on our drafting board, the owners hired Allen Gilliland of OneSkyHomes, a Passive House Institute certified consultant and builder, to analyze the design of the house using the Passive House Planning Package and make recommendations. The PHPP is a software tool that comprehensively models the energy use of a building, including its skin, insulation, windows and doors, air sealing, ventilation, and mechanical systems. Allen thought he had an easy task ahead of him, but it turns out that modifying an existing ranch house is a tougher challenge than designing a new house.
The ranch house in question has typical 2x4 stud walls, and their thickness didn’t provide enough space even if high performance sprayed foam insulation was used. Luckily, the owner wanted to remove the existing stucco wall finish and replace it, so amending the wall thickness was conceivable.
Likewise the roof is framed with 2x4 rafters, and even though the attic provides a large space for insulation, the edges where the roof comes down to meet the walls are inconveniently slender. And, as it happened, the house’s roof had been replaced recently so the owners were naturally disinclined to rip the roofing off to add more insulation on top.
The other inconvenience is that the existing house is oriented to face east and west, just the opposite of ideal north-south orientation. East and west glazing is hard to shade for summer cooling, and doesn’t face the warming sun of winter, so the project had a strike against it from the start.
Triple glazing in our mild climate seemed silly to me (and to the window suppliers who laughed at the owner when he asked for triple glazed windows to be priced), but Allen explained that it wasn’t just energy control, it was comfort. Triple glazing ensures that the interior surface of the glass stays within a comfortable temperature range – one doesn’t feel cold radiating out and therefore feels more comfortable.
Allen’s creative powers were challenged but he came up with several good alternative packages for balancing insulation value, cost, and constructablity. We are looking at combinations of spray foam and densely packed cellulose insulation within framing spaces, extra layers of polystyrene foam on the outside of the shell, and taped plywood sheathing for air control. We are now awaiting pricing of these options before completing our construction drawings. The owner is prepared for a certain amount of sticker shock (these improvements may not pay for themselves in a strictly economic sense for a long time), so we may end up with a very good but not certifiable Passive House design.
Buildings contribute about 30% of all greenhouse gases emitted. On the other hand, Passive Houses are exponentially friendlier to the environment because of their minimal energy & fossil fuel consumption.
Posted by: sustainable architects | October 22, 2011 at 10:41 PM