Well, the energy model confirms it, we’re going PASSIVE! After months of design and plenty of feedback from the Passive House consultants at Hammer and Hand, we’ve finalize the design and submitted for permit. Construction is set to being this summer, with H&H also acting as the builder.
So what is the big deal about Passive House?
Passive house is today’s highest energy performance standard, saving up to 90% in heating energy costs. In order to meet the standard, a building (not just a house) must be exceptionally air tight and well insulated, with a high-efficiency heat recovery ventilator. These factors allow the house to gain the majority of its heat passively, from the sun and from internal gains such as people and appliances.
Our client’s directive on this project has been to realize the extraordinary energy savings of Passive House with no more than 10-15% additional investment over a conventional house. So far we think we can do it, and apply some valuable lessons to future projects.
To our surprise, achieving the passive house criteria wasn’t too different from what we’ve already been doing on our projects for the past several years. With some minor modifications to the way we normally do things, we were able to reach the passive house standard. The 3 categories below are the key components in achieving a high-performance building:
- Insulation (and eliminating thermal bridging)
Most of our projects use advanced framing techniques to save on lumber costs and reduce the problem of thermal bridging. In the case of the Madrona Passive House, we employed this strategy again but with an extra 4” of insulation on the outside of the walls and some extra insulation at the slab and roof. The added wall thickness, visible from the exterior only, will really be the only visual clue to the building’s high performance design.
An additional improvement to the usual insulation techniques has to do with the use of high-performance windows, and the way the windows are placed in the wall and insulated. The walls are extra thick and the windows will be placed roughly in the center of that thickness where they perform best (away from interior and exterior convection currents on the surfaces of the walls). Also, because we are using European style tilt-turn windows (they open in, not out like traditional American casement windows) we are able to bring the added exterior insulation into the window opening to cover the face of the window frame and eliminate a huge thermal bridge (see below).
- Heat recovery
We use HRV’s in many of our projects, but in the case of a passive house, this takes on added importance due to the required air-tightness (no leaks allowed). An air tight envelope and extra insulation, in combination with an HRV, allow the structure to retain and recycle all that passive heat gain. Fresh air is brought in by the HRV constantly at a low volume, and as it passes through the HRV it is tempered by the outgoing, heated air.
The focus on air-tightness has been the biggest difference between a passive house and a conventionally built house. A few measures we’re employing to achieve air-tightness:
Window installation: In most houses we allow for ¼” on all sides of every window opening, to allow just enough space to set the window in the rough opening. Such a tight space can be difficult to properly seal. In the case of Madrona Passive House, we’re allowing for a full ½” space all around, which will allow for easier installation of expanded foam insulation.
Liquid applied membrane: We usually use a building membrane that comes on a roll, such as felt, or in some cases today’s improved version of felt, Vaproshield or similar membranes. For the Madrona Passive House we’ll be using a liquid-applied building membrane, which is essentially painted onto the exterior of the plywood sheathing and into the window and door openings. This material eliminates the need for cutting and taping a membrane and in general makes for a tighter building.