NW Net Zero Residence
Why SIPS: SIPs were ideal for Net Zero energy projects, have a proven performance in Net Zero applications, and are ideal for timber frame/cathedral ceilings for projects like this.
The owners of this new residence requested a Northwest Lodge-style house of durable commercial-grade construction to bear up to the elements and stand the test of time. Significantly, their goal is net zero energy usage and net zero water usage, which means that all the energy and water the family uses inside the house will come only from the sunshine and rain that fall on its roof. The home rests on a gently sloping corner lot almost fully encircled by existing trees and foliage, providing privacy from the adjacent streets and neighbors. Save for the clearing required for driveway access at the southwest, this buffer was preserved. Placement of the home on the site required a delicate balance in order to remain inside the setbacks and existing tree drip line while being far enough to the north for optimal solar exposure. Digital modeling of these conditions
served as a valuable tool during design.
The house is oriented just 3.5 degrees west of due south. The form of the house is a main gable covering 8 timber frame bays with the double-height living room bay extending out beyond the main volume as a cross-gable portico. This simple structure is well suited to true timber frame construction as well as SIP panel walls and roof. The attached workshop and garage are advanced framed structures with truss roofs. The timber frame is crafted from Douglas fir, using modern power tools to fabricate traditional mortise
and tenon joints fixed with oak pegs. Each member was numbered at the factory, delivered to the site, and assembled into large bents as wide and tall as the house. Each bent was then raised by a crane and fastened in place by the crew. When complete, the timber frame formed the structural skeleton of the house. Its strength and beauty are exposed to the view of the finished interior. Enclosing the timber frame are Structural Insulated Panels, made from plywood and OSB skins and an EPS foam core. These too were fabricated at a factory and then raised on site using a crane. The SIPs are 8-1/4" thick and provide R-30 insulation at walls and roof. Their joints are splined, which eliminates most thermal bridges from the interior to the exterior, a source of significant heat loss in typical houses. SIPs are extremely airtight as well, and special attention was paid during construction to seal all gaps that might leak indoor air to the outside. The tightness was confirmed at 1.7 ACH50 using a blower door apparatus and a thermal camera.
Windows are made from Douglas fir with an extruded aluminum finish on the exterior. The house is capped by prefinished steel roofing which is both durable and specially formulated to reflect unwanted solar heat in summer, as well as providing the best surface from which to collect rainwater. The siding is cedar shingles recovered from stumps, relics of century-old logging. True stone accents the exterior of the living room and faces the fireplace and chimney.
Interior finishes feature cherry flooring, cherry cabinets, and Douglas fir trim, all certified by the FSC to be sustainably harvested. Most walls are finished with integrally colored Venetian plaster that never needs painting. Ceilings in principal rooms are FSC-certified Douglas fir. Flooring in the wet rooms is slate tile.
Targeting net zero energy required first reducing the demand to 10,000 kWh annually and then installing an 8.6 kW PV array to supply that demand. Energy Star appliances and equipment were selected throughout. LED and compact fluorescent lighting are used for all but three fixtures. Motion sensors also serve to minimize wasted watts. The house is served by a dedicated ventilation system filtered to surgery room (HEPA) levels. Heat and hot water are generated by heat pumps which are 3 times as efficient as
electric resistance devices. The house is wired with an energy monitoring system that continuously logs usage from two dozen sensors throughout the house, providing the homeowners with real-time feedback on how they are tracking towards their zero goal.
The rainwater harvesting system will meet the net zero water goal. In addition to irrigation, toilet flushing, and laundry uses, the rain will also provide water for bathing, cooking and cleaning, and drinking. A 5000-gallon buried tank will store winter rain to last through the dry summer. Rain falling on the metal roofing will pass through three progressively finer filters before being sterilized with UV light to render it potable for the 80,000 gallons the family will use annually.
- Achieved Net Zero energy status
- 10kWh annual usage with 8.6 KW PV array
- 80K gallon rainwater collection
- Air tightness at 1.7 ACH50 using a blower door/thermal
- LEED Platinum
- Energy Star Certified
- Built Green Certified
- Rainwater collection
- PV System
- All panels had plywood skins with custom splines