Joshua Greisen thinks he’s found an ideal building lot in Yakima, Washington, a city in the south-central part of the state in Climate Zone 5B. Now, can he find a design for a zero-net-energy house to go with it?
Working with a limited budget, but on a south-facing lot ideal for passive solar gain, Greisen is looking for a cost-effective way of reaching his goal. “I’m by no means a rich man,” he writes in a Q&A post at GreenBuildingAdvisor, “and can only afford to do what has a return on investment that will be realized within a decade or so.”
The single-story home he’s planning has a footprint measuring 26 feet by 38 feet, about 990 square feet in all, with two bedrooms, a bathroom, laundry, kitchen, and great room. His plans currently call for 2×6 exterior walls insulated with fiberglass batts and a layer of continuous rigid foam insulation on the exterior — probably 2 inches of extruded polystyrene (XPS).
Other features include triple-pane windows with low-e glass, a heat-recovery ventilator, and an air-to-water heat pump with radiant-floor heating and an air handler to provide summer cooling. He’s aiming for a blower-door test result of 1.5 ach50 or better. He’s also planning on 8-foot-tall doors and 10-foot-high ceilings.
Greisen has a number of questions as he searches for the “sweet spot” for R-values in the ceiling, walls, and slab.
“Code [minimum] in Washington state is R-49 in the ceiling, R-21 in the walls, and R-10 in the slab and stem walls,” he writes. “I have read theR10/R20/R40/R60 rule, but that seems overkill, perhaps, given all the other factors in play that I have noted above.”
His research has uncovered a presentation on R-40 walls showing that 2×6 walls with an exterior layer of XPS…