Colorado Builders Learn to Speak German Standards
Oakwood Homes, LLC, has been building Energy Star — certified houses since 2002. Through construction of their homes, they have been committed to educating construction managers and trade partners about house-as-a-system building science. Year by year, they've gained greater confidence in the performance quality of what they have built in their houses through the testing and consulting provided by their energy-rating firm, Lightly Treading, Incorporated.
This past fall, the leadership team of Oakwood Homes asked itself three questions:
Are we doing enough to have our houses be homes that will withstand the test of time?
Can we build a cost-effective net zero energy house (nZEH)?
Could someone seven generations from now buy a house that we built in 2011 and have it meet their expectations?
These questions prompted a meeting between Oakwood Homes' leadership team, including Frank Walker, vice president of operations for Oakwood; Paul Kriescher, principal of Lightly Treading, Incorporated; and Francisco Reina, new-homes manager of Lightly Treading, Incorporated. It had been described as a design meeting to address ideas for building a nZEH, but it quickly moved into a discussion of the three broader questions listed above.
After hearing Oakwood's leadership team list their questions, I said, "The answer is Passive House." The Oakwood team said that they wanted to include my input on passive-solar design for their intended net zero home. "This is beyond passive solar — this is a Passive House!" I said.
As the conversation progressed over several meetings, Lightly Treading analyzed and computer modeled many different strategies and materials that could be used to achieve the home's goals for a budget of $115,000 - 300,000 — one that the average homeowner could afford.
Oakwood Tries to Build a Passive House
With Passive House in mind, Oakwood began with the walls. There were several options considered, including SIPs, Insulated Concrete Forms, and double-frame walls. In the end, a double wall with staggered studs was determined to be the most cost-effective/high- performance option. The 2 x 4 exterior wall is separated from the 2 x 6 interior wall by a 4-inch gap, and the interior wall carries the structural load. The gap is filled with fiberglass Blown-in Blankets (BIBs) to deliver a center-of-cavity R-value of 46. This was done not just to meet the letter of code law but to deliver a truly high-performance home by aligning the thermal barrier with a truly effective air barrier.
But the walls were only the beginning. The attic was constructed with 2 x 4 joists 24 inches on center. The spaces between the joists were filled with 15 inches of blown-in cellulose, to deliver a center-of-cavity R-value of 54. For the foundation, Oakwood installed 2 inches of extruded polystyrene (XPS) under the basement slab, for an R-value of 10; and 4 inches of XPS on the exterior side of the 9-inch concrete walls, for an R-value of 20. This served to keep the thermal mass of the concrete inside the envelope. Finally, Oakwood installed triple-pane windows with three layers of low-e coating throughout the house. Most of these windows have a U-value of 0.19 and a solar heat gain coefficient (SHGC) of 0.22.
During a predrywall blower test, the house tested at 1.86 ACH50 (or NACH of 0.17 winter / 0.10 summer, which were calculated in REMrate energy rating software following RESNET protocols to include building height [stack] and wind exposure).
Lightly Treading has had the opportunity to air-leakage test an all-cellulose house prior to drywall installation in the mountains and about half a dozen all-foam houses at predrywall; this house was tighter than the cellulose house and nearly as tight as the foamed houses. The final blower door resulted in 335 CFM, 0.69 ACH50 RESNET, 0.79 ACH50 Passive House Planning Package, and a NACH of 0.06 winter / 0.04 summer. Although the house falls just short of the Passive House standard of 0.6 ACH50, it is still more than 10 times tighter than the International Energy Conservation Code (IECC) 2009 requirement of 7.0 ACH50.
During design and modeling evaluation of the 3,339 ft2 house, and upon agreement the envelope was finalized, we turned our attention to what was inside the house, beginning with the mechanical system. A tiny ground source heat pump for forced-air heating and cooling was determined to be the most cost-effective option, and the easiest one for the homeowner to maintain.
With design and installation consultation from Al Wallace, president of the Energy and Environmental Corporation of Centennial, Colorado, the 1.5-ton Bosch system was installed. The system has a COP of 3.3. It was installed with 2,100-foot vertical loops within the over-dig area for the basement. This installation strategy can be used only when there is at least R-20 of insulation between the ground and the foundation walls, to avoid thermal coupling between the loop field and the house.
The ground source heat pump was at first intended to provide domestic water heating, but this option was quickly ruled out because the system was too small. Instead, a GE air-to-water heat pump water heater was installed. The decision to install this unit sealed the deal that the house could be an entirely electric house. Neither natural gas service nor utility company monthly metering fees would be burdening the homeowner. This decision also eliminated any possibility of combustion safety issues associated with natural gas or propane furnaces or water heaters.
All of the fixtures in the house are fitted with CFLs, and the house includes an Energy Star refrigerator and dishwasher.
In such an airtight house, mechanical ventilation was a must. Based on research done by John Cheney, project manager at Oakwood Homes, it was decided to install a Venmar AVS EKO 1.5 heat recovery ventilator (HRV). An HRV was considered preferable to an energy recovery ventilator because Colorado's high-plains desert climate results in consistently low humidity. This means that there is no need to worry about removing or capturing moisture from the outdoors as there is in so much of the rest of the United States. The HRV exhaust is pulled from the baths, kitchen, and laundry room.
After doing all of this applied building science, Oakwood Homes took the final step by buying a solar-lease arrangement for the future homeowners through Solar City. The 6.7 kW system comprises 32 Kyocera panels. Each panel is rated for 210 watts, and has a 95% efficient inverter, meaning that the panels convert about 20% of the potential energy in the sunlight striking them into electricity.
For more information on Oakwood Homes, LLC, visit www.oakwoodhomesco.com.
To learn more about Lightly Treading, Incorporated, go to www.lightlytreading.com.
Although the house falls just short of the Passive House requirements (air leakage less than 0.6 ACH50, annual heat requirement 4.75 kBtu per square foot or 15 kWh per square meter per year), and primary energy 38.1 kBtu per square foot or 120 kWh per square meter per year), it sets a precedent. It represents the first step on the path to Passive House performance for all future homes. Oakwood Homes and Lightly Treading are confident that they have built what may well be the standard house of the 22nd century. The house scores 8 on the HERS index when a score as high as 85 results in Energy Star compliance. But unless the occupants leave the lights, home entertainment equipment, and computers on all day; run their clothes dryer nearly continuously; and leave the windows open in the middle of winter, Lightly Treading is confident that the house will achieve net zero energy consumed versus generated by the PV system yearly. The house has just come available for purchase, and Oakwood's leadership team is looking forward to seeing how it performs. With actual performance data in hand, Oakwood plans to incorporate many or all of these features into all the houses in its new Belle Creek development in Henderson, Colorado. And "within five years," Frank Walker says, "Oakwood Homes wants all the houses it builds to meet this level of performance."
At press time, the occupants of the home had used their solar system for exactly one month. Over that time period, it had generated 977 kWh and the net meter has a credit of 221 kWh. The house thermostat was set to 70°F during the hottest August that the city of Denver has experienced since weather records have been kept.
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