Deep Energy Retrofits—Eleven California Case Studies

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This research documents and demonstrates viable approaches using existing materials, tools and technologies in owner-conducted deep energy retrofits (DERs). These retrofits are meant to reduce energy use by 70% or more, and include extensive upgrades to the building enclosure, heating, cooling and hot water equipment, and often incorporate appliance and lighting upgrades as well as the addition of renewable energy. In this report, 11 Northern California (IECC climate zone 3) DER case studies are described and analyzed in detail, including building diagnostic tests and end-use energy monitoring results. All projects recognized the need to improve the home and its systems approximately to current building code-levels, and then pursued deeper energy reductions through either enhanced technology/ building enclosure measures, or through occupant conservation efforts, both of which achieved impressive energy performance and reductions. The beyond-code incremental DER costs averaged $25,910 for the six homes where cost data were available. DERs were affordable when these incremental costs were financed as part of a remodel, averaging a $30 per month increase in the net-cost of home ownership.

Building enclosure performance was poorer than expected, though the average HERS (2006) score was 49. Air leakage was greater than 5 ACH50 in seven homes, and only five projects installed insulation beyond 2008 California Title 24 code minimum levels. Increased airtightness was the most obvious place for improvement in most homes. 50% energy reductions were proven possible in Northern California climates without superinsulation or extreme airtightness, but these measures allowed for greater variability in user behavior while still achieving deep energy savings. Some DERs used overly complex, custom engineered HVAC solutions, which did not perform as expected, and sometimes required replacement or major service. These features cost more, used more energy and resulted in comfort issues. DER should target current energy code requirements in new homes for envelope and equipment.

Indoor environmental quality in the DERs was mixed. None of the project homes were verified as meeting all requirements of ASHRAE Standard 62.2-2010, and only four out of eleven projects provided whole house continuous mechanical ventilation. While all homes installed kitchen and bathroom exhaust fans, failure to meet 62.2 airflow requirements occurred in 10 out of 20 bathroom fans and three of nine kitchen systems. Indoor temperatures were also extremely variable. Some homes maintained very consistent, comfortable temperatures, and others actively used cooler winter temperatures as a way to reduce energy use. A number of homes spent significant portions of the year above the recommended 60% relative humidity limit, though no specific moisture issues were observed. DER should comply with ASHRAE 62.2 requirements.

Average post-retrofit net-site energy, net-source energy and carbon dioxide equivalent emissions (CO2e) were 9,552 kWh, 18,453 kWh and 4,480 pounds, respectively. Average reductions relative to a typical CA single family home were 52%, 49% and 52%. Five DERs with preretrofit data achieved weather-normalized average reductions of 15,966 kWh (58%), 16,918 kWh (43%) and 6,423 pounds (54%). Homes with pre-retrofit net-site usage <15,000 kWh had average absolute reductions of 6,546 kWh, whereas those using >30,000 kWh pre-retrofit averaged a reduction of 22,246 kWh. High usage pre-retrofit homes were much more successful at achieving large absolute net-site reductions, despite having higher average post-retrofit usage (13,797 vs. 6,314 kWh). Net-site savings >60% did not guarantee satisfactory net-source performance in homes that switched from natural gas to electricity. Net-source energy increased 12% in one case and was only reduced by 7% in another, while net-site reductions were 31% and 61%, respectively. Furthermore, even without fuel switching, homes experienced negative changes in relative rank going from net-site to net-source energy, if net-electricity made up more than 45% of their total net-usage. DER should be assessed in terms of source energy and CO2e emissions, in addition to site energy, preferably on a regional basis. Per house or per person, not per square foot metrics should be used.

For homes where heating and hot water were disaggregated, usage averaged 2,088 kWh and 2,031 kWh, respectively. Average appliance usage (2,446 kWh) was greater than either disaggregated heating or hot water, and plug loads were just slightly lower (1,717 kWh). Lighting was on average 916 kWh. Combined HVAC-hot water averaged 6,444 kWh (54%), and combined plugs-lights-appliances averaged 4,856 kWh (46%). Combined HVAC-hot water exceeded combined plugs-lights-appliances only in those homes with either very low heating energy or exceptionally high appliance usage and low heating energy. Baseload electricity consumption averaged 203 Watts, for an estimated 1,778 kWh per year, or 22% of total average net-site consumption. Baseload was a clear opportunity for deeper reductions in nearly all homes.

Based on these results, the following basic approach for DERs is recommended:

  1. Bring building envelope to current IECC requirements for project climate zone.
  2. Tighten building envelope to reach <3 ACH50 if replacing interior or exterior cladding, or <5 ACH50 if not replacing cladding. If applicable, either insulate and air seal forced air ductwork, or bring it into conditioned space or eliminate it.
  3. Change all water fixtures to low-flow.
  4. Install simple, non-customized, high efficiency systems for heating, hot water and ventilation. Ventilation systems should comply with ASHRAE 62.2. Commission and verify performance.
  5. Replace all lighting with either CFL or LED.
  6. New appliances to Energy Star or better.
  7. Manage plug loads with power strips, whole house off switch, etc. Consider post-retrofit electricity audit.
  8. Install PV system aiming for zero-net electricity (optional).
  9. Provide feedback to occupants for whole house energy use.

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