Green Building Strategies for ADUs in California
Author: Napa Sonoma ADU Guest Blogger Hannah Bruegmann, Build It Green
This blog post is a light overview of green building strategies for accessory dwelling units in California. It should be helpful to you in thinking through design and building decisions for your ADU on the front-end of your project. Feel free to use it as a guide when you speak with your professional team about your hopes and dreams for your ADU!
Smaller buildings by their very nature use fewer resources, however they are not “green” or “efficient” by default. There are a number of “green” building choices you can make when designing and building a new accessory dwelling unit, or retrofitting an existing building. These range from more affordable to more expensive, and address everything from energy efficiency and water conservation to indoor environmental quality and overall comfort.
Many of these techniques are the same no matter what size home you are building, like orientation to the sun to take advantage of passive heating opportunities or making sure your ADU is fully air sealed to prevent air leaks. Other areas impact ADUs differently, because of their small footprint, like insulation and wall thickness. We will cover both at a high level in this overview.
You can break down how you think about building a “greener” (more sustainable, efficient and comfortable) ADU into three areas: where you put it, how you build it, and what you put into it.
Let’s look at some of the considerations for each of these areas:
WHERE YOU PUT THE ADU
Orientation to the sun: The cheapest and most efficient heating or cooling system is one that takes advantage of the sun and wind, or natural air flow. On sites where there is enough space to consider different orientations of your ADU, position the building so that it maximizes windows on the southern side to allow lots of sunlight to enter during cooler months and heat the interior. If you plan on installing solar photovoltaic (PV) panels to your roof, design it to maximize for easy installation — consider the angle of the roof and where things like skylights and vents go to leave plenty of space for panels. Learn more about building or retrofitting for passive heating here.
Solar shading: In tandem with building orientation, adding solar shading for south-facing windows can reduce glare and heat gain to a building from daylighting windows. This can be done by adding fixed 16-24” overhangs or dynamic window shades that move throughout the day. Overhangs increase a dwelling’s durability by protecting it from the elements and help reduce the amount of rain and sunlight striking the walls and windows. If you are converting an existing building or space, try to do this as much as possible given your existing site and orientation — add or move windows and window overhangs to make the most of what sun exposure you have available. For more on solar shading, see here.
Water run-off and land disturbance: In addition to energy use, consider if the site of your building will interrupt water run-off (creating moisture problems for your building, as well as impacting your local water table and landscape). Also try to avoid removing mature trees or other established plants providing shade, habitat for birds, butterflies, bees and other species, water filtration and preventing soil erosion.
HOW YOU BUILD THE ADU
Air tightness: Air leakage can account for up to a third of the heat loss from a home. This occurs primarily around the attic floor, doors, windows, ducts or any other openings into or out of the building. When these items are installed, make sure they are properly fit with seals, insulating weather barrier strips and caulking. To get a measurement of exactly how air tight your ADU is and where the air leakage is occuring, you can get a blower door test. If you are building a brand new ADU (not converting an older building), Title 24 energy code includes requirements for maximum air leakage.
Insulation: To keep conditioned (warm or cool) air inside to maintain a comfortable temperature in your ADU, insulation is critical — both the type you choose and how much of it you choose to use. In general, the thicker the insulation the better it will be at maintaining the temperature you want (insulation ability is measured by R-values). However, ADUs are smaller and the difference of a few inches matters. California’s 2019 Title 24 code requires new construction to have a minimum of R-19 insulation in walls with 2”x6” wall thickness. There are many types of insulation, ranging from non-renewable and non-recyclable products, like spray or rigid polyethylene foams and fiberglass, to renewable and recycled products, like recycled denim, cellulose (usually from recycled paper), hempcrete or even straw bale. When selecting insulation, consider both how much space you have (and how thick your walls can be) and the material composition (Is it recycled and/or from a renewable source? What is the R-Value? How durable is it?). To learn more about insulation types go here, and proper installation, go here.
Materials: The materials you choose to build or retrofit your ADU with also impact how green it is. Consider both materials that come from renewable, especially rapidly renewable, sources: think softwoods like pine, locally made tiles to reduce transportation energy, or reusing items like solid wood beams, flooring or fixtures. If you have the budget, consider double or even triple paned windows with wood frames, which better insulate and reduce air leakage. Also, try to minimize the use of materials that will off-gas chemicals into your ADU, creating unhealthy indoor environmental quality. Look for low or no-VOC paints, stains, carpets, counters, flooring, etc. There are lots of helpful lists of good materials to use, like our Green Product Directory, this one or this one.
WHAT YOU PUT INTO -- OR ON -- THE ADU
Efficient electric hot water heaters and heating ventilation and air conditioning (HVAC) systems: Water heating and air heating/conditioning are two of the largest uses of electricity in a home. About a third of all energy used in a home in California is to heat or cool air, so making an energy efficient and durable choice for these items can pay off both financially and sustainably. If you have space for a heat pump water heater or an air source heat pump to both heat and cool the air in your home, they can be three times more energy efficient than a traditional appliance. Ductless mini-split air heating and cooling systems are also a good choice for a small space like an ADU. They don’t require installing ducts so you minimize air leakage. When shopping for energy efficient appliances, look out for those marked with the ENERGY STAR label.
Go all-electric: There are countless reasons homes are going all-electric, and why you should consider making your ADU all-electric too. It costs more to add a gas line to a detached ADU, it is healthier for both the occupants (no carbon monoxide present from combustion appliances), and it moves us closer to reaching state and national climate action goals in reducing greenhouse gas emissions. Find more information on requirements, financial resources like rebates, and contractors who specialize in electrification at the Switch is On.
Efficient electric appliances: After HVAC and water heating, large appliances like refrigerators and clothes dryers use the remaining bulk of energy in a home. Buying the most efficient appliances (again shop for ENERGY STAR labels) that meet your needs and budget will reduce energy consumption, and your utility bills. Because some of these appliances cost more up front, many utility companies, towns and counties offer rebate incentives for ENERGY STAR products to offset the investment. If you can’t use a clothesline to dry your clothes, try an efficient heat pump dryer or even a combination condensing washer/dryer. Consider buying an electric induction cook stove. There are two-burner and other small models available that work especially well in ADUs. Additionally, many manufacturers offer compact versions of full-size appliances, perfect for ADUs. Potential appliance rebates can be found here.
Lighting: “Better” lighting includes going 100% LED, customizing lighting quality to apply to functional needs and reinforcing the architectural design through reduced fixture count, and limited recessed ceiling lights. Choose fixtures that are easy to repair and come with a higher Enervee score. For a deep dive on lighting design for very low energy-use homes, see this guide.
Furnishings and fixtures: The materials that are used to both build and furnish your home impact how green it is. When selecting products opt for durable fixtures - think metal over plastic and wood over laminate - that won’t need to be replaced and thrown away for a long time. Also consider whether these objects contain volatile organic compounds (VOCs) that are harmful chemicals that will off-gas into the home. Many furnishings contain fire retardants, adhesives, sealants, etc that release toxic chemicals into interior spaces. Opt for furnishings and finishes that declare they have no or low-VOC content for a healthier living space. Again, reusing, refurbishing, upcycling, and other techniques will help you save money, use fewer virgin resources, and increase the overall sustainability of your ADU. Many reuse stores and marketplaces exist. Online, try Ebay, Facebook Marketplace or Craigslist. In person, look for Habitat for Humanity ReStores, local antiques, thrift or reclamation stores.
Solar: Solar is a renewable, plentiful and sustainable power-source that can reduce your monthly utility payments significantly. If you are building a “new-construction” non-manufactured ADU (i.e. not an alteration or addition of an existing space or garage conversion) you generally need to install solar panels on it. There are some exceptions, but here’s what the State Housing and Community Development department says:
● Newly constructed ADUs are subject to the Energy Code requirement to provide solar panels if the unit(s) is a newly constructed, non-manufactured, detached ADU.
● Per the California Energy Commission (CEC), the panels can be installed on the ADU or on the primary dwelling unit.
● ADUs that are constructed within existing space, or as an addition to existing homes, including detached additions where an existing detached building is converted from non-residential to residential space, are not subject to the Energy Code requirement to provide solar panels.
Please refer to the CEC on this matter. For more information, see the CEC’s website www.energy.ca.gov. You may email your questions to: title24@energy.ca.gov, or contact the Energy Standards Hotline at 800- 772-3300. CEC memos can also be found on HCD’s website at https://www.hcd.ca.gov/policy-research/AccessoryDwellingUnits.shtml
And the CEC says, “The 2019 Energy Code PV system prescriptive requirements under Section 150.1(c)14 apply only to newly constructed detached accessory dwelling units (ADUs) for which the permit application was submitted on or after January 1, 2020. Smaller ADUs will require smaller PV systems, or may be exempt from the PV system requirements if they meet Exception 1 to Section 150.1(c)14:
• No PV is required if the effective annual solar access is restricted to less than 80 contiguous square feet by shading from existing permanent natural or man-made barriers external to the dwelling, including but not limited to trees, hills, and adjacent structures. The effective annual solar access shall be 70 percent or greater of the output of an unshaded PV array on an annual basis.
The PV system can be installed on the ADU, or on the main house. In the event that panels are added to an existing PV system to satisfy the PV system requirements for the ADU, those additional panels must be part of the ADU’s permit application. The PV system requirements do not apply to alterations or additions to existing homes, including detached additions where an existing detached building is converted from unconditioned to conditioned space (this is an addition per the Energy Code definitions).”
If you want to install panels, you can choose to either purchase them upfront or get a solar power purchase agreement (PPA). To read more about which choice might make sense for you, go here. Learn more about solar panel installation costs, options and contractors here.
To go even deeper on any of the areas above, there are many resources that outline how to build a Passive Solar or Zero Net Energy home you might explore. There’s also a very in depth guide on how to fully electrify a home retrofit from Redwood Energy here (not focused on ADUs specifically, but many compact or applicable concepts).
For more detailed information on how Title24 and CalGreen energy codes impact ADUs, see this webinar that the Tri-Counties Renewable Energy Network (3C-REN) hosted in March 2021 (coming soon).
You can also learn more about Title 24 and ADUs from another guest blog post, right here.