Tiny House in My Backyard: Meeting emissions goals while reducing housing stress by THIMBY

Many thanks to both reviewers for their thoughtful comments and suggestions on our proposal. We have grouped the comments/suggestions into categories and have summarized our responses (and associated changes) below:

• "More detail on actual energy use in summer and winter compared with output of PV system would strengthen case. In many regions, winter PV output may be a third of summer output, when more energy is needed, so off-grid viability can be a challenge. However, if teamed with an EV (mentioned but not clearly incorporated into analysis), then EV could 'import' extra power in cloudy, cold weather and 'export' when excess generation"
• Summary of changes (Section 2): Details were added about anticipated electrical use (4-5 kWh/day) and generation (6.9 kWh/day in December, 11.9 kWh/day in June). Furthermore, we have included a plot of the state of charge of our battery based on modeled electricity generation and demand, demonstrating that our photovoltaic and battery system is sized well to allow the house to function off-grid throughout the year.

• "Your 3-4 year emission payback period is conservative: if the occupants would have built a conventional home, embodied emissions would be higher than your 18 tons, so your savings may be bigger than claimed."
• "It would be helpful if the authors could provide some reference data for a 'typical' Tiny House to more clearly establish how this project will improve emissions over what would otherwise be built. It is potentially misleading to compare the performance of this off-grid Tiny House to only a conventional 2-person household because a significant percentage of the benefits could be attributable to the choice of a Tiny House alone (reduced square footage), regardless of energy sources and water reuse. Thus, the proposal could be strengthened with additional discussion about the status quo for currently constructed Tiny Houses and how this demonstration project would amplify the benefits of going 'tiny'."
• Summary of Changes (Section 5): 

1. CCC estimate of natural gas + electricity emissions recalculated at city level rather than zipcode level, to be more representative of target users. Result is ~7% higher estimated CO2e for average 2-person household.

2. CCC does provide estimates of embodied emissions per square foot (using the Economic Input-Output Life Cycle Analysis, EIO-LCA approach on data from the U.S. Census American Housing Survey). These estimates are now included for “on-grid” houses of both standard (1750 sq ft.) and comparable (192 sq ft.) size in order to more clearly isolate the benefits achieved through space efficiency and those achieved through energy efficiency and local PV generation.

3. Two emissions payback periods are now estimated, such that the reader can understand the payback period of the house relative to new builds of conventional tiny houses and a previously built house (i.e. with 0 additional construction-based emissions). Those two payback periods are:

• “Relative” emissions payback, whereby the additional embodied energy associated with PV production, storage, and various efficiency technologies is offset due to the lack of grid electricity and natural gas use. This serves to compare the emissions benefits of this home relative to a conventional, “on-grid” tiny house build.

• “Total” emissions payback, whereby all of the embodied energy is offset due to the lack of grid electricity and natural gas use. This serves to compare the emissions benefits of this home to an existing “on-grid” build.

• "Experience shows that independent water and waste systems can face problems when people go away on holiday or have guests - bugs can die or be overloaded, system limits can be exceeded."

• Summary of changes (Section 2): We have included a paragraph detailing back up plans for the water system and living wall for when the users have guests or go out of town.

• "It could be useful to provide an update on the percentage of the $75,000 project cost has been raised to date."

• Summary of changes (Section 7): Total estimated project cost has been adjusted from $75,000 to $65,000 due to budget reevaluation. Information about funds raised to date has been added; our team has raised close to $42,000 and has received about $10,000 in materials donations, together covering 80% of our total estimated project cost including labor.

An addition to the responses listed above:

• "In many climates, a small, highly insulated home with limited thermal mass can require summer cooling - no mention of this, although this is when there is usually plenty of PV generation!"
• Summary of changes (Section 2): Several sentences were added justifying our use of ventilation, ceiling fan, and phase change materials within our roof assembly to adequately cool the house during summer in the East Bay. Additionally, we indicated that the sizing of our electrical system to meet winter heating needs (w/ lower PV production) creates excess electricity in the summer that could be used to power a standard window A/C unit should the house be relocated to a Climate Zone with hotter summers.