Results of the first year at 8 zero net energy possible homes

The eight affordable homes that South Mountain Company developed, designed, and built at Eliakim's Way in West Tisbury for the Island Housing Trust have now been occupied for one year, and two of the eight achieved net zero energy. I've written a detailed report on the data and what we've learned. It's full of interesting key insights. The full report is downloadable on my home page at www.energysmiths.com.

Comments

[David Fay]

Great report just full of insights. But there's one figure I don't understand: Figure 11. It seems to contradict the graphs in the Monthly Data section. For example, it shows that House 8 (assuming bars are ordered by house number) used maybe 85 kWh of radiant panel electricity in August. Yet Figure 20 shows no usage of radiant panels by any house in June, July or August. What am I missing?

David

[Marc Rosenbaum]

Wow, you're a careful reader! That's a typo on my part - it's heat pump energy, not electric radiant panel. Thanks for letting me know!

[John Semmelhack]

Great report. It will be interesting to see if the two families are able to maintain net-zero energy use in future years...and if the other six families will incorporate your feedback to lower their energy usage.

[David Fay]

Sorry, but I'm still not following. If I'm reading the report correctly, Figure 11 shows the electricity usage of heat pumps and radiant panels by house and month, and Figures 19 and 20 show the same data, although separated into two graphs.

Yet the numbers don't seem to match up. For example, House 5 in January shows ~385 kWh for the heat pump and ~40 kWh for radiant panels in Figure 11 but ~280 kWh for the heat pump in Figure 19 and ~425 kWh for the radiant panels in Figure 20.

[Marc Rosenbaum]

Hello again David

Two separate things happening. The first was a typo caused by me labeling one column the wrong color. The second is more systematic and there is no way you can know about it. Figures 10 and 11 are in a different house order, because they divide by two bedroom and three bedroom rather than house number. So the graphs are not directly comparable. For our purposes, to show trends, this wasn't important. The first four bars in each month in Figure 11 are the 3 BR houses.

[Bob Lemaire]

I'm curious if any of the homes have clothes dryers. That would account for at least some of the plug load not appearing as internal gain. Also, with a net-zero competition, it would be advantageous to maybe do the weekly laundry at a laundromat. Looking at the radiant panel vs heat pump breakdown in two vs three bedroom homes, it seems like there is less overall heat provided by the heat pumps, and significantly more by the radiant panels. Would it make more sense to use a ducted mini split system, notwithstanding their decreased efficiency, so as to increase their utilization vs the radiant panels?

[Marc Rosenbaum]

I think they all have dryers. The 2 BR homes use lower amounts of radiant panel heat. A ducted system serving all rooms would eliminate the need for the radiant panels but would cost more than the system we chose. On a recent house, we could get four separate wall cassettes for less than a fully ducted single one system! Ductwork done well (well sealed and insulated) costs a fair bit.

[j chesnut]

Thank you for sharing in detail the results of a nice sample of net zero homes. I appreciate the factual presentation. Nice to read something without marketing pitches.
From the perspective of an operator trying to reduce energy usage do you see any advantage trying to limit the use of the HRV during periods when outdoor condition are amenable (i.e. turn it off during the swing seasons)?
I'm just starting to get a feel for energy units. A 30-35W draw isn't much but if constant doesn't it add up? I would have liked to have seen the ventilation consumption separated from the other plug loads to understand the HRVs yearly impact.

[Marc Rosenbaum]

Turning off the HRVs for the summer would save maybe 100 kWh/year. We chose the no controls strategy both for minimum cost, and to ensure that the ventilation was on in the cooler weather when windows are closed.

[Joel]

What prevents condensation on the inside of the sheathing in this building? Thanks for any insight.

[Marc Rosenbaum]

The sheathing, as in almost all frame walls that don't have exterior rigid insulation, is below the winter dew point of the interior air. Strategies to prevent condensation are mechanical ventilation to control indoor relative humidity, and an extremely airtight shell to prevent air migration into the wall. Nonetheless, double stud walls are thought to be less bombproof than walls with exterior insulation.

[wall foam insulation]

Transforming off the HRVs for the summer season would save maybe 100 kWh/year. We decided the no manages technique both for lowest cost, and to make sure that the air flow was on in the chilly climate when ms windows are shut.