What’s the Secret Recipe to get to Zero Net-Energy

A SECRET RECIPE TO GET TO NET-ZERO???

One of the reasons why many homebuilders shy away from building a zero net-energy home is that it sounds so complicated.  The various experts don’t help with their barrage of technical advice on building design, insulation strategies, computer modeling, inspections, auditing, testing, etc.  Furthermore, a discussion of incremental costs is frequently lacking. — Instead, homeowners just want someone to tell them in simple terms how to get to net-zero, and what it will cost. What’s the secret recipe? 

The best recipe that I’ve currently found is provided in the book The New Net Zero, by William Maclay and Maclay Architects.  Their recipe is based on experience, so I expect it works quite well in the Northern New England climate zone. 

Anyhow, the general overall strategy (in northern New England) to get to zero net-energy is summarized as follows:

• Use passive solar heating and cooling strategies as may be reasonably practical.
• Use day lighting strategies using natural lighting as much as possible.  This usually requires using an open floor plan.
• Insulate to very high R values.
• Reduce air leaks to very low levels.
• Significantly reduce electric loads for appliances, lighting, heating and hot water.
• Add a renewable energy system (usually solar).

Notes: 

• The above R values for the two recipes are not totally comparable as MACLAY’s figures represent the actual R value of the assembly (i.e. the roof or wall in its entirety, which includes both the insulation and the structural members). Whereas the HAYFIELD HOUSE’s R values represent the nominal R value of the insulating material only. The MACLAY’s R value metrics are a more accurate and stringent measure of the R value of a roof or wall and requires more insulation to reach the same R value when compared to the nominal R values shown for THE HAYFIELD HOUSE. 
• EUI is an estimate of how efficient the house is in its use of energy, with a lower number being better.  Without such a metric, one could ‘cheat’ by building a house that is an energy hog and merely reach zero net-energy by loading up on renewable power. For a more detailed explanation of EUI, see Performance Metrics.

A comparison of the above two recipes to reach net-zero shows that the recipes are different, with the MACLAY recipe requiring more insulation. This would seem to indicate that THE HAYFIELD HOUSE couldn’t reach zero net-energy (with an acceptable EUI). However, THE HAYFIELD HOUSE did reach these targets.  The likely explanation is that the MACLAY figures are probably for mid to upper New England, versus THE HAYFIELD HOUSE’s design metrics which are for a more southerly and maritime location on the North Shore of Massachusetts.

As one can see, there is no single secret recipe to reach net-zero.  The recipe changes depending on the geographic location of your project, your building site, your budget, and other factors.  In addition, although we refer to a house as being net-zero, it isn’t really the house that is net-zero. Instead, it is the combination of the house, the number of occupants, and how the occupants use the various heating, cooling, and other systems. For example, THE HAYFIELD HOUSE has only two occupants, with both the heating and cooling systems used very judiciously to conserve energy. The same house with more occupants would have a higher EUI. Conversely, if the house had the same number of occupants (2) but those occupants wanted to heat their house to a higher temperature, then the EUI would also be higher.

If you are contemplating building a net-zero house, you can browse the case studies of actual houses built and reported to the Net-Zero Energy Coalition.  These studies show the location of the house, the R values, the air infiltration rates and other relevant metrics, including the HERS rating.  This information can be used to gain a general sense of what building strategy produces what HERS rating by climate zone.   The Rocky Mountain Institute also has a useful report on “The Economics of Zero-Energy Homes”.

After having lived in THE HAYFIELD HOUSE for nearly five years, I tend to think the insulation values of the house are a little low and the air leakage rate a little high. I was surprised that a post-construction energy model that I did of the house indicated that the biggest energy loss was from air leakage (even though the air leakage number is just below 1 ACH@50, which is a respectable reading). — In hindsight, THE HAYFIELD HOUSE’s target EUI arguably should have been a little lower (i.e. more energy efficient). Larger families with children using THE HAYFIELD HOUSE might be a little disappointed in their resulting higher value of the EUI of the house.

If I were to re-do my house, I would make the following changes as I suspect these choices would have a favorable cost/benefit.

• Minimize the use of double-hung windows as my double-hung windows are leakier than I’d like. Casement style windows tend to be tighter sealing windows.
• Insulate the interior edge of my concrete slab to R20 instead of R10.
• Use higher solar heat gain coefficient windows on the southerly side of the house’s first floor. This would have allowed more passive solar heating during the winter. This extra heat should not be a problem in the summer due to the southerly porches shading my house.

If I were to re-do my house, I might make the following changes, but I’m less confident about the cost/benefit tradeoffs.