How Do I Meet Net-Zero Energy Requirements?

By Buz Artiano, BuildX | October 2025

Massachusetts energy codes have changed everything about how we build ADUs. The whole net-zero energy thing can be a bit intimidating. You have to know about HERS scores, blower door tests, and air changes per hour. When a lot of people hear the term "net-zero", they picture these crazy complicated systems that add $100,000 to their project. But it's not really like that. You just have to understand what the code actually requires versus what gives you the real energy independence you want.

We built the first net-zero house in Massachusetts. Our insulation partner, Jesus Barosso over at Advanced Spray Foam, did that over 20 years ago. Since then, we have done so many of these high-performance ADUs around Massachusetts that get HERS scores in the low 40s. The Massachusetts code only requires 55. The most recent project we did in Marshfield was measured at 0.7 air changes per hour, which is almost enough for a Passive House certification. And we've never failed a blower door test.

At BuildX, we believe energy efficiency is about a lot more than just checking boxes. It is about building ADUs with lower energy bills that stay at a comfortable temperature all year-round, and hold their value when the state energy standards get more rigid. Every ADU we build in Massachusetts goes beyond the code requirements. If all we are doing is meeting minimum standards, we are leaving money on the table for homeowners.

What Does Net-Zero Actually Require?

Net-zero means your ADU produces as much energy as it uses in a year. The Home Energy Rating System (HERS) uses a scale where lower numbers are actually better. A HERS score of 100 is about what you could expect to get from an older home, maybe from the 70's. A score of zero means the building produces all the energy it uses. Your ADU doesn't need to get to zero to be within the code, only to 55. But understanding the process helps you decide what score you want to go for.

Massachusetts requires different HERS scores depending on what you use for fuel. After the February 14, 2025 code change, ADUs became their own category where the scores don't have to be as high as in a single-family homes. Here is how the scores break down:

Solar panels don't count toward your HERS score. You have to get to the 55 or 52 by having a good building envelope and mechanical systems. Solar can get you closer to the actual net zero level though.

Why Does Air Sealing Matter More Than Insulation Alone?

A typical older house experiences four to five air changes per hour. What that means is that all the heated or cooled air inside escapes and gets replaced five times every hour, that is a lot of wasted energy. BuildX ADUs average around one air change per hour and we even had a couple ADUs hit 0.7 to 0.86 ACH50, which is almost Passive House level.

The blower door test is what you use to prove you built it correctly. Somebody will come out and pressurize your ADU and measure how quickly the air escapes. Massachusetts requires it for every ADU. We've never failed one because we always deal with any air leakage before the walls are closed up.

The most common spots that air can leak are:

• Sill plates where the frame meets the foundation (we spray foam these)
• Bottom plates where walls meet subfloor (we caulk every one)
• Electrical boxes and penetrations (we foam around every opening)
• Window and door openings (we tape and foam these carefully)
• Ceiling-to-attic transitions (we put on special tape before drywall)

Getting down to 0.5 ACH50, which some designs require if there are a lot of windows, usually means spray foaming everything plus adding Aeroseal technology. Aeroseal pressurizes the ADU and then it sprays a fog of this special sealant that finds and fills any gaps that are left.

What Insulation Strategy Achieves Net-Zero Performance?

Jesus Barosso recommends kind of a mixed approach for net-zero ADUs: two inches of closed-cell spray foam against the sheathing, then fill the remaining cavity with open-cell foam. That will give you R-30 level insulation in a 2x6 wall and give you vapor control and air sealing in one system.

The closed-cell layer does all kinds of stuff. It stops any air movement through the wall. It also works as a vapor barrier so you don't need plastic sheeting. And it gives you that high level R-value insulation quality without being super thick. The open-cell layer fills the rest of the empty space, it trims flush with the studs, and it adds to that R-value without adding a bunch to the cost with that 5' thick closed cell stuff.

Each part of your ADU's "envelope" contributes to the HERS score. Here's what a net-zero score requires for the average 900-square-foot ADU:

What Mistakes Prevent ADUs from Reaching Net-Zero?

The most dangerous mistake you can make is doing a double vapor barrier. If your contractor installs plastic sheeting inside the walls AND your exterior sheath is a vapor barrier, moisture gets trapped between them. Jesus has opened walls that were filled with mold from exactly this mistake.

Flash-and-batt (a thin layer of spray foam with fiberglass batts) can work but creates problems when done incorrectly. Every nail and fastener penetrating the foam layer becomes a problem. If the foam layer is too thin, condensation forms on the cold sheathing behind the fiberglass. Then the fiberglass holds the moisture against the wood causing mold and rot.

Other common failures include:

• Skipping blower door testing. If you don't measure it, you have no idea what needs to be fixed
• Not sealing top and bottom plates before insulation
• Leaving electrical punctures with no foam
• Installing windows without proper tape and foam at rough openings
• Assuming R-value alone determines performance, the air sealing is the main thing.

Is Net-Zero Worth the Investment for Every ADU?

But we don't always recommend building a net zero ADU. It makes the most sense to do it when you plan to hold the property for a very long time or when the person living there pays for their own utilities.

If you're building a short-term rental unit where utilities are included in the nightly rate, the energy savings go right back into your pocket. For long-term rentals with separate metering, the tenant gets the benefit, which usually means you can charge more rent. For aging-in-place ADUs, you need those lower utility costs when you are on a fixed income.

We don't recommend putting in the extra money if you plan to sell the property within three years or if you are trying to get the most out of a tight budget. Code-minimum construction with the HERS score of 55 still makes for a very efficient building. The extra $15,000 to $25,000 for net-zero performance usually takes 10 to 15 years to pay for itself through energy savings.

That being said, energy codes will tighten over time. Buzz believes that existing homes will eventually need HERS ratings when they go up for sale, with penalties for bad scores. If we build to exceed the code now, it covers you for future changes.

What Do Net-Zero ADUs Actually Cost to Operate?

Our ADU at 72 Jordan Road has had 3 people living in it for over a year, in 900 square feet. The ADU is all electric, all heat pump, and their average monthly electric bill runs $175 to $250, which covers heating, cooling, hot water, cooking, and all appliances. That's a lot better than the $300 to $400 a lot of people are paying for gas and electric combined in a similar-sized space.

The people living in these ADUs notice the difference immediately. One of our clients over in Marshfield said that he was standing inside during some very high winds and he felt nothing. No drafts, no cold spots, and no temperature swings. That sealed envelope works both ways, it keeps the temperature controlled air in and the outside air outside where it belongs.

Map Your Path to Net-Zero

Net-zero is not a mystery or some crazy high building standard. It only needs three things: a tight building envelope, good quality mechanical systems, and solar to offset the electricity you use. Massachusetts code only gets you partway there. If you want to go all the way to net-zero value, it costs $15,000 to $25,000 more but gives you much better comfort, lower energy bills, and better resale value.