What Energy Code Does Your ADU Have to Meet in Massachusetts?

By Buz Artiano, BuildX | May 2026

Watch: The Good, The Bad & The Ugly Truth About Energy Ratings w/ Andrew Popielarski | BuildX Podcast #04

If you are planning to build an ADU in Massachusetts, you might have heard that the energy code changed. You might have been told that the ADU has to be completely electric, and that the cost to insulate it is twice what it was. Or that you have to have triple-glazed windows just to pass inspection. Because of this real confusion, it is costing families money before they even start to build. When the rules are confusing, budgets get bigger, projects get delayed, and homeowners lose confidence in the whole building process.

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need, we handle design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We deal with septic constraints, zoning issues, and the state's new ADU laws. We work directly with HERS raters, insulation contractors, and the Department of Energy Resources rules so that every ADU we build meets code and doesn't cost more than necessary.

This is the basic energy code article for those who want to build an ADU in the state of Massachusetts. It will tell you exactly what HERS score your ADU needs to have, what changed on February 14, 2025, and how those changes will affect your insulation, ventilation, and choices of energy. If you are trying to set a realistic budget, this should be where you start.

What HERS Score Does My ADU Need to Pass Inspection?

Massachusetts ADUs have to have a HERS score of 55 if all-electric, or HERS score of 52 if mixed fuel. The Massachusetts Department of Energy Resources (DOER) made this change on February 14, 2025, and created a new energy code category just for accessory dwelling units. Before this change, ADUs had to be built to the same standard as full-size new construction homes which was HERS 45 for all-electric and HERS 42 for mixed fuel.

The HERS Index (Home Energy Rating System) measures how energy-efficient a home is compared to a certain specific rule. The lower the number is, the more efficient the ADU. For example, a standard code-built home might have a score of around 100. The previous ADU requirement of HERS 42 to 45 meant you had to install a lot of insulation, very tight air sealing, and sometimes triple-glazed windows. The new ADU-required scores of 52 to 55 make the requirements more in line with what the previous codes required, so builders and homeowners have more affordable ways to meet them.

This change comes under the IECC 2021 with Massachusetts amendments. The code itself did not change but the HERS score range changed for ADUs only. You can review the full ADU rules at mass.gov.

What Changed on February 14, 2025, and Why Does It Matter for My Budget?

In July 2024, Massachusetts put into place the most recent version of the stretch energy code with state amendments. That code stated that all new construction be HERS 45 (all-electric) or HERS 42 (mixed fuel). The DOER then decided that making a 600 to 900-square-foot ADU come up to the same standard as a 3,000-square-foot home would cause unreasonable costs. A small ADU has a higher proportion of exterior envelope to the volume of the inside and that makes getting extremely low HERS scores more expensive per square foot.

The February 2025 change made a separate ADU category with HERS targets of 55 (all-electric) and 52 (mixed fuel). To put it simply, this means your insulation, window and air sealing requirements are not as stringent as a full-size home must have. That means you will have lower costs for materials and a shorter time for construction.

We saw the difference this made immediately on our ADUs that were being built. Before the rule change, getting a HERS 42 on a small ADU would often mean R37 walls using five inches of closed-cell spray foam, triple-glazed windows, and blower door results that were below 0.5 air changes per hour. Those regulations could add tens of thousands of dollars to the insulation and window budget by itself. With the new HERS 55 target, we could be in compliance with approximately 3 air changes per hour on the blower door test. This is doable if we use a skilled combination of insulation and standard double-pane windows that have the right glass specification. That is a large reduction in the cost that doesn't give up comfort or durability.

Do I Still Need an ERV Even Though the Code Relaxed for ADUs?

Yes. The ERV (Energy Recovery Ventilator) or HRV (Heat Recovery Ventilator) rules did not change. Even though the HERS score target was lowered, mechanical ventilation is still required for every ADU built under the current Massachusetts code. The adjustment of the code just changed the HERS score range for ADUs. It didn't take away any of the ventilation, air sealing, or equipment requirements.

An ERV and an HRV both trade out stale indoor air for fresh outdoor air and recover energy while doing it. The difference is in how they deal with moisture. An ERV keeps around 50 percent of the moisture in the air that's going out, which makes the humidity inside comfortable during the winters in New England winters without needing a separate humidifier. An HRV has a metal core that condenses and drains moisture, and that dries the air inside. With the climate in Massachusetts, if you are using a heat pump that doesn't have a humidifier, the ERV is a better choice. If your system has a humidifier, the HRV makes more sense because the humidifier will put back the moisture the HRV takes out.

For ADU-sized homes, we've been using through-wall ERV units that fit in a six-inch wall space and set high on the ceiling. They will give you the necessary ventilation and won't take up closet space or need their own ductwork. That keeps the open floor plan and the mechanical space small.

Can I Have a Gas Stove or Propane Appliances in My ADU?

Yes, with the current code you can. If your ADU has propane or gas for heating, cooking, or hot water, you will use the HERS 52 mixed-fuel target instead of HERS 55 that is for all-electric. The mixed-fuel choice will let you have gas stoves, propane-fired heating, and gas water heaters as long as the home is properly structured by a HERS rater and meets the 52 guideline.

One of the most common questions we hear from families planning an ADU is if it must be all electric. Many homeowners moving a parent into an ADU hear that everything must be all-electric and think that gas cooking is not an option. That was never entirely true, and the February 2025 code change made the mixed-fuel plan even more possible for ADUs.

There are limits. The stretch code limits gas and propane to heating, hot water, and cooking only. A propane fireplace is not included in this restriction, which means you can have one without it counting as mixed fuel. Backup generators are also excluded. If you want a gas stove but want to keep everything else electric, the energy model can accommodate that as long as the overall HERS score hits 52.

How Does the New HERS Target Affect My Insulation Budget?

The difference between HERS 42 and HERS 55 is partially affected by the type of insulation you use. Back when you had to get a HERS 42, a lot of builders were installing R37 walls using five inches of closed-cell spray foam in a two-by-six wall cavity. That's very expensive and way overkill. Some of them were even adding one inch of rigid exterior insulation and filling the cavity with R20 as an alternative, but either way, the insulation budget for a small ADU took up a much bigger chunk of the ADU budget than it should have.

At HERS 55, the insulation requirements aren't as ridiculous. A hybrid insulation approach, which uses two inches of closed-cell spray foam against the sheathing and fills the remaining cavity with open-cell foam, is what we feel is the best way to do insulation for ADUs. The closed cell gives you that vapor barrier and extra structure. The open cell expands to fill any gaps, which gets rid of the holes that cause moisture problems in pure closed-cell installations. The installer then trims the foam flush with the studs, so there's a nice clean surface for drywall. That combination gets you the results you need without the cost of a full five-inch closed-cell package.

Windows are the other major variable. ADUs tend to have a lot of windows for such a small structure because homeowners want natural light in a small space. Every window is a point at which we can lose heat or cooling. At HERS 42, having all those windows would push builders toward triple-glazed windows, which can add tens of thousands of dollars to a project. At HERS 55, double-pane windows with the right glass specification can be all you need. If you go with glass that lets in more of the sun's heat, it can help your energy score and keep you from having to upgrade to triple-pane windows, which gets expensive fast. We coordinate with the HERS rater before purchasing windows on every project, because the type of glass you choose will affect a lot of decisions that come after that.

Should I Talk to a HERS Rater Before I Start My ADU Project?

Yes, and the earlier the better. A HERS rater (Home Energy Rating System rater) is the professional who rates your home's energy performance and certifies that it meets code. If you get their opinion at the very beginning, it helps you decide which insulation strategy, window specification, and mechanical system will get you to compliance for the least amount of money.

The most expensive mistake we see is homeowners or builders who buy the windows before talking to the HERS rater. Once you are locked into a window type, it determines what insulation you'll need, how big your HVAC system has to be, and air sealing strategy. If your windows don't give you the right numbers for insulation and how much sun they let in, you'll have to compensate for that in other areas. That usually means spending more money on insulation or upgrading to expensive triple-pane windows to make it work. Always talk to the HERS rater before you decide which windows you buy, it can save you thousands of dollars.

At BuildX, we always bring in the HERS rater at the beginning, during the design phase. We do it in this order because it keeps insulation and window costs down while still making sure you get to that HERS 55 score.

Is the Relaxed ADU Energy Code Always a Good Thing?

The HERS 55 is definitely cheaper to get to, but that doesn't mean you should only build to those minimum standards. If you're building an ADU for a parent who's going to be living there year-round, or if the ADU will eventually be rented out, investing in better insulation and windows pays for itself in lower heating and cooling costs over time. The code is more relaxed now, but remember, it's just the bare minimum.

We also see situations where the lot constraints, foundation type, or mechanical layout make it a no-brainer to make sure the ADU is sealed extra tight, regardless of code minimums. A crawl space that has storage and mechanical equipment and everything else in it would benefit a lot from better insulation whether the code requires it or not. The code is just the minimum. The ADU should be designed for how the space will actually be used.

If you're really trying to save as much upfront as possible, that HERS 55 gives you a lot of potential savings. If your priority is saving money in the long term, building closer to HERS 45 with a hybrid insulation package and high-performance windows could still be the better investment. We help families work through that decision based on their specific property and goals.

What Should Your Next Step Be on Energy Code Compliance?

The February 2025 code change gave Massachusetts ADU projects a more affordable way to stay within the code. Your ADU needs to hit HERS 55 if all-electric or HERS 52 if mixed fuel, with mandatory ERV or HRV ventilation regardless of which one you pick. The insulation, window, and mechanical decisions you have to make to get that score will depend on your specific lot, your floor plan, and how the space will be used. The right time to figure that out is during the design phase before any materials are ordered.