What Kind of Insulation Do ADUs Need?

By Buz Artiano, BuildX | October 2025

Every homeowner that is planning on building an ADU will at some time ask the same question: "What kind of insulation should we use?" Maybe it sounds simple, but the answer can affect your energy bills, comfort, and your ability to pass Massachusetts energy code inspections for the future.

At BuildX, we have put installation into dozens of ADUs all across the state of Massachusetts, so we have seen what works and what will cause problems down the road. Our projects consistently get HERS ratings in the low 40s, which is well below the required 55 for ADUs that are all electric.

We use a well thought out combination of spray foam inoculation and blown-in insulation that combines energy performance with practical buildability. Here is exactly what makes up an ADU that is well-insulated and why each part of it matters.

What Insulation Does BuildX Use in ADUs?

We use a combination approach which means spray foam for the walls and crucial air-sealing areas, and blown-in insulation for all the attic spaces.

Here is how it normally breaks down:

• Exterior walls: We will use open-cell spray foam to fill every stud cavity. This will seal up all the gaps around electrical boxes, plumbing, and framing joints that batt insulation won't work for.
• Attic spaces: The entire attic floor or the roof deck will have blown-in fiberglass or cellulose insulation. For vented attic assemblies, we will insulate the floor of the attic and for unvented (hot roof) types of construction we will spray the roof deck directly.
• Foundation and sill areas: We will use spray foam insulation on the bottom sill plate and rim joist because that is where most air leakage occurs. We also caulk the bottom plate before the foam.
• Interior walls between conditioned and unconditioned spaces: If your ADU shares a wall with a garage or breezeway, we use fiberglass batts in those inside partitions.

This combination gives the homeowner the air tightness that is necessary to comply with codes while also keeping material costs reasonable.

Why Do We Prefer Spray Foam Over Fiberglass?

Spray foam will cost you more than fiberglass batts, but it will also work better on the metrics that are most important to comply with the energy code.

• Air sealing: Spray foam will fill every crack and crevice. On the other hand a fiberglass batt may stop heat transfer but will do nothing to stop the movement of air. One insulation contractor we work with put it this way: spray foam doubles the air tightness of the house compared to just fiberglass by itself.
• Performance on blower door tests: The Massachusetts energy code requires ADUs to meet certain air-tightness standards that are measured by blower door testing and spray foam insulation nearly always performs better on these tests.
• Moisture management: There are two types of spray foam insulation to choose from: open-cell that allows moisture to move through it, and closed-cell that acts as a vapor barrier. We select the one that's best based on the assembly location and the moisture conditions in the particular area.

The price difference between spray foam and fiberglass isn't as wide as it used to be. Fiberglass is produced by only three major manufacturers (Johns Manville, Owens Corning, and Knauf), so the cost stays high. In the meantime, spray foam contractors are becoming more common which has created competition in the pricing of spray foam. So for many projects, the cost difference is no longer as large as it once was.

What R-Value Does an ADU Need?

The minimum R-value depends on your target HERS score and whether you are building an ADU that is all-electric or one that is mixed fuel.

• Wall assemblies: Most ADU walls are framed with two-by-six lumber, which will give you a 5.5-inch cavity. If you fill this with open-cell spray foam it will provide about R-20. If you are working toward a target of net-zero you will need high-performance insulation so some builders will add continuous exterior insulation to reach R-30 or higher.
• Roof assemblies: Vented attic assemblies will usually need R-49 to R-60 of blown-in insulation but unvented (hot roof) assemblies with spray foam can achieve almost the same performance with less depth.
• Foundation and floor assemblies: Crawl space floors and basements need R-15 to R-20 depending on the assembly type and ground contact.

For our Massachusetts ADU projects, we design wall assemblies between R-20 and R-30 depending on what the homeowner's overall energy target is. This keeps us well below the HERS 55 threshold that is needed for all-electric construction.

What Are the Massachusetts Energy Code Requirements for ADU Insulation?

Massachusetts adopted an updated energy code in July 2024 that at first required all new construction to have HERS 45 for all electric or HERS 42 for those with mixed fuel. These standards were difficult for smaller building projects like ADUs.

So in February 2025, the Massachusetts Department of Energy Resources (DOER) made a separate category for ADUs with more achievable targets:

• All-electric ADUs: Must have HERS of 55 or better
• Mixed fuel ADUs: Must have a HERS of 52 or better

This change recognizes that small buildings like ADUs have unique challenges such as a high ratio of outside surface area to conditioned floor space and that makes energy goals more difficult to achieve. ADUs also normally have more windows per square foot than a large home does, which affects the HERS calculation.

Our BuildX ADUs almost always achieve HERS ratings in the 40 to 42 range, and that is well below either threshold. This gives our clients a comfortable margin in order to qualify for code compliance.

You can verify current ADU energy requirements on the Massachusetts DOER website: https://www.mass.gov/info-details/building-energy-code

What Is the Difference Between Open-Cell and Closed-Cell Spray Foam?

These two types of spray foam serve different purposes when constructing an ADU.

Open-cell spray foam uses water as its blowing agent. It expands to fill cavities, provides excellent air sealing, and has minimal off-gassing after installation. It is permeable to moisture vapor, which can be an advantage or disadvantage depending on the assembly. We use open-cell foam in most wall cavities because it is cost-effective and can be removed if repairs are needed later.

Closed-cell spray foam uses a petroleum-based blowing agent. It is denser, provides higher R-value per inch, and will act as a barrier to vapor. On the other hand, closed-cell foam blocks nearly 100 percent of radon transmissions, so it is the right choice for a space in areas that have concerns with radon. The trade-offs will be a higher cost and more off-gassing during application.

Massachusetts now requires waterborne spray foam to have a lower VOC content than the formulas that are approved for use in other states. This change happened about five years ago and makes spray foam safer for ADU construction.

Spray foam releases fumes when it is being applied so it's required that the work area must be vacated and those that are installing the foam have to wear respiratory protection. After 24 hours of curing, the foam is harmless and the building is safe for occupancy.

How Does Insulation Affect Air Sealing and Blower Door Tests?

The R-value of your insulation is only half the equation. Air sealing will decide whether your ADU passes the blower door test which is necessary to pass energy code compliance.

Blower door testing measures air changes per hour at 50 Pascals of pressure (ACH50). And this will tell you how leaky the building envelope is. If you use a spray foam insulation package that covers the sill plate, rim joists, wall cavities, and the roof assembly, you will be able to get approximately 1.0 ACH50, which is far better than the code requires.

Our normal air-sealing process involves five steps:

1. First, we spray foam the bottom sill plate at the place that it meets the foundation because this is where most air leakage starts.
2. Second, we caulk the bottom plate of the wall framing before we apply the spray foam.
3. Third, we spray open-cell foam into all outside wall cavities.
4. Fourth, we apply special tape along the ceiling perimeter before we install the drywall.
5. Fifth, we will add the blown-in insulation to the attic space.

This order of doing things addresses air leakage systematically from the bottom to the top.

When Might Fiberglass Be the Right Choice?

We mostly install spray foam because of its superior air-sealing performance, but fiberglass also has recognized applications.

Fiberglass batts work well for interior walls between conditioned spaces where air sealing is not really necessary. We use fiberglass between the ADU ceiling and a garage below, or inside walls between rooms within the ADU because these locations do not affect how tight the building envelope's air is.

Fiberglass also makes sense if you are working with a very tight budget and plan to add aggressive caulking and taping at all penetrations. The cost of labor for this approach usually offsets the savings for the materials, but it can work for certain projects.

If you are building in an area that has few radon concerns and excellent drainage of the site, you might also think about using fiberglass in floor assemblies over vented crawl spaces. Although we normally recommend using spray foam in these areas, the decision depends on your specific site conditions.

Ready to Plan Your ADU's Insulation Strategy?

The right approach to insulation will depend on your site conditions, energy goals, and your budget. For most Massachusetts ADUs, spray foam walls with blown-in attic insulation will give you the best combination of performance, durability, and cost.

If you are considering an ADU project and want to understand how your choice of insulation will affect the overall cost and energy performance, download our ADU planning checklist or schedule a site consultation with BuildX. We will walk through your specific situation and help you make informed decisions before construction begins.