Accessory dwelling units have become one of the most practical solutions for multigenerational housing. Whether you are adding space for aging parents, adult children, or rental income, the construction approach requires careful planning. Season 47 of This Old House showcased two very different ADU projects: a full attached ADU built onto a 1970s ranch in Walpole and a compact attic conversion in Dorchester. Both projects offer valuable lessons. Before diving into structural decisions, it helps to review key building tips from podcast discussions that address the everyday details of framing, finishing, and weatherproofing.
Transforming the Exterior and Site for an Attached ADU
The Walpole ranch project began as a single-family home and needed to become a dual-unit property for homeowners who wanted an attached ADU for their parents. General contractor Kevin Smith showed Kevin O’Connor several site changes that made the property function as two residences while keeping a unified look. A new hidden driveway provided dedicated ADU access without disrupting the front yard. The team added a front porch, installed green siding, trimmed all windows in white, relocated the electrical service, buried the propane tank, and planned a large backyard deck. These changes demonstrate that saving original wood features during renovations requires balancing preservation with the infrastructure demands of a second unit.
Open Cell versus Closed Cell Spray Foam for Garage Walls
Insulation choices in ADU construction are rarely one-size-fits-all. Insulation professional Jesus Barroso demonstrated why he used both open-cell and closed-cell spray foam in different parts of the Walpole project. The corner walls of the unconditioned garage required particular attention because they experience greater temperature swings. For a broader look at construction sequencing, Dirt To Done Episode 3 covers foundation-to-finish decision making on similar projects.
Closed-cell spray foam provides a higher R-value per inch and acts as a vapor barrier, making it ideal for exterior cavities where moisture control is critical. Open-cell spray foam has a lower R-value but is more flexible and cost-effective for interior applications where vapor permeability is acceptable. The table below compares both types for ADU applications.
| Property | Open-Cell Spray Foam | Closed-Cell Spray Foam |
|---|---|---|
| R-Value Per Inch | 3.5 to 4.0 | 6.0 to 7.0 |
| Vapor Barrier | No (breathable) | Yes |
| Best Use | Interior walls, conditioned spaces | Exterior walls, rim joists, unconditioned spaces |
| Cost Per Board Foot | Lower | Higher |
| Sound Dampening | Good | Moderate |
| Expansion Rate | Low, gradual fill | High, can push framing |
Understanding when to use each type saves money and prevents moisture problems. For unconditioned spaces, closed-cell on exterior walls and open-cell on interior partitions offers the best balance.
Fire Rated Wall Assemblies and Shared HVAC Safety
When two living units share a structure, fire safety becomes code-critical. General contractor Jared Ruggieri showed the fire-rated wall assembly separating the main house from the ADU. This wall uses multiple layers of fire-rated gypsum board and mineral wool insulation to achieve the required fire-resistance rating. Mineral wool is the material of choice because it is non-combustible and withstands temperatures above 1,000 degrees Celsius without melting. Unlike fiberglass batts, mineral wool does not sag or settle over time, keeping the fire barrier intact for the life of the building.
The project also included a fire damper in the shared HVAC ductwork. When a single system serves both units, ducts must pass through fire-rated partitions. A fire damper is a spring-loaded shutter that stays open during normal operation but slams shut when a sensor detects heat from a fire, sealing the duct and preventing flames and smoke from traveling between units.
- Check local codes for the required fire-resistance rating between attached units, typically 1 or 2 hours
- Use mineral wool insulation in the fire-rated wall for better heat resistance than fiberglass
- Install listed fire dampers in any shared HVAC ductwork that penetrates the fire-rated separation
- Seal all penetrations with firestop sealant, not standard caulk
- Inspect the assembly before closing up the wall to avoid costly rework
Hydro-Air Heating and Whole-House Water Filtration
Mechanical systems can make or break ADU comfort and efficiency. In Walpole, Richard Trethewey and plumber Bob Connolly reviewed the hydro-air system installed for the ADU. This system combines hydronic heating with forced-air distribution. A gas boiler heats water that circulates through a heat exchanger in the air handler. A fan blows air across the exchanger and delivers warm air through standard ductwork. The same boiler supplies domestic hot water for both units, consolidating the load into one high-efficiency unit rather than duplicating equipment.
The team also installed a whole-house water filtration system in the basement. It treats all water entering both the main house and the ADU, removing sediment and chlorine before water reaches any tap. The system includes a sediment pre-filter followed by carbon filtration. For shared ADU water supplies, a single whole-house filter is more cost-effective than separate point-of-use filters in each unit. The filter must be sized based on peak simultaneous demand from both dwellings, which requires a larger tank and higher flow rate than a standard single-family unit.
Cold Joint Waterproofing Between Old and New Foundations
One of the most common failure points in attached ADU construction is the connection between the existing foundation and the new foundation. Sean Hayward demonstrated the technique used at the Walpole project to seal the cold joint where the ADU foundation met the original house foundation. The gap between old and new concrete creates a path for water infiltration unless properly sealed.
The method involved injecting polyurethane foam into the joint through ports drilled at regular intervals. A specialized injection gun forces the liquid polyurethane under pressure into the gap. The foam expands as it cures, filling every void and creating a watertight seal that accommodates minor differential movement between the two foundations. The process follows a reliable sequence:
- Clean the joint surface and remove loose debris
- Drill injection ports at a 45-degree angle intersecting the cold joint, spaced 6 to 12 inches apart
- Install injection nipples and seal the surface between ports with hydraulic cement
- Inject polyurethane foam starting from the lowest port and working upward
- Allow the foam to cure for 24 hours, then remove nipples and patch the holes
This injection method is more reliable than surface-applied sealants because the foam penetrates deep into the joint. It also self-heals if minor cracking occurs later, as the flexible foam expands to re-seal the gap. For basements finished as living space, this step is essential before any interior wall framing or insulation goes in.
The Dorchester Attic: A Compact Attic ADU Transformation
The second ADU project took place in Dorchester, where homeowners Cara and Paul wanted to turn their cluttered attic into a living space for their son Tom. General contractor Gareth Barr led the transformation, proving that an ADU does not require a full addition. An underutilized attic can become a complete dwelling unit with careful space planning.
The attic ADU included a private entrance, a code requirement for independent dwelling units. The interior featured a compact kitchen, a combined laundry area, and a bathroom with heated flooring. Heated floors are a smart choice for attic spaces that run colder due to roof exposure. Storage was integrated into the low-eave areas using custom cabinetry designed for the sloped roofline. The main living area felt bright and spacious thanks to strategic window placement and light-colored finishes. Mechanical systems were hidden behind accessible panels for a clean aesthetic with easy maintenance access.
- Verify ceiling height meets local minimums for habitable space before beginning design
- Install a separate entrance to satisfy ADU code requirements
- Use sloped eave zones for built-in storage rather than wasting that volume
- Choose heated flooring for attic bathrooms to offset colder subfloor temperatures
- Hide mechanical equipment behind accessible panels to keep the living area uncluttered
Both the Walpole attached ADU and the Dorchester attic conversion prove that accessory dwelling units can take many forms. Whether you are building a full addition or converting unused attic space, the same principles apply. Plan for separate access, invest in proper insulation and fire separation, design efficient mechanical systems, and seal every possible water entry point. These real-world projects offer a reliable blueprint for anyone ready to take on an ADU of their own.