A deep energy retrofit is transforming a century-old home on a busy residential block in Johnson City, Tennessee. Built in 1910, this house had survived more than 100 years without any comprehensive overhaul save for minor electrical upgrades and surface-level remodeling. Remarkably, the structure remained in good condition, retaining original plaster walls, hardwood flooring, and the kind of craftsmanship rarely found in modern construction. Lost Cove LCA, led by President Jamie Constantopoulos, took on the challenge of converting this historic residence into a net zero energy home while preserving its character. The project aims to meet the EnerPHit standard, the Passive House Institute’s rigorous retrofit certification. For homeowners considering their own efficiency upgrades, understanding when to replace an existing assembly versus retrofit over it is a critical first decision, much like evaluating metal roof options over existing asphalt shingles when balancing cost and performance.
Planning a Two-Phase Retrofit for Historic Homes
From the outset, the owners of this 2,400-square-foot Johnson City home had clear ambitions. They wanted EnerPHit certification before they had even met Constantopoulos, which helped establish realistic expectations about budget and performance from day one. With that clarity, the team planned a two-phase approach to manage costs without compromising the long-term goal of net zero energy.
The first phase focuses entirely on the building enclosure. By making the walls, roof, and foundation as airtight and well-insulated as possible, the team can dramatically reduce heating and cooling loads. This allows them to install a much smaller mechanical system than the original house would have required. A modest 1,200-watt photovoltaic array offsets the energy used by this downsized HVAC equipment. Once the owners have the resources, phase two will add a larger solar array and further measures to bring the home down to net zero emissions. This staged strategy mirrors what many commercial buildings adopt, and an HVAC retrofit guide for upgrading commercial systems offers similar principles of right-sizing equipment after envelope improvements.
The EnerPHit Standard and Passive House Retrofits
EnerPHit is the Passive House Institute’s certification program specifically designed for existing buildings. Unlike the Passive House standard for new construction, EnerPHit recognizes the constraints that come with retrofitting older structures and offers adjusted performance targets that remain ambitious but achievable. For the Johnson City project, the team is targeting an airtightness level of 1.0 air changes per hour at 50 pascals (ACH50), along with rigorous insulation and thermal bridge-free construction requirements.
One of the most unusual challenges Constantopoulos encountered was the legacy of coal heating. Homes in this Tennessee neighborhood were originally coal-fired, leaving decades of accumulated soot lodged in every crack and crevice of the framing. Performing a standard blower door test, which places the house under negative pressure, would have sucked that soot into the living space. Constantopoulos learned this lesson the hard way on a previous project where an HVAC startup triggered a catastrophic soot discharge that led to an insurance claim. For the Johnson City retrofit, the team has kept the house under positive pressure throughout construction. Only if the data suggests they are hitting EnerPHit targets will they perform a negative pressure test and deal with the necessary cleanup. Understanding the full range of exterior insulation retrofit details and material choices is essential for anyone pursuing this level of performance in an existing structure.
Wall Assembly Design for Deep Energy Retrofits
The original wall assembly of the 1910 home contained zero insulation. The structure consisted of extra-thick 2×4 oak wall panels with original siding and a later layer of vinyl siding installed over it. Constantopoulos noted that the vinyl siding had trapped moisture against the original wood, accelerating rot. The team’s solution was to design a high-performance wall assembly that would bring the thermal resistance up to an average R-30 while managing moisture properly.
The retrofit wall assembly follows this sequence:
- Strip all existing siding down to the original sheathing
- Wrap the structure in INTELLO X, an intelligent air barrier and vapor retarder membrane
- Build a 2×8 Larsen truss system with OSB plates every 16 inches screwed into the existing wall
- Rest the truss system on a double-ledger board running the full perimeter
- Fill the truss cavity with high-density fiberglass batt insulation (chosen over mineral wool due to supply shortages)
- Wrap the assembly in SOLITEX MENTO 1000 and 3000 weather-resistive barrier
- Install new clapboard siding matching the original design profile
Fiberglass batt insulation was chosen not only for its thermal properties but also for its acoustic performance. Because the home sits on a busy street, the sound-dampening quality provides significant comfort benefits. The principles applied here share similarities with other envelope retrofits, such as adding a shed dormer retrofit to add space and light, where the junction between old and new assemblies must be carefully detailed to maintain continuity.
Overcoming Structural Challenges in Retrofit Construction
Every deep energy retrofit reveals surprises, and this Johnson City project had its share. The roof system proved to be one of the most challenging assemblies. Constantopoulos had hoped to perform a chainsaw retrofit, where protruding rafters are cut flush with the exterior wall plane to maintain a continuous air barrier between wall and roof. However, this was impossible because the overhangs were cantilevered: the attic floor system extended past the top plate, and the rafters rested on that extension rather than on the wall frame. The team had to wrap the INTELLO membrane all the way up under the soffit, creating a complex air barrier with many folds.
The existing roof was built with 2×4 rafters that could not support a thick enough layer of continuous insulation to reach the modeled target of R-60. The team explored extending the Larsen truss over the roof, but the structural engineer determined the existing system could not bear the additional weight, and the city also rejected the idea. The eventual solution was to use EPS foam boards with a deck on top, striking a balance between lightweight construction and improved R-value.
The windows presented another unexpected challenge. When the team peeled back the INTELLO wrap at each window opening, they discovered that none of the original openings contained headers. This meant they had to slice open the air barrier at every opening, install a header, and carefully tape everything back up. Luckily, the team consolidated many of the 32 openings into larger windows manufactured by Wythe Windows. Some spans were as wide as 13 feet, requiring structural engineering input to ensure the added roof load from the new assembly would not crack the original plaster below. The process of fitting new windows in an out-of-square old house demands this same attention to structural integrity and careful air sealing.
The following table summarizes the key challenges and solutions encountered during the retrofit:
| Challenge | Root Cause | Solution Implemented |
|---|---|---|
| Coal soot in framing | Historic coal heating left decades of soot in every crack | Positive pressure throughout construction; no blower door test until final verification |
| Cantilevered roof overhangs | Floor system extended past top plate, rafters rested on cantilever | INTELLO wrapped up under soffit; complex air barrier detailing |
| Insufficient roof insulation | 2×4 rafters could not support thick continuous insulation | Lightweight EPS foam boards with deck on top |
| Missing window headers | Original 1910 construction had no headers at openings | Sliced open air barrier, installed headers, re-taped each opening |
| Mineral wool shortage | Supply chain disruption during construction | Switched to high-density fiberglass batt insulation |
| Owners living on site | Family remained in home throughout renovation | Staged construction: upstairs first, then downstairs |
Mechanical Systems and Net Zero Energy Planning
Johnson City sits in climate zone 4A, where summers are humid and winters can bring occasional extreme cold snaps. The HVAC system was designed to operate efficiently down to 13 degrees Fahrenheit. Heating and cooling are provided by an outdoor Chiltrix heat pump that circulates water to four indoor fan coils and also supplies domestic hot water. Because the region experiences subzero temperatures every few years, an auxiliary electric heater was included as backup.
A key component of the electrical system is the SPAN Panel, a smart electrical panel that allows the homeowner to program which circuits receive power during an outage. This load-shedding capability integrates seamlessly with battery backup and solar management. Constantopoulos updated the entire electrical system to support this modern infrastructure, ensuring the home is resilient as well as efficient. The phased approach to renewable energy means the initial 1,200-watt solar array offsets the small HVAC system, while a future larger array will bring the home to true net zero. Commercial-scale projects follow similar logic when selecting equipment, and the VRF retrofit strategies used in commercial HVAC modernization demonstrate how right-sizing after envelope improvements yields better performance.
Constantopoulos offers several practical recommendations for teams undertaking similar work:
- Overestimate your budget, especially on retrofits. Unexpected conditions will arise and plans will need to change.
- Place dryers outside the building envelope or use a ventless heat pump dryer, since venting through an airtight assembly defeats the purpose of the air barrier.
- Cut tape and membranes longer than needed and fold carefully. Tight corners are where air leaks develop.
- Build physical mockups of tricky connections rather than describing them verbally. A visual aid on the jobsite is worth hours of explanation.
Lessons for the Next Generation of Retrofits
The Johnson City retrofit demonstrates that historic homes can achieve modern energy performance without sacrificing the character that makes them special. By preserving original plaster walls, flooring, and layout while wrapping the structure in a high-performance enclosure, the team proved that deep energy retrofits and historic preservation are not mutually exclusive. The two-phase financial model offers a realistic path for homeowners who cannot afford all the work at once but want to commit to a net zero trajectory.
Constantopoulos expects the house to last another 100 years. That longevity depends on getting the fundamentals right: a continuous air barrier, adequate insulation, appropriate mechanical systems, and careful detailing at every junction. The same principles that guide school construction toward exemplary performance, as seen in the Lady Bird Johnson Middle School net zero standard, apply equally to residential retrofits. When building enclosure improvements come first, mechanical systems shrink, energy bills drop, and the path to net zero becomes achievable one phase at a time.