Cherrywood Home Earning Net Zero Energy Certification

Written by Sunshine Mathon

Stop by our Open House and see the first Living Building Challenge’s Net Zero Energy certified building in Texas on Saturday, May 3, 9-11 a.m., 1706 E. 40th St.

Cherrywood Net Zero ENERGY House. Photo credit: Sunshine Mathon.

Cherrywood Net Zero Energy House. Photo credit: Sunshine Mathon. See the full collection of photos on Flickr.

When I learned we were going to have our second child, in addition to being simultaneously excited and daunted, my thoughts turned to the idea of a new house. Our old house was wonderful and full of character, but the size and layout were going to be a challenge with two kiddos and it didn’t lend itself to graceful expansion.

By the time we made an offer on this house I had explored almost every possible avenue of development – an existing house that needed no work, a scrape and re-build, new construction on an infill lot, and every other iteration. That we finally decided to rehab a small existing house and build an addition was as much as accident of circumstance as anything else. We were expecting a child soon and wanted to have a solution finalized. Given the context, the success of our search still surprises me.

Beyond the tale of net zero energy, we landed in a diverse, established neighborhood with a deeply rooted character, a large City park and an unusually functional and balanced elementary school within a ¼-mile walk, a beautiful urban creek (a certified Wildlife Habitat, in fact) that coursed through the neighborhood on the other side of our street, and a 2-1/2 mile bike ride from the heart of downtown.

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Set on a slope, six feet above the street frontage, the original house was built in 1948, one of many in the neighborhood built during the post-WWII housing boom. Most of the houses in the area, both duplexes and single family, were built with the durable, somewhat innovative, and readily available material of the time, concrete block. With an unassuming white stucco exterior, lathe and plaster interior wall finishes, and black steel casement windows carefully located throughout the well-proportioned form, the house modestly hinted at the mid-century modern design ethos that was emerging at the time.

At just over a thousand square feet, the original central portion of the house transitioned from a small entry living area to a connected dining room at the back. After entering the house, just off the living area, an immediate right lead to two bedrooms, becoming the eastern face of the house, with closets and a single shared bathroom separating the bedrooms. Further into the house, the kitchen turned to the west off the dining room, pinned between the back yard and a single car garage that was well-integrated into the building’s form.

The entry to the house, an 8-ft by 14-ft extension of the slab foundation, was held between the forward-shifted footprints of the bedrooms and garage. The roofline carefully covered the slab with clear understanding of its southwest orientation. Whomever designed the original house thought quite carefully about the depth of roof overhangs and entry shading. The sun merely trims the edge of the porch slab during the searing summer months, yet during the winter, the sun is welcomed, carrying deeply into the interior of the house during the afternoon hours.

Nonetheless, for all the character and thought brought to the simple design, from an energy perspective, the house performed quite poorly. The concrete block envelope had no insulation. The original single-pane, steel-framed windows did not close properly and provided near zero thermal performance. The insulation on the attic floor was 3”-4” thick at best. Though the attic was vented reasonably well, the central HVAC air handler and ductwork were located in a space that likely reached 160º+ in July and August. The AC system and gas heat were a decade old, and though in acceptable condition, were far from efficient. The gas hot water heater was 30+ years old and on its last legs.

All told, from an energy consumption perspective, the house had only three redeeming facets to its character, all of which are actually critical to the current success of the recently amended house, the well-chosen solar orientation, the well-designed overhangs, and the thermal mass of the concrete block.

When we purchased the house in early 2012, a previous owner had brought some care to what I am told was a deteriorating structure. The roof had been replaced, though with basic black composite shingles, the wiring and electrical service had been largely replaced, central AC and gas heat were added, the living room and dining room interiors had been sheathed in sheetrock and wired for low-voltage MR16 lighting in the ceilings. Further, a local architect had been hired to re-design the feel of the entry and the living room. Burnished steel plate was wrapped around the central fireplace with layers of steel shelving winging to the walls on either side. In addition, a hand-welded steel door, with double-paned, fritted glass infilling the sparse frame, newly marked the entry. On clear winter afternoons, the sunlit glazing casts a powerful glow throughout the front of the house.


As a designer, I tend to cast myself to the inside initially, considering the character of and relationships between interior spaces. I then work my way out, evaluate, and iterate my way back inwards. Throughout this process, given my pragmatic predisposition to strive to achieve more with less, I continuously catechize the design through the lenses of space, cost, material, energy, and water efficiencies.

Given that I was designing a house for myself, my wife and our (soon to be) two children, the basic spatial goals for the addition were clear – a master bedroom with master bathroom and a porous, flexible space that could act, at different times, as home office, guest room and children’s playroom. As such, I originally hoped to limit the addition to 300-400 square feet.

Three aspects of the site and existing house ultimately expanded the size of the addition. First, with 11 feet of grade change over the 115-foot length of the site from front to back, the topography constrained opening up the house very far to the rear. Second, as much as I tried to re-work the existing living room to meet our family’s needs, I couldn’t make it work. So I chose to add a second living room. And third, the first two factors culminated in a loft as the flexible space that then introduced stairs and a double-height space to maintain the desired porosity. While I do not regret the evolution of the addition, what might have been 400 square feet turned into almost 1,100 square feet.


By the time the space plan for the addition was settling to its final form, I had wrestled extensively with how far I was going to intervene in the envelope and systems of the original house. I had achieved clarity on a few critical points.

WINDOWS: The windows needed replacing. I was working with a concrete structure, however, so I chose not to modify the openings. I had also decided that the new guise of the house would reflect the original character. As I specified the new fiberglass windows, I maintained the divided-lite ratios in similar casement forms. I was not striving for historical accuracy, but alignment with the original design intent.

Specifically, I chose Pella double-paned, fiberglass-framed windows. With a U-value of 0.30, from a thermal performance perspective, the windows were sufficient, but nothing extraordinary. More critical, however, in our hot, humid climate, is the Solar Heat Gain Coefficient (SHGC) to control solar insolation. As a higher SHGC commonly translates to a lower visible light transmittance, I chose to specify different low-E coatings depending on shading and orientation of the window. An inclusive solar study of the house revealed which windows would be subject to significant solar exposure during hot, sunny periods of the year. These windows were specified with a 0.18 SHGC low-E coating; all others received a 0.24 SHGC coating.

The choice of the fiberglass frame was intentional. Fiberglass frames have similar excellent thermal resistance and rot-resistance properties as vinyl, similar rigidity and strength as aluminum, and a far better thermal expansion relationship to the glass of the window than any other framing material. As windows heat and cool, they expand and contract. When the expansion rates are dissimilar, such as between glass and vinyl or wood or aluminum, the seal between divergent materials eventually becomes compromised, affecting performance. Fiberglass expands and contracts at a very similar rate to the glass it frames, mitigating this issue.

ATTIC: As the attic was the only practical location for the air handler and ensuing ductwork, and the cost of a ductless minisplit was prohibitive, I sealed and insulated the attic with 8 inches of open cell spray foam at the roof plane (~R-28). Although R-38 is the rule of thumb goal for attics in our climate zone, it assumes attic on the insulation floor and no ductwork in the attic.

NATURAL GAS: To position our house to achieve net zero energy, I disconnected the gas line. Two of the previous gas uses were straightforward to switch. The gas cooktop changed to an induction cooktop; and an air-source heat pump version of the air conditioning system replaced the gas space heating.

The hot water, however, posed a puzzle that I still continue to engage periodically. To be honest, I had immediately assumed solar hot water was the clear answer. Professionally, I had overseen the installation of a great number of solar hot water heaters in Austin and had an established, trustworthy relationship with a local installer. However, part way through construction, I read an article by Martin Holladay that caused me to reconsider. His basic argument was that the combination of inexpensive solar electric and high efficiency heat pump water heaters challenged the decades-old assumption that solar hot water represented a more effective cost-payback proposition.

However, Martin did caveat his assertion with a few considerations. One of the most pertinent to our household related to how much hot water gets utilized in the household. The less hot water used, the less effective efficiency heat pump hot water heaters exhibit as heated hot water sits for extended periods and has to be reheated. Knowing that I was installing 1.0 gallon per minute showerheads and very efficient aerators on all the faucets, even though we are a four-person household, I suspected we’d use less than most households.

Ultimately, I chose to move forward with the solar hot water install, but the decision largely hinged on being far enough down the construction path to make alteration challenging. The original gas water heater was located in the garage. Maintaining this location would have made the plumbing runs to the new master bathroom, including laundry, quite long. So I had designed a new hot water closet close to the center of the house with a clean vertical run to the solar hot water panels on the roof of the second story loft, thereby improving the efficiency of that system. Providing sufficient makeup air to a heat pump water heater in this location would have been challenging at best.

For sake of comparison, I have two hot water panels on the roof, taking up the equivalent square footage as ~1.25kW of solar electric. Given the performance of the rest of my solar electric array, these panels would have produced ~1,725 kWh annually. Using estimates from both and GE’s heat pump websites as references, a heat pump water would have consumed 1,800-2,200 kWh in the same year. Given that my solar hot water array utilized 187 kWh for circulation pumping and backup electric resistance heating on streaks of gray days, the scenarios are roughly equivalent from a simple performance perspective. Surprisingly, the comparison is also very close from a cost perspective. After local rebates and federal tax credits were accounted for, the solar hot water solution was marginally more cost effective.

The installed solar hot water drain-back system, with an 80-gallon storage tank, typically gets our household through three days of gray weather. In Austin, with an average of 300 days of sun a year, the electric resistance backup element energized only a handful of days last year.

CONCRETE: The concrete block envelope of the original house had zero insulation. If there is one phrase that echoes in my mind from my classes with Paco Arumi, a theoretical physicist turned architecture/building science professor at UT Austin, it is this, “Always place your thermal mass inside the thermal envelope.” We applied his computational models to run scenario after scenario and the results were always striking.

For many people familiar with the concept of thermal mass in buildings, the idea is often conflated with ‘70’s “passive solar” architecture using southern-facing windows and Trombe walls, or equivalent, to capture and hold heat from day to night, or with vernacular architecture from arid regions that use stone or earthen mass to mitigate significant diurnal temperature swings. In both cases, though the ideas are sound, of course, they do not speak to the hot, humid climate and condition of central Texas.

Every wall in the existing house was built with concrete block, interior walls included. My goal was to pull all that mass inside a newly applied thermal envelope. With the contractor I had hired to manage construction, I explored numerous methods for mounting, properly draining and flashing a significant amount of insulation outside the concrete structure. I considered ZIP R-panels, which, incidentally, were used on the stick frame construction for the addition on the house, but that was going to provide only R-6.6. I considered two layers of polyiso rigid insulation, but the attachment methodology and labor costs were both challenging. My research lead to Quad-Lock’s R-ETRO, a system specifically designed to mount pre-cut, interlocking EPS foam blocks to the exterior of a concrete structure. I chose to install the R-18 4-¼” foam blocks. This choice proved extremely impactful on the performance of the house as a whole, I believe.

I make the claim of importance for the thermal mass in part because the envelope of the 1,100 square-foot addition is satisfactory, but not extraordinary. The addition was framed with 2”x4” studs using advanced framing techniques to minimize wood and maximize insulation. The stud cavities were filled with open cell spray foam and sheathed with ZIP R-panels, impregnated OSB sheathing that functions as an integral weather barrier with an additional sheet of polyiso insulation laminated to the interior side of the sheathing, thereby breaking the thermal bridge of the wood studs. The result is a wall with a stud cavity R-value of 12.6 plus a continuous insulation R-value of 3.3. The insulated roof, the vaulted ceilings of the addition, follows the same methodology, just more of it. The ceiling joist cavity has an R-value of 26.1 with a continuous insulation R-value of 6.6. Satisfactory, but not extraordinary.

Nevertheless, both the raw energy data and the subjective experience of living in the house convey an environment that wholly exceeds my performance expectations.


When I aligned the orientation of the new master bedroom with the existing bedrooms, I did so with two clear rationales. I spent most of my summers growing up sleeping outside in the woods of Vermont, waking up to the sun’s glow through leaves and canvas. Almost without thought, given the natural orientation of the existing bedrooms, I simply extended the eastern bedroom block. This choice also provided effective spatial consistency to zone the heating and cooling system.

COOLING: Properly sizing an air conditioning system in Austin’s hot, humid climate is critical. Moisture management is the goal. Some estimates I’ve seen suggest that roughly half the energy consumption from air conditioning in our climate just addresses the latent load – pulling the moisture from the air. This issue is even more critical in buildings with high performance envelopes as the sensible load, the simple heat in the air, is more passively managed by an effective envelope.

In our climate, the most common malady in air conditioning system design is an oversized system. When a system is too large, the air is cooled very quickly – too quickly – and moisture lingers, resulting in an uncomfortable cool, sticky condition. Occupants then typically turn the temperature down further to compensate and efficiency suffers, as does the equipment from over-cycling and increased wear and tear.

Professionally, I have had experience with variable speed heat pump systems, minisplits and the like. The enormous potential variable speed systems have in our climate is that they adjust their effective sizing, and the amount of energy consumed for both latent and sensible load control, as conditions evolve. The challenge in variable speed systems lies in the first costs.

I contemplated a few variable speed solutions, from a ducted whole house air handler, with no zoning, to multiple non-ducted air handlers with independent zoning control. The cost, unfortunately, was prohibitive. In the end, I specified a 19-SEER Carrier Infinity heat pump system with similar, yet limited, properties to variable speed systems. Though the outdoor unit is technically 3-ton system, it is capable of stepping down to a 2-ton capacity when the loads are small. Anecdotally, I’ve found the system runs in 2-ton mode most of the time.

The indoor air handler is also variable speed and is ducted to the three zones throughout the house, each with an independent programmable thermostat control. On summer nights, as one zone, the temperature and humidity in the bedrooms is controlled to a comfortable sleeping level, typically 80º and 52% (or less) relative humidity. The other zones, the daytime living spaces and the loft, are left unconditioned.

During the day, when we’re home, the conditions are reversed. The bedrooms stay unconditioned and the daytime living spaces are managed, usually set to 78º or 79º, depending on level of activity, with the same relative humidity level.

FANS: In all cases, however, ceiling fans play a critical role. Though somewhat anecdotal, I estimate that ceiling fans allow us to keep the room temperatures 2º-3º higher than would otherwise be comfortable.

Not all ceiling fans are made equal, however. Knowing we would have ceiling fans spinning through much of the year, I searched extensively for affordable fans with induction motors, which can often push 2-3 times as much air for the same energy compared to more basic Energy Star-rated fans.

VENTILATION: The final aspect of the mechanical system for a high-performing house regards ventilation. Given that I was replacing windows, providing an opportunity to flash and seal them properly, and was adding so much insulation to the exterior of the existing structure with careful attention to air sealing details, I suspected that the overall house would be relatively airtight, though not extraordinarily so. Such an envelope requires effective ventilation management to be sure that ample fresh air is provided, yet without a huge energy penalty to condition it.

Energy recovery ventilators (ERV) quasi-passively transfer the embedded energy in conditioned air to fresh outdoor air as the two air streams, one out and one in, are blown across either side, respectively, of a heat and moisture exchanger. The fresh air is effectively pre-treated as it enters the house. During the summer and winter, when windows are rarely opened, the ERV operates during the most energy-beneficial portions of the day or night, depending on the season. In all seasons, the ERV acts as the bathroom fan, via timer switches, to manage high moisture loads in those specific spaces.

ENGAGEMENT: Having spent much of my childhood in Vermont chopping firewood for the woodstove and, at times, hand pumping our water from a well, I gravitate towards engaging my home as a place that compels tend and care, providing comfort and shelter in return. I pay attention to the weather two to three days out, strategizing when to open the windows to cool or heat the thermal mass. I pay attention to the rain, planning when to do a load of laundry so that it can hang dry before the rain arrives. I pay attention to a forecast of two or more days of cloudy weather, planning the use of hot water carefully. Some might find these efforts burdensome. I find them cathartic and cyclically engaging, as by the season.


With slight modification at the contiguity of 1948 and 2012, the existing roof profile was simply stretched to cover the addition. This low laying roof is delightfully shaded by mature Pecans and Cedar Elms, rendering solar energy capture impractical. However, the design decision to loft the flexible office-guest room afforded a largely unhindered roof plane for solar.

The profile of the loft roof iterated numerous times as I attempted to achieve a balance between form and solar-bearing function. The result was a rare moment of compromise in the house. The architecturally ideal roof form, deduced from the existent roof lines, would have supported only 2.25 kW of solar electric. On the other hand, a fully oriented shed roof would have swelled solar electric potential to 7.75 kW.

As a designer, a fundament of my effort resides in realizing a deep affiliation between form, guise and utility. Nonetheless, there are always exceptions, always opportunities to break self-imposed rules. Architecturally, I eventually acquiesced to the final roof profile, a shifted pitched roof with gabled ends, in recognition that utility bore the greater respect in this design moment.

Though I didn’t know how much solar the house would need to achieve net zero energy, based on my experience in our climate, I had hopes it could be achieved with the 4.5 kW array that was ultimately installed.


The other exception to the rule of balance in the house is the front door that the previous owner had commissioned. Hand-crafted from hollow black steel and translucent glass, the entry to the house is not simply beautiful, it communicates a moment of singularity. The front door, through the care and craft of its design and hand-built nature, claims simultaneous expression and character. The decision to keep the door, though it’s thermal performance is very poor, was not difficult to make.

PECAN: In the 1940s, freshly cut boards of dense, variegated Pecan, striated with shades of rust and soil and limestone, were laid atop a fifty-year old, worn warehouse floor in Waco, TX. Decades later, smothered in layers of dull paint, the floor was saved during the demolition of the warehouse. The planks for planed and sanded and planed and sanded. The rust and soil and limestone are exposed as stairs and flooring throughout the addition to the house.

STAIRS: The stairs spatially frame and connect distinct regions of the common space of the house – living room, art space, children’s corner. The treads, six-quarter slabs of hundred-year old Longleaf Pine, rest atop a rising pattern of climbing Pecan, turning vertical, horizontal, vertical, reaching and growing upwards towards the loft.

STEEL: Thin plates of blackened steel adhere to an open wall, skewed to the left and then to the right and then back again. Separated by equidistant ranges of white paint, the horizontal plates create a wide, flat, seven-foot tall ladder on the wall. My photographs, mounted on varying magnitudes of black foam core, magnetized to the steel, span black, white, black.

BEAMS: Where once concrete block ended the rear of the house, cautiously open spans were broken through. At the meet of Pecan and Oak on the floor, woods of the same era, squared Cedar trunks bridge above, carrying the seam of old and new. Though the story is lost, the texture of saw teeth and sheer dimensions imply a stout and functional history.

I am almost always compelled by craft, compelled by the result of a careful creation of a thing, a somatic, sometimes speculative, articulation of an imaginable function.

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