Image Credit: TK Exterior walls are insulated with 7 1/2 inches of expanded polystyrene insulation. Rafter bays are insulated with closed-cell polyurethane foam, sprayed in from the top. The new roof assembly includes a layer of sheathing below the I-joist rafters. CertainTeed says its AirRenew drywall will absorb formaldehyde in the air and convert it into an inert compound. A structural ridge supported at each end by a steel post holds up the new roof. Above the OSB sheathing are I-joists rafters insulated with closed-cell spray foam. Rafter bays below the sheathing are insulated with fiberglass batts. One challenge was finding craftspeople skilled enough to tackle the Victorian details inside the house.
In 2014, I learned about net-zero energy at the Minneapolis Home & Garden Show. Shortly thereafter, my wife and I purchased our retirement home and interviewed five sets of architects to compose the core of the design team. We settled on Marc Sloot of SALA Architects and Sean Morrissey of Morrissey Builders in St. Paul, both with considerable experience in sustainable design and construction. The result is our net-zero Victorian, showing that a standard city house on a standard city lot in chilly Minneapolis can be renovated to be net-zero in energy with no sacrifice in attractiveness, space, or comfort.
The design and permitting took more than a year, beginning in August 2014 through the end of 2015. We sought a variance in local zoning since the exterior walls on one side of the house were already over the side-yard setback from the property line, and then added 1 foot of exterior insulation, which moved the exterior wall surface even closer to the property line.
This kind of variance was unusual for the Minneapolis community economic planning and development board. We also requested approval for a 500-square-foot addition — just within the legal limits for the lot. After four months (late 2015-early 2016), approval was granted for both.
Construction took 15 months (twice as long as we anticipated), from September 2015 through December 2016, although we were able to move in by late October 2016. Delays were due to adjustments in what needed to be done, scheduling problems, difficulties of constructing the envelope in the middle of winter, the level of detail required, particularly for interior Victorian style woodwork, and an insufficient number of skilled craftsmen.
The exterior walls required multiple layers — OSB, EPS blocks, window frame boxes, plywood sheathing, furring strips and lap siding — all of which generated a more extended period of construction noise than is usual.
Four legs of the net-zero stool
Efficient insulation, effective air and moisture barriers, reduced energy consumption, and sources of renewable energy for both heating/cooling and electricity were the biggest drivers for the design and energy outcome of the home.
Typically, old houses are insulated by tearing out the interior plaster and putting in fiberglass batts, or punching holes in the walls and filling the cavities with cellulose. This house was insulated on the outside by attaching a 7 1/4-inch-thick layer of rigid insulation board (see Image #2 below). When added to the fiberglass insulation already in the stud cavities, we ended up with R-40 walls. The rigid foam used was expanded polystyrene (EPS), with 1,000 times less environmental impact than extruded polystyrene (XPS).
We also sprayed closed-cell spray foam between the rafters of a new roof (see Image #3 below). In tandem with fiberglass batt insulation below, the R-60 worth of foam gives the roof assembly a total R-value of 80. A new foam chemistry has a much lower global warming potential than earlier spray foam formulations.
A tight house also needs to keep heat in the basement from leaking out. Typically, the foundation wall is insulated on the inside — not very effective — or by digging a deep, wide trench on the outside, which makes a huge mess. Our renovation was one of the first in Minneapolis to use a relatively non-invasive method of insulating the foundation wall from the outside. Kinzler Construction Services removed a 4-inch-wide slice of dirt all the way down to the footings, then inserted a 1-inch foam sheet and 3 inches of sprayed closed-cell foam. The net insulating effect for the basement was R-30 while removing very little dirt.
The house has triple-glazed Andersen A-series windows throughout, further reducing heat loss.
Air and moisture tightness
Much heat and moisture is lost through leaky walls. We wrapped the house in a sticky membrane made by 3M (3015 air moisture barrier). This product is a vapor barrier. Twenty-five percent of the insulation is inside the 3M barrier, so moisture will evaporate back into the living space where humidity averages a comfortable 40-50%.
Seventy-five percent of the insulation is outside the 3M barrier but inside a Tyvek barrier. Air (and moisture) are allowed to circulate freely under the siding without entering the house. It’s not quite the recommended 60%-40% split, but we’ve had virtually no moisture on window interiors through two pretty harsh winters
Our house is almost five times tighter than the building code, measuring 0.63 ach50. It is ventilated by an ERV (energy-recovery ventilator), drawing exhaust air from the kitchen and bathrooms. When the range hood operates, makeup air is drawn from outside past the refrigerator coils for more efficiency.
Formaldehyde is poisonous gas given off by some types of particleboard and some forms of insulation. Formaldehyde is a threat to health, and our house is so tight that inadvertent leakage would do nothing to reduce the threat. To combat this, we installed CertainTeed’s AirRenew wallboard, which absorbs formaldehyde and renders it inert for a promised 10 years (see Image #4 below).
Reduced energy consumption
The house uses three heat pumps: a 3-ton geothermal (ground-source) heat pump, with a COP of 5.0; a heat-pump water heater; and a heat-pump ventless clothes dryer. We disconnected our natural gas line and made the house all-electric. For heating and cooling, four linked geothermal wells were drilled in the 30-by-40-foot backyard. The wells are 250 feet deep, in a diamond pattern 10 feet on a side.
One lament of owners of old homes is that they must run the faucet a long time to get truly hot water from their centrally located water heaters. Our hot water typically arrives within five seconds, saving water and energy. To accomplish this, our plumbers installed a recirculation loop of thickly insulated (R-4) piping inside the building envelope. A pump quickly moves hot water from the tank to the sinks. To save energy, the pump is activated by motion detectors — people entering the kitchen or bathrooms.
All lighting is LED, whether in traditional fixtures, cans, or in other locations.
Recycling heat is another way that we reduce energy consumption. In this house, heat is recycled in three ways. 1) The heat exhausted from bathrooms and the kitchen is used to warm incoming outside air through the ERV. 2) Excess heat from the geothermal heat exchanger pre-heats water in a pre-heat tank for the domestic water heater. 3) Our water heater is driven by a heat pump which sucks heat from basement air. It’s three times more efficient than an electric-resistance water heater.
Energy is also saved through a new dryer design. Conventional dryers waste energy by heating with an electric coil and then venting warm exhaust outside at the rate of 135 cubic feet per minute. Our Whirlpool dryer (WED99HEDW) generates warm air with a heat pump. Instead of exhausting the warm air to the outside, it condenses the moisture from wet laundry into liquid. The warm liquid water then flows down a drain. No heat is lost to the outside air.
Renewable energy source
The house has 54 photovoltaic modules (42 on the house and 12 on the garage, at 315 watts each) for a 17 kW capacity. This system was sized through modeling, which initially projected an annual consumption of 19,000 kWh. The actual 2016-2017 output was 17,000 kWh, more than enough to cover 12,000 kWh consumption in the first year.
The 42 PV modules on the house weigh 1,500 pounds. To support them, a new roof was built over the old crooked and weak roof. The new roof consists of I-joists and the space between them is filled with 10 inches of closed-cell spray foam. Including the 6-inch batts installed between the rafters of the old roof just below, the roof assembly achieves R-80. This spray foam employs a recently introduced blowing agent with a far lower greenhouse gas potential (GWP is 1) than other products.
Supporting all this new construction is a massive laminated beam running the length of the attic, which workers installed by hand (in six 300-pound pieces) rather than by crane. The beam rests upon steel posts which run down invisibly through the walls to thick footings in the basement (see Image #5 below).
The home’s operation requires little involvement on a day-to-day basis. The ERV refreshes the interior air 20 minutes per hour. No thermostat setback is needed at night, and humidity remains at comfortable levels (40%-50%).
An eGauge system was installed to allow the internet-based monitoring of 24 individual electrical circuits throughout the house, while the performance of the solar system is also monitored over the internet through SolarEdge software.
Our aim was to create a low-maintenance natural Minnesota environment. The yard is bordered by hardscape frame — Versalok blocks capped with New York bluestone (which also is used for the front walk and terracing). Minnesota perennials, mostly drought-tolerant, cover 2,500 square feet of the yard. A three-zone drip irrigation system is installed to carry these 800 plants through a dry summer.
One-fifth of the yard is planted in Kentucky bluegrass, with no installed irrigation. Managing precipitation is another priority. The house keeps runoff — even from the heaviest of rains — from reaching the street and storm drains. The water is channeled into FloWells, perforated plastic barrels buried underground. These hold the water until it percolates back into the ground.
We have the satisfaction of knowing that we have rescued, renewed, restored, and repurposed a perfectly viable Victorian treasure in a way that preserves its character and demonstrates that a neighborhood can be revitalized without being reduced to a brownfield.
The house has achieved — and surpassed — net zero. The energy-conserving measures described here have kept the electricity consumption to 12,000 kWh for the first year — far less than the 17,000 kWh we generated from solar panels during the same time frame. Our house is therefore “net positive,” producing more energy than it uses.
This extra energy is sold to our local utility, Xcel Energy, at an effective rate of approximately 20 cents per kW hour. In our first year, the system produced about $3,000 worth of electricity, yielding a return of about 7% on the initial investment of $40,000 (after the federal tax credit).
This post originally appeared at the Zero Energy Project and is republished here with the permission of the author. Stewart Herman also is the author of Decarbonizing Your Retirement as You ‘Age in Place’