How much does it cost to perform a deep-energy retrofit at a 100-year-old single-family home? Thanks to a recent study in Utica, New York, we now know the answer: about $100,000.
The research was sponsored by New York State Energy Research and Development Authority (NYSERDA), an agency that administers programs funded by public benefit charges tacked onto electric utility bills. The program paid for deep-energy retrofits at four wood-framed buildings in Utica, New York.
The project manager for the study was NYSERDA engineer Greg Pedrick. At the recent Better Buildings by Design conference in Burlington, Vermont, Pedrick gave a presentation, “Research Findings and Momentum for Deep Energy Retrofits,” explaining the scope of work and summarizing the costs of the retrofits.
A big fan of deep-energy retrofits, Pedrick explained, “I want to see a fatter house with a smaller mechanical system.”
An ambitious goal
Pedrick’s team selected four wood-framed buildings; brick buildings were deliberately excluded. All are owned by low-income families who had applied for weatherization assistance. Three of the buildings are single-family homes; the fourth is a duplex. All of the buildings are about 100 years old.
The work was paid for by NYSERDA; there were no out-of-pocket expenses for the building owners.
The researchers’ goal was to reduce energy use by 75%. To achieve this goal, the plan was to retrofit slab floors to R-10, below-grade walls to R-20, above-grade walls and roofs to at least R-40. The windows would be upgraded — either with low-e storm windows or new windows — to achieve a maximum U-factor of 0.25. The airtightness goal for the homes was 0.15 cfm @ 50 pascals per square foot of surface area.
The basement insulation was installed on the interior
To insulate the basement floors, the contractors first installed a layer of on top of the existing concrete slabs, followed by R-10 rigid insulation and a layer of Durock cement board. The Durock was not fastened down; it just floats over the XPS, held in place by gravity.
The basement walls were insulated with two different types of rigid foam. Next to the foundation walls, the workers installed 2-inch-thick Dow Perimate (XPS with vertical drainage grooves). The second layer was 2-inch-thick Thermax, a type of rigid foam that can be installed without a thermal barrier (that is, without gypsum drywall protection). The Thermax was held in place with cap screws.
Above-grade walls were insulated on the exterior
All of the existing siding was removed from the above-grade walls. Once the walls were stripped to the sheathing boards, contractors installed a layer of with taped seams as the air barrier. Then they installed two layers of 2-inch-thick polyisocyanurate with staggered seams, followed by vertical rainscreen strapping and new siding.
New fiberglass-framed windows from Serious Energy (R480 series) were installed in most of the buildings. The windows had Heat Mirror glazing consisting of two layers of glass with a plastic film suspended between the panes (a type of glazing that performs like triple glazing). The windows were installed as “outies” in new window bucks that projected 4 3/4 inches out from the old wall sheathing. Many of the existing exterior doors were replaced with new insulated doors.
Two of the buildings got new metal roofing installed over 4 inches of new polyisocyanurate. The roof strapping was extended to increase the width of the roof overhangs. Two of the buildings had roofing in very good shape, so those buildings didn’t get new roofing. At these buildings, it made sense to install insulation on the attic floor. After all of the existing insulation was removed and discarded, the exposed lath-and-plaster ceilings were sealed from above with a thin coat of closed-cell spray polyurethane foam to seal air leaks. Then a deep layer of cellulose was installed.
Using a tankless gas water heater to provide space heat
The design heating load of these renovated buildings is less than 40,000 [no-glossary]Btu[/no-glossary]/h. The existing forced-air furnaces in these buildings were all removed, and new hydro-air heating systems were installed.
The hydro-air systems use a natural-gas-fired Rinnai tankless water heater to supply heat; hot water is circulated through heat-exchange coils in an air handler, and the space heat is distributed through the existing ductwork. The same Rinnai heater also supplies domestic hot water. To be sure that the Rinnai’s limited output of hot water is adequate, each unit is connected to a 12-gallon electric water heater (with the electric resistance element removed) that acts as a buffer and storage tank.
According to Pedrick, construction crews encountered “a lot of unforeseen conditions,” including undersized electrical service, damp basements with improper drainage, failing black-iron sewer pipes, and lead water-supply pipes. (They also encountered lead paint, but that wasn’t unforeseen.)
Of course, the project’s goal was to create safe, energy-efficient, code-compliant homes. “Anything we found that needed to be fixed — we fixed it,” said Pedrick. Correcting unforeseen conditions at the four buildings cost $81,680 — an average of $20,420 per building.
Deep energy savings
The energy retrofit work greatly reduced the air leakage rate at all four buildings; final results ranged from 2.2 to 5.0 ach50. The homes had impressive levels of energy reduction; however, the energy-reduction goal of 75% was not met. Overall energy use (including space heating, domestic hot water, and electricity) was reduced by 60% to 65%. Electricity use in the four buildings actually went up. (Among the new appliances that added to the electricity load were the homes’ mechanical ventilation systems.)
The bottom line
This was a very valuable research project. The retrofits resulted in very significant energy savings, and the gathered cost data are extremely useful. Before the retrofit work, the homes were drafty, uncomfortable, and out of compliance with local building codes. After the retrofit work, all of the homes have new siding, and some of the homes have new roofing and windows. All of the homes are safer, more comfortable, and less expensive to operate.
However, the energy savings alone can’t possibly justify the very high costs of this type of retrofit. The average cost for the work was $112,000 per building, or $89,783 per housing unit. The average annual energy savings was 393 therms of natural gas (11,486 kWh) per housing unit. Since the cost of natural gas in Utica is $1.65 per therm, the average annual energy savings are $647 per housing unit.
In other words, the simple payback period for these retrofits was 139 years.
If the same amount of money ($89,783 per housing unit) were invested in a photovoltaic (PV) array instead of a deep-energy retrofit, you could buy a 20-kW PV system with an annual electrical production of 22,401 kWh (worth $3,248 at the local electricity rate of 14.5¢ per kWh). The value of the PV electricity would be 5 times the savings achieved by the deep-energy retrofit.
[Postscript: The September 2012 issue of the Journal of Light Construction includes an article, “Tightening Up a Two-Family Home,” that describes a deep-energy retrofit project at a century-old wood-framed duplex in Massachusetts. The cost of the work was $275,000, or $137,500 per unit.]
Last week’s blog: “Carl and Abe Write a Textbook.”