Deep Energy Retrofits
The two of us met in 2007 in a HERS rater training class. Paul was a remodeling contractor interested in developing ways to quantify the impact of his remodeling company’s projects. Mike was an electrical engineer deeply concerned about energy and environmental issues and looking to make a career change into the world of building energy efficiency. Three years later, we started a consulting partnership, DEAP Energy Group, to assist project teams in the development of Passive House, net zero energy, and deep energy retrofit projects.
Paul’s remodeling company, Byggmeister, Incorporated, hires Mike frequently to assist in the planning and evaluation of Byggmeister’s deep energy retrofits. Over the course of the last couple of years, Paul and Mike have collaborated on retrofit projects of (among others) a single-family home, a two-family home, and a three-family home, all in the metropolitan Boston area. These three projects were all made possible in part by utility incentive programs—NSTAR in the case of the single-family retrofit, and National Grid in the case of the other two. Case studies of these three projects offer some useful and interesting lessons for planning, implementing, and commissioning deep energy retrofits on wood-frame residential structures.
All three projects shared the same overall prescriptive goals: R-10 basement floor; R-20 basement walls; R-40 above-grade walls; R-60 roof, 0.2 windows; air leakage of roughly 1 ACH50 (0.1 CFM50 per square foot of shell area); and ventilation meeting ASHRAE 62.2. Heating fuel choice was a function of the utility incentive program; if the gas company was providing funding, it would be hard (ungrateful, even) to make a case for switching the house from gas heat to heat pumps, for instance. In any event, we were not able to reach all the prescriptive goals in each case, but we never missed by much.
The following are the project specifics.
The single-family home is a typical ranch style, with about 1,200 square feet of living space on the main level, and another 1,000 or so square feet in the walk-out basement. It has outstanding solar access, and in fact a year after the insulation retrofit, the homeowners installed an 8.1kW PV system.
In this house, we were able to reach R-10 on the basement floor by installing 2 inches of polystyrene insulation on the floor and a layer of subflooring on top. There was sufficient height to add this 2½ inches of thickness to the floor and still have 7 feet 3 inches of headroom. The basement floor slab was in excellent condition and was reasonably level and flat. This meant that we could float the floor—just lay the rigid foam and subflooring down, with minimal mechanical fasteners into the concrete. The basement walls were insulated with approximately R-20 Thermax HD panels glued to the foundation walls. This was an easy enough approach to the basement insulation that the work was done by a group of weatherization volunteers who wanted to learn about deep energy retrofits.
The above-grade walls had the old siding material stripped, the cavities were dense packed with cellulose, and we added two layers of 2-inch polyisocyanurate to yield an overall R-value of about 40. The sheets of wall foam were cut to fit around the rafters and installed to extend up to the roof vents. The attic floor was air sealed and the attic was filled with cellulose—about 18 inches of it in the center and less, of course, at the eaves owing to the slope of the rafters. The attic access hatch was moved from a hallway ceiling to a gable end to eliminate a major penetration through the ceiling, where we were trying to establish the thermal/air boundary. Finally, we replaced the windows with Paradigm vinyl units with a U-factor of 0.2. Mike’s modeling showed that we had reduced the modeled peak heating load from about 65 MBH to about 18 MBH—a reduction of around 75%, which is typical for these deep retrofits.
Although this project was an enormous success overall, three things about it disappointed us.
The first had to do with our attempts to get a good air seal while leaving the rafter tails in place—in other words, we couldn’t do a “chainsaw” retrofit. Our air barrier in the basement was the rigid foam on the floor and the walls; these joints were easy to seal. Then the air barrier transitioned through the sill to the layers of exterior foam on the above-grade walls. This transition through the sill is a tricky area that we discuss in the next (two-family) case study. At the eaves, the air barrier transitions from the exterior rigid foam through the rafter tails and top plate area to the plaster ceiling (that is, the attic floor). We were pretty confident we could achieve good continuity of the air barrier through this complex area by spraying open-cell foam into it from the attic side. We were wrong. Although we didn’t do badly, the overall result was a leakage rate of 380 CFM50, or about 1.13 ACH50. Some very aggressive air-sealing work was needed to achieve even this level of airtightness, and we spent a good deal of effort on the eaves. Before we started the project, we had felt confident that we could get it down to 1.0 ACH50.
Bottom line: It’s tough to get really good results if your air barrier takes a convoluted path. Either simplify the path at the design stage if you can, or lower your expectations accordingly.
The second disappointment was our inability to find an installer who could work with us to develop a really clever heat pump strategy. We had been quoted over $20,000 to bring natural-gas service into the house. Fuel oil and propane seemed poor choices, partly because the electric utility was providing some of the funding for the project, and partly because really good air source heat pumps would make electricity as the primary heating fuel source cost-competitive with oil or propane—not to mention the fact that we’d be getting a cooling system at the same time. So air source heat pumps were the way to go, and we started talking with a few installers, all recommended by local reps for the equipment manufacturers.
Although we had brought the peak load down to about 18 MBH, we ended up with a system of seven wall-mounted cassettes with an aggregate capacity of over 60 MBH. There was one cassette in each of the three bedrooms, one in the kitchen and dining area, one in the living room, one in a guest room (a converted garage), and one in the basement. This seemed excessive, although at some level it was understandable. The installers’ concerns had to do with heat distribution. If the bedroom doors are all open on a cold winter night, one cassette at that end of the house may keep the bedrooms all within a comfortable range. But close a door to a room with no independent heat source, and even given the superinsulated shell, the room may cool down too much to be comfortable. No installer was prepared to risk an uncomfortable occupant. Thus, the seven cassettes. Truth be told, the system works well. Total heating energy usage has been 14% less than what the REM model predicted. It is 7.5 MMBH, or 2,200 kWh, per year; and the homeowners (a couple with two young children) have been very happy with the comfort levels. The one downside was the initial installation cost, which was about 50% more than the cost of a conventional gas-fired ducted system.
The last disappointment with this single-family project was the ventilation system. Until recently, Paul’s remodeling company has paid little attention to household ventilation, beyond making sure that bathroom and kitchen exhausts were installed and functioning. The most sophisticated we got was to put bath fans on a timer switch to improve the odds that the fans would be used properly. There are two reasons why we paid relatively little attention to ventilation. First, most of our projects—almost exclusively involving old, leaky homes—just did not include extensive, systematic air-tightening measures. And second, it was also a consequence of the fact that we lacked a way to evaluate ventilation effectiveness.
The net result of our inexperience was that we did not have an established relationship with a good energy recovery ventilation system installer, did not know the equipment that well, and were not really able to tell the difference between a good system and a bad one—until, that is, the occupants moved in, started using the house, and found that it got pretty rank inside pretty fast.
We brought Mike in to troubleshoot the system. He found that the flow rates were less than half what they should have been. Some problems were the result of bad ductwork installation. A bigger problem was a defective energy recovery ventilation (ERV) unit. But the ductwork was fixed (reducing supply duct leakage from 80 CFM25 to 22 CFM25); the ERV unit was replaced; and the system now works fine.
Bottom Line: We should have commissioned the ERV system right after it was installed, rather than starting it up and waiting to see what happened. We also should have developed a clear HVAC strategy and conceptual design before soliciting quotes and hiring an installer.
The clients for this two-family retrofit project represented three generations: a couple and their children in the upper floors, and the grandparents on the first floor. Each household was downsizing from a larger old (and inefficient) home, and both households wanted to reduce their carbon footprint by using less energy.
The two-family home they bought for this purpose was a poster child for deep energy retrofits. It had no cavity insulation at all, and it needed new siding, new roofing, new windows, and new mechanical systems. The house also had a very simple geometry and had no real ornamentation, so applying exterior insulation would pose few challenges.
There were two porches. The rear two-story porch was in very poor condition and needed to be replaced. The front entry had a curved roof over a simple landing at the front doors to the two units.
Porches and decks can pose challenges to an exterior insulation retrofit. If you leave them in place and try to fit the insulation around them, you can end up not only with a thermal bridge, but also with a water management nightmare. For this two-family retrofit, we chose to rebuild both the front and the back porch completely. This provided an opportunity to add one layer of 2-inch rigid insulation, and then a ledger board bolted through the foam into the framing behind. Because the 2 inches of foam behind the ledger created a sheer condition, we needed to use more substantial lag bolts. We then packed out the 2 x10 ledger with ½-inch plywood to yield a 2-inch overall thickness that would match the thickness of the surrounding second layer of 2-inch rigid foam, and carried our water-resistive barrier right over the ledger.
An alternative strategy that we’ve used on subsequent projects (including the three-family case study described below) is to support the deck or porch completely independently of the house by installing additional piers and posts to eliminate the need to carry it off the house itself. We can then add 4 inches (or more) of foam, continuous between the porch and the house, and not worry about sheer strength. We do need to provide some connection between the porch and the house for lateral stiffening, but this requires fewer attachments than are required for vertical support, and the foam is not an issue. One caveat with this strategy, however: The additional piers have to be within 4 feet or so of the existing foundation, so you may have to go deeper than you anticipate to reach undisturbed soil.
On this two-family project we were participating in a National Grid deep energy retrofit pilot program. We were able to achieve all the insulation levels required by this program—except for the level of basement floor insulation. The headroom in the basement was 6 feet 6 inches—too tight to accommodate much of any added floor insulation, let alone the R-10 the program expected. The program administrators cut us some slack on the floor insulation (although they reduced the project funding commensurately). We did install a perimeter drain to manage bulk moisture, but we did not gain the capillary break or condensation control that floor insulation would have provided. We monitored indoor conditions for 18 months after project completion using Onset HOBO data loggers. We tracked the moisture content in the air (in grains per lb, or gpp) in a couple locations in the basement, in various locations throughout both units, and in the outdoor ambient air, and found that (at least for that 18-month period) the basement floor slab was not an appreciable source of moisture.
We also monitored CO2 levels in various locations in the house. Terry Brennan, of Camroden Associates, pointed us to research that indicates that an indoor CO2 level of about 1,100 ppm is a pretty reliable sign of healthy indoor ventilation rates. Using a Telaire monitor and a HOBO logger to track CO2 levels, we worked with the homeowner to adjust the ERV run time so that it yielded CO2 measurements consistently in that range. This level of ventilation turned out to be about 60% of our ASHRAE 62.2 calculations.
We also installed an eMonitor to measure electrical consumption. We did this in order to evaluate the impact of our work and the validity of our modeling. The homeowners were interested in the eMonitor as well; they saw it as a feedback mechanism for learning how best to use the high-performance house our project had given them. Achieving really low energy use in a house is always the result of a partnership between the house and the occupants.
One intriguing bit of information we gleaned from the data that the eMonitor provided was that the pumps used for the solar-thermal system for domestic hot water (DHW) were significantly bigger than was necessary. When the sun is heating the collectors, the pumps draw 160 watts. It is estimated that with high-efficiency pumps, 20 watts would be sufficient.
The net result of the superinsulation retrofit, and the highly motivated homeowners, was that in 2011 the house met the lofty goals of the Thousand Home Challenge—an ACI initiative with the goal of reducing energy use by 70% to 90% in a thousand homes in North America.
The three-family home posed more challenges in an interesting way. It already had pretty decent energy performance, with an estimated HERS index in the mid-80s.
The project’s genesis was peeling exterior paint. The clients started getting quotes for a new paint job, and received consistent feedback that what was really needed was new siding—the existing cedar shingles were too far gone to hold new paint for long. So they started getting quotes for new siding, and learned that a re-siding job was a good time to upgrade wall cavity insulation. In the course of researching the best way to get that insulation done, they learned about National Grid’s deep energy retrofit pilot program—and quickly decided to participate.
Meeting the program requirements for the basement floor (R-10), basement walls (R-20), above-grade walls (R-40), and windows (R-5) was pretty straightforward. Reaching R-60 for the roof posed a bigger challenge, although the building had a slate roof in good condition.
Slate is a beautiful, durable roofing material. You don’t strip and completely replace a slate roof every few decades, as you do with asphalt shingle. Typically, you replace a few slates every year or two as they break or wear thin. So a slate roof replacement happens gradually, a dozen slates at a time, over the course of a century or so. Which means that most of the slate roofs on older homes in our area consist of slate shingles over ancient felt or rosin paper over board sheathing with ¼-inch to ½-inch gaps between the boards. Generally the rafters are uninsulated, and the thermal boundary (to the extent that it exists) is at the attic floor.
These assemblies do leak, but generally only a small amount of water at a time. This water is absorbed into the board sheathing and then released back into the outside air when the rain stops and the sun comes out.
In this three-family, there was a finished third-floor apartment right under the roof. To allow for drying (and occasional drainage), we installed 1½-inch foil-faced rigid-foam vent chutes right under the roof sheathing, spaced 1½ inches down from the sheathing. These chutes extend up past the sloped section of ceiling and down to the soffit vents. They create an air space that will allow the sheathing to dry to the bottom if it takes on small amounts of water, but will also allow for drainage if for some reason the roof assembly starts to leak a lot of water. The signal to the homeowners that they might have serious roof problems will thus not be a stained third-floor ceiling, but will be water coming through the soffit vents.
The clients on this project had installed a FreeWatt gas-fired cogeneration system a couple years before embarking on the retrofit project. Unfortunately, a full year of concurrent energy use and production data were not available, but using six months of overlapping data, and six months of data following the period with utility bill data produced a reasonable estimate of total use. Calculating energy use by this method, we arrived at a total of 7,197 kWh per year, of which 3,782 kWh (52%) was generated by the FreeWatt system.
Modeling the FreeWatt in REM/Rate posed some challenges. The FreeWatt system consists of a motor generator and a 95% annual fuel utilization efficiency boiler. The boiler is used only when the space-heating load exceeds the heating output of the motor generator (about 12 MBH for an input of 18.5 MBH, ~ 65% efficient at generating heat). Given that REM/Rate is an annual (as opposed to hourly) simulator, it is not possible to use REM/Rate to determine the annual run time (and so electricity production) for the motor generator pair. There are ways to work around this issue, but we did not address them, since a REM/Rate model was not a program requirement.
We benefited greatly from the additional experience we gained from the prior two projects in planning and installing ventilation systems. We worked very closely with the HVAC contractor on this project in laying out the system and incorporating the ductwork installation into the other phases of the work. Mike was instrumental in the postinstallation commissioning. As a result, it took only six hours to commission all three ventilation systems, and there were no issues that prevented adjustment and verification of appropriate air flows at all registers.
Our experience is that every deep energy retrofit has its challenges. Houses have an irritating but interesting way of fighting back. Here are some general lessons that we learned.
Baseline energy usage.There are a number of reasons for establishing baseline, preproject energy usage. In our projects, one reason was that the utilities that were helping to fund the work wanted to be able to verify that the projects were, indeed, achieving deep reductions. Another is that the contractor, Byggmeister, has set a companywide goal of reducing the overall energy usage of its portfolio of projects (not just the deep energy retrofits) by at least 40% over time, and so needs to know the baseline for each project. Finally, the homeowners had a keen interest in the numbers. These projects represented a huge commitment of time and resources for them, even with the generous NSTAR and National Grid assistance.
But as readers of Home Energy are well aware, it’s not always that easy to get baseline information. In two of our cases, our clients were purchasing the homes and were doing the retrofits before moving in, and it proved impossible to get credible preproject energy usage information from the prior owners. In these instances, we relied on Mike’s REM modeling to estimate a preproject baseline. In the third case, our clients had installed a co-gen system (FreeWatt) in their owner-occupied three-family, and although it was possible to calculate net usage for the building by looking at all three sets of utility bills in the aggregate, it proved a little trickier than anticipated to calculate gross energy usage.
Roof treatments and air leakage reductions. On the two-family project, we were able to do a chainsaw retrofit: completely remove the overhangs at the eaves, wrap exterior insulation up the wall and onto the roof, and then apply a new eave detail on top of the foam. The three-family had a slate roof in good condition, and the “attic” was in fact the third-floor apartment—fully finished space. So we could not add insulation to the roof or remove the eave to do a chainsaw retrofit, since removing and reinstalling the slate would have been prohibitively expensive; and we had limited opportunities to add insulation to the inside. On the one-family, the roof was in poor shape, but the owners were at their budget limit for the project and would need to defer the new roof for at least a year, at which point they planned to install PV as well. Fortunately, the attic was not very suitable for storing luggage or holiday decorations, but it was perfect for storing cellulose insulation, so we decided to achieve our desired R-60 at the attic floor rather than at the rafter line.
What we found was that it’s very difficult to get a supertight result if you don’t do the chainsaw retrofit. Comparative numbers were 0.9 ACH50 for the chainsawed two-family; 1.13 ACH50 for the one-family; and 2.37 ACH50 for the three-family. To be sure, other factors besides the eave treatment play a role here, but it’s clear from the details associated with each building that the eave treatment is a significant factor in achieving airtightness.
HVAC strategies. The one-family started off with oil heat and hot water and two bath fans for ventilation; it ended up with heat pumps and an electric-resistance heater for hot water and an ERV for whole-house ventilation. The two-family started off with one old gas furnace, one old oil boiler, two gas water heaters, window units for air conditioning, and bath and kitchen fans for ventilation. It ended up with two gas furnaces for heat, central air conditioning, solar thermal with electric-resistance backup for DHW, and two ERVs for ventilation. The three-family had had a FreeWatt system installed for hydronic heating and shared hot water about a year before our project planning started, and it was clear we were not going to change that. The cooling strategy was to replace old window-mounted units with new Energy Star units; and we added kitchen fans for the gas ranges, and ERVs in each unit for general ventilation (see Table 1).
You can read more about Case Study 2 at the Thousand Home Challenge web site: http://thousandhomechallenge.com.
All of these strategies had pros and cons and trade-offs. Being limited to one particular fuel type either by utility program expectations or by the fact that new high-performance equipment had recently been installed that no one wanted to replace any time soon led to some compromises. Embarrassing as it is for Byggmeister’s owner (Paul) to admit, another factor was the contractor’s inexperience at retrofitting high-quality ventilation systems. A bigger problem was the general lack of HVAC contractors in our area who know how to design and install systems for high-performance homes.
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