In 1983, the Heron was one of five cookie-cutter models offered in Bay Country, a tract housing development in a suburb of Baltimore, Maryland. Perhaps a little ahead of its time, the design featured walls with 1-inch-thick expanded polystyrene (EPS) insulation over the standard 2x4 walls filled with fiberglass batts.
Many years later, the original green aluminum siding was faded, and the cold leaky windows whispered that it was time for a new approach to “green” — or at least an energy-efficiency makeover. When an old house needs new windows and siding, it is a great time to add exterior insulation and to tighten it up against air infiltration. That was the basic approach we took to reduce our all-electric home's energy bill.
It seemed to me that the house was destined to mature into a new phase, more modern and energy-efficient: able to take advantage of the amazing, timeless, natural power of passive solar energy.
The floor plan is small, although it was enough for a family of four for many years. The exterior lines are fairly simple, so that the wall area exposed to the weather is not large compared to the volume enclosed. The long side of the house faces 20 degrees east of solar south, and there are no windows on the west wall, keeping unwanted heat gain from the late afternoon summer sun to a minimum.
The fairly open floor plan was ideal for the use of highly efficient ductless minisplits to replace the aging central heat pump system and its bulky air handler and dusty ducts.
Two smiley faces are better than one
Since we completed our energy retrofit work, our house hasn’t received any awards. (It may qualify for some sort of award if I ever become interested in such things.) Instead of receiving awards, the house has provided many rewards: comfort, quiet, very low electric bills, and general satisfaction.
The closest thing to an energy-efficiency award the house has received is a letter from the local electric utility titled “Home Energy Report.” The report compares the electricity use for the previous two months with that of the closest 100 occupied houses. In the section called “How you’re doing,” we were informed that our house is doing "great." This was followed by not one, but two smiley faces.
Do awards really get any better than that? The colder it gets, the better our house performs compared to neighboring houses. According to the Two-Smiley-Face Letter, in January and February of this year, the house used 57% less electricity than the most efficient neighboring houses.
The sun was my inspiration
The earth and every house on it are powered by the sun or other stars, mostly with energy sources that are really just solar energy converted into some other form. Oil, natural gas, and coal are natural storage batteries of solar energy absorbed by the earth long before our time. Wood and cow pies are other examples of solar energy storage batteries that some people use to heat their homes.
Solar energy also drives wind and hydro power. We currently harness nuclear energy split from radioactive elements heavier than iron, believed to have been made mostly by supernovae — explosions of stars. Our electricity is converted inefficiently from one of these solar storage batteries. The batteries are not particularly easy to recharge, so that it seems best to use solar energy directly from the source.
One cold and sunny winter day, before I started the energy retrofit, I was lying on the rug in front of the south-facing sliding glass door with my dog. With the sun pouring in, I had never felt so naturally cozy. The old door may have been leaky but the glazing was clear, and it was collecting a lot of heat.
While I was lying there, I decided that at some time in the future it would be possible for people to live very comfortably in homes completely powered directly by solar energy (using passive solar energy, photovoltaics, or some other direct form), coupled with advanced storage techniques to carry the house through the night and cloudy weather. I also then decided I would retrofit this house to maximize energy efficiency, using the classic, purist approach:
- First, reduce the energy footprint as much as possible; all techniques to conserve energy, big and small, add up.
- Second, use the sun as much and as directly as possible.
This was a DIY project
Almost every part of the house was reworked, and I did all the design and trade work myself, except for the installation of the minisplit heat pumps. Energy modeling was initially done with Energy Plus, but since it was so cumbersome to use, I was happy to see the introduction of the easier-to-use BEopt software.
Before I started, I knew it would be a huge amount of work, particularly since I had very little experience with any of the trades or design. Now we’re almost finished and we’ve been very happy with the results, with big improvements in energy efficiency and aesthetics.
Fiberglass batts were added in the attic
The house has two separate conventionally ventilated attics with access hatches. As built, both attics had about 12 inches (R-30) of blown-in fiberglass insulation. No attempt had been made to seal air leaks at the hatches.
The attic was the first part of the house to receive an insulation upgrade. I used canned spray foam to seal every ceiling penetration I could find. The hatches were covered with several layers of rigid foam board, and I installed foam weatherstripping tape at the perimeter of the hatches.
As part of the retrofit, three layers of R-19 fiberglass batt insulation were rolled out over the existing loose fiberglass, with each layer perpendicular to the one below. Nominally, this adds up to a total of R-87. Of course, the attics are not really R-87, since the three layers do not fit all the way to the edges near the soffits, and because of the inevitable gaps between rows of rolled fiberglass.
The estimated actual overall attic insulation level used in our energy simulations is R-70.
Adding exterior polyiso to the walls
The original walls consist of 2x4s, 16 inches on center, with R-13 fiberglass batts between the studs and 1-inch-thick EPS tacked to the exterior of the studs, for a nominal R-value of 17. Considering the thermal bridging of the closely spaced studs, the actual R-value of the original walls was about R-14.
Since new siding was needed, the old aluminum siding was stripped away, revealing the aging EPS. By now, many builders know that EPS panels can shrink over time. I discovered gaps between almost all the edges of the EPS foam boards, some as wide as 1 inch. Most of this original foam board was left in place, refastened to the studs as needed. I sealed all of the gaps with spray foam before taping the seams with construction tape.
The original fiberglass batts were mostly left in place, with any obvious, accessible problems corrected. It was easy to see that the house had been overly and randomly ventilated in places, but at least the air had been filtered (joke), as evidenced by some dirty fiberglass appearing every so often as the siding came off.
After those relatively minor repairs, the insulation value of the walls was increased, basically following the REMOTE “outsulation” method that originated in Alaska. I installed two layers of 2-inch-thick polyisocyanurate foam board on the exterior side of the existing EPS. All of the polyiso seams were staggered and the seams were sealed with canned spray foam and construction tape.
The taped polyiso is the air barrier for the walls of this house. The polyiso is also the water-resistant barrier (WRB) to which the flashing for the windows is sealed. I attached the two additional layers of polyiso, along with the furring strips, to the studs using 7½-inch Fastenmaster construction screws. It is not as difficult as it sounds to run the screws through all that material thickness and hit the studs.
The original plan included leaving a gap between the furring strips so that the back side of the siding would be ventilated. We decided to install vinyl siding, however, and vinyl siding manufacturers want absolutely no liability for a wavy finish on the siding. They want nothing but a perfectly flat (planar) substrate for siding installation, and most manufacturers state that requirement in their installation instructions. Rather than risk wavy siding or a fight with the manufacturers, I filled in almost all of the gaps between the furring strips with ¾-inch extruded polystyrene (XPS) rigid foam before I installed the new vinyl siding.
According to our original calculations, the total R-value of these modified walls would be about R-40: R-14 for the fiberglass batts and EPS plus R-26 for the 4 inches of polyiso (assuming that polyiso is rated at R-6.5 per inch).
At first, I wasn't familiar with the R-value testing protocol that specifies that insulation products be tested at a mean temperature of 75°F. This seems like a useless temperature to assess cold-weather performance. In cold weather, polyiso performs poorly — more like R-5 per inch rather than R-6.5 per inch. Therefore, the modified walls of this house are approximately at R-34 — or maybe R-35, because the inner layer of polyiso probably stays a little warmer in the winter since it is insulated by the outer layer.
Installing more south-facing glazing
Windows are amazing. We all like natural light. We all want it to look and feel like we are outside in beautiful weather, even when we are inside. Windows help do that, but with our current state of technology, they provide very little insulation; they are like holes in the thermal envelopes of our houses.
For the energy-conscious person, windows are one of those things you think of when you say, “You can’t live with them and you can’t live without them.” We all like windows because they feel good; they connect us to our natural environment. That natural feeling is dramatically enhanced when windows are used for intelligent energy collection from the sun, our natural energy source.
The original windows of our house were double-hung windows with aluminum frames and clear double glazing (air filled).
The south-facing area of this glazing was 7% of the total floor area, so the house was “sun-tempered,” according to the language of passive solar design. Sun-tempered means that a non-trivial amount of the winter heating demand of the house was being supplied by the sun.
Seven percent solar glazing is an important quantity in passive solar design: it is considered the largest percentage of south-facing glazing that should be used without adding extra thermal mass to the interior of the house. All houses include some internal thermal mass — for example, drywall and furniture — and that thermal mass is enough to temper the heat introduced by south-facing glazing up to the 7% value, in a way that avoids overheating.
The original windows' contribution to the home's winter heating demand was noticeable. It could be felt by someone familiar with the concept. No attempt was made to quantify, either by calculation or modeling, the heating contribution made by the original south-facing windows.
The plan to maximize the passive solar energy contribution to help meet the heating load basically involved adding as much glazing to the south side as seemed both possible and reasonable. After adding more south-facing windows, we now have solar glazing amounting to 9.3% of the total floor area of the house. New, deeper window bucks were installed to accommodate more and larger windows on the south side, and to bring all of the new windows out flush with the 4 inches of new foam board on the walls.
We love our passive solar house
These days, people's opinions on the use of passive solar energy range from love, to hate, to anywhere in between. We love it.
We love the brilliant winter sun that seems to blind some other people. We don't care about color fading. We have no problem opening and closing thermal curtains, on a daily basis, if needed.
We have never felt what we would characterize as "overheating" due to passive solar gain in this house. The passive solar features sometimes provide a little more heat than wanted during parts of the shoulder seasons in this climate. On those very few days, opening the windows is easy, and we welcome the cool fresh air.
Solar fraction calculations
Calculations and computer modeling indicate that the new windows provided about 54% of the heating demand for the house during the winter of 2013-2014 (the coldest winter in this area in the past five years). The minisplit units were installed in February 2013. Thanks to the south-facing glazing, the downstairs unit has spent a lot of time sampling the air and looking for something to do during the heating seasons.
Most of the calculations needed to determine the solar fraction, or percentage of the heating load supplied by the sun, are easily performed by readily available computer simulations, such as BEopt and others. For various reasons, these calculations are not scientifically precise, but they provide a useful estimation of heat loss and gain. The calculations I used to determine the solar fraction were as follows:
- The heat loss formula for determining transmission losses through floors, roofs, and walls is Q = A • U • ΔT. In other words, the rate of heat flow through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the U-factor (in Btu/ft² • h • F°) of the assembly times the ΔT (in F°).
- For our house, I calculated that the total heat loss of the house is about 190.7 Btu/h x delta-T in F°. I calculated that number using the known insulating ability (R-value or its reciprocal, U-factor) of walls, ceilings, floor and windows, the geometric areas of those features, and the infiltration rate of outside air (either estimated or measured).
- With the rate of heat loss known, heating degree days (HDD) can be used to determine the amount of heat loss over a period of time. HDDs do not have to be calculated; it is a statistic monitored by weather services and is based on historical temperature data. The HDD is a number that represents how much (in F°) and for how long (in days) the outside temperature was below a base temperature during that time when the house needs heating. The base temperature usually used in the U.S. is 65°F. Some simple examples of how HDDs are determined are as follows: If the outside temperature is 10 F° below 65°F for one day, then that is 10 HDDs. If the temperature is 5 F° below 65°F for two days, that is also 10 HDD. Of course, the outside temperature varies over the course of any winter day. It is not just simply and constantly five or ten degrees, for example, below 65°F. So, the average of the hourly temperature values (the ones below 65°F) that occur throughout the day can be used for HDD determination. The units for HDD are F° x day.
- Now the total amount of heat needed to keep the house warm over the winter season can be calculated. The amount of heat needed is 4,577 Btu / (day x F°) x HDD. There were about 5,019 HDDs for this area during the winter of 2013-2014. So, the entire heat loss of the house for that particular winter season was: 4,577 Btu / (F° x day) x 5,019 (F° x day) = 22,971,963 Btu.
- The idea is to keep the house at 65°F during the heating season, so it is necessary to supply enough space heat to balance the heat loss. The heating load is typically considered to be only the amount of heat required to achieve 65°F because the heat to raise the other 5°-7°F degrees needed to reach the 70°-72°F “standard” comfortable indoor temperature is considered to be provided by such things as household appliances, lighting, and bodies.
- So, the heat load for the 2013-2014 heating season was about 22.972 million Btu. How much of that heat was provided by the solar glazing on the south side of the house? To determine the contribution by the solar glazing, the primary information needed is the area of the glazing, the solar direction it faces, the solar heat gain coefficient (SHGC, a standard window specification), and the average amount of sunshine (insolation) for the area during the months of the heating season. Historical insolation by month and geographic area are readily available from the National Renewable Energy Laboratory (NREL) website. Window heat gain calculations that provide fairly good results are not difficult, but there are many steps and iterations, so computer programs really shine at the task. One online calculator I have recently discovered that appears to deliver reasonable results, when reasonable information is entered, is offered by the Sustainable by Design website. The author of the calculator “estimates (rather subjectively) that the overall calculation is accurate to within about 20% as an average value.” After using this calculator, I believe it is far more accurate for my situation than 20%, because its results are similar to those I originally got a few years ago, using the more sophisticated Energy Plus computer program. The online calculator determines, for this house and location, that about 92,500 Btu of solar energy per square feet of solar glazing is collected during the average heating season. There are about 134 square feet of solar glazing in the house, so approximately 92,500 Btu/sq. ft. x 134 sq. ft. = 12.395 million Btu were collected over the heating season. That’s about 54% of the total heating demand for that heating season: (12.395/22.972) x 100 = 54%.
The solar fractions for all other, warmer, winters within the past few years are somewhat higher. This result is somewhat modest; solar fractions in excess of 60% have been reported by some passive solar energy designers for years.
The best windows make bad walls, with respect to thermal insulation. However, it was established as fact years ago that south-facing double-pane windows will typically supply more heat than they lose over the course of a heating season, in most heating-dominated climates that enjoy any appreciable sunshine in the winter. If the heat loss by all the windows and doors in this house, both north- and south-facing, are broken out of the heat loss calculations, that loss amounts to about 8.191 million Btu for the 2013-2014 heating season. Based on that, the solar glazing collected enough heat that season, the 12.395 million Btu, to offset its own losses as well as the losses of all the other windows and doors in the house.
More thermal mass
Additional thermal mass in the form of extra layers of drywall, cement board, and thick veneer brick was added to the walls of the family room, which is the space on the first floor with the majority of the total solar glazing. Although passive solar design principle calls for somewhat more thermal mass than was added, it was considered impractical to install additional thermal mass to the flooring.
There has been no obvious overheating of the house. Mechanical air conditioning is a modern “must-have” in this climate, but there has been no need to turn the AC on any earlier in the cooling season, or to keep it running any later in the season, than before the retrofit work. However, solar glazing certainly does add to the cooling load, although it only adds a relatively small amount. Recently, we added ComforTrack cellular thermally insulated shades that seal at all edges to the south-side windows. The shades insulate both against unwanted heat and cold from the outside.
Fiberglass-framed windows
The new Accurate Dorwin windows have fiberglass frames which expand and contract about the same amount as glass while insulating better than aluminum. The similar coefficient of thermal expansion shared by the glass and the fiberglass frame presumably contributes favorably to the quality and life of the glazing seals.
The north-facing windows have triple-pane IGUs filled with argon, and the window area was reduced on this side of the house. On the south side, the windows have double-pane, argon-filled glazing with a solar heat gain coefficient (SHGC) about as high as can be readily obtained in this modern world in which high SHGC glazing has fallen out of vogue.
Casement windows were specified for the operable windows, since they seal against air infiltration better than double-hung windows. The number of operable windows was reduced slightly all around; fixed windows seal better and have a higher SHGC than casement windows. The single original east-facing window was removed.
The large window area added to the south side of the main section of the house features new permanent overhangs. The depth of the overhangs is consistent with passive solar design principles, minimizing the heat gain from the sun traversing high across the sky in summer while allowing winter solar heat gain in the winter, when the sun tracks lower in the sky.
SketchUp software was used to geometrically and visually model the entire house, and was used to confirm the design of the solar shading of the windows. When the geographic location is set in SketchUp, and the geometry and orientation of windows and overhangs are drawn accurately, this free computer drawing program depicts solar shading at this location very accurately, whatever the season or day of year. This tool has been tested against reality multiple times at this location with excellent results.
Construction of the overhangs was difficult and time-consuming, but was worth it, in the opinion of the designer, energy consultant, carpenter, roofer, siding mechanic, and homeowner.
Ductless minisplits for space heating and cooling
In 1983, the brand new house came with a central air conditioning system based on a 3-ton heat pump. As a young first-time homeowner, I’m sure I was impressed by that. I was the owner of a powerful central AC system, after growing up in a fairly poor family with no AC at all. My summers as a teenager sleeping in the Maryland humidity involved a massive window fan right next to my bed. Since I’ve always loved airplanes, I felt fortunate to sleep in the cool, wet, noisy prop wash. One night back then, the outer edge of a hurricane came through, and I honestly didn’t know it until the next day.
When it was time for the house to get new siding and windows, it was also time for a new heat pump. Since the house is small with an open floor plan, and we had made the decision to superinsulate and reduce infiltration, it seemed an ideal time to specify ductless minisplits. The minisplits would also save interior space. The old air handler and return ducting took up a lot of space in the utility room. That equipment was removed and sold as scrap.
A new heat-pump water heater was installed in the space created by the removal of the air handler, while the old water heater location under the steps became a wine rack and an extension of the family room.
I performed a conservative heat load calculation when sizing the heating system. This was prior to the blower-door test. Actually, the only reason a blower-door test was done was because of the extreme curiosity to know the results, and because the local electric utility offered the test at a reduced rate.
I calculated that the peak heating load at the 12°F design temperature was 13,970 Btu/h. Based on this value, I chose the Fujitsu 12 RLS2 for downstairs. Fujitsu claims this unit can put out about 17,000 Btu/h of heat. I chose the Fujitsu 9 RLS2 for upstairs, not to help meet heating demand but for cooling.
There have been no regrets with these choices. The upstairs unit has never been used for heating, even when the temperature dropped to 4°F last winter, but it has proven invaluable for summer cooling. More recent peak heating load calculations, based on more accurate input including the results of the blower-door test, show that the peak heating load is less than initially determined: It is 190.7 Btu/h x F° x (65-12)F° = 10,107 Btu/h. This heating load could probably have easily been met with one 9 RLS2, but the 12 RLS2 seems to have been a good choice to meet the cooling demand of the entire downstairs, about 960 square feet.
The new minisplits and the dramatically reduced heating and cooling loads provided some of the best fun of the entire project. It was fun to realize that the house would be comfortable using about one half of the former heating/cooling capacity. It was fun to feel the real heat put out by minisplit units; as everyone knows, the air flowing from the registers of a central heat pump system never feels warm unless the supplementary electric resistance heating coils are running. It was fun to make more space and scrap money by hauling away the old air handler, ductwork, and outdoor unit. It’s fun to experience the good job the minisplits do.
Of course, they cannot respond to large changes as quickly as the big central units can, but when they reach steady-state conditions, the temperature variation throughout the house has only been about 5 degrees, maximum. Typically, the variation is only about 3 to 4 degrees.
Finally, it was fun to experience the reaction of the HVAC contractor chosen to install the Fujitsu units.
The HVAC contractor was skeptical
When I called the HVAC contractor, I told him only that I needed an estimate for a new system. When he arrived for the estimate, we initially stood outside the house talking, and I asked him what he thought the required heating/cooling capacity would be for the house. I had previously read that HVAC contractors used rough estimates of capacity based on a quick look at the exterior dimensions. This contractor did just that, and replied “about 2 ½ to 3 tons.”
I see nothing at all wrong with that initial approach, particularly when there was a whole neighborhood full of 3-ton units, but times are changing a little. When he saw the smile on my face as I looked over at the corner of the house where the side wall was sticking out several inches past the brick veneer front, and that it had been trimmed out with plastic trim board, he said, “Hmmm.”
Of course, I then took him on a tour of the house and told him I already knew what I wanted, and that I just needed a contractor to do the work. Still, the contractor had only limited experience with minisplits, and no experience with energy retrofits, so he was very skeptical. The quotation he gave me for the price to install the minisplits contained the following passage:
"The specified equipment has been requested by the owner for its electrical efficiency. This system will result in uneven heating and cooling throughout the home due to the total lack of any type of air distribution system. The construction materials and insulation ratings of this home exceed the engineering abilities of our standard load calculation software; as such [the HVAC contractor] does not guarantee this system’s ability to heat the home. The new system will offer no supplemental or emergency heat."
Well, under normal climatic conditions, perhaps the new minisplits are the supplemental heat, as backup to the passive solar primary heat source. Normally, when it gets cold and snows here, that’s even better because it will typically be clear and sunny afterward, so that even more sunshine is reflected into and collected by the solar glazing.
As for emergency heat, the house never before had any emergency heat that wasn’t dependent on electricity. So now it could be said that the house has more emergency heat than it ever had before, as provided by the solar glazing, and the much improved ability to retain heat.
In this climate, the need for emergency heat supplied by an alternate source is not as critical as it might be in a colder area of the country.
Extending the roof overhangs
How do you add 4 inches of foam board insulation to the exterior of the gable end walls when the original walls are already flush with the edge of the roof? One answer is that you extend the roof a bit. Doing this required unweaving some shingles away from the existing edges so I could weave in new ones all the way to the new, extended edges.
The roofs were extended by first lagging regularly spaced blocking made from 4x4s to the original verge rafters, and then nailing new barge rafters onto the spacers. I also added widths of new roof sheathing. That was a painful process, but was needed for two of the three gable end walls, and it was one of the biggest challenges of the entire project.
Of course, the gable walls at the level of the attic have no need for insulation, but it seemed easiest to just continue the plane of the foam board all the way up to the peak. In the end, this proved a useful base for the addition of solid sheathing, a requirement for the plastic shake siding that was installed at the gable ends.
More shingle weaving was necessary when I removed our old metal fireplace, along with the entire upper part of its chimney.
Foundation and floor insulation
The foundation consists of concrete footers, concrete block frost walls, and poured slabs. Originally the foundation had no insulation. Using a rented excavator, I excavated the entire perimeter of the foundation down to the footers. Two layers of 2-inch XPS (totaling R-20) were added all around the exterior of the frost walls, extending vertically from the top of the footers to the bottom of the 2x4 sole plates. I installed sheet metal flashing, bent by a roofing shop, over the foam for its protection. The metal flashing extends well below grade.
The concrete slab is in two parts, with a split-level effect. The upper part supports the kitchen and living room. This part of the slab is still not insulated; it only has the R-20 perimeter insulation. The kitchen and living room floors had been upgraded with a mix of hardwood and tile just before I was bitten by the energy efficiency bug. There was no way that I was going to take up this brand-new flooring to add floor insulation.
The other part of the slab sits under the family room and utility room, and supports two stories. This larger section of slab was completely covered with a framework of 2x4 sleepers, with the long edge set vertical, and the framework was filled with XPS foam board. The XPS has a nominal R-value of 17.5, before subtracting a derating factor due to the wood framing.
Net zero ready
The building used 6,772 kWh of electricity between May 2014 and May 2015. That demand could be offset by a properly oriented 5.5-kW PV array. In fact, within a few weeks the house will have a new PV system.
Now that my work is almost done, I'm really happy that I took the approach of reducing the energy footprint of the house as much as I could before adding the solar panels. It would have felt completely unnatural to me to employ PV back when my electrical load was a little more than double what it is now. Enough PV to meet the former load would not even have fit on the roofs.
Now the house will be net-zero, with some electricity left over to charge an electric car in the future. To me, the approach hints at, and shows some measure of respect for, the beauty of efficiency found in nature.
