A Quality Measurement began offering City of Portland Home Energy Scores a short time ago. In our latest blog post, factors that are, or are not, considered by the score modeling software were discussed. Sometimes our assessors are surprised when a home that is expected to score above average (over a five, meaning it uses less than average energy for a US home) scores below average, or when the opposite occurs.
The City of Portland Home Energy Assessor Training indicates that conditioned square footage has the greatest influence on the scoring software, and on energy usage in general. But we wonder what other factors also affect the score and to what extent? The Home Energy Report that accompanies the score, provides a list of recommended improvements to raise the score a few points. The suggested upgrades are generally based on repairs that would pay off in under ten years, but what are the underlying factors that make one house score well and another seemingly similar home score poorly in comparison?
To answer these questions, we put together an experiment using a hypothetical baseline calculated to represent an “average” US home. The baseline characterizes average in every way including size, age, and so on. Specifically, it is a Portland home built in 1982 with two levels. It features 2,100 square feet with eight-foot ceilings and three bedrooms. The structure has not been professionally air sealed; it has an unconditioned attic with insulated heat ducts and a vented crawlspace with insulated ducts. There is a typical amount of insulation in the walls, ceiling, and attic (for its age). The south facing building has wood siding, a composition roof, and a normal amount of updated double pane vinyl window area with insulating low-e coating. There are no skylights. A typical efficiency 10 SEER electric air conditioner, an induced draft common 82% efficiency furnace, and a typical gas storage water heater are installed.
Using the Home Energy Scoring Tool, the baseline home scored below average with three of ten. The software recommended duct seal and air seal now, followed later by replacement of the furnace, water heater, and air conditioner with Energy Star label units to raise the score to a six and to save an estimated $394 per year.
It is surprising that this home scores below average, yet it is average in most ways including systems, age, and size. To find out why, we ran more than 25 tests, changing one element at a time to see what underlying factors influence the score most on this hypothetical Portland home.
|Repair Now, Not Repair Later||Sealed ducts and professional air seal as recommended under “Repair Now” of the Home Energy Score Report and skipping the “Repair Later” recommendations increased the score by two points. (Not the three points that were possible if all the recommended improvements were completed.)|
|Year Built||The control house is an average age built in 1982, but changing that to 1900 with no other adjustments surprisingly reduced the score by one point, while the score after improvements remained the same. This means that the modeling software is making some subtle underlying assumptions about a home based on the year built. However, do not assume that if you have an older home, it will score poorly. It really depends on the other factors discussed below. Also, if the home is “newly constructed” (never occupied), the City of Portland Home Energy Score Report will not provide any recommended improvements, regardless of the score. Not all homes are built the same. Some newer high performance or green homes score a nine or ten. Typical newer homes might score a six to eight, but some newer homes may score much lower.|
|Bedroom Count||Changing from three to four bedrooms increased gas and electricity use estimates slightly, but the score and recommendations remained the same. Changing to a five bedroom also did not move the score, but again increased utility costs slightly.|
|Ceiling Height||Changing the average ceiling height from eight feet to nine feet reduced the score by one point. It is easy for a two-story foyer on a typical home to increase the average ceiling height for the entire house by one foot. A greater volume of air requires more energy to heat.|
|Conditioned Space||Adding 500 square feet of conditioned space to the subject reduced the score by one point, but surprisingly, the control home could still score the same with improvements. Conversely, subtracting 500 square feet increased the score by one point.|
|Direction House Faces||The control house faces south, with most windows on the front of the house. Southern orientation of the windows is usually desirable in Portland allowing sunlight to heat the house in the winter when the sun is lower in the southern sky. Completely flipping the orientation to north reduces the score by one point, but does not change the score with improvements. If the windows on the house are optimized for direction at the time of construction, then the orientation should not matter.|
|Number of Levels||Changing from two levels to one level construction doubles the area in the ceiling and the floor where heat can escape from a house. Since it would be unusual to see a one level home with half the ducts in the crawl and half in the attic, our test was run with all ducts in a vented crawl space. This results in a score reduction of one point. The score with improvements was also reduced by one point in this scenario. Notably, the recommendations no longer include air sealing probably because pressure differentials are greater in two level homes, making air sealing more important. When the one level home was tested with ducts in the unconditioned attic, the score remained the same as a one level with ducts in vented crawlspace, but the energy use went down slightly having ducts in unconditioned attic.|
|Attic Insulation||The control home has six inches of blown fiberglass insulation resulting in R-19. Doubling that to 12 inches increases the R value to a much higher R-37, but the score remains the same with a one-point increase predicted following improvements. Conversely, if insulation of the baseline home had been insufficient to begin with, adding attic insulation would increase the score. Hypothetically removing all attic insulation results in a decrease of two points with the score report suggesting insulating to R-60.|
|Floor Insulation||The control home has typical R-13 floor insulation for its vintage. It is common to see older homes in Portland with no floor insulation. Removing floor insulation does not change the score in this case.|
|Wall Insulation||The control home has typical R-7 insulation in the walls for its vintage. It is common for older Portland homes to have no wall insulation. Removing all the wall insulation of our test home reduces the score by two points with the score report suggesting insulating to R-13. That results in an increase of one point after improvements.|
|Window Area||The control home has a generous amount of window area (400 square feet) for its size. Although the control has double pane insulated windows, walls are more effective insulators, so reducing window area by half was expected to increase the score. However, the score and the associated recommendations did not change. Had the windows of the control been single pane, the results of this test likely would have been different.|
|Low-E Glass Coating||Removing the insulating low-e glass coating, but retaining clear double pane windows results in a one-point decrease of the score, but the score after improvements remains the same. If insulating low-e glass is changed to solar control low-e glass (more common in hot climates), the score is also reduced by one point from the baseline. In this case, the score after improvements also remains the same.|
|Single-Pane Window||Exchanging the double pane low-e windows for standard wood frame single-pane windows results in a two-point drop of the score, while the score with improvements drops by one point.|
|High Efficiency Gas Furnace||The control home has a common gas induced draft furnace with 82% efficiency. When that is swapped for a typical Energy Star 98% efficient condensing furnace, the score is raised by one point and (surprisingly) the score after improvements is increased by one as well.|
|Electric Furnace||The control home has a typical (for age) 82% efficient gas furnace. Changing the gas heat to a typical electric heat pump with 7.1 HSPF (no change to AC efficiency) does not alter the score today, nor later after improvements. Having electric heat does not mean that you will receive a low score. In addition, electric heat does offer high efficiency options for future upgrade.|
|Oil Furnace||The control home has a typical (for age) 82% efficient gas furnace. Changing the gas heat to a middle efficient retention head burner furnace (also 82% efficient) results in no change to the score. Having oil heat does not mean that you will receive a low score. Oil furnaces can also be upgraded to a high efficiency oil furnace later.|
|Air Conditioning||Removing a typical efficiency (10 SEER) AC unit, and electing for no air conditioning increases the score by one point and increases the score after improvements by one point.|
|Ducts Insulated||The control home has 50% of its heating ducts in the unconditioned attic and 50% of ducts in the vented crawlspace. Removing the insulation from the ducts results in a one-point drop of the score. This factor is likely going to have a larger influence on the score depending on the percentage of ducts in unconditioned area and if the home has air conditioning.|
|Heating Ducts in Conditioned Space||The control home has all the ducts either in the vented crawlspace or in the unconditioned attic. Although the ducts are insulated, if this home had been built with the ducts inside the conditioned space of the structure, the score increases by three points, and after improvements it increases by one. It turns out that this is the most important factor affecting the energy efficiency of the control and of actual homes that our company has scored. If these homes had been designed with ducts in the conditioned space or with ductless heating systems, the energy efficiency would be dramatically increased. Unfortunately, it is not usually practicable to change the location of heating ducts after a home is built, but attics and crawlspaces can be insulated and conditioned for both energy efficiency and health benefits.|
|Solar PV System||The control home has no solar system. Adding a typical 4kw system increases the score by one point and by one point after improvements.|
These tests give a good idea of which factors influence the City of Portland Home Energy Score most and by how much. The features that tend to have the greatest influence also tend to be unfeasible such as changing the location of ducting or the overall size of the home. With this hypothetical example, it is easy to make a few improvements to bring the score from below average (less than 5) to above average (greater than 5). However, given the underlying limitations of the structure, with features like ducts in unconditioned space, it would be difficult for this home to receive an eight or higher without solar or other expensive modifications.
Remember that this is just one hypothetical house. There are many more variables that could alter the outcome of your individual score. Also, remember that if your home receives a low score, it does not mean that it is a bad home that no one will buy. Luxury SUVs get much worse fuel economy scores than a compact car, but people still buy SUVs and pay large amounts of money for them. Energy efficiency is just one factor in a complex home purchase decision.
If you want to learn about your home specifically, just have it scored. An assessment today may cost a couple of hundred dollars but, it could save you hundreds per year. Due to the City of Portland’s rule, eventually you will need an assessment to sell your home, so why not get it now? Click here to schedule a Home Energy Score today. Our staff will provide a list of improvements you can make to your home and the savings associated with those improvements, but we will not try to sell you anything.
Do you have input, or did you want me to run another test on the control home? Let me know in the comments below.
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Did you know, Gary Kristensen is an Approved Trainer with the City of Portland Home Energy Score program? He can speak at your real estate office, and can offer a unique perspective as both an expert on appraising energy efficient homes and as a certified home energy assessor.
Thanks for reading,
Gary F. Kristensen, SRA, IFA, AGA