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How Is a Home’s HERS Index Calculated?

It’s time to pull back the curtain to look at the algorithms used to come up with a HERS Index score

Posted on Jun 10 2011 by Martin Holladay

Anyone involved with the Energy Star HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. program has probably heard of the HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Index, a method of scoring the energy efficiency of a new or existing home. A Web page maintained by the state of Arkansas, for example, explains that the “EPA requires a house qualifying for Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. to be built with best practices, tight ducts, and at least 15% more energy efficient than code as shown by a HERS Index score of 85 or less as determined by a HERS Rater.”

Knowing that the HERS Index measures a home’s energy efficiency is a good starting point. But it’s useful to dive a little deeper, to understand how the Index is calculated and exactly what it measures.

Defining the HERS Index

The HERS (Home Energy Rating System) Index was established in 2006 by the Residential Energy Services Network (RESNET), a California-based national association of home energy raters and energy-efficiency mortgage lenders. The lower a home’s HERS Index, the more efficient the home.

To calculate a home’s HERS Index, a rater uses a computer program — most commonly, REM/Rate. (RESNET has also approved three other software programs for calculating the HERS Index.) After the rater has entered data about the home into the program, REM/Rate compares the home being rated to a “reference home.” The reference home is an imaginary home of the same size and shape as the home being rated. In other words, the size of the reference house is not fixed; when a house is being rated, it is always compared to a reference house of the same size as the rated house.

The reference home does not have the same window area or window orientation as the home being rated; instead, the area of the windows in the reference home is assumed to be 18% of the floor area of the rated home. The windows of the reference home are assumed to be evenly distributed on the four orientations of the home.

The reference home is assumed to barely meet the 2006 International Energy Conservation Code; if the home being rated has thicker insulation or better window glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. than the imaginary reference home, those improved specifications will result in a lower (better) HERS Index.

Lighting and appliances are accounted for

The HERS Index accounts for on-site energy production, if any, and energy used for lighting and appliances. The reference home is assigned a budget for lighting and appliances; if the home being rated includes energy-efficient appliances or lighting fixtures, these items can help lower (improve) the home’s HERS Index.

The imaginary reference home is assigned a HERS Index of 100. Most existing homes have a HERS Index over 100, since most existing homes fall short of the requirements of the 2006 IECC. If a rated home gets a HERS index of 100, it can be expected to use the same amount of energy as a code-minimum home of the same size — a home equipped with “typical” lighting and appliances that are operated according to average American usage patterns.

If a rated home gets a score below 100, it will use less energy than a code-minimum home. A score of 0 corresponds to a net-zero-energy home.

Each 1 point decrease in the HERS Index corresponds to a 1% reduction in energy consumption compared to the imaginary HERS Reference Home. A home with a HERS Index of 70 uses 30% less energy than a code-minimum home of the same size and shape.

“The HERS Index measures the relative performance of your home with respect to a home of equal geometry that is constructed exactly as the HERS reference home is constructed, using a standard set of appliances that are operated according to a standardized set of operating assumptions,” explained Philip Fairey, deputy director of the Florida Solar Energy Center. “The reference house has the characteristics of the energy envelope requirements in the IECC. But the IECC addresses only heating, cooling, and hot water, while the HERS index addresses all energy uses.”

The HERS Index rewards efficiency, not conservation

The REM/Rate software assumes a certain pattern of appliance use. “A house without a dishwasher is rated as if it had a standard dishwasher,” notes Fairey. “The rated home has essentially the same appliances as the reference home, unless the rated home has more appliances. The reference home has one refrigerator. If the rated home has two refrigerators, that can hurt you. The reference home has one clothes dryer and one clothes washer. But you don’t get a credit for building a home without a clothes dryer. The software still assumes that a clothes dryer is there, even when one isn’t. You’ll just end up using less energy than the software would predict.”

Although this method of defining the reference house rewards the use of electrically efficient appliances, it does not give full credit to those adopting a simpler way of life. “The HERS Index does not encourage Americans to downscale their lifestyles,” says Philip Fairey. “It does not reward conservation; it rewards efficiency.”

For Richard Faesy, an energy consultant for Energy Futures Group in Bristol, Vermont, the fact that the HERS Index is conservation-neutral makes him a little uneasy. “It has always rubbed me the wrong way that you could have a 10,000-square-foot ‘energy efficient’ home,” says Faesy. “That’s possible because of the way the reference home is set up — in effect, it compares the house to itself. It’s an issue that we have tried to tackle in the Vermont green building standards and with LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. . The philosophy there — and it’s a philosophy that came out of Vermont — is, if you have a big home you should have to work harder to achieve the green label.” This philosophy is not reflected in the HERS Index.

To Faesy, conservation is not a dirty word. “As we push down towards zero-energy homes, we need to be addressing this issue,” he says. “It is not only about how much are you using compared to how much you could use — it’s also about making some choices to do without.”

The HERS Index can’t predict energy use

The HERS Index is a useful metric; however, it doesn’t tell you how much energy a home will use. Of course, it’s a good sign if a home has a low HERS Index — but just because your house has a low HERS Index doesn’t mean that your energy bills will be low.

For example, if you have a very big house with a low HERS Index, all you know is that your house will use less energy than the typical very big house. But a huge HERS 70 house can still use more energy than a small HERS 100 house.

A 1997 Home Energy magazine article, “Home Energy Rating Systems: Actual Usage May Vary”, looked at the HERS rating, a precursor to the HERS Index. Under the old (obsolete) HERS rating system, high scores indicated that a house was energy-efficient, and low scores indicated that a house used a lot of energy — the inverse of the current HERS Index scale. In spite of the fact that the current HERS Index has inverted the scale, the two scoring systems share the same fundamental approach to rating homes.

In the Home Energy article, author Jeff Ross Stein wrote, “As a research project for Lawrence Berkeley National Laboratory, Alan Meier and I compared home energy ratings with actual utility billing data for about 500 houses. … In general, we found that HERS can be remarkably accurate at predicting average annual energy costs for groups of homes. Predictions for individual homes were less impressive. Some individual ratings significantly overpredicted or underpredicted energy costs, especially for older homes. Furthermore, there was no clear relationship between the rating score of an individual home and actual energy cost. … Technically, rating scores only measure a house’s individual potential for energy improvement; they are not designed to be used to compare different houses in the same way miles-per-gallon ratings are designed to compare cars. However, many consumers and HERS-related financing programs assume that houses with higher [better] scores will have lower energy costs. Unfortunately, houses with higher [better] scores, even when compared to houses of similar size, did not tend to use any less energy than houses with lower [worse] scores. … Occupant behavior is probably the single most significant determinant of actual energy use. HERS have the difficult task of making assumptions based on typical occupant behavior. Reality can easily diverge from these assumptions; predicted energy use or energy cost can be off by 50% or more due to occupant behavior.”

According to Philip Fairey, it's unfair to criticize the HERS Index for doing a poor job of predicting energy use. After all, there's more to a home energy rating than the calculation of the HERS Index. “The HERS Index is NOT the only requirement of a Home Energy Rating,” Fairey noted in a recent e-mail. “The RESNET Standards quite explicitly specify what is required to be included in a a Home Energy Rating Report, as follows: ‘303.3.2.3: The estimated annual purchased energy consumption for space heating, space cooling, domestic hot water, and all other energy use, and the total of these four estimates; 303.3.2.4: The estimated annual energy cost for space heating, space cooling, domestic hot water, and all other energy use, and the total of these four estimates.’ So, you see, a Home Energy Rating requires not only an estimate of the energy use of the home but also and estimate of the cost of that energy use.”

The HERS Index doesn’t take occupant behavior into account

Researchers who study residential energy use have long known that occupant behavior explains much of the variation in energy use from one house to another.

Speaking at the March 2011 NESEANorth East Sustainable Energy Association. A regional membership organization promoting sustainable energy solutions. NESEA is committed to advancing three core elements: sustainable solutions, proven results and cutting-edge development in the field. States included in this region stretch from Maine to Maryland. conference in Boston, building scientist John Straube noted, “Energy use variation between the lowest and highest energy users — the bottom 10% of users and the top 10% — varies by a factor of 2. A study of 17 identical homes in Oklahoma showed a factor of 3 variation in electricity used for purposes other than space heating or cooling. The high users consumed 3 times as much electricity as the low users. Even in Germany, where we expect everyone to always follow the rules, the ratio of domestic hot water use varies by a factor of 4 from the low-end users to the high-end users. That’s why REM/Rate and HERS Index numbers are just predictions.” Your actual energy use may vary.

Limitations of the HERS Index

Many energy raters have noted that it’s easier for a large house than a small house to get a low HERS Index.

Duncan Prahl, an energy researcher at IBACOS in Pittsburgh, described the problem in a technical paper, “Analysis of Energy Consumption, Rating Score, and House Size.” Prahl wrote, “One artifact of this method [HERS] is that as houses of a given occupancy get smaller, the energy consumption for space heating and cooling is lowered, and domestic water heating becomes a larger relative component in the final rating score. This is due in part to the fact that as houses get smaller, the ratio of envelope area to floor area increases. This has the tendency to make it more difficult for smaller homes to achieve the same score as a larger home, provided both houses have the same number of bedrooms.”

According to a Boston architect and building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. consultant who posts comments on the JLC Web forum using the name Ted S., “As the house gets bigger, the HERS rating goes down. I get failing small houses in my state's energy efficiency program all the time, but the bigger ones pass with the same insulation, windows, mechanicals, etc. I'm not supposed to promote that little fact.”

Another criticism of the HERS Index concerns the way that REM/Rate conjures up the reference house. Posting on the Q&A page at the Web site, architect Jesse Thompson wrote, “If we design a house that has 18 bay windows and dormers (chock full of difficult-to-air-seal, difficult-to-thermally-break and difficult-to-insulate construction transitions), that house then only gets compared to a better-built version of that same 18-bay-window-and-dormer house, not a simpler, less-surface-area-to-volume, easier-to-build house that will use less energy by design. So, two buildings with HERS 50 can end up with vastly different energy use, size, constructability, and final energy use. They won’t, in the end, be comparable to each other, even though that is the goal of the tool.”

Other energy rating systems

While the HERS Index attempts to provide a score to describe the energy-use characteristics of a home’s envelope, appliances, and lighting fixtures — an approach sometimes called an “asset value” scoring system — other home-scoring systems use historical energy bills to get a handle on a home’s energy consumption — an approach called an “operational value” scoring system. One example of an operational value scoring system is the EPA Home Energy Yardstick.

In an article discussing the merits of the two different scoring approaches, Philip Fairey and David Goldstein explain that the EPA Home Energy Yardstick is “a web-based statistical analysis tool that compares the reported energy use of a given home against data provided in the 2001 Residential Energy Consumption Survey (RECS) to determine the home’s percentile energy use compared against the reported RECS data, adjusted for climate, number of occupants and house size. … The Yardstick and the HERS Index have completely different (and complementary) uses and purposes. Neither one is necessarily correct or incorrect – they simply measure different things. The Yardstick uses utility bills to characterize the energy use of the home as it is used by its occupants. The HERS Index provides a measure of the relative energy use of the home based on its physical characteristics and a standardized set of operating characteristics. … An energy rating label based on the previous occupants’ energy use habits would say little to nothing about the physical energy attributes of the home and would make it virtually impossible to compare the physical energy attributes of one home against another.”

Energy consultant Allison Bailes has reached a similar conclusion to Fairey and Goldstein. In a comment posted on the Q&A page of the GBA site, Bailes wrote, “The HERS protocol looks at the physical parameters of the house and mostly disregards occupant behavior. It uses a set of standard operating conditions so that one house can be compared accurately to another. … The Home Energy Yardstick proposed by the EPA does the opposite. ... It’s great information to have, but not the same as the HERS rating at all. The two are complementary systems and would go well together. It’s not an either-or situation.”

Recent changes to the HERS Index

Because new data on residential energy use for appliances and electricity have become available, RESNET has voted to change some elements of the algorithms used to calculate the HERS Index. “There’s an amendment to the RESNET standards that has recently been adopted by the board of directors,” Fairey explained. “It changes a number of things in the current standards, which change the results some. Larger homes don’t have as big of an energy budget for appliances and miscellaneous energy use as they used to, but reasonable-sized homes — 1,600-square-foot homes and 1,800-square-foot homes — are about where they were before.”

The issue of whether the HERS Index should be tweaked and recalibrated as energy use patterns change is contentious. “The HERS standards assume that 10% of a home’s lighting energy use is from fluorescent lighting — tubes, CFLs, whatever,” Fairey noted. “If incandescents become illegal, I don’t know where RESNET would go. These are difficult balancing acts that RESNET has to concern itself with. If you change the basic lighting budget now, within a few minutes, a builder will call you up and ask, ‘Why is my house design no longer compliant with Energy Star?’ or ‘This house used to get a HERS 60, and now I only get 65.’ ”

According to Fairey, it makes sense to keep changes to the HERS Index to an absolute minimum. “RESNET has not seen a need to increase the efficiency of its reference case,” said Fairey. “If anyone wants to raise the bar, it is very easy to do: just aim for a lower HERS Index.”

Last week’s blog: “New Air Sealing Requirements in the International Residential Code.”

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Jun 10, 2011 8:00 AM ET

Edited Jun 10, 2011 8:11 AM ET.

Reference Home Windows
by John Brooks

The Reference home "shell" may have the same size, perimeter and surface area as the home being rated.
It does not have the same window area or window orientation as the home being rated.

I believe the reference home has 18% window to floor area and the windows are evenly distributed in orientation.
An easy way to improve the HERS index is to design less than 18% window area and to carefully orient and shade the windows.

AND... your design will consume less energy

Jun 10, 2011 8:26 AM ET

Edited Jun 10, 2011 8:34 AM ET.

Wall and Roof Orientation
by John Brooks

I have recently started playing with REM/Design (very similar to REM/ of the software models used with the HERS Index)
The software does account for window orientation and window shading...
But as far as I can tell it does NOT consider the orientation or shading of the Walls or the Roof planes.

I think the REM/Design software does seem to be very useful... but not perfect

Maybe it will be a good tool for designing the HalfassiveHaus
perhaps REM/Design can be used as the HHPP(HalfassiveHausPlanningPackage)

Jun 10, 2011 8:29 AM ET

Bailes' Misunderstanding
by David Fay

Allison Bailes' conclusion that the EPA Home Energy Yardstick "ignores the physical characteristics of a house and looks only at the effects of occupant behavior" perpetuates a misunderstanding of the Yardstick. The Yardstick most certainly does reflect the physical characteristics of a house: if I add insulation to my house while keeping my behavior the same, the Yardstick score will get better. The important point is that the Yardstick reflects both physical characteristics and behavioral choices, whereas the HERS score holds the behavioral choices constant, or at least tries to.

A more important difference between the EPA Yardstick and the HERS score is that the former measures actual energy usage, while the latter is based on predicted energy usage from an engineering model of the building. But that prediction is only as good as the energy model and, as Fairey and Goldstein note, the HERS energy model is not well calibrated to actual energy usage.

David Fay

Jun 10, 2011 8:40 AM ET

Edited Jun 10, 2011 8:45 AM ET.

Response to John Brooks
by Martin Holladay

Thanks very much for correcting my misunderstanding about the window area and window orientations in the reference home. I have edited the text of the blog to reflect your correction.

Jun 10, 2011 8:48 AM ET

Version 3
by John Brooks

It looks like ES version 3 is changing the glazing parameters for the reference home

Jun 10, 2011 8:58 AM ET

Not the Same Shape and Orientation
by John Brooks

The Reference home may have the same surface area and perimeter as the home being rated...
It does not have the same shape and orientation

Jun 10, 2011 9:18 AM ET

Edited Jun 10, 2011 9:21 AM ET.

Another response to John Brooks
by Martin Holladay

I just got off the phone with Philip Fairey. We discussed the question of whether the reference home and the rated home have the same shape and orientation. The short answer is that they do.

The long answer is, "It's complicated." Not all RESNET-approved software programs use the same energy modeling engines to calculate the HERS Index. Differences in these energy modeling engines sometimes require different approaches to modeling.

Some models allow the reference home to have windows that face a different orientation than the walls in which the windows are set; other models do not. For the models that allow the reference home to have windows facing a different orientation than the walls -- a situation described by Phil Fairey as "models which don't require the building to close from a geometrical perspective" -- the windows are abstract mathematical constructs rather than windows that sit in a wall. The wall may face southeast, and be modeled as a southeast wall, but the window is assumed to face south -- without any explanation of how that is geometrically possible. It's a mathematical equation -- it's not a real house.

Other models will occasionally change the orientation of the reference home (but not the rated home) by 45 degrees or some other rotational amount in order to force the windows to face the true cardinal points.

Most RESNET-approved software programs allow but do not require raters to enter all of the data required to be sure that the reference house is the same shape as the rated house. Raters in a hurry sometimes simplify the house shape to simplify the data entry -- an approach which is evidently allowed.

Jun 10, 2011 9:33 AM ET

HERS Change
by Doug McEvers

Early on the HERS best score was 100 (lowest energy use), then the rating system was inverted so a 0 is the goal for efficiency. I believe this came about with the introduction of the Zero Net Energy concept.

Jun 10, 2011 1:30 PM ET

The limited nature of compliance tools
by Brennan Less

When I used to build a lot of energy models in REM/Design for retrofit analysis, I quickly learned the tricks of fast data entry. All above grade walls are entered as a single input. The analysis does not change at all when the walls are broken up into their actual proportions, because, as someone else mentioned, there are no orientation or shading inputs for the walls. If you want to apportion heat loss or heating/cooling energy use to a given wall, just calculate its proportion of total wall area. The calculations/simulations in REM/Design are not very detailed from what I can tell, nor are they dynamic. I don't believe it is an hourly simulation model, which means it is using some sort of degree-day bin analysis to pump out an annual energy use number, based upon numerous simplifications. You are not even provided, say a monthly energy use description, which would provide a little bit of useful info. Equipment sizing and design loads I'm sure are just based on ASHRAE 98.5% conditions. Unfortunately, none of this helps a building designer to understand the integrative, dynamic impacts of the decisions they make. You don't know what the temperature is doing in a given room with specific orientation, adjacencies, internal loads ,etc. This is the crux of the problem, REM/Design is not a design tool, it's just a compliance tool. Those looking for nuanced answers to complex problems will be befuddled.

A somewhat better option, I believe, is Energy Gauge by FSEC. Energy Gauge at least does hourly simulation using a DOE2 back end, which means that it is running through legitimate weather files, and has the ability to give hourly outputs for energy, temperature, etc. These features can make it more useful, while stil performing HERS score analysis. Sill other simulation tools are more useful, but do not provide HERS analysis. Of course, from what I can tell, none of these programs have reasonable models for natural ventilation/infiltration, which makes them next to worthless for analyzing dynamic ventilation options.

Jun 10, 2011 1:40 PM ET

A question for Brennan Less
by Martin Holladay

You wrote, "All above grade walls are entered as a single input." Are you saying that REM/Design can't distinguish between a long skinny house and a cube? Both houses are modeled the same?

Are you saying that the rater doesn't even enter individual inputs for the areas of the south elevation, the east elevation, the north elevation, and the west elevation?

Jun 10, 2011 3:38 PM ET

Edited Jun 10, 2011 3:39 PM ET.

Bailes's misunderstanding...or lack of coffee?
by Allison A. Bailes III, PhD

David Fay, you're absolutely right. I must have written that comment (nearly two years ago) before I'd had enough coffee to connect my writing with what I actually know. Of course the Yardstick takes into account the physical characteristics of the house. I really don't know how I could've written otherwise, even early in the morning.

Jun 10, 2011 3:42 PM ET

Wall inputs
by Allison A. Bailes III, PhD

Martin, yes, that's correct that walls get entered without orientation and without any indication of the shape of the house. The only way a rater can affect that would be to change the framing factor for a particular wall type. Windows get orientations and are assigned to walls, but if a house has only one type of wall, there could be only one wall input, and it would look the same to REM if it were a cube or a long skinny box or a complicated mish-mash of shapes.

Jun 10, 2011 3:43 PM ET

Response to Allison Bailes
by Martin Holladay

Thanks for your comment. To reflect your wiser, caffeinated view of the topic, I have edited the paragraph quoting you.

Jun 10, 2011 3:48 PM ET

Edited Jun 12, 2011 9:15 PM ET.

Another response to Allison
by Martin Holladay

My own shorthand term to describe a house that you refer to as "a complicated mishmash of shapes" -- what Jesse Thompson refers to as a house with "18 bay windows and dormers (chock full of difficult-to-air-seal, difficult-to-thermally-break and difficult-to-insulate construction transitions)" -- is a "crenellated house."

It is quite surprising that REM/Rate can't distinguish between a crenellated house and a cube. They certainly perform differently.

[Later edit: Actually, I see that REM/Rate does distinguish between a crenellated house and a cube, albeit crudely. REM/Rate knows that a crenellated house has more exterior surface area per square foot of indoor conditioned space than a cube, and therefore loses more heat on winter nights per square foot of indoor conditioned space than a cube. However, it seems as if REM/Rate doesn't account for the other problems associated with crenellation.]

Jun 10, 2011 4:14 PM ET

I like it!
by Allison A. Bailes III, PhD

The crenelated house. That's a good one!

REM's lack of input detail may be surprising, but it's true. There's a lot of work going on in the energy modeling world now, so I imagine we'll see some improvements in the future.

Jun 11, 2011 2:06 AM ET

Edited Jun 12, 2011 8:23 AM ET.

"It's Too Complicated"
by Kevin Dickson, MSME

Let's see, it takes Martin Holladay, one of the best explainers in this industry, 5000 words to explain HERS ratings. I'm just a small local homebuilder. How the in the heck could I ever hope to explain it to a homebuyer?

Since I want to build homes and not spend time explaining a rating system to customers that haven't taken the coursework needed to understand HERS, I will just reject HERS and LEED -H..

I can imagine a performance-based rating system based on actual home energy usage and temperature data. A rating cannot be made on an unbuilt house, so it favors builders that have actually built some homes and taken data. It would be a simple dollars per year figure like an EPA rating for appliances.

A custom home's unbuilt rating would have to be interpolated from other similar homes by that builder.

More details at

Jun 11, 2011 5:19 AM ET

Edited Jun 11, 2011 5:30 AM ET.

Response to Kevin Dickson
by Martin Holladay

I agree with you that actual energy use (measured in kWh, BTU, or dollars) is a much more useful measurement than HERS or any system (including Passivhaus) based on conditioned floor area. Area-based metrics allow large houses to be rated highly even if they use a lot of energy.

Good discussion on the blog page that you linked to. Thanks.

Jun 11, 2011 2:54 PM ET

Edited Jun 11, 2011 3:00 PM ET.

Excellent article, we can explain the variances if we try...
by Ted Clifton

Martin did a great job, even for 5000 words, of explaining the HERS Index Rating. He is correct that actual energy use can be all over the map, in relation to the HERS Index Rating. Those variances can usually be explained in relatively mathematical terms, if you look closely at the occupant loads and habits. They can be quantified, and will usually show that the estimated energy use predicted by the RemRate (or similar) program will be pretty accurate.

I recently ran numbers for our below-net-zero-energy home at the Painted Hills unit of the John Day Fossil Beds National Monument at Mitchell, Oregon. This is a Ranger Cabin at a National Park, and it is occupied by one person. The HERS score was -15, indicating that the house should produce 15% more energy over and above its usage than what a house built to the 2006 IECC would use. By subtracting the predicted energy use from the predicted energy generated by the PV, I calculated how much was surplus. That surplus would represent 15% of the usage of the 2006 IECC model house. I used that number to calculate the energy use predicted for the Code house, and from there I could generate a lot of other useful data. One thing I was able to calculate is what the HERS score would have been without the PV installed (yes, I could have just asked the HERS rater to provide that, but I just love crunching numbers..) Then I used the actual electricity production and actual usage numbers from the first four months of operation of the house to generate some interesting numbers.

Some things we learned:
1. The house would rate HERS 43 without PV, but is operating as if it were a HERS 27.
2. The house is rated HERS -15, but is operating as if it were a HERS -41.

On the surface, it looks like the HERS rating is really screwed up, but not so fast! The house is only being occupied by one person, and that person is not home at all during the day, except on weekends. The assumptions in the RemRate program would have this home occupied by at least two persons, and that would double the amount of laundry, showers, cooking, and many other functions of living. If we look at the difference in the HERS ratings without PV, and compare the predicted energy use for the occupant load related items, it is easy to see that for this home, each occupant would use the energy equivalent of HERS 16, and the envelope (for heating and cooling) would use HERS 11. Subtract 16 from the predicted 43, because there is only one occupant instead of two, and this house is performing exactly as designed.

Regarding the extra surplus energy generated, remember that approximately HERS 16 of that surplus is because there is only one occupant. I believe the rest (HERS 10 worth) is because the solar panels have been producing more energy during the cool spring months of the year, and that generating capacity may actually decrease during the summer. We are using February through May data at this point, so we need another eight months of data before reaching any final conclusions about the energy production. It is common to have year over year differences in production of more than 20% for PV systems on the same house simply due to weather variations.

In conclusion, I think the best way to do actual energy-use comparisons is to relate them to the HERS prediction, then use measurable occupant behavior to explain the difference. This can be done in a totally objective manner, and delivers repeatable results.

Jun 12, 2011 6:59 PM ET

by James Morgan

"Crenellation - a rampart built around the top of a castle with regular gaps for firing arrows or guns".... "A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval European architecture." So not a general word for architectural doodads, and only indirectly affecting energy use - parapet walls are prone to leaky flashings and generally more trouble than they're worth if you're not inclined to get into bow-and-arrow fights with the neighbors. I agree it would be useful to have a word for arbitrary complications of building forms that negatively affect their energy performance - name the beast in order to tame it. This one doesn't quite fit the bill though. Wish I had a better suggestion. Anyone?

Jun 12, 2011 9:08 PM ET

Edited Jun 12, 2011 9:12 PM ET.

Response to James Morgan
by Martin Holladay

You're being too literal, James. It's a poetic leap -- an attempt to sum up a concept in a single word.

(I can assure you that I know what crenellation is.)

I'm always open to the suggestions of my fellow poets and word-coiners, however. Invent away.

Jun 13, 2011 6:49 AM ET

by James Morgan

might hit the metaphorical spot a little nearer. Indicates an eruption of the primary enclosure, conveys a nicely opinionated sense of undesirable and ugly, no possibility of confusion with the primary meaning.

On the related topic: I'd like to suggest that not all dormers and bays are bad, if that's implied. I'm very familiar with the excess of these features in many spec plans, however I've found they can be useful additions to the architectural palate when used sparingly. The energy penalty need not be great when executed properly, in fact can often be less than adding additional square footage to achieve the same functionality. Example: a bay window we added to a room to be used as yoga studio provides needed occasional storage for students' belongings as well as esthetic enhancement. The alternative would have been to extend the full width of the room for two feet, not only at the studio level but also in the bedroom suite below. I should probably add that we do not glaze the sides of the bay when we project a window like this, and the light reflectance offered by the deep flank walls is glorious.

Obviously architectural complications like this make the HERS analysis even harder. We try to make our homes easy to build well but would never sacrifice lifelong functionality for constructional convenience. Similarly I'd hate to design a home based on how easy it is to calculate some abstraction of relative energy performance.

Jun 13, 2011 7:03 AM ET

Vocabulary proposals
by Martin Holladay

"Pustulated" will work. I like it.

Or one could go further -- how about "bubonic"? That is, covered with buboes.

If I'm in a fulminating mood, I can combine them all -- "This crenellated, pustulated, bubonic monster is an energy disaster!"

Jun 13, 2011 2:05 PM ET

More on REM and other modeling
by Brennan Less

Allison, thanks for dropping in and adding to my post, responding to Martin's questions about modeling surfaces/walls in REM. Martin, it should not be surprising that these tools don't account for all of the real world interactions of a crenelated home. They are simplified engineering models, which work best under steady-state conditions, in those situations where the building most directly reflects the underlying logic of the model itself. I think that this is why thermal performance in highly-insulated homes conforms well with modeling efforts, because the actual assemblies perform much more like the engineering model assumes they do. I would guess that REM assigns some boiler plate aspect ratio to the wall area that is entered, and performs some sort of weighted analysis to attempt to account solar insolation effects...maybe that's wishful thinking. Of course if the whole purpose of the tool is the production of an asset rating, which is a comparison to a baseline home that doesn't have the same orientation as the real home, then who really cares about the orientation of the individual walls...accuracy, to some extent, has already been thrown out the window. Again, this is why I prefer other modeling tools, whose sole purpose is not the production of an asset rating.

Jun 17, 2011 1:40 AM ET

HERS is a moving target!
by Kevin Dickson, MSME

Note that the same exact home rated in 2006 will have a different score in 2012. What?

If you have a hard time grokking that, then just take it to a logical end point:

Eventually the IRC code may require all homes to be zero net energy cost. At that point the reference home will have a HERS score of 100 and Zero at the same time. Then you will have one worthless standard rating system.

Jun 17, 2011 5:14 AM ET

Response to Kevin Dickson
by Martin Holladay

Your statement, "the same exact home rated in 2006 will have a different score in 2012," is not correct for most homes. The only adjustment to the HERS Index since 2006 was a recent minor tweaking of the budget for appliances and miscellaneous energy use for large homes. Most homes still score the same as when the HERS Index was first introduced in 2006.

Do you have special information about an impending change in 2012? If a major change is in the offing, that would be a surprise to me and Philip Fairey.

Note that changes to the IECC and IRC do not affect the HERS Index, since the HERS Index remains anchored by the 2006 International Energy Conservation Code. There is no automatic mechanism that changes HERS Index calculations as the IECC changes -- nor is there any political movement to make that happen.

Jun 17, 2011 12:15 PM ET

Energy Star
by Kevin Dickson, MSME


Thanks for setting me straight. Energy Star is tied to the IECC/IRC, and changes as the code changes. Then I made an incorrect leap to HERS. Is LEED-H anchored to 2006?

Jun 17, 2011 12:52 PM ET

Second response to Kevin Dickson
by Martin Holladay

As far as I know, the energy points awarded under the LEED for Homes program are tied to a home's HERS Index. Of course, nothing prevents the USGBC from changing the energy requirements or scoring system used in LEED for Homes in the future.

Jun 20, 2011 12:36 AM ET

Who came up with this?
by Philipp Gross

HERS does not make sense to me.

1) Why give a number with no unit, if it must (or should) be based on physics and calculates energy usage (kWh).

2) Houses with nicely orientated windows do get a benefit, good shapes do not.

Obviously it is hard to predict the real energy consumption with any software because it is so dependent on occupants behavior. My only explanation why someone came up with the HERS system is that they were afraid of getting sued. Anyway my opinion is: Use kWh/ (SF a) and have a penalty if the ratio SF/Person is high. I also hope to see a Net-Zero movement. We know its possible and the up charge is not as much as most people think.

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