Problems with the HERS rating system
GBA Editor
| Posted in Energy Efficiency and Durability on
I’ve been hearing a lot of talk of HERS score deficiencies lately. Mostly that it is easy for a large, not-so-efficient homes to obtain low scores.
Does anyone want to tackle the problems of the HERS rating system?
Is it time for a more comprehensive method of ratings homes?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
While it may be an overstatement to call the inaccuracy of the HERS rating system a problem there is no doubt that smaller, high efficiency homes score poorly as compared to plus-size moderate efficiency homes. The goal of the program is to get more builders to build with an eye to the thermal bypass checklist and to build homes that perform better for energy efficiency and by that measure the Energy Star HERS rating system has been a huge success.
Jim Sargent, one of the advisers here, has been collecting the energy usage data on his homes for years and they consistently perform better than predicted by the Energy Star software. This approach to measuring true energy consumption is the only way to accurately respond to the biases of the HERS system and to collect the data we will need to improve it.
The Green Homes section of this site has a place for reporting the actual usage of the homes and the potential to systematically compare real world results to HERS predictions. Let's all start collecting the data on the actual usage of the homes we build that we think out-perform the HERS predictions so that we can make that information available to those who are working to improve the scoring system.
Michael,
I have to disagree....smaller high efficiency homes can score very well compared to large moderately efficient homes....
It is not really the size that matters .... it is the parameters of the "reference house" compared to the house being rated that drives the "score"
Understanding your "reference house" can lead to better scores and offer insights into better performance...... In your case... I am guessing that triple glazing would boost big.
I find it interesting that Energy Star is moving towards actual Performance reporting with the proposed (HEY)Home Energy Yardstick and meanwhile the Germans have already gone full circle and are ranking their homes based on Energy Modeling of predicted Useage instead of actual Utility Bills.......
The Germans have confidence in their energy modeling and think that Utility Bills are too biased by Occupant Behavior....very interesting
John
I do understand that I can improve my HERS score by paying attention to the way it values different aspects of the design. For me the issue with the windows is more solar heat gain than U-value and I have been addressing that with exterior shading whichis not considered by their software. They also use only one "primary" heating system when multiple redundant heating systems are implemented so my solar radiant floors and air tight wood stoves are disregarded in favor of the 16 SEER heat pump back-up. No doubt the sheer quantity of glazing in relationship to the square footage area works against me with my particular set of homes but the point I was trying to make is that my larger homes inevitably have less glass per square foot and more volume inside the heated envelope so they tend to score better than my smaller homes of similar design.
I'm not all that bothered by this, which is why I say it's not really a problem. Still, I'm putting in Truvien energy monitors in a couple of recent homes to find out how they perform in comparison to the model so I can make more informed decisions going forward.
Michael,
I'm sure that you know this is a friendly conversation.
Exterior shading IS considered by the software...
One of the big reasons that HERS scores and performance do not match up is because the Energy Raters either do not know how to fully use the software or they do not want to use all of the features.
I have found that most raters are concerned with pass fail only and beyond that they do not want to waste their time tweaking the model.
The Reference house is considered to have no shading ....
If you are shading your windows and not getting credit then you are leaving point reduction on the table.
If you do not have a Ratings Standards document ..let me know and I can email one to you.
I am not looking to game the system ... but rather to understand it and use it as a tool to improve my homes.
If you are not scoring "low" because of your glazing ratio then glazing ratio may be a weakness in
reality.
I am not saying to reduce your glazing ratio...but maybe dial down your wall R-value and Dial up your window R-value and see what happens.
You may have tools to make more informed decisions now .. but may not be taking full advantage.
You might offer to pay your rater extra for the extra hassle .. I wore my rater out.
I need to find a new one and pay him/her better.
Michael,
Why do you say your window issue is more solar heat gain than U-value?
I might understand this if you lived in Austin.
The first source I googled showed that
Chapel Hill ,NC has 4,631 HDDays and only 741 CDDays
I would think that heating is your bear.
Are you experiencing a significant cooling load?
Maybe (un-doubtedly) I'm not reading the details right and need to do a lot more data mining but my sense is that the passive solar homes are pretty easy to heat but the glass is killing me on the cooling side. Thus the shading.
I'm very aware that folks who choose to "lean on" their raters can get lower scores than folks who just let the chips fall as they may. To me, so long as I beat 70, I'm not gonna fight it but some are hooked on beating 50 and collecting the tax credit. The windows I'm using are U.30 / SHGC.30 so it gets pretty expensive to dial it down much from there.
I think that at this point I really want to dig into some data logging and see where my assumptions are falling short. Bottom line is that I don't really trust the model.
Brett, the Home Energy Rating System (HERS) standards don't have a significant bias toward large homes. There are some parts of the calculations that can benefit larger homes, but it's not a given. For example, the rated home's window-to-floor area ratio hurts the HERS Index if it's above 18%, so a larger house might be able to perform better in that regard. I haven't seen much evidence of that, however. Most larger homes have more window-to-wall area than smaller homes, so just adding more floor area doesn't help much.
The big issue here is the misunderstanding of the purpose of a HERS rating and what the Index means. 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. This is the equivalent of the EPA mileage ratings that cars have. Just because a car's label says it gets 32 mpg, however, doesn't mean that's what any particular driver will get. Same goes for houses.
The Home Energy Yardstick proposed by the EPA does the opposite. It ignores the physical characteristics of a house and looks only at the effects of occupant behavior. 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.
RESNET, the organization that oversees the HERS industry, just came out with a document discussing the difference between 'asset value' labels and 'operational value' labels, and you can download it from their website (resnet.us). It's on the bottom of the page under Hot News.
Having said all that, I do believe the HERS rating protocol isn't as good as it could be, but that's why it's constantly under review and gets revised occasionally. But for now, it does a decent job of modeling the energy use in homes and serves its purpose well.
Also, on the issue of raters, raised by John and Michael, most certified raters have gone through a one-week training course and then do three practice ratings to get certified. Their proficiency with the procedures and the software is usually pretty weak until they've got a bunch of ratings under their belt. For the complexity entailed in energy modeling of homes, that's not much training, especially when it's someone who comes from an entirely unrelated background and has little experience with houses.
Allison,
I liked your description.
An interesting aspect of the HERS index ..... it is not really comparing your house to another house of similar size.... It is comparing your house to a geometric twin of your house.
Certain design strategies will not be reflected in the HERS index.
An 1800 sf house with lots of corners and extreme exterior surface area could score just as well as another 1800 sf house that has a very simple shape and modest exterior surface area.
The Simple 1800 sf house will likely use less energy ....yet will not score better.
I agree that HERS and HEY combined would be better than one or the other.
You've hit on one of my problems with the HERS technical standards. I've taught the HERS rater class 11 times in the past 18 months, and every time we discuss the reference home, we tell the students that it's the same size and shape as the rated home. And that's true, insofar as the reference home is analyzed using the same limited inputs as the rated home.
The lack of data inputs for the shape of the house, though, reduces the accuracy, and your example highlights that perfectly. Two 1800 square foot houses may look the same to the software but in reality perform very differently.
The number of corners and the number of openings can have a huge effect on the results, but we aren't required at this point to specify those data. I think it's going to come eventually, however. ENERGY STAR 2011, with its numerous checklists, will drive the HERS community to look more carefully at details that we're now ignoring.
REM/Rate, one of the two accredited software tools for doing ratings, assumes, for example, a framing factor of 23% for walls framed at 16" on center. A rater can go in and adjust the framing factor manually, but then we get back to the training issue as well as the how-much-to-charge issue.
In case folks have not seen it, Duncan Prahl from IBACOS in Pittsburgh did one of the more-referenced papers on this subject back in 2000: "Analysis of Energy Consumption, Rating Score, and House Size." Available online at: http://www.usgbc.org/Docs/Archive/MediaArchive/310_Prahl_PA772.pdf. The paper point out:
Our experience in developing standards for Summerset at Frick Park in Pittsburgh PA lead us to investigate the impact of a single energy performance standard for a range of house sizes. In the current method (RESNET 2002) used to determine Home Energy Rating System (HERS) scores, the thermal shell component areas of the reference home (i.e. walls, windows, ceilings, etc,) from which the point score is derived are the same as to the rated home, except for windows.
One artifact of this method 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.
As an architect who strives to build low-energy homes, HERS is a frustrating measurement device. It took some time engaging with the Passivehouse standard to understand the consequences of a rating system that compares a house design with a "reference version" of itself. Our problem with HERS is that it greatly discounts the efforts we make as designers to create energy efficient homes before a wall system or mechanical system are decided (including building form, siting, building orientation, etc.)
In other words, if we design a house that has 18 bay windows and dormers (chock full of difficult to air seal, thermally break and 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, constructibility and final energy use. They won't, in the end, be comparable to each other, even though that is the goal of the tool.
By comparison, having an absolute performance target (4.75 kBTU / ft2 / yr for example...) exposes all those design decisions we make early on in an explicit way, and allows design to help create the solution, instead of merely relying on swapping out different construction methods.
We see the same problem with the standard ASHRAE / LEED "percent better than code" measurement system. Absolute numbers by building type would be much more valuable.
Jesse Thompson
http://www.kaplanthompson.com
Jesse,
Your concerns and frustrations with the "reference house concept" are much the same as mine.
Kohta mentioned OCCUPANCY
I like the Passivhaus standard EXCEPT for the failure to recognize number of OCCUPANTS and CLIMATE.
It looks like Sweden is at least taking Climate into account.
The proposed new Energy Star standards seem to be offering a loophole for McMansions based on the number of "bedrooms"
I can imagine that we will soon be seeing McMansions with closets and egress windows provided for all of those popular McMansion rooms...
theatres,craft rooms, gfit wrapping rooms,butler's pantry....
A Cigar room with a closet and an egress window.
Hi. I share many of the frustrations of the members who have posted on this topic. As a veteran user of REM Rate and a provider of HERS rater training, I have much experience with the software and the HERS system. In my experience, larger homes definitely have an advantage when it comes to the HERS Index. The good news is that ENERGY STAR's Version 3 has a variable HERS Index target that makes it harder for large homes to qualify. I like the Passivhaus program except for the fact that it is a "win / lose" program. There is no in between.
For all its faults, Energy Star Homes is an excellent program that's getting better all the time.
For those who haven't yet, check out the new Version 3.0, which includes HVAC commissioning and water management, as well as envelope and appliance, standards.
http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/2011_Fact_Sheet.pdf
Very good to see the size adjustment factor showing up in Energy Star. It's amazing how much better and rigorous Residential Energy Star is than Commercial Energy Star (especially the actual performance testing of the constructed building instead of modeled behavior!).
KISS rule tossed to the bureaucracy wolves
mountains of paper, raters and irs agents lurking behind bushes,
endless classes to keep your rating badge
great system. Just tax the hell out of fuel and let the market respond.
What is wrong with simple... oh.. I know.. no goverment or education jobs created!!!!!
We've been infected with yet another anonymous nincompoop ideologue. Someone get the vaccine! or the pesticide!
And see my response to this idiocy at https://www.greenbuildingadvisor.com/community/forum/general-questions/17618/secret-hers-software-and-ratings-reports
And see my post on another simple way to eliminate unemployment and all our fiscal problems here: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/18988/water-wonder-and-danger