# Practical Design Advice for Zero-Net-Energy Homes

First of all, thank you very much to all of you who contributed to this thread. I agree with all of you completely.☺ Remember, the reason Internet discussions are so acrimonious is because the stakes are so low…

Last week, I published an article titled “Minisplit Heat Pumps and Zero-Net-Energy Homes” on GBA. At the end of the article I asked readers to submit questions on topics that they’re looking to learn more about so I could provide a “mini-consultation” and answer their question while hopefully helping others with the same questions. Here are answers to the five questions that I picked.

Q. How do you decide when ducted minisplits (rather than ductless minisplits) are worth the time and expense to install them?

Answer 1 — I’d like to combine the questions included in Comments #1, #5, and #13. The essence of the question is: When do we have to transition from a point-source heating strategy to a strategy that provides each room with its own source of space conditioning? (I say space conditioning because we may ask this question just as well about providing cooling.)

The first question we should ask is, are doors open or doors closed? And a subset of “doors open” might be a small transfer fan moving air from a warmer space where a heater is located to a room without a heater. (Robb Aldrich at Steven Winter Associates has done great work on this strategy.) Let’s look at “doors closed” first.

Say a 120-square-foot room has contact area through the floor with a heated room. (Let’s assume that the room under consideration is above the heated room.) Let’s assume that the room has 150 square feet of wall area that faces the upstairs hallway, which we’ll assume is open enough to the first-level heated room that it is at the same temperature as that room.

Each of those areas is has a thermal resistance of about R-3. So there is a UA of (120+150)/3 = 90. This is a room with a pretty high area of contact with the heated room. For every °F difference across these assemblies, there is 90 BTU/hr of heat transferred. Say you want that room to be 68°F when the heated room is 71°F. The available heat transfer is 270 BTU/hr.

If the room to be heated has a 15-square-foot window that has a U-factor of 0.2 (a good triple-glazed window) and a wall area to outdoors of 150 square feet with a U-factor of 0.03, a ceiling of 120 square feet with a U of 0.02, and an air leakage rate of 4 cfm, then the room UA to outdoors is (15×0.2 + 150×0.03 + 120×0.02 + 4×1.08) = 14.2 BTU/hr.

If we divide 270 BTU/hr by 14.2 BTU/hr, we find that (when the room is unoccupied) there is an energy balance when the indoor/outdoor temperature difference is 19 F°. With a desired indoor temperature of 68°F, that’s 49°F outdoors. Not impressive — and this is a superinsulated house.

Now, add some internal gains and things change. Put a person in there with an iPad and maybe an LED reading light and you more than double the available heat, and now you have a balance at 45°F to 50°F outdoors. But if you’ve been gone all day, and it’s been 30°F outside, you’re not coming into a warm room if you left the door closed.

What about if it’s open? That helps a lot. How many cfm move through an open door? I think it’s different depending on where the door is. If it is up a floor, it has more air flow through it than if it is on the same level — the difference in height between the heated room and the door opening helps that movement of air and energy. I noticed this recently in a house with a minisplit in the main living space, and an open door to an adjacent bedroom, and open doors on the second floor to two other bedrooms. The first-floor bedroom was noticeably colder. It had less conductive contact area too, and more exterior area, so all things weren’t equal; but it was striking how much cooler it was. (The main level was 72°F, the upstairs rooms were 68°F, and the first floor room was 63-64°F.)

I tend to think of an open door as being equal to a decent bath fan — it’s worth 50-100 cfm. So at the low end, 3°F temperature difference buys you 150 BTU/hr, and at the upper end it’s double that.

So Aaron’s 3,000 BTU/hr bedrooms aren’t going to be heated by a point-source heater, at least to satisfy most people’s comfort criteria. My guideline is that if people will tolerate 4°F lower than the heated space (which in my mind means 72°F heated space, 68°F bedroom) and they leave the doors mostly open, then a point-source heater is viable when the heat loss is 1,000 BTU/hour and the room is occupied; 1,500 BTU/hr is kind of my soft cut-off for considering it. Beyond that, I’ll provide some electric resistance backup in those rooms.

A couple of other thoughts to confuse the issue:

• How hot is the heater? The plume of very hot air off a wood stove creates a significant layer of very warm air at the ceiling level that drives more conduction and more convection. A wall mounted minisplit is the opposite extreme (well, maybe a radiant floor is) because it mixes the air in the room and there is little stratification.
• Who is going to live there? In speculative development with multiple units, there will be someone who hates you because you didn’t put heat in their room. I speak from experience.
• Cooling is harder. A cooling source on the main floor won’t cool the upstairs.

Q. The thermostat for our Mitsubishi ducted system only goes down to 63°F. Do you know of any way to reset the thermostat to have a lower minimum temperature?

Answer 2 — This question is an easier one. In Comment #12, Eric asked about his four-year-old Mitsubishi multi-zone system and how to set the temperature lower than the 63°F limit. Today we’d use the controller Honeywell provides to Mitsubishi, the MHK, which allows communication over the Web, and much wider setpoints. I re-confirmed today with Mitsubishi engineers that this control is unfortunately not retrofittable to the older system.

So my suggestion is that you try just shutting the second floor ducted system down at night completely, and let the main floor wall cassette do the work overnight. This would work well when you’re away for a longer period of time.

Another way, perhaps a bit cumbersome, is to figure out a way to put a tiny bit of heat right at the thermostat and fool the sensor into thinking it’s warmer than it really is. We’re talking about two bulbs worth of Christmas lights… or mount your iPhone charger right below the thermostat overnight.

As to your question on the flexible duct work — flex duct has a bad rap because it’s easy to install so badly. In your case, where the runs are short, it virtually assures you that there is very little duct leakage, and it’s not long enough to cut down on the air flow unless it’s been installed such that it is crushed or otherwise improperly installed. I’d leave it, as long as the rooms being served are being adequately heated and cooled. (Disclaimer: I just put flex duct in my own system — using a 5 or 10 foot flex duct runout to the register boot makes the system quiet — just stretch it out and don’t crush it.)

Q. When your heating and cooling loads in a net-zero home are already smaller than the capacity of anything but a single wall-mount ductless minisplit, how do you balance correct sizing with adequate distribution?

Answer 3 — Leigha’s question in Comment #17 is a variation of the first question in a climate that will need cooling as well as heating. The house sounds like a single-story. With the bedrooms to the north in this very well insulated passive solar house, I’m assuming that they don’t have a ton of glazing, which in this case drives heat gains from the sun, as well as heat loss. So I might be comfortable using just a single unit in the main space if the occupants understood that the bedrooms will be cooler than the main space in winter and warmer in summer, and if the rooms met my 1,000 BTU/hr criterion mentioned above.

Having said that, the system you mentioned that incorporates a wall cassette for the main space, and a ducted unit for the bedrooms would be a step up in occupant satisfaction for many people, because the bedrooms are now able to have their own setpoint. A ducted unit with the temperature sensor built into the air handler (the stock setup) averages the return temperature from the bedrooms to decide how much heating or cooling to provide. You also have the opportunity to incorporate some good filtration in that system.

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Here’s another approach: do the whole house as a single-zone ducted system. With a strongly passive solar design, the whole-house ducted system can serve to redistribute air from the warmer side of the house to the bedrooms.

As to limits to the length of ducts: these units don’t have the static pressure capability of traditional American central forced-air systems, so looking carefully at the unit specification is important, as well as good duct design. For example, Mitsubishi’s SEZ air handler (used on the Mr. Slim models) has a top setting of 0.20 inches of external static pressure — that’s low! The Fujitsu RLFC series has a capability of 0.36 inches on the smaller air handlers, and lower on the largest one in that series. So you have to check.

Q. Our current energy load calculations show around 50,000 BTU/hr for heating and cooling. We were considering forgoing cooling, but with the Daikin Altherma system we could possibly have it as an added bonus to domestic hot water and radiant floors. Unfortunately, the initial cost has us leaning towards a condensing gas water heater.

Answer 4 – Eric’s question in Comment #4 is about the economics of comparing a gas condensing water heater with a Daikin Altherma, which can also drive a radiant floor and make domestic hot water, and can provide chilled water for cooling in the summer. Before I address this, may I comment that a design load of 50,000 BTU/hour for a house measuring 2,100 square feet in Zone 5 seems way high — at 24 BTU/hr/sf, maybe even higher than a code-minimum home today. My quicky guideline that I tell people is to set a target of 10 BTU/hr/sf in Zone 5. So make sure your load has been properly calculated; and if it has, first spend your money on reducing that load.

It sounds as though the base solution of the gas water heater has no cooling, but even though the Altherma offers it, you don’t need to implement it, especially since it means a second distribution system — added to the quite costly concrete/steel floor hybrid, you’ll need a ducted system for the cooling.

To begin looking at costs and benefits, you need to compare fuel costs. If the gas available is natural gas at current fuel prices (let’s say \$1.25/therm) and the heater operates at 90% efficiency, then the heat costs about \$14/million BTU. Say your electricity costs are \$0.15/kWh. To get the same cost per BTU, the Altherma needs to operate seasonally at a Coefficient of Performance of about 3 — three units of heating for every one unit of electricity used. I don’t have data on Altherma units other than a couple of retrofits with very extenuating circumstances, but from the published engineering data I think an annual COP of 3 in Zone 5 in a low temperature hydronic radiant system is plausible.

So at that point you’ve spent more in capital cost than the gas system for similar annual cost.

What the heat pump offers you is the possibility on making your own energy with onsite renewables, which you can’t get with the gas. Buying the renewables is a separate economic decision: What does electricity cost where you live, what’s the history of price inflation, what are incentives beyond the Federal 30% tax credit, etc.? The prices on solar electricity continue to drop, the systems are long-lived and reliable, and you’re investing in your own energy source. And even if you choose the gas heating system, you’ll still be using electricity, so that choice doesn’t preclude using onsite renewables; it just makes it hard to get all the way to net zero.

But please first work on getting that 50,000 BTU/hr down…

If it’s OK, I’m going to pull Peter’s question in Comment #9 into this, because he asks to compare minisplits with ground-source heat pumps (GSHPs). Peter, I approach this question in two different ways. The first is to recount some analysis done by the team who designed the Putney School’s 16,000-square-foot Net-Zero Field House. Being in southern Vermont, they felt that air-source heat pumps were perhaps on the edge of their applicability.

Fortunately, the HVAC engineers Kohler & Lewis from Keene, New Hampshire, had switched their office building to minisplits a couple of years earlier and had seen their system operate down to -15°F even though there was no published data at those temperatures. And then Andy Shapiro led the team through an interesting analysis. They asked, if the GSHP system could truly achieve a full point of COP higher than the minisplits (I can’t recall what values they picked for the two COPs, just that they were separated by 1!), what would the cost be of each heat pump system plus the cost of the solar electric system necessary to provide the energy consumed by the heat pump system? (In essence, the cost of the heating system cost plus its associated fuel cost over the years.)

If I recall correctly, the minisplits, even after conceding a higher COP to the GSHPs, came in something like \$4/square foot less than the GSHP system and the smaller associated solar electric system. And as Dana said in his post, we now have some really excellent hard field data about the COP of minisplits in cold climates that is really encouraging.

The second way I approach your question is to say that one thing I really like about the minisplits is how they are packaged systems from a single supplier, and are highly engineered as a system and therefore very reliable. GSHP systems are, at least where I have practiced, essentially custom engineered and installed, usually by several entities who have a shared responsibility to make sure the systems perform. Knowing that this will cause howls of au contraire to arise, I will say that GSHPs have been the most problematic HVAC technology I’ve worked with, and so I choose the Japanese air-source equipment without a second thought at this point (and I have worked with GSHPs on projects ranging from zero-net-energy homes to buildings up to 70,000 square feet).

Q. How exactly are these companies determining the capacities of these units? Is there a way to actually calculate, or even estimate, how much heat these units will actually output at -13°F?

Answer 5 – On ratings and Rheannon’s question in Comment #14: the only way through this is to get published engineering heating capacity from the manufacturer and then vet it through their representatives. If there are contradictions, pursue them until you are satisfied or until you reach a point where your confidence is so shaky you abandon consideration of that product.

For example, Dana mentions the Daikin Quaternity, and yet I can’t seem to ever find a capacity table below 14°F. That’s not low enough to make me comfortable in specifying it. I know some Daikin VRF machines have been observed operating at -15°F, and I’ve seen the Mitsubishi Hyperheats operating at -22°F at a school in west central New Hampshire. In Zones 6 there are increasing numbers of these heat pumps being installed. (We did some Fujitsus north of the White Mountains this year.) In Zone 7, I’d be looking at some backup heat. It all depends on the client’s appetite for risk.

### Tidbits on some of the rest of the posted questions

Machines that offer separate sensible cooling and humidity setpoints almost always will do that at a cost in efficiency, because to maintain a relative humidity setpoint without over-cooling a space they will use some form of reheating the air. This means they cool the air down to wring the moisture out, then heat it back up to avoid over-cooling.

They may do this cleverly — like using the rejected heat of the compressor for reheating — but this is not an efficient operating mode. It may be more efficient than operating a separate dehumidifier, but the two options would need to be carefully compared to make that determination. I know from experience that using the Dry mode in the Mitsubishi VRF equipment uses more energy than running that equipment in the Cooling mode.

I would tend to agree with Dana that most buildings with minisplits won’t need additional dehumidification, but there are always conditions where there is no cooling load but there is a moisture load, and if you can’t tolerate occasional excursions out of the ASHRAE comfort envelope (60% RH limit, if I recall correctly), then you need some independently settable humidity level. Scott, the best way to assess whether the minisplits work well in Dallas is to ask people who have been using them in similar applications to yours.

On point-of-use demand electric water heaters: Say you start with 45°F water and want a 1.5 gpm shower at 110ËšF — that load is just a smidgen under 15 kW, or 60 amps. So you need to consider electrical panel capacity first.

By comparison, a typical electric tank type water heater is 1/3 of that. You can buy the demand heater for about half the cost of a really well insulated electric tank type, and installation is probably a bit lower in cost. If you need two demand heaters, probably there aren’t savings over the tank type and no worries about trying to take a shower when someone starts the dishwasher or clothes washer or just turns the kitchen sink tap wide open.

When might a point-of-use electric be a reasonable choice? Small urban single-occupancy units where the laundry is in the basement. Basically, when only one person needs to decide which uses to satisfy with the single unit.

On EcoCute machines (Japanese air-to-water heat pumps): Yes, it would be great to get them into the U.S. I wonder if the barrier is a refrigerant that operates at 1,500 to 2,000 psi — What will UL say about that?

Marc Rosenbaum is the director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon and teaches a 10-week Zero Net Energy Home Design course as part of the NESEA Building Energy Master Series, you can test drive the course for free here.

1. | | #1

Thanks Marc.

Unfortunately your statement: "So at that point you’ve spent more in capital cost than the gas system for similar annual cost." is where I found myself when I started researching our mechanicals. I would really like to get away from gas, but for now and probably until my ROI year of a heat pump/solar system I think the annual costs will be similar.

As for our energy load.... I came to this number using rough calculations from a radiant floor heat design website and a manual J excel spreadsheet I found online. Obviously a professional will be doing these calculations in the near future, but with 4200 sq. ft. of livable space (2100 main floor & 2100 basement) I think I am not too far off (42000 btu/hr = 50000 btu/hr)? We are planning on doing PERSIST wall construction with R-30 walls, R-60 roof, and hopefully a less than 1 ACH which will hopefully equate to much lower energy needs.

Thanks again for the very informative Q&A on a topic/product that, I think, will be gaining a lot of momentum over the next few years.

2. | | #2

Thank you Marc and GBA
Thank you Marc and GBA

This blog follow up idea is the best thing to happen here at GBA.

And the subject for me is covering knowledge I am very much in need of.

Question to any of you that calculate energy use via UA units. Resheck which I just used yesterday for a log home I am the draftsman for is a very easy to use program that outputs au units. It also note that one has passed code and by what percentage. The UA units are listed.

Tell us what formula to use in converting UA units to btus lost per a given delta T for either the whole project UA and for the individual UA units it calculates and lists (walls, floors, windows, etc)

3. GBA Editor
| | #3

Response to AJ Builder
AJ,
Your questions about AU units have me confused. But I'm guessing that you really want to know about UA, not AU.

UA is used to measure whole-building heat loss in Btu/h°F.

Area x U-factor = UA

UA is defined as the "overall average heat transmission of a gross area of the exterior building envelope."

Multiply the surface area of each exposed component of the building envelope by its U-factor to get its "UA" value. Add up all the UA values to get a total building UA value.

UA units are Btu/h°F.

[AJ: I notice now that you have edited your original question, changing "AU" to "UA." That's less confusing.]

4. Expert Member
| | #4

But Martin, multiplication is commutative...
...thus UA=AU, and shouldn't be a cause for confusion, nicht wahr? :-)

Rescheck notwithstanding, what most of us really care about during the design phase is the AUF- total of A x U x °F, where °F is the delta between the conditioned space temp and the 99th percentile temperature bin for the site location, since that's the stake in the ground that the heating mechanicals and internal heat sources need to meet.

5. | | #5

design temperature is wrong
Dana, I will never use lowest winter temperatures to determine BTU use. To really save energy that is nuts. Use winter average daytime temperatures.

Don't argue this point. No super insulated home needs to combat the lowest temperatures. Homes can coast till warmer daytime temperatures.

Aj

6. | | #6

Great Post
Thanks Marc!

7. Expert Member
| | #7

99th percentile bin isn't coldest by any means (response to #5)
Meeting the heat load at the 99% outside design temp is often required by local code and vendors, even though the temperature decay time of high-R houses is such that undersizing by a small amount won't create comfort issues.

In an average year there will be 88 hours of temperatures BELOW the 99% bin, and in colder than average years 100s of hours below the 99% bin. The coldest temp in a given year can easily be 15-25F cooler than the 99% bin, or in a mild year 5F above the 99% bin. During this season's cold snaps a few locations have seen daily high temps that did not reach as HIGH as the 99% outside design temperature for more than 48 hours. If sized EXACTLY to the 99% bin it doesn't take much in the way of auxilliary heating or excess plug load to be comfortable in a high-R house, but the notion that you'll "coast" through that with a heating plant undersized by 10-15% is a bit optimistic.

8. Expert Member
| | #8

Daikin Quaternity is fine for Dallas (response to Marc)
On the prior blog my suggestion (in response # 7 to Scott Tenney) to look into the Quaternity series isn't much affected by a +14F low end of the capacity tables, given that in Dallas TX the 99% outside design temp is +24F, fully 10F above that number. Even the TX panhandle has design temps in the high-teens. Any location under the high-humidity influence of the Gulf of Mexico would be a candidate.

I would be a bit reluctant to recommend it for New England locations, but it's not out of the question for high-R houses in coastal NJ or Long Island NY where the 99% design temps are in the mid-teens, where the 1% design temps are only in the mid to high 80s, yet with summer time dew point averages in the high 60s.

9. | | #9

Standard A/C vs. Ducted Mini
The efficiency on the ducted mini will always be lower than a ductless mini but I wanted to know is it better to go with a DUCTED mini-split or a STANDARD A/C & Heat Pump setup ? What advantages are there with a ducted mini-split over a standard A/C with ducts?

10. | | #10

re: std AC vs ducted mini
Advantages of ducted mini vs std AC

1) inverter-driven compressors in mini's part one. Inverter-compressors allow widely variable outputs to match load as opposed to two-stage std AC high-end compressors, which improves efficiency and COP's in most operating conditions.

2) inverter-driven compressor in mini-part two. The inverter allow the compressors to "over speed" during extreme low-temp conditions. This allows the mini's to create more heat at lower temperatures than standard units and still have an acceptable COP. This in-turn allows minis to have no electric resistance backup heat for those extreme cold temperatures. Note Marc's anecdote about successful mini-operation down to -15 deg. F.

3) ducted mini's are available in sizes 3/4-ton and up, which better match many NZE home loads than std AC's which start at 1.5 tons and in the higher eff. units, frequently are 2 tons and up.

Disadvantages of ducted mini's vs std AC units

1) lower static pressure capacity, which means short, large ducts, large air filters, etc. They don't retrofit well to an existing std AC duct system. Many US HVAC contractors don't yet "get" mini-ducted units and don't know who to lay out the ducts/grilles to their best advantage.

2) mini's use proprietary thermostats. You can't just go down to Lowe's and pick up a standard programmable T'stat or get other 3rd party control systems. Note that this is slowly changing. Some manufacturers are now incorporating BACNet control protocol options in their units, which allows the units to communicate and be controlled over a very common commercial networked digital control protocol used by numerous control manufacturers. This flexibility could eventually trickle down to the residential level

As an EV owner, too, I see the mini/STD AC comparison similar to comparing a purpose-built EV vs an EV built on an existing ICE platform. The purpose-built EV has everything optimized for efficient electric operation (weight, aerodynamics, battery configuration, etc" vs a EV drive-train that has been thrown into an existing automotive platform. It just works better for doing what the primary goal is - optimal efficiency and comfort by breaking some traditional pre-conceptions on the design.

11. | | #11

Design temperatures that lower costs and energy use +
Dana, I hear you... aside from the codes. going green is going with less. at least to me.

The straight forward way to use less energy is to use less.
The straight forward way to less is to install less HVAC.
The same with water and water heating energy.
The smaller the tanked water heater installed the less water and energy used.
Same with all the rest of the home. A smaller home is greener, basic math and science.

Not you necessarily Dana, but crazy talk to anyone who buys the latest SUV so that their kids and family aren't embarrassingly the talk of the town from dropping off kids at school in an unapproved whatever lessor vehicle. Seems much of this site's discussions center around the needs of the SUV owning 3500sqft homeowners that somehow want to go "green" because it's in the news thinking they should save. How does one justify saving \$500 a year in energy costs when the rest of their spreadsheet is filled with multi thousand dollar expenditures to live the Jone's life in comfort riding on heated leather seats in their SUV that starts remotely while checking one last time to see that our hair is perfect before stepping out the door to be... to be a well off "green conscience" approved member of one's community.

Less is more is simple green.
There a hundreds of ways to deal with a few colder than normal hours or days. Going green means doing anything except sizing an HVAC for it, code or otherwise.

That's my idea of green, though I know not that of most.

12. | | #12

AJ Builder
While smaller is a form of being greener, one must also be careful not to cast stones. There is always someone more "greener" than you. They can find fault even with your minimalist approach. We should embrace all forms of being energy conscience and not cast aspersions that one persons green approach is better than someone else. We can find faults in all approaches and the homeowner with a 3,500 sqft home who is trying to build an efficient home shouldn't be disparaged for doing so.

So yes, less is more and a form of being energy efficient but there are people who even take a deeper approach and could fault you for not being green enough and accuse you of not being a minimalist. They might live in a 500 sqft home and they find your 1,000+ sqft home to be grossly oversized.

The point is that the green/energy efficient movement shouldn't self-cannibalize and shouldn't cast stones at those looking to be more energy efficient.

13. | | #13

To Keith Ritter
Thank you for the information. Would a ducted mini be able to handle 3 medium sized rooms (16x16) or is that too much static pressure for a single ducted mini to handle?

14. | | #14

Less is more and there is no way around that
Peter, I am not against large homes. Just stating what I see as an obvious fact. Smaller homes, less children, less vehicles etc., is the lowest cost most effective way to being more green than most anything one can do.

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• ## Is NIST Serious About Net-Zero-Energy Homes?

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