As I sit here on a nice cool May evening, getting ready for my trip to New England tomorrow, I'm thinking about sweat. And that makes me think about air conditioning. It may seem like it doesn't make sense, but when you twist your mind around in just the right way, it does. Really! And what I'm thinking about air conditioning is that maybe we just need to stop talking about how many tons of air conditioning a house needs. Because there's a serious flaw hidden in that kind of talk. Allow me to explain.

See that sweaty back above? She'd benefit from some conditioned air, right? Notice that I didn't say 'cool' air. I said 'conditioned' air. The difference is important. An air conditioner, you see, does two jobs. It reduces the temperature of the air, and it removes moisture from the air. In my part of the world (the southeastern US), that second job happens to be pretty important. For the woman with the sweaty back, it's important, too.

Even though we all acknowledge the importance of an air conditioner's dehumidification capacity, almost no one talks about how much of that capacity their home needs and whether they have the right air conditioner installed to accomplish the job.

First, though, let's get the HVAC lingo right. We don't call an air conditioner's ability to dehumidify its dehumidification capacity. Nope. We call it latent capacity. If you've ever had a chemistry course, you may know why. Remember learning about latent heat of fusion and latent heat of vaporization? Those are the energies associated with phase changes, when a substance changes between a solid and liquid or between a liquid and vapor, respectively.

The latent capacity of an air conditioner tells you how much water vapor it can turn into liquid water. It accomplishes that phase change by passing your home's air over the evaporator coil, which happens to be cold enough that it's below the dew point of the air. The photo of the coil above shows this process in action. The cold refrigerant passes through copper tubes connected to the closely spaced aluminum fins you see in the photo. The condensation collects on the fins, drips down to the pan, and is removed from the house when it drains away.

Voilà! The sweat from the woman's back magically evaporates into the home's comfortably dry air, cooling her in the process. The sweat-turned-water vapor then finds the cold coil and turns to liquid again. The beautiful woman is cool and dry now...as long as her air conditioner was sized properly according to both the sensible and latent loads in the house.

So, let's get it right. You don't have a 3 ton air conditioner. You have an air conditioner with a 27,000 Btu/hr (2.25 tons) of sensible capacity (the AC's ability to lower the air temperature) and 9,000 Btu/hr (0.75 ton) of latent capacity. The air conditioner does two jobs, and there are two numbers to describe how well your air conditioner handles them.

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Photo at top by kencf0618 from flickr.com, used under a Creative Commons license. Photo at bottom by Katie Tegtmeyer from flickr.com, used under a Creative Commons license.

Smart people in the home-building industry have a saying about codes: A code-built house is the worst house allowed by law. The implication behind that statement is that if all you're doing is meeting the code, you're probably short-changing the people who will live in the house. The folks at the International Code Council (ICC) are doing their best to make sure that that barely-legal house is worth living in.

Consider this

If I told you that your new home had to meet the following requirements, would you call that a high-performance home?

• Be pretty darn airtight (3 ACH50 for all but the two warmest climates, zones 1 and 2, and 5 ACH50 for them) and be verified with a Blower Door test
• Have almost no total duct leakage (4 cfm25 per 100 square feet, the same as ENERGY STAR Version 3 for leakage outside the building envelope), as verified by a duct leakage test
• Include a mechanical ventilation system if the airtightness is less than 5 ACH50 (which catches all new homes except those that come in at exactly 5 ACH50 in climate zones 1 and 2)
• Increase attic insulation from R-30 to R-38 (climate zones 2 and 3) or R-38 to R49 (climate zones 4 and 5)
• Increase above-grade wall insulation from R-13 to R-20 (climate zones 3 and 4)
• Insulate above-grade walls to R-20 in the cavities plus R-5 continuous (e.g., 1/2" XPS foamboard sheathing) or R-13 cavity plus R-10 continuous in the cold climates of zones 6, 7, and 8
• Install really good, double-pane, low-e windows

I'd definitely call a home built like that a high-performance home. Those specifications are part of the 2012 International Energy Conservation Code (IECC), which you can read more about in Martin Holladay's 2012 IECC overview. The 2009 IECC was a big step up from 2006, too. When we adopted IECC 2009 here in Georgia, we instituted mandatory airtightness and duct leakage testing, so we're a step ahead on that requirement.

The big question

So, if the energy codes are getting so much better, what will happen with energy efficiency programs like ENERGY STAR? Since the mid-1990s, more than a million new homes have qualified for the ENERGY STAR label. The reason the numbers are so high is that the program set its requirements to an attainable 15% better than the energy code. As the codes get more stringent, ENERGY STAR gets more stringent, and home builders have begun to reassess their priorities.

Is it possible that for the many builders who have participated in ENERGY STAR and had their homes labeled with the well-known brand, satisfying the energy code will become sufficient? Will only the extreme programs, like Passive House, survive?

I answer that question with an emphatic NO. It boils down to two words: adoption and enforcement. The IECC is a model code, meaning that it's just a suggestion from the ICC. If no one adopts it, it's meaningless. For those jurisdictions that do adopt it, the code is only as good as its enforcement.

Even if the requirements were exactly the same, I'd take an ENERGY STAR Version 3 home over an IECC 2012 home. The Home Energy Raters who qualify ENERGY STAR homes, on average, have more training, knowledge, and skill in the process of verifying compliance with energy efficiency requirements and in finding building science problems than those who do building inspections for code. They are also subject to quality assurance rules that has someone looking over their shoulder and checking their work.

That's where we are right now anyway. The ICC is pushing hard, and states are getting their acts together. I hope that eventually the people who verify energy codes will have to meet similarly stringent training and quality assurance requirements as HERS raters.

If and when we get to that point, my answer to this big question may change. What do you think?

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We're making progress! With the all the emphasis on energy codes and energy efficiency programs like ENERGY STAR New Homes, more homes are getting Manual J heating and cooling load calculations these days. The intent is that the heating and cooling systems installed will be sized properly because oversized systems have problems (poor dehumidification, short cycling...). But just because an HVAC contractor does a Manual J, that's not a guarantee that the system is sized properly.

It's getting hot!

One of the ways that many HVAC systems get oversized is by someone using the wrong design temperatures in the load calculation. Here's the skinny:

• Homes lose heat to the outside in winter and gain heat from outside in summer.
• The rate of heat loss or gain depends on the temperature difference between inside and out.
• ASHRAE (a bunch of engineers who know a thing or two about HVAC) has a table of outdoor design temperatures for winter and summer.
• ACCA (the trade association for air conditioning contractors) bases its Manual J load calculation procedure on the ASHRAE design temperatures.
• Contractors running load calculations regulary override the recommended design temperatures.

The temperatures inside and outside your home are constantly changing. Sometimes the amount of heat your home loses or gains is low, and sometimes it's high. Even over the course of a day, the loads change. For example, we might hit a low of 70° F at night in the summer and a high of 96° F, and all through the day, the loads keep changing.

If we're trying to keep the indoor air at 75° F, the temperature difference is close to zero in the morning and rises to about 20° F in the afternoon. As the temperature difference (ΔT) changes, the cooling load changes. As it turns out, though, most air conditioners installed in homes cannot change the amount of cooling they provide when the temperature conditions change. Yes, some systems do have variable capacity, like mini-split heat pumps, but most are fixed capacity.

The 99% and the 1%

Enter ASHRAE's design temperatures. It turns out that if you have HVAC equipment with fixed capacity, this temperature will provide optimal performance. But what exactly is the ASHRAE design temperature?

Winter: 99% design temperature. This is the outdoor temperature that your locations stays above for 99% of all the hours in the year, based on a 30-year average. Turning it around, the outdoor air where you live is going to be colder than this temperature for only 1% of the hours in a year. That happens to be about 88 hours per year. In Atlanta, the 99% winter design temperature is 23° F.

Summer: 1% design temperature. Your location will go above this temperature only 1% of the hours in a year, again, based on a 30-year average. Here in Atlanta, that number is 91° F, so we go above that temperature for only about 88 hours in a year.

Occasionally, you'll run across something called the 97.5% and 2.5% design temperatures. It's not what you may think. Those numbers were defined differently and are not used anymore. Because the definitions were different, the temperatures are pretty close to the 99% and 1% design temperatures.

The difference between the outdoor design temperatures and the indoor design temperatures (70° F and 75° F) is the ΔT that should go into the load calculation.

In defense of HVAC contractors

HVAC contractors don't like to get called back because of comfort complaints, and oversizing solves many comfort complaints. Of course, HVAC contractors don't have control over the building envelope and usually don't know how much infiltration a house will have. Phil Mutz, one of the smart HVAC guys at Moncrief Heating & Air Conditioning in Atlanta, recently wrote about this issue:

Homes are rarely built as designed, meaning blocking/insulation/sealing almost always fall short of design. When the home is hot mid July months after a customer has moved in, the customer is calling and yelling at the HVAC guy... not the insulation guy.

With the testing and verification required in Georgia now, we should get a lot better at controlling some of these other issues for all homes. Programs like ENERGY STAR have required inspections, testing, and equipment sized to Manual loads for years now, so we do know this can work. As we get better data about the envelope, the loads in new homes will match the load calculations better, and HVAC contractors will stop being afraid to put in systems that they think are too small.

Check the numbers

Still, anyone getting a new home or doing extensive remodeling should insist on accurate Manual J load calculations and equipment that matches the loads. If you're a builder, homeowner, or HERS rater checking the Manual J reports, one of the first things you should look at are the outdoor design temperatures. The should match the 99% and 1% design temperatures from ASHRAE, which are also given in Table 1A in Manual J.

If the 1% summer design temperature is 91° F, as in Atlanta, a Manual J report showing that the load calculation was done with 97° F will result in an oversized air conditioner. Get your equipment sized for design loads, not extreme loads. Maybe it's getting hotter in recent years, but it's still best to use design temperatures based on 30-year averages — not the temperature it got to last summer for 3 hours.

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Photo at top by Steve 2.0 from flickr.com, used under a Creative Commons license.

I write mostly about buildings and the people who fight about them: the crazy things I find, the good things I find, the super-secret Building Science Fight Club, how I don't need no stinkin' Building Science Summer Camp. Just your standard energy geek fare. Occasionally I talk about peak oil and the Long Emergency. Aside from the few articles I've written about the green home I built, with its greywater system, reclaimed materials, and passive solar features, I haven't said much about sustainable living, though.

It's not that I don't believe in sustainability. Well, to be honest, I usually have doubts about anyone who uses the word 'sustainable' too much. If they marry that word to 'growth,' the doubts change to antipathy, and I start gnawing on the nearest sheetrock. Part of the problem is that the word 'sustainable' has become almost as meaningless as 'green' because people use it to mean whatever they want.

Reducing my environmental impact is something that I've strived for pretty much all of my adult life. The only car I've owned that had more than 4 cylinders was the first, and I had it for only 6 months. I lived in Florida for 9 years and almost never slept a night with the AC on. I can't bear to see organic matter or recyclables go into a trash can. Kermit may think it's not easy being green. I think it's not easy not being green. I have to do these things.

Another Allison has me beat on all of this, though. Let me introduce you to Allison Adams, a friend of mine I first met at the Georgia Organics conference in 2005. She lives in Decatur, bicycles and takes public transportation to work frequently, grows organic fruits and vegetables in every spot she can, keeps chickens in her backyard, and uses rainwater from her roof to provide water for all of her gardening.

She also knits, cans, plays music, and forages for wild berries. If you're thinking this sounds like someone right out of the Foxfire books, well...you're right, sort of. She grew up in Rabun County, Georgia and got a chance to write part of those back-to-the-land guidebooks when she was in school there.

The reason I'm writing about her here today is that I just read a wonderful article she wrote for Emory Magazine called Recalculating the Cost of Living. In it, she lays out one of the most cogent rationales for sustainable living that I think I've ever read. I couldn't do it justice if I tried to recount it here, but I will say that she does an amazing job of tying together the social, environmental, and personal factors. She makes it real. She shows that it's attainable. And she does it oh, so eloquently. There's a good reason she's able to make a living writing and editing.

I've been a subscriber to her blog, The Southern Urban Homestead, for a couple of years now, and I love her posts. She's written about many of the topics I mentioned just above (not above above, when I was talking about my stuff, but above - I know it's confusing with two Allisons in the same article). Some are practical, this-is-how-I-did-it articles. Some describe her philosophy of urban homesteading. Some are just fun.

An example of one of her practical articles is Slide Down My Rain Barrel, a description of the evolution of her rainwater catchment system. She originally had four rain barrels that just weren't up to snuff, so she described the solution that allowed her to water her garden only with rainwater all of last year. Just recently, she updated the story with a post about her addition of a solar-powered rain barrel pump.

On the fun side, she totally caught me last year with her April Fools' Day article. I love a good April Fools' prank, and my post this year on the new USGBC requirement that LEED-certified homes would have to be all glass was a lot of fun. I heard that I may have even snared a couple of folks at Building Science Corporation. That kind of stuff is in my blood, so you get a lot of points for tripping me up.

Her post was brilliant. She wrote that she was adding to her backyard menagerie. In addition to chickens, she had just recently gotten a bison and was going to lead the new backyard buffalo movement, as she's been a leading proponent of what she calls chicks in the city. Hey, why wouldn't I believe it?! That's exactly the kind of thing she would do!

If you haven't done it yet, you need to get over there and:

1. Read Recalculating the Cost of Living
2. Subscribe to The Southern Urban Homestead

Would I steer you wrong? OK, maybe sometimes, but if you subscribe to only one blog, it should be Allison's. And if you subscribe to two, it should be Allison's and Allison's.

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Photos of rain barrels and Nellie Oleson, the backyard bison, used with permission from Allison Adams.

It kills me when I see homes getting their windows replaced. No, I'm not referring to the FTC's recent slamming of the window industry for their overblown claims of energy savings. I'm talking about how a large number of window replacements miss a big opportunity that would help ensure they reduce energy usage as much as they can.

If you're getting a window replaced completely, not just replacing the sashes, the trim comes off and the old window comes out entirely. When the installer sets the new window in the rough opening, the opportunity presents itslef...but too often is missed. That gap between the window frame and the hole it went into needs to be air-sealed.

If you have old windows, they can hurt you on heating and cooling in several ways:

• Single-pane windows transfer more heat through conduction.
• Metal-framed windows are superhighways for heat conduction.
• Without low-e coatings, old windows allow a lot more of the Sun's heat to penetrate into the house (good for heating, not for cooling).
• The sashes of old windows are often loose in the frame and allow air to leak in.
• Air leaks in around the window frame in most older installations.

Replacing an old window with a new, insulated glass, low-e coated window can help with the first 4 of those problems. The last one in the list is the focus of this article. We've known for a long time now that we need to air seal homes and that it's just a myth that "you shouldn't make your home too tight." Yet a lot of new windows still get installed as shown below.

This was an older home getting some of their windows replaced (not the same house shown above). The installer thought it was sufficient to stuff bits of fiberglass insulation into that gap. The truth, though, is that fiberglass is not an air barrier and should not be used there. Low expansion foam is probably the best option for sealing that gap.

I'll leave the question of whether you should even bother replacing your windows for another article. (The quick answer is that it depends on what you have now and on your budget, but you probably shouldn't if you're just looking to make your home more energy efficient.) If you are replacing them, though, make sure to seal that gap properly. You're not likely to have that opportunity again for a long time.

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