Net-Zero Homes, Part 2: How to Get to Zero
Choose the most cost-effective ways to lower your energy demand
In part two of this episode, the Net ZeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations. conversation gets real as Phil and I talk about how to think about the balancing act of increasing your building performance, decreasing your energy demand, decreasing your mechanical costs, and optimizing your cost. Of course you have to have the target of net zero in mind and the whole team has to be on board. We've even included a handy spreadsheet that Phil's office uses to help run the options and find that “sweet spot.”
In Part 2 of this episode, we discuss:
- The client is part of the team. The success of a net-zero home requires commitment from the owner.
- Know the simple strategy and stick to it: Reduce energy demand and make up the difference with renewables.
- Seal it up — bring down that ACH50. Phil chats about using a fogTo fog a room or building is to use a fog machine during a blower door test, revealing locations of air leaks where the fog escapes. The fogging material is usually a glycol-based solution, completely non-toxic. machine as a tool.
- Windows and doors — and daylightingUse of sunlight for daytime lighting needs. Daylighting strategies include solar orientation of windows as well as the use of skylights, clerestory windows, solar tubes, reflective surfaces, and interior glazing to allow light to move through a structure.
- Proper insulation — Remember the 10-20-40-60 rule.
- Control solar gain.
- Systems — find the one that's right for the house.
- Natural ventilation.
- What's your 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., before renewables and after renewables? (After renewables, it should be zero).
- Renewables make up the difference — likely solar, because residential wind is difficult.
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Remember to check back in later for Part 3, where we really get into the details of making net zero cost-effective. We'll also share some ideas, and Phil shares a great song from the new Bright Eyes album. And of course part one tells you how to make our cocktail of the episode.
This is Part Two of the net zero series, and we’re going to focus on how to get to zero. We’ve talked about what it is and who’s doing it. We’ve talked about our cocktail, the Irish-American. It has been refreshed and we’re ready to move on.
Chris: Phil, how do we get there? If a client walks in and says, “Hey, I’ve heard about net zero, and I want to be net zero” — what would be your next sentence?
Phil: Right on! Whether you asked me or not, we were headed that way anyway. And it is simple. We know how to do it, but it’s about commitment. We’re doing the things we already know how to do and what you and I have been talking about forever. It’s less about the electrical loads and lighting, and more about reducing our energy demand as much as possible
Chris: Yeah, we’re usually on the big three: air sealing, windows and doors, and insulation. It’s all about buttoning up and reducing your demand. I always like to throw in working with the sun.
Phil: That’s part of it. Here we’re dealing a little bit more with human behavior, but we’re also dealing with our electrical load. Net zero is simple, but a lot of it is up to the person. The best clients are the excited, educated ones — excited about doing something green — and we’re getting more of those.
Chris: Everyone is moving in that direction regardless of whether they want to or not. The goal is to reduce the energy demand as much as possible, and make up the difference with renewables. It’s really that simple. Jamie and I were doing a presentation and he pulled out a great spreadsheet — I have a similar spreadsheet — where you’re playing the “what if” scenario. The envelope is this, and the demand is this; therefore your mechanical system is this. You can almost chart where you start to go overboard with insulation, and you can chart where you can find that sweet spot.
Phil: It’s all about the sweet spot. We can find it in a net-zero optimization spreadsheet. The goal is to make it and put it in a PHPP spreadsheet and make it another tab. All it is, is an Excel spreadsheet.
Chris: It maximizes the optimization of your insulation to the reduction of the mechanical system so that the cost for your mechanical system is low and the cost for your insulation and other features is not that high. That’s the sweet spot.
Phil: And the n the numbers will show you need $160,000 worth of renewables to get to net zero. Then you’ll bring it up to a R-40 wall and R-60 ceiling, and your renewables will come down to $40,000. Here’s the cool thing. I’ve had this conversation with Martin Holladay — at some point you should just buy more PVs and make it easier on yourself.
Chris: Usually that conversation happens when you’re at the Passivhaus level. And you have the Passivhaus versus net zero discussion.
Phil: Let's go through the list. To reduce the energy demand, let's get airtight. There's the "big three," and number one is "airtight."
Chris: That’s a wave of construction quality that is important. People know ACH50 now. If I say "ACH50" in a group of peers, 25% of them know it.
Phil: Here’s a cool thing. We’ve only done it with one house, but I see people advocating for it: fog machines. You want to see a builder get on board quick: fill the house with fog, and send him on the outside of his house where he can see where his house is leaking. It’s kind of a commercial trick. The fog machine showed us the usual culprits around the doors that weren’t well sealed yet.
Chris: You’re doing the opposite of the blower door. With the blower door you’re depressurizing the house and measuring how quickly it re-pressurizes. You get digital numbers at the door. With a fog machine you’re getting a fog machine on the inside and filling the place with fog, pressurizing the house and seeing where it leaks out. You stand at the outside of the house and you go, "Holy cow."
Phil: Airtightness. Windows and doors. We can say a little more about net zero. We can talk about optimized 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..
Chris: You’re trying to optimize day lighting so you’re minimizing the electrical usage of your lights. With net zero, every watt counts.
Phil: We add daylighting to the list. It’s not just optimizing your glazing. It’s also daylighting. It’s kind of different, but it’s a corollary. Moving on to proper insulation.
Chris: If you’re doing net zero, you’re at least doing 10/20/40/60 — the recommendations from the Building Science Corporation guys. Real quick: R-20 at the basement walls, R-40 for regular walls, and R-60 for whatever is above, and R-10 for under the slab. It’s not standard, but it's a good rule of thumb.
Phil: We find that people are often curious how we came up with that. When we do our cost-optimization spreadsheets, we generally end up in that area. R-40 keeps popping up — it’s kind of a sweet spot. Control of solar gains is more important to net zero south of us. They worry about overheating; you basically want to stick a big umbrella over the house in the summer.
Chris: Clever architects figure out a way to do cool shading, and get rid of it in the winter and bring it back in the summer. Deciduous trees are great. I don’t know if we talked about this in our windows podcast, but you must balance your desire to see out your windows. With your systems you’ve reduced your demand and you’re trying to find the system that is your perfect match.
Phil: That’s when we had our conversation about heat-pump COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. : coefficient of performance. You have a high COP, then you could do a heat-pump scenario. We keep talking about getting off of fossil fuels. Heat-pumps are a great way to do that. They’re taking the mechanical world by storm. There’s a great article in the latest issue of Fine Homebuilding about how to heat a low-energy home. It’s a Martin article.
Chris: Are we at the point when we talk about renewables?
Phil: The last thing I want to mention is natural ventilation. We talked about how different it is down South. Maximize comfort and minimize the need for mechanical equipment. AC is important, but if you can catch those prevailing winds, you may not need it as much. Know your site and have rooms that have access to that air. In the South, AC is necessary some of the time, and in the North we have to heat our houses. It’s not within the human comfort zone
Chris: Let’s talk real quick about HERS. It is the energy score from the 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. system. It’s a home energy rating system, and the closest thing we have to mpg for houses. In our climate zone, zone 6, if you get a HERS Index of 80, congratulations — you get an Energy Star label. You are 20% better than the code-compliant house. If I’ve got a HERS 40, you’re in striking distance of starting to use renewables and getting to net zero, with options of PV or wind. We’ve yet to do a podcast on wind, because residential wind is hard. You’ll likely only qualify for wind if your climate is too windy and brutal to be outside. Wind guys, e-mail me and tell me where I am wrong, and we’ll do a podcast on it.
Phil: If you’re going to do renewables, you’re probably doing PV. If you want to get your heat or AC low, start considering solar hot water, biomassOrganic waste that can be converted to usable forms of energy such as heat or electricity, or crops grown specifically for that purpose., geothermal. Those things all help out, but they’re not really helping your electricity load.
Chris: Biomass can be carbon-neutral but not net zero. All right, let’s call this Part Two and we'll wrap it up.
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