Green. Clean. Sustainable. Organic. Eco-[insert any word]. It’s hip to be environmentally conscious, and these days you can’t go anywhere without advertisers slinging these meaningless phrases at you to make you feel noble buying their products. OK, organic has a legal definition, but only if it’s food under USDA laws (or the equivalent in other countries). Anyone who tries to sell you an organic floor should be roundly ignored.

Instead of blindly aiming for a “sustainable design” or a “green solution”, I want you to determine your true target. So, here’s a little dictionary of legitimate eco-phrases you’ll encounter while planning your next project. Like the construction glossary, it’s alphabetical and (I hope) entertaining.

Carbon footprint: For you, it’s how much CO₂ your lifestyle releases into the atmosphere. (Here’s a great place to calculate it.) For a building, it’s the ongoing CO₂ emissions required to keep the structure running. The quantities of CO₂ released to build the structure in the first place are usually classified separately as “indirect” carbon emissions, though that makes them no less harmful. (See “embodied energy” below.) If something is “carbon-neutral”, then its carbon footprint is zero: it doesn’t release any CO₂ at all, or it sequesters as much as it uses.

Embodied energy: The amount of electricity required to manufacture an item. It’s a useful concept for determining whether a product meant to reduce emissions actually does. If a solar panel produces 20 million kWh over its 30-year lifetime, then it’s pointless to consume more than 20 million kWh to manufacture the panel in the first place. (In fact an average roof-mounted solar panel takes less than 3 years to recoup its embodied energy.) The concept is also applied to building materials like dimensional lumber and insulation. Some architects and builders seek to minimize embodied energy in their structures; conscious customers should request it.

Energy-efficient: Using less fuel than an equal alternative. If you hear that something is energy-efficient, you should always ask, “Compared to what?” A Prius is energy-efficient compared to a Hummer, but not compared to a bicycle.

Net-zero: Producing at least as much energy as it uses. It’s generally calculated on an annual basis. My old client Colin has solar panels that generate insufficient electricity to run his house on a cloudy winter day, but more than enough on a sunny day in June, so averaged over a year his property is indeed net-zero. (Importantly, Colin uses no propane or natural gas – all his heating comes from electric sources.) The concept is similar to carbon-neutral but uses a different measurement.

Passive house: A dwelling (someone told me never to use a word in its own definition) that takes advantage of building materials and natural patterns to use as little energy as possible. Walls are highly insulated, and materials with a high thermal mass store up heat during the day and release it at night. A heat recovery ventilator captures the temperature of exiting air and returns it to the building through incoming fresh air. The Passive House Institute publishes standards that a building can meet for recognition. Not to be confused with “passive solar” – see below.

Passive solar: A technique that provides certain comforts without help from machinery or the occupants. For example, a room with south-facing windows might use passive solar energy to maintain room temperature in the winter sans central heat. Skylights and mirrors can provide daylighting to otherwise dark areas, reducing electrical needs.

Payback period: The time it takes a technology to recoup the additional expense you incurred for it. If triple-glazed windows cost you $10,000 up front and reduce your heating bill by $1000 a year, then they have a payback period of ten years. Payback period is an important test of whether a technology is viable: the shorter, the better. You may also consider the “energy payback period”, which is how long it takes a technology to recoup its embodied energy. For some reason the energy payback period hasn’t caught on yet.

Renewable energy: A source of power that’s unlimited or easily replaced. Solar, wind, and hydro are renewable; so is wood if it’s harvested properly. Coal, oil, and gas are not renewable – there’s a finite amount on Earth and the reserves would take tens of thousands of years to replenish.

Takeback: The concept that when energy costs are reduced, consumers will use more energy than they would’ve used otherwise. Efficiency Vermont and other organizations have measured takeback, and found that for most people the increase of use is insignificant. In other words, cheap energy is a good thing.

Thermal mass: The ability of a material to store and release heat energy, moderating temperature fluctuations in an enclosed area. Technically it’s equivalent to heat capacity, but it’s rarely measured. Houses made of concrete or adobe or straw bales (yes they exist, and not just in the Three Little Pigs) are better than stud-wall houses at passive heating and cooling, because these materials have a higher thermal mass than conventional insulation.it to