There has been a lot of debate about which "sustainable" products are the best "value". The answer to this question may be influenced by utility cost, natural resource availability, whether the building is new or preexisting, and the quality of the construction. The debate includes technologies like radiant floor heating, geothermal, photovoltaic (solar electric) and hydronic solar water heating systems (solar thermal), wind turbine technology, methane accumulators for septic systems and other biomass based energy collectors, etc. I am going to focus on what is the most prevalent "alternative" system for homes: geothermal and radiant heating systems. It seems that geothermal systems are the first building block that people are using with their sustainable houses.
At this point you may be thinking, "What is a geothermal heating system?" Geothermal heating systems use stored heat from the earth's crust in conjunction with heat pump technology to move and amplify heat instead of making heat. The crust of the earth maintains the average temperature of that location annually, usually between 42 and 75 degrees Fahrenheit. Now to clarify, I am not talking about active geothermal (often called geothermic) heating systems, like they have in Iceland and other areas where there is thermal activity right near the surface of the Earth's crust. Conventional geothermal is using latent heat stored in the crust of the Earth from the sun. Yes, I said it; the geothermal systems essentially use solar energy in conjunction with electricity to heat the house.
Why invest in geothermal? A geothermal system will save 50-75% of the heating bill per year (varies by utility rates), and in cooling it will be in the range of a 30-50% decrease in cooling costs. This turns into real money every year coming back to the buyer; we are talking thousands of dollars! The cost of a fully installed geothermal system can be $12,000 - $15,000 higher than a conventional heating and cooling system (vertical well systems will be higher, and it always depends on the size of the house). But when we talk about the return on this investment (ROI) we are generally in the range of 5-10 years. I have had instances where, based on the fuels available, my clients may come in at 3 years, but that is rare. From my research these systems have the most consistent ROI of any of the systems we deal with.
Geothermal systems are available in forced air or hydronic units, making them quite versatile for either a conventional forced air, ducted system or for use in a radiant floor heating system with forced air for cooling. Most people are familiar with forced air heating and cooling systems. Radiant is becoming more popular every year because it is typically about 30% more efficient as a delivery method when compared to forced air. The reasons for this are very easy to understand.
We have always learned that heat rises right? Well, yes and no. Hot air rises. Heat travels through convection, conduction, or radiation. If you are heating a room with forced air there is generally not a consistent temperature in that room. There are going to be warm spots and cold spots AND it will be warmest at the ceiling. With a radiant floor, there is more consistency in the heating because the "hot air" is not rising. You are heating the floor, which heats the objects in the room thus heating the space. Most of the heat is traveling via conduction and radiation and not convection (hot air rising). The heat is kept lower spatially, generally where the people are, and you can usually keep the room temperature lower and "feel" warmer because the heat is literally everywhere. It's a pretty easy concept. Radiant heating also incorporates the mass of the floor like a battery of stored energy. Although these systems are more expensive to install than forced air systems, people who have lived in a radiant house seldom build their next home without radiant. It is not about the money, it's about the comfort.
There are several different ways to put radiant heating into the floor. Tubing can go in concrete, a lighter gypcrete, over the floor with furring strips, or into a ready to lay product like Radiant Panels. All of these types of radiant systems are generally compatible with a geothermal system. The general rule is that if the tubing is above the subfloor, it can be used with an "alternative energy" system whether that is geothermal or solar thermal. (Of course, always consult design software or a professional to be sure that your radiant floor can keep up with your heat loss). If you are putting the tubing in a "staple up" or below the sub floor type installation, there may be issues with meeting the heat loss through the sub-floor material with the lower delivery temperatures that the alternative energy systems can reach. Depending on construction type and room load requirements, you may have to choose the proper type of radiant based on the circumstances.