Okay, we ended discussing those items that had respectable K-Values. Now for those of you who are just joining us, K-Value is the ability to retain heat, and R-Value is the ability to resist heat transmission. (We talked about this more last week, so get a copy of last week's Step-by-Step.) Thermal mass is the relative measure of an object's ability to store heat. In my opinion the best item on the market today that has excellent thermal mass is concrete masonry. There is a growing trend to build passive solar homes from straw bales and even rubber tires that are filled with rammed earth. Personally, I think that it's great to have people look at these options, but can you imagine what your banker might say when you tell him?
"Excuse me, sir, but I want to apply for a loan."
"Well, that's great. What are you building your home out of?"
"Rubber tires filled with dirt."
Need I say more? Or how about your insurance agent - "I want to apply for flood and fire insurance on my house. I just built it, and it's really a great energy-efficient home."
"Oh, really? Let's see now, fire insurance... what's your home made of?"
"Bales of straw." (I think that the best thing that you can offer your insurance agent in good news is that you don't live on a ranch.)
Kidding aside, there are some positive aspects of straw bale homes, especially in extremely cold areas, because they do have a good R-Value. Face the facts - your home and the land you own are quite an investment, and many of these new ideas are great for the environmentally conscious, but I tend to believe that there is not a good resale market at this time for homes made of tires and straw bales. You need to be wise in looking at all aspects of selecting your building material.
Now, what about passive solar with concrete masonry? The simplest method of solar heating with masonry is called "Direct Gain." (See illustration of solar-direct gain.) Imagine what happens on a day when the sun shines through the skylights. Where does it go? It hits the wall in the back of the home. The back wall is storing the heat from the sunlight, and during the night, it will gradually release it, keeping your home a lot more comfortable, During the summer, you put shades on the skylight, preventing the back wall from storing heat, allowing the wall to release heat during the night and release cool air during the hot days. When materials with high-heat storage capacity are used for walls, floor and even ceilings, the performance and efficiency are increased because of both storage capacity and radiation ability. Shading devices on windows can affect the amount of solar gain, and heat loss can be stored by most insulation systems. Imagine if you will, a greenhouse attached to a regular home, with all of that glass space, and the dividing wall is a thermal storage wall of masonry. This system works on the same principle as direct gain and heat release, but the difference is that you have a much increased space for heat gain, allowing for greater heat storage and release.
Finally, there is the famous Trombe wall, developed by Felix Trombe in Odeillo, France. The Trombe wall, or vented thermal storage wall, was first tested in 1967, and was based on being a mass solar collector with a series of vents along the top and bottom.
We'll close early because I want to leave room for the diagram, we'll finish this article next week. Until then, see ya'!
Originally published in the Daily Courier March 30, 2000