Q: I want to build a passive solar home and have read some good books on the issue. My question deals with the need for thermal mass, which I understand is important because it stores heat and helps keep the home from overheating. I know ceramic tile or stone floors would be good for thermal mass, but I want to have wood floors (reclaimed pine). I read somewhere that several layers of sheetrock would be good for thermal mass. Is this true? What else could I use for thermal mass to avoid overheating? I also plan to have a fairly large stone fireplace, which might have some thermal mass benefit. Also, I will build the home in West Virginia, where I live. Thanks! (Dan, I love your Natural House book!)

A: Thermal mass is always a challenge and you may want to check out my new book, The Solar House: Passive Heating and Cooling, to read up on it. The chapter on solar design will walk you through the process of determining how much thermal mass you need and where to place it. It also gives some ideas for materials that are useful.

I'd almost have to see your floor plan to help you out. But here is some advice that may help. (Sorry if you already know this.) First, if your south-facing glazing (glass area) is 7 percent or less of the total square footage, you shouldn't need any additional thermal mass. Drywall, wood floors, framing members, furniture, etc.--all parts of your interior--will suffice. I call this incidental mass. It is only when you exceed the 7 percent glazing amount that additional thermal mass is required. Here are some ratios you can use to figure out how much mass.

For thermal mass in the floor (slabs, tile, etc.) in direct contact with the sun, you need about 5.5 square feet of four inch mass per square foot of solar glazing over the 7 percent limit. This mass, by the way, should not be covered with carpeting. For floor mass not in direct contact with thermal mass, you need 40 square feet for every square foot of solar glazing. This also pertains to uncarpeted thermal mass.

For wall mass, you need about 8 square feet of four-inch thick thermal mass for every square foot of solar glazing. To best effect, you need about 4-inch thick mass, which is why drywall isn't that great.

Two other rules: you need to distribute the mass throughout the building, for optimal comfort, but you also need to concentrate mass in areas that are in direct contact with the sun. This mass is much more effective at absorbing heat than mass not in direct contact. You might consider some interior masonry walls. If you have some divider walls, you may want to make them out of masonry, rammed earth, cob, or adobe. You could also place tile over the floor by the windows, then cover the rest of the floor with wood. Planters in the path of incoming sunlight also make for good solar mass. If there's a front entry way that receives sunlight, tile would be a good choice here. Your stone fireplace will serve as mass. Consider that wall mass. If it is in direct contact with sunlight during the heating season, all the better.

You might want to get a hold of a copy of Energy-10 software, or find someone who has it. They can run an energy analysis of your design that would be helpful in determining how the mass you have will perform.

Q: Also I am trying to determine the optimal number of windows for their traditional house. Right now, for a house that has 4990 ft^2 of floor space (first and second floor), they have a total of about 188 ft^2 of glazing. By setting the clear day solar gain equal to the heat-loss, I have solved for the optimal glazing area and determined it to be about 534 ft^2 using January as the design month. Does this sound reasonable to you? How have you determined optimal window area in the past?

A: As a rule, for solar home south-facing glazing (window area, not including window frames) should range from 7 to 12 percent of the total floor space, assuming you have a good amount of thermal mass inside the home to absorb the solar heat. The more solar heat you need, generally the more south-facing glazing. North- and east-facing glazing should not exceed 4 percent of the total floor space. West-facing glass should not exceed 2 percent of the total floor space. These are general recommendations that seem to work well in many climates.

Q: I am designing a solar passive in New Zealand, on a beach site where the abundant material is beach sand. Thoughts on using it as thermal mass would be appreciated.

A: (Kelly) Sand can be utilized as mass in many ways. I used quite a bit of the sand on my building site for this purpose, by placing it in earthbags, and then stacking the bags to fashion stairs, landings, parts of walls, etc. My sand is rather fine and slippery, so care had to be taken to keep it from wanting to shift its form. The sand can be "stabilized" with a bit of Portland cement to make it stay where you want it...this would also enhance its thermal mass property. Sand it also a major component of adobe, cob, rammed earth, cast earth, etc. so if you can find some local clay to mix with it, these techniques become possible. You could build your entire house with sand, especially if you live in a climate of moderate temperatures.

Q: I would like to create a passive solar home using a large aqua-sink as part of my thermal mass. The question is....can I use the aqua-sink as a cistern as well. This would be 4000 gallons in a concrete tank. Minnesota application.

A: Sure you could use a cistern as your thermal mass. Water, as you probably know, absorbs more heat per unit volume than concrete or other masonry mass. It might be tricky to use a cistern as thermal mass, however. You'd want the cistern inside, of course, and the cistern would have to be designed so that sunlight does not actually penetrate the tank. If it does, you'll very likely have a problem with algal growth in the cistern. Also, the cistern would have to be sealed. No open tanks, which present a danger to young children and pets (they can drown in tanks). A sealed tank also helps reduce moisture problems. A third problem is that a 4000 gallon cistern is very large and you will lose a lot of square footage to it. So the idea might work, but with these caveats and precautions. I don't think I'd try it, personally, but you may want to give it a try.

A: (Kelly) I might add that 4000 gallons of water might actually be too much mass for the situation, in that it might never really get warm enough, especially if it is used for domestic purposes where it would be regularly replenished with cooler water. If the main purpose were to keep the space cool, this might work, but otherwise it might not do what you want it to do.

Q: My question relates to the assumption that most (all?) of these designs incorporate wall mass to store heat and release it later. We are contemplating a build in Fiji, where the night-time temps do not cycle down like they do in the desert for instance, and the seasonal variations are small as well. Would I be correct in assuming that as well thought-out as your site's home designs are, to adapt for a Fiji climate, I need more consideration than just removing the mass?

A: (Kelly) The thermal mass incorporated in the passive solar designs at www.dreamgreenhomes.com should pose no problem for your climate. Such mass really just tends to stabilize temperature fluctuations, which would be enjoyed anywhere on earth. What would need to be altered in your climate would be the extent of glazing and shading devices to assure that you do not get too much solar gain, and perhaps increasing the potential for general ventilation.

Q: We would like to use ICF's to build a partially earth bermed home (West and North walls under ground). Will the ICF's work as a thermal mass if I expose a 4'X 18' "window" on a 10' X 20' wall built with ICFs? How much migration of heat in and out of this window might be expected? We will have direct solar exposure on this wall as well as an adjacent block wall.

A: The window you propose should let in a lot of sunlight, but I've found that ICFs don't provide any thermal mass. It's locked up by the foam. I toured a passive solar home built in Taos, NM from ICFs and the owner found that it didn't perform well at all. You can go this route, but will have to build mass in some other way -- for example, floor mass.

Q: Can there be TOO MUCH internal thermal mass? I'd say NO - it can only help to stabilize temps and reduce daily hi and lo cycles. And if it is all sitting on the solar slab, it becomes a "wing" of it.

A: Opinions are split over this issue. Some say "Yes, you can have too much mass" while others say, "No" for the reasons you give. I live in a passive solar home with 20 tons of thermal mass, and I say "Maybe."  Remember, when it cools down, it takes a long time to heat up. According to my sources at NREL, only the first three or four inches of mass is actually beneficial. You don't get much higher performance out of more mass.

Q: I am thinking of building a trombe wall into my house design in Galway Ireland. I saw on a green forum that if you are in a location where sunshine is intermittent, ie periods of no sunshine for several days, trombe walls will take the heat out of a solar house. Can you clarify this issue for me, as this issue of breaks in regular sunshine was not a con in your book The Solar House. By the way, the book is excellent.

A: A trombe wall will draw some heat out of a house if the sun doesn't shine for lengthy periods of time in really cold climates. If that's the case, I recommend that builders design some way to block the glass (with rigid foam insulation). I discuss this in the book.

Q: I really enjoy your books! I am working on 2 alternative passive solar home floor plans - a single story, and a more compact 2-story. My question is on the glass-to-mass ratio calculations. The first floor of each home would sit on an insulated concrete slab. The single story slab would be 1200 sq-ft. The 2-story slab would be 600 sq-ft with a 600 sq-ft standard framed wood 2nd floor.

I understand the single story glass-to-mass ratio calculations, but I am not sure how to interpret the calculations for a 2-story case. My instinct tells me to treat the 2 stories separately. The first floor south wall glazing could be as high as 12% of 600 sq-ft (72 sq-ft). I think the second floor should be treated as a "solar tempered" home with no more than 7% of 600 sq-ft (42 sq-ft) glazing on the south wall (unless I add more thermal mass material to the 2nd floor). Please let me know if this interpretation is correct and if there are any other special considerations for a 2-story home.

A: Your instincts are correct...so long as you can prevent heat from rising (through stairwells, for example) from the first to the second floor. This is exactly how I'd determine the glazing.  You get an A+...Sorry, I just got done teaching a class at Colorado College and am still in the teaching mode.

Q: Our unglazed Trombe wall built in 1978 is covered on the inside by drywall, has four vents (two top, two bottom ) and is 4-5 feet from the outside greenish, plastic panel (approx 8ftx 20ft). The area is roofed and has footings with stucco on the non-paneled areas. It has never worked since we bought the house in 1998. Where can I find information on dismantling this unworkable, eyesore? There is no floor in the 4 x 20 space, just rock.

A: What a mess! Based on your descriptions, this isn't really a Trombe wall. It is just a mass mall. And it sounds as if the person who designed this structure had no idea what he or she was doing. I don't think you will find information on dismantling this disaster. Any remodeller worth his or her weight in roofing nails should be able to assist in dismantling it. Alternatively, you might also want to send me some pictures. Maybe there's a way to make it work.

Q: How much does a direct-stick bamboo floor negatively affect the thermal mass properties of a concrete slab?

A: I don't know. I've never seen any data on this. Here are some thoughts, however. Wood and bamboo do provide some thermal mass and will conduct heat to a concrete slab beneath it. I don't know the R-value of bamboo, but the R-value of wood is around 1 per inch. I would expect bamboo to be pretty similar -- maybe a little less.  What that means is that wood and bamboo flooring will transfer heat to a slab below it. Nonetheless, I would expect some reduction in heat flow into a concrete slab but much less than if the space was covered with carpet or laminate flooring. The best product to apply over concrete slab used as thermal mass in a solar home is tile.