Thermal Mass and Glazing

Ken Haggard, formerly an architecture professor at California Polytechnic, is an architect and principal in the San Luis Sustainability Group. Since the late sixties, Ken has designed more than 200 solar buildings, from homes to large commercial and institutional buildings—as well as the first permitted straw bale building in California. An active member of the American and International Solar Energy Societies, he received the Passive Pioneer Award from ASES in 1999 and was made a fellow of ASES in 2000. His office and home—in Santa Margarita, California—are passive solar, off grid, and straw bale. With David Bainbridge, Ken wrote Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting and More Using Natural Flows, published by Chelsea Green in 2011.

Questions and Answers

Q: I understand a wall is part of a building envelope - a system. A wall has various breaks in it that allows faster or slower transfer of heat thru the wall (doors, windows, vapor barriers, cracks in stucco, etc,). If one were to eliminate all wall penetrations and compare a 2 x 6 frame/stucco wall to a 2 x 4 wall to an ICF wall to any other wall system, which wall, overall resists the flow of heat best? The answer should assume Tucson, AZ or Houston/Austin, TX for the comparison. Mention is made of a shortcoming of an ICF system having its mass centered within the wall rather than on the living side of the wall. Is there an ICF system that places or allows placement of the concrete mass closer to the living side of the wall?

A: I can't say too much about ICF since I don't use them because they are not a thermally optimized system. Generally the mass as you mention, is insulated from the interior where it can work for you . The most optimized system in this regard would be an uninterrupted straw bale wall.This way you get R-30 to R-35 insulation with the mass ( the stucco on the interior surface of the straw bales) where it can do some good. Also the optimal thickness of distributed thermal mass of this sort is 2 inches which is close to what you get in stuccoing a bale wall. I've found some prejudice against straw bale construction as being "hippy dippy" but from a optimization viewpoint it is a highly sophisticated and thermally optimized system with a host of other advantages as well.

C: (Kelly) There are some other manufactured systems that do place the thermal mass on the inside. See thermalwall, cfi, or SSP.

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: (Daniel Chiras) 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: (Daniel Chiras)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: (Daniel Chiras) 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: (Daniel Chiras) 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: (Daniel Chiras) 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: (Daniel Chiras) 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: (Daniel Chiras) 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: (Daniel Chiras) 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: (Daniel Chiras) 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.

Q: Can we substitute an interior masonry mass wall with a wall covered with tile, like the fluted ceramic tile, that have an airspace inside?

A: (Daniel Chiras) A solid masonry wall always provides the best thermal mass, but tile will be better than drywall...

Q: I am building a room 18x16 and will not be putting heat in it, what kind of walls should I use, sheetrock or paneling, I live in the North where the temperature can get very cold.

A: (Kelly) An unheated room in a cold climate will be cold no matter what you put on the walls, unless you are able to capture some sunlight in there, in which case sheetrock might hold that heat a little longer.

Q: I would like to build a thermal mass wall inside my new home. The wall is south-facing and is located approximately nine feet away from the front of our home. The wall will span two rooms and will stand roughly 12-feet high. Both clerestory and casement windows will allow solar gain through to this wall. Is plaster a good, lower-cost option for thermal mass than a masonry wall?

A: (Daniel Chiras) I'm not sure what you mean by "Is plaster a good, lower cost option...than a masonry wall?"  Were you proposing to plaster over sheetrock? If so, you probably won't end up with very much thermal mass in the wall. A solid masonry wall would work the best.

Q: I am planning on building a 1600 sq ft strawbale cabinet shop with 175 sq ft of south facing window and 36 sq ft of north and west facing windows. The floor is concrete slab and I am wondering if 1600 sq ft of 4 in concrete is too much thermal mass. Should I isolate a portion of it so that I can actually utilize this mass?

A: (Daniel Chiras) It is sometimes a bit difficult to answer specific questions such as yours without seeing plans (including dimensions) and especially knowing where your building is being built. If you are building in a sunny climate, you will probably be okay. In other words, I don't think you have overmassed your home. I don't think you would need to isolate mass. However, be absolutely sure to insulate under the entire slab and around the perimeter as well. Two to four inches of rigid foam (XPS) should be sufficient. I'd seriously consider vertical and horizontal (wing) insulation around the perimeter, as in a shallow frost-protected foundation for best performance. Also be sure that thermal mass closest to the south-facing glass is unobstructed by carpeting and furniture or equipment to the maximum extent possible.

Q: I am student of architecture in France and this semester, as a project, we are working on sustainability and water. I'm trying to design a facade which can store the rainwater and during this process acts as a thermal mass wall. I want to ask you if it's possible having the water in the facade and serve as a thermal mass? And if yes which thickness at least is required?

A: (Daniel Chiras) Good question. Folks have been experimenting with water as a thermal mass, but mostly given up on the idea probably for aesthetic reasons. It requires large storage columns that must be placed behind south-facing windows.

That said, I could envision -- and perhaps you can as well -- ways to store water in Trombe walls (which were invented by a French engineer, by the way). Trombe walls are typically built from masonry materials like poured concrete, brick, adobe, or concrete-filled cement blocks. There's no reason why the Trombe wall couldn't be filled with water.

As for the thickness, I have no idea. My guess is that 8 to 12 inches might work. You'll need to work with an engineer to calculate the heat storage capacity of the water and the release of heat into the adjacent room to determine proper thickness. There are other ways to approximate the thickness. You could determine the thickness of a masonry Trombe wall, then scale it up based on the difference in density and heat storage/transfer of water vs. masonry.

Q: We are building a strawbale, passive solar home near Toronto. Are glass bricks suitable for thermal mass use? We also have seen a kitchen counter/bar made with garden bricks; will they also be suitable as thermal mass? The bricks we saw were shiny, so painted with a clear finish. Will that effect the retention of heat too much as they are reflecting light and therefore heat?

A: (Daniel Chiras) Are glass bricks suitable for thermal mass use? No. Not at all. If the bricks are thick, they can serve as thermal mass. If the bricks are shiny and do reflect light, they won't be very useful. You need a darker colored brick. A rougher surface will absorb more heat than a shiny, hard-finished surface.

Q: What are the best stones for heat storage and passive solar heating systems? I know soapstone is good but its expensive (at least around southwestern Ontario).

A: (Kelly) Virtually any stone will act as pretty good thermal mass...the denser and heavier the better. Volcanic stone is not very good if it is lightweight or full of air pockets. Dark colors are good if the stone is positioned where the sun strikes it. Beyond that, it is a matter of aesthetics.

Comment: My experience with thermal mass in houses is that if the mass doesn't have a means of gaining heat, it will always be colder than the room it's in and thus a net loss in warmth for the space. Thermal ballast, as I like to call interior house mass, does greatly help reduce temperature swings. It doesn't however make you feel warm if it's colder than the room. Thus, my rule of thumb is if it doesn't see sun or have hot water or air circulating through it, it's not such a good idea for heating, though it may prove valuable for keeping cool in the summer. It really depends on where you're from and most of my solar engineering has been either for Colorado or western Washington.

Response: (Kelly) I agree that the right amount of mass in any given house is a delicate matter, with many factors that would govern what is appropriate. If there is too much mass it can definitely rob the room of needed heat, but if it is well balanced then it will just help stabilize the temperature at the desired level. I still think that the best text on all of this is Ed Mazria's classic book on passive solar design.

Q: I will be using ICF's for my passive solar house project in Northern New York and am wondering how a full brick veneer on the interior of the ICF walls may positively or negatively affect the homes thermal performance.

A: (Kelly) I think that adding a brick veneer to the interior of an ICF house is an excellent idea as it will add considerable thermal mass on the inside where it is needed. The ICF's themselves have thermal mass in the concrete in their cores, but this is isolated from doing much good by the foam wrapping them.

Q: I have a sunroom that works quite well for the amount of glass that is there. Now the sun shines quite strongly in the winter into the room. I want to create some sort of large thermal mass that will capture some of that solar heat and warm the house. I think a large concrete bench would work, but would be too heavy for my floor. It may be an oxymoron, but do you know of a lightweight thermal mass material that I could use?

A: (Kelly) There are no light-weight thermal mass materials. But you might be able to distribute the mass over a large area, such as using floor tiles, to accomplish the same goal.

Q: I cannot seem to find the answer for an effective form of solar mass under the floor. I have in my personal inventory an amount of dry heart-wood black walnut that I wish to use for the floor, The color is a deep, natural dark chocolate, without any white sapwood. It is magnificent. Do U think that it would absorb the heat of the light shinning on it? But again, the problem is the solar mass for long term heat storage.

A: (Kelly) I have heard of hydronic floor heating installations that do use wood, but this is not common, and certainly would not be as efficient as truly thermal mass materials. One approach you might consider is using a cement board (like Hardi-backer) under the wood to help retain more heat. The fact that the walnut is dark is to your advantage, but since walnut is not especially dense, this is a disadvantage. I can certainly understand your desire to use it for flooring though, as it would make a gorgeous floor!

Q: It's time for me to pick the tile floor and coverings for the thermal wall that extends from the sun room into the living and foyer area, as well as the back of the thermal wall in our master bedroom. Do I need to use dark colors for maximum heat absorbing?

A: (Kelly) Yes, the most efficient solar thermal mass is dark, since it tends to absorb and store more heat... so dark tiles would be a good idea.


Home       Site Map       STORE

For Email contact go to About Us

See this page to read our Privacy Policy

Established in 2001, GreenHomeBuilding.com is primarily a labor of love. Kelly, and the GreenHomeBuilding team of experts, have answered thousands of questions for readers over the years, and we continue to publish up-to-date information about increasingly important sustainable architecture. If you feel moved to assist us in this work, your kind donation would be much appreciated; this can be easily done through our PayPal account:
Custom Search


  [Natural Building Blog]      [Earthbag Building]     [Dream Green Homes]

Disclaimer Of Liability And Warranty
I specifically disclaim any warranty, either expressed or implied, concerning the information on these pages. Neither I nor any of the advisor/consultants associated with this site will have liability for loss, damage, or injury, resulting from the use of any information found on this, or any other page at this site. Kelly Hart, Hartworks LLC.