Thermal Mass and Stones

Jose Garcia has been a landscape contractor for 24 years and has gravitated to doing a lot of rock work. He has built innumerable retaining walls of timbers, boulders, drystacked and mortared stone. He has built foundations out of stone and mortar and put rock veneer around the base of a straw bale building to raise the level of waterproofing. He lays about 20 tons of flagstone a year in patios and walkways. Over the last couple of years he has built a half dozen mortared flagstone staircases. In Colorado we are blessed with a wonderful red flagstone with great tensile strength that he uses to make benches. He tries to work with the stone's shape as it comes, and can generally lay out a patio with a minimal amount of cutting or chipping, and the benches are free form and distinctly shaped. Mostly he's out rolling boulders and flipping flagstone on a daily basis.

Questions and Answers

Q: I want to build a stone cottage in Plattsburgh, N.Y. (60 miles south of Montreal and wish to construct it with Solar Heat. Where can I find information on Stone Homes with Solar Heat?

A: (Kelly) The principles of passive solar heating are basically the same regardless of the materials the house is built with. Care must be given to make sure the insulation and thermal mass and windows are in the right places and in the right proportions. I recommend Daniel Chiras's book The Solar House: Passive Heating and Cooling to learn about these basic. Building with stone is a bit tricky because it is a lousy insulator and and excellent thermal mass. This means that it works well inside an insulated envelope. In order to use it on the outside, the stone should not be a solid wall through to the inside; some form of thermal break, with insulation or an air gap should be provided in the wall.

Q: I am going to build exclusively with stone because it is abundant. I want the solid walls and the ability of this structure to blend in.

A: An issue with solid stone walls in most climates is that they are thermally inefficient; they pass both the cold and the heat right through to the interior. If you want to be comfortable, you will need to provide a thermal block in the wall somehow. This can be done by building a double wall, with air or insulation sandwiched in between, or by putting insulation over the rocks on one side. The best side to insulate would be the outside, so that the stones would be open to the interior to take advantage of their thermal mass...but this defeats the natural rock appearance on the outside. Another approach would be to insulate the inside, and then add more stones in strategic places (such as fireplace surrounds) to provide the thermal mass.

Q: We have owned for the past 2 years a 150 year old stone house in northwest Illinois. As I am writing this it is 0 degree and the winds are at about 30 miles per hour. The cold seeps through the 20" stone walls that are plastered inside and out. We have just had one room re-plastered inside and the cold still somewhat radiates through the walls. Should these exterior walls be somehow insulated to protect from the extremes of cold and heat, to conserve interior energy?

A: (Kelly) You are experiencing one of the laws of thermal dynamics: thermal mass materials (like stones) will take on the average ambient temperature, and hold that temperature for quite a long time. This is a great advantage if the stones are on the "inside" and have a nice warm blanket to keep the cold outside from penetrating, but without this insulating blanket they will always be sending more cold to the inside. So yes, you should put a good layer of insulation on the outside of your home if you want it to be comfortable. With 20" walls it might take quite awhile for all of that mass to warm up, even with a warm blanket, so perhaps a good time to do this would be towards the end of the warm season when the walls have already warmed up. Or, if you add insulation sooner, you may need to be patient to feel the effects. Even just a couple of inches of a rigid panelized foam insulation can make a big difference; this can have a stucco plaster over it.

Please tell me more about this exterior insulation.  Does it cause a problem with moisture build up within the walls or on the inside walls? 

This sort of insulation should not create such moisture problems. It will seal the wall from any moisture that might come in from the outside, and it will provide an insulated blanket to keep the stone wall warm enough to keep it from reaching the dew point required for condensation of moisture arriving from the inside. 20" of stone will not pass very much moisture anyway.

How do you attach it? 

It would need to be attached physically with anchors in the mortar. Sometimes you can actually nail into mortar effectively...sometimes you need to drill into it and insert anchors that receive screws.

Do you need to put chicken wire over it then plaster? 

Yes, this is commonly done with stucco applications and should be familiar to a local stucco crew.

How do you deal with the windows with the extra inches of material when they are flush with the walls?

This could be the trickiest part of such a project, and would involve some expert attention to the specific situation. It might be that some additional rock work would be best, or it might be that the stucco can be effectively finished in a satisfactory manner.

Another question came up about the stucco and the insulation expanding and contracting at different rates.  The local stucco supplier would not warrant the stucco over this type of insulation in this climate.  Do you have any more details on this problem? 

That is interesting. I would not expect the rigid insulation to expand or contract at all, and since the stucco is not physically bonded to the insulation, this should not be a problem. I have seen stucco applied over rigid insulation by professional crews in Washington state, where it does occasionally freeze. You might ask around and get some other professional opinions.

Q: I have been planning to build a 3,500 sq ft house entirely out of stone, with the outside wall being ~ 10-12 inch thick. But recently my friend has pointed out that house like that would require much more effort than a regular house to cool down or heat up due to stone not being a very good insulator. Is this true?

A: (Kelly) Solid masonry homes, such as stone houses are notoriously hard to keep comfortable year round. The reason is that with so much uninsulated thermal mass, the house tends to take on whatever the average temperature is outside and just stay there...so in the winter it will be very cold, and in the summer it might be very hot. The way to overcome this is to add a layer of insulation in the shell somehow. This can be done in various ways, such as making a double stone wall with an insulated cavity in the middle, or by putting insulation over the stone (preferably on the outside) and applying a stucco or plaster over this.

Q:  I'm planning on building a house and using stone masonry is one of the choices for making it; limestone to be more specific. I live in Cozumel Island at the Mexican Caribbean and the weather is hot and humid, very much so! I want the house to be fresh, well ventilated and fresh, BIG TIME FRESH! The problem is that I want to stay away from using 'air conditioning', although electric fans will have a 'green light' and then, maybe a pond and/or a fountain, the strategic allocation of plants and trees, the position of the house with regard to the sun and air currents, the type, location and size of doors and windows and so on. The roof will be made of natural thatch. The structure most likely will be reinforced concrete and the walls could be made using limestone masonry and/or perhaps cement blocks insulated with expanded, water repellent perlite. Now here comes the big question: How good are the thermal insulation properties of a limestone wall? Or stone masonry in general?

The island is a limestone shelf, so limestone is obviously available. As far as I know, the stone stores heat throughout daytime and then that heat goes inside the building during the night. I'm told that probably foot thick stonewalls work better. So, I don't really want to learn how to build a stonewall, for that I will be using the help of a professional stonemason, but first I need to know if stone walls have good insulation properties and how thick should they be. I don't care if its ashlar, slipform or rubble masonry as long as the walls keep the heat out. It'd be great if you could give me some advice or tell me anything that could help me.

A: Virtually all stone is best considered a thermal mass material rather than an insulator. It absorbs heat, holds it for a period of time, and then passes it on. However, if the walls are thick enough then it can act like insulation in that it takes quite awhile for the heat to penetrate. The trouble is that if it is hot all of the time, then the stone will eventual get quite warm and just continually be passing this into your home. So to  avoid this, you need to place some form of insulation (preferably on the outside of the shell) to block the heat from entering the stone. If you do this, then the stone will help keep the space cool because it tends to stabilize the temperature at a constant level. You can enhance this effect by berming or earth-sheltering your home, if this is feasible given water tables and such.

I sounds like you already have a pretty good idea for strategies to keep your home fresh, such as with air movement using fans, evaporative cooling from a pond or fountain, use of trees and plantings, good ventilation, etc. You might look into the Middle Eastern concept of having a "wind catcher" that creates air current through having a tower that draws air upward through the home...and this can be past a pool of water.

Q: What would be the insulation factor of a wall built of 2-foot thick solid rock? Most of the rock in Oklahoma is limestone so that would be the material used.

A: (Kelly) The insulation value of solid rock is negligible, since it is really considered a "thermal mass" material that absorbs heat and cold readily, and then holds it there for quite awhile. Even though the wall might be two feet thick, it would still be passing the heat and cold on through it. Old rock castles are notoriously uncomfortable places to live because of this.

Q: You say that exterior stone walls should be insulated from the inside. I am assured by my son, who has just started a BEP TAH (Architecture course in France), that you normally insulate from the outside. Can you clarify, please?

A: (Kelly) I don't believe that I have ever advocated insulating stone walls on the inside. Your son is right that it best to isolate stone as thermal mass so that it is exposed on the inside. In some instances, a double stone wall can have insulation in between in order to preserve the appearance and durability of the stone on the outside.

Q: We want to do slip form stone walls and insulate on the outside with rigid foam board insulation so that the interior rock walls will act as thermal mass for a passive solar design. Regarding the outside "skin", would it be faster to do a double rock wall while doing slip form so that rocks are also on the outside as well and sandwich the insulation between two layers of rock, OR would it be faster to mortar the rocks on the outside later? If we used stucco on the outside (if that is faster) could we then later put rocks over it for appearance? If we did not do rock on the outside, how could we attach board and batten wood to the rigid foam board?

A: (Kelly) I think that your basic concept is excellent, to make an insulated stone building with the bulk of the mass on the inside. I have heard of people slip-forming stones with embedded insulation, so I know that this is possible. What is easiest, is hard to say. Putting the insulation on the outside, and then stuccoing it is certainly an option, but if what you really want is the look of stone, it seems like more work and expense to do this. You could possibly dry stack stone over the insulation on the outside, since it is mainly for appearance If you wanted to do boards and batts over the insulation, then you might insert wire ties periodically in strategic places, to which you could later attach wood stringers or nailers horizontally.

Q: I have a stone cottage in Tuscany. The house is very very cold in winter and I want to know whether I can insulate it from the outside - that is, over the stone. I had been thinking of cob, but from reading the other questions and answers I have learned that cob is not a good insulator. I already have thermal mass from deep stone walls.

A: (Kelly) You're right that it is best to insulate the stone walls from the outside so that all of that thermal mass will eventually get warm and stay warm. There are several possible ways to do this: use commercial rigid insulation panels that are attached to the exterior and then plastered over; frame the outside with wood and fill the voids with fiberglass insulation, as is done with conventional construction, then sheath this for the exterior; stack earthbags filled with an insulating material (such as volcanic stone, rice hulls, or perlite) on the outside and then plaster these to protect them for the finish; do the same thing with bales of straw. Several of these options would require additional foundation work to support them.

Q: A man who's lived most of his life in France and retired to Nepal for the last decade is about to become the first builder of a Mas de Provence (French provencal style, stone house) about 15 kms from Kathmandu, to be built on 400 sq. meter plot. We cannot place insulation on outside if we want that Mas de P look, but we can use lime and mud like they do here for the joints. Then double wall sure...all using irregular stone (no cut stones) and placing whatever you recommend on the inside, ..plywood, and/or rockwool, and/or whatever insulation we can find here, and add brick and stone on inside if we must so that house will be cool in summer and warm in winter. If we do that, with double wall, how thick should the exterior wall be? how thick for insulation?? and how thick for the inside stone/brick wall? Then I have been advised to KEEP the house off the ground in some way, so no humidity or cold comes up .....and to unite the wall with concrete on the floor, really tightly. What is your opinion and recommendation for this pioneer in French style stone houses the first of its kind in Nepal?

A: With the climate in Nepal, I'm sure that the house will be more comfortable with insulated walls, and I can see that for aesthetic reasons what you propose might be the best way to build the walls. The best material for the insulation is something easily poured or placed into the cavity that will not degrade over time, such as the rock wool or lightweight volcanic stone (pumice or scoria). I would think at least 6 inches for such insulation, and then the stone walls can be as thin as is practical given the available stone and the structural needs for stability.

For the floor you can first place a moisture barrier to keep water from migrating into it, and then a layer of insulation to isolate that mass of the floor from the soil. This insulation might be more volcanic stone gravel or commercial rigid panel insulation.

Q: I'm interested in renovating and extending an old house where the first floor has solid stone walls. It will be preserved, the upper floor and roof will be demolished and expanded with a modern steel framed structure with lots of natural wood and glass. This is located in South Eastern Europe in a mountain terrain, where in the winter it easily drops to -20C, summer is around 30-35C. Old people living there say these types of homes stay uncomfortably cold even in the summer. Following your other advice I will opt for insulating from the outside but want to keep the stone VISIBLE! The only thing that comes to mind is to use an insulated glass, probably triple glazed utilizing low emission glass, rather than regular clear glass. It will be aligned with the upper floor glazing to form a smooth finish. An air gap will be left between the glazing and the stone wall. Internally the heating will be wood burner (just for character) and geothermal/solar system. What is you opinion on this ? Do you see any obvious drawbacks with thermal insulation ? How about condensation between the glass and stone cavity? Should the glass panels be fixed or able to open? Is triple glazed/low emission glass an overkill?

A: (Kelly) These are interesting questions. What you describe with using glass outside thermal mass is essentially the same concept as has been used for many years as a passive solar technique called a "Trombe wall". To do this on such a massive scale would probably result in considerable overheating in the summer, since the glass will trap more heat in the stone wall and pass it through to the interior. In the cooler season it might provide a comfortable interior space however. If you could shade the walls exposed to the sun with seasonal plantings, like trees that lose their leaves in the winter, this might be a solution.

I can imagine some challenges to doing this however, and condensation inside the glass is certainly a potential. That air gap will be relatively warmer than the surface of the glass, and this could easily lead to condensation. For this reason a dual or triple pane window would be best. Low-e might be beneficial in the summer, but not in the winter.

My suggestion, if you want to try this concept, is to run some trials using various types of glass, ventilation schemes, air gap distances, etc, over a period of time and observe the effects.

Another approach that might not be practical or desirable, but would work, would be to add a new stone wall on the inside, leaving a space for insulation between the two walls. This might actually preserve more of the character of the building.

Q: What are the unique properties of stone, and how is it most effectively used as a building material? I’ve read that it is a poor insulator, despite it having a high thermal mass. What are the best methods to get the highest efficiency out of it?

A: (Kelly) Stone is very dense and heavy, attributes that make for good thermal mass. It is able to store a great deal of thermal energy, so if it located inside a house, surrounded by insulated outer walls, then it will hold and gently radiate that warmth over time. Some possible uses for interior stonework include fireplaces or surrounds for other types of stoves or heaters, decorative partition walls, benches, and planter beds.

Q: I have a 217 year old home in the Poconos. It has issues. The main living space is a stone structure, not insulated, with parged walls, and with dirt from the crawl space going right up underneath the one layer of wood floor. The ceilings are about 18 feet high, also not insulated. There is no soffit space that I know of and the building is extremely humid. When I light the wood stove in the winter, the heat causes the parged walls to sweat copiously, with water running down the walls in rivers, ruining the wood moldings. The window, though double paned, also gather much moisture in the winter. There is a wooden addition to the house, where the kitchen is; the walls are drywall and they are always dry, even though attached to the stone wall. I want to use more sustainable approaches to redoing the house, keeping it warmer and dryer and less humid. I'm thinking of having the earth removed under the basement and having the parge taken off/remortaring the interior stone and having a drywall built to channel the moisture. Are these good ideas? What would you suggest?

A: (Kelly) The problems you are having puzzle me a bit, especially the fact that the parged walls sweat so much when you light a fire. This is most likely due to condensation because the warmer air brings more humidity while the walls stay cold. I doubt that replacing the parge with dry wall will solve the problem; in fact the dry wall could get damp from this treatment. My suggestion is to find a way to insulate the stone building on the outside to keep that thermal mass at a comfortable temperature so it won't promote condensation. This will make the whole house more energy efficient and comfortable. There are various foam panel products that can be applied to the outside of masonry walls, and then this is protected by stucco or other siding. Certainly getting the soil away from the wooden floor is is a good idea, and perhaps putting a moisture barrier over the soil to limit humidity from entering the house from the ground.

Q: I am into my first year of architecture and we have been given the task of presenting about insulation materials. So I choose stone, but I am very confused about the process of insulating stone walls. Many websites speaks about "thermal mass"... yes stone is a material with high thermal mass, so could you please explain to me the insulation process of a stone wall and heat transferring process?

A: (Kelly) Yes, stone is definitely a thermal mass material, not insulating. In order to create an insulated stone wall it would be necessary to add some other insulating material in either of two ways:
1) Add a layer of insulation on the outside of the wall, preferably, so that the thermal mass of the stone can be fully utilized on the inside of the room.
2) Create a double stone wall with a cavity between the two walls into which insulation is placed.

Q: My property has the remains of an old stone farmhouse built into a hillside. There’s about four feet of wall above ground on three sides. I’d like to make a little guest cottage here with the old stone wall being the first floor. I understand insulating the outside of the stone wall is preferable because of the thermal mass benefits of stone as an internal surface. If I went this route and stucco over everything, am I targeting a specific R value, or just creating a thermal break between the outside environment and the stone?

A: (Kelly) It partly depends on the climate where you live, but generally, the more insulation the better. I suggest a minimum R-10 for this application.

Q: I'm planning on building a stone home in middle TN (cold, below freezing winters, hot summers, and moderately high humidity all year round). The house I want to build will be limestone on the exterior and exposed limestone on the interior, with about 20% of the exterior wall surface in glass. A story and a half, 14' tall stone walls will start at 18" at grade and taper to 14" at top. My concern is sweating. What are your thoughts on being able to use slip-form on the interior of the stone walls to create an air gap that can then be insulated with rock wool or similar? I'd have to create tie-stones between the interior and exterior halves to preserve the structural integrity. Is there any strategy to apply in locating the air gap in the center of the wall thickness, or more towards one side (interior) or the other (exterior)? Is there a less labor-intensive design that will preserve the stone surface on both sides of the walls?

A: (Jose) I would guess that your thickness might be a little low without knowing any particulars about your situation. Skip the air space and lay the whole wall up against blue board. Keep the thermal mass within the house. Oh jeez I just read your email again and now I think you want to build the whole structure above ground. Why would you want to build with stone above ground? This is complex and inefficient in the most gentle terms. Stone works very well at the ground level and is wonderful as a medium to get your structure up off the ground and splash  zone but as a decorative feature it is inherently inefficient and I can’t see how a reinforced air space solves that. 

A: (Kelly) Having a hollow space for insulation seems like the best way to achieve your goals, and I expect this could be accomplished with slip forms, but I do not have experience with doing this. I would tend to favor the void either right in the middle, or closer to the outside, which would provide more thermal mass for the interior.  Using stones to tie the two walls together would be effective, but could be awkward when using the slip forms. I would expect that some other material to tie the walls together might be easier and equally effective. Perhaps stainless steel brackets, or even some strong durable synthetic material could be used.


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