Building with 'thermal mass'. Are the claims bogus?

[alchemysa]

Every second episode of Grand Designs seems to extol the virtues of 'thermal mass' as a means of home temperature control, particularly in winter. But is it all its cracked up to be?

My neighbor spent about a million dollars (seriously) building an architect designed home with massive thermal mass. He had thick concrete floors with built in heating, concrete and foam sandwich walls inside and out, double glazing, super thick roof insulation and every other energy saving trick. All this was supposed to make the home very 'energy efficient'. (Which I take to mean 'cheap to heat').

I asked his wife recently how the underfloor heating was working out. The answer was a shock. "We can't afford to run it . We are using blow heaters to warm the place when we get home at night. And that's a pretty slow process in a house that's almost all concrete." They would have been far more comfortable in a 'lighter' home with a good reverse cycle air conditioning system that can rapidly heat or cool the place.

It seems to me that underfloor heating and huge thermal mass might be a good idea if you live and work at home. But its totally uneconomical and pretty useless if the house is empty for most of the day. My neighbors will never make energy savings to justify the money they spent on this... errr ... fad.

Obviously much depends on your weather conditions and lifestyle, but I wonder if other TBN'ers have thought much about, or have experience with balancing 'thermal mass' and 'practicality'.

 

[stephan]

The center of my house is a 2 foot thick concrete wall full of soil. Works a treat. Slow combustion heats the wall and as the days go by less heat is needed to make the house comfortable.

In summer the wall is always cool to the touch.

Got my vote.

 

[Baby Grand]

The use of a thermal mass to retain and evenly distribute heat is an old idea that has been very successful. Southwestern Adobe construction is an example evolved to moderate extreme temperatures. The Russian Stove is another example - developed by people who had long, cold winters in areas where firewood was at a premium. In both cases the designs persisted through history because they worked well. It sounds like either the architect on this job didn't know what they were doing, or the homeowners insist on using the house in a way that it was never meant to be used.

 

[JimRB]

The homeowner comment of we need to use auxiliary heater to warm up the house when we get home. There are studies that show some houses do not do well with setback thermostats. One would assume that thermal mass houses would not like rollback thermostats. The school I went to had in concrete floor heating. That was rather nice.

An energy use audit would be required as well as an inspection of the house to see if it was built as designed, used as designed and that the design was correct.

 

[dave1949]

My floors are 8" concrete plus I have a good bit of 8" thick concrete interior walls. But I also have lots of south-facing glass for solar heat gain. That makes for a very stable thermal mass temperature. The air temperature will make some wild swings on a sunny day; reaching 78*F - 80*F is not uncommon. When the sun begins to set, the air temp drops down to the temperature of the concrete fairly quickly.

We supplement the sunshine with a wood-fired masonry heater and, if we have to we can use the radiant floor heat. We don't use that often. I can see that if I removed the solar gain from my house, the propane to keep all that concrete warm enough for comfort would be very expensive. On the other hand, our house will never drop below 55*F to 60*F in winter with no added heat.

Our house also has full earth berms on three sides. The exterior side of the perimeter concrete walls are insulated, plus I put down a 2" thick by 12' wide horizontal styrofoam band about 6" below the surface on top of the berms. This keeps the frost from penetrating the soil next to the concrete foundation walls. The result is my house is sitting in earth that never gets very cold (compared to the outside air), and the earth berms also have zero air infiltration.

My guess is that your neighbor's house stabilizes at a temperature that is not comfortable and it takes many btu's to heat the mass of concrete. If his floor and home in total is truly well insulated, he should set and forget the radiant heat temperature, not try to heat it in large temperature swings.

It will probably be just as well to program the thermostat to run maybe three-four times a day for the period of time that results in comfortable temps. For example, say he wants 70*F, so he sets the thermostat to 75*F but only allows the heat to run for one hour, four times a day. That would drive us out of our house. Once or twice a day for 1 to 1.5 hours is plenty for us. The thing is, you won't feel a thick slab getting warmer until some time has passed. But if you keep pulsing it with a bit of heat, it will reach and stay at the desired temperature.

Since there is a large time lag from when heat is applied to the thick slab until it warms the air enough to satisfy the thermostat, if you do that, it will constantly overshoot the slab temperature and waste fuel. Some people embed a temperature probe in the slab for the thermostat control.

Thermal mass works, but it takes some tuning to the local climate.

 

[alchemysa]

The use of a thermal mass to retain and evenly distribute heat is an old idea that has been very successful. Southwestern Adobe construction is an example evolved to moderate extreme temperatures. The Russian Stove is another example - developed by people who had long, cold winters in areas where firewood was at a premium. In both cases the designs persisted through history because they worked well. It sounds like either the architect on this job didn't know what they were doing, or the homeowners insist on using the house in a way that it was never meant to be used.

Perhaps those examples support my theory: Thermal mass works well for 'traditional' lifestyles but is economically risky in a more modern situation.

I don't believe that the benefits of 'thermal mass' are a total fantasy. But I am wondering if its somewhat 'oversold' in shows like Grand Designs.

 

[charlz]

Is the house designed at all to for passive solar radiation such as Dave mentions? What Dave describes is the same concept used for adding solar mass to green houses or cold frames. Picks up heat during the day, radiates it back out at night. If the house is just a huge heat sink that you have to pay to heat that doesn't sound very good. On the other hand maybe it will stay very cool all summer.

 

[aczlan]

Apparently they didn't consider their lifestyle when they chose that heating system. Have they tried setting the thermostat to one temperature and leaving it alone?

Aaron Z

 

[dmccarty]

I designed our house to have passive solar and thermal mass. It works but it is not a silver bullet. Our passive solar is limited by the location of our septic field and money. The south side of the house is our living room and study which is where we spend all of our time except for sleeping. The passive solar will add at least 5 degrees to that side of the house, IF it is not cloudy. We have colored and finished concrete floors for thermal mass. We thought long and hard about putting in radiant heat in the floor but the cost did not make money sense for our winter climate. The house would have to have a heat pump and primary heat is from a wood stove so having a third, expensive heat source did not make money sense. If we were up Nawth, it would be a different situation. The wood stove is on a brick and granite hearth that is about 18 inches off the floor. The stove sits in a corner and the wall as solid brick that are opened to the study and our bedroom. The wood stove heats both the air, the concrete floor and the brick walls which really do hold the heat. You can really feel the difference on the concrete floor even 10 feet from the stove.

Here is the kicker about thermal mass. It moderates temperature changes meaning it holds a temperature and slows down temperature changes. This can be good or it can be bad. If we are home and either running the stove or AC to maintain a comfortable temperature, thermal mass works real well. The problem is if we go on vacation and let the house get too hot in the summer or too cold in the winter, it takes a bit more time to get the house comfortable again because of the thermal mass. I seem to notice this more in the summer than the winter since I notice the heat more than the wifey.

You can also use the thermal mass to your advantage. If the temperatures are going to fall over night, say into the low 30s or upper 20's but the high for the day will be in the 60-70's, I can run the stove to heat up the house. The fire can burn out but we can easily have the house in the mid 70's for the day. Usually when the outside temperatures allow this, the 60-70s will stay for a few days and we won't have to run the stove. Or, like yesterday I did not start a fire even though it was cool, cloudy and icky outside. The house stayed around 75 all day but this morning it was down to 70. I turned on the heat pump to exercise it and to kick the heat up in the house. The temperatures are going to be in the 70s and maybe touch 80 this week so heating will not be needed much if any this week.

In the cooling seasons, I will cold sink the house by opening the windows at night if the outside temperature is comfortable. The mass will soak up the cool temperature and we won't have to run the AC until the afternoon. If the humidity is up, well the AC has to run.

Later,
Dan

 

[LD1]

Right or wrong in my theory of "thermal mass", I'll offer my :2cents:

Thermal mass cannot create heat. It can only store it. SO in the wintertime, when temperatures outside never warm enough to NOT necessitate heat, thermal mass dont save anything. The "mass" can only give off the heat that was put into it. And that heat had to come from somewhere.

I agree that a setback thermostat probably isnt good on a thermal mass setup. You spend all night heating and storing that heat. Then go to work during the day and dial down the t-stat. That mass is giving that heat off keeping the place warm, only to have to spend all night storing that heat again.

It likely isnt costing any more, or saving any, as opposed to just leaving the t-stat alone. Cause once the mass is heated, the heat should kick off and on just as it would if you didnt have all that mass. Only it may do it less frequently, but when it kicks on, it stays on longer. But total percentage of run time should be the same. Again, thermal mass cannot create heat.

Think of it in terms of batteries. If you have one battery, it will only last so long and then require charging. Add a second battery, it will last twice as long. But it will also require twice as long to charge the system back up.

NOW....the benefit of solar mass is when outside temps warm up during the day to the point heat isnt needed. BUT still drop at night where heat is required. All day long while it is warm, you are storing free heat given off by the sun. And at night you may not need heat at all. Whereas without alot of stored heat in the mass, the heat may have to kick on. Thermal mass, like insulation, will balance out the temperature swings of the daytime highs and nighttime lows. But again, when the daytime highs might only reach 30F and drop to 10F at night, thermal mass doesnt save anything. (But it dont cost any extra either).

 

[s219]

What are his concrete floors sitting on? If on ground level, are they well insulated from below? An uninsulated slab on ground level will want to drive interior temperatures towards the ground temperature, and it can be a very powerful heat sink. I can see that being good in summer, but in winter it seems like you'd be fighting to counteract the effect.

 

[aczlan]

NOW....the benefit of solar mass is when outside temps warm up during the day to the point heat isnt needed. BUT still drop at night where heat is required. All day long while it is warm, you are storing free heat given off by the sun. And at night you may not need heat at all. Whereas without alot of stored heat in the mass, the heat may have to kick on. Thermal mass, like insulation, will balance out the temperature swings of the daytime highs and nighttime lows. But again, when the daytime highs might only reach 30F and drop to 10F at night, thermal mass doesnt save anything. (But it dont cost any extra either).

Correct, where thermal mass helps is where you can setup for passive solar heat when building (ie: south facing windows that let the sun warm your thermal mass), you can get "free" heat to warm your thermal mass and reduce the amount of "paid" heat (be it gas, oil or electric) that has to be added.
You can get a fair amount of heat that way on a sunny day even if its only 30F outside.

Aaron Z

 

[CobyRupert]

Think of it in terms of batteries. If you have one battery, it will only last so long and then require charging. Add a second battery, it will last twice as long. But it will also require twice as long to charge the system back up.

Or think of it in terms of a flywheel.

 

[TnAndy]

I've been using it in my home since 1985. Thick, insulated concrete floors, massive fireplace in the center, double 2x4 exterior walls. In the winter, we can heat the whole place with a small wood stove, lot of south facing glass. In the summer, the temperature stays cooler inside until very late in the afternoon, south glass shaded by good overhang and deciduous trees on that side. Our AC is an 18k BTU mini split in the main living area, and a 12k one in the master bedrm.

My BIL came over to feed animals one winter when we went on a trip for couple weeks, and when we got back, his comment was our house was warmer with no heat on than his trailer was with it going full blast.

Mass works. And has my vote.

 

[CalG]

I can't do much better than those "flywheel" descriptions, and restate that thermal mass does not make heat. THERE IS NO FREE LUNCH!

But if you can find free heat in solar, or in burning efficiency, thermal mass will capture the heat when it is not needed and return it when it is (or vice versa)

The "traditional" Russian stove is just that. Efficient burn technology (no smoldering smoke maker in warm temps) that captures the heat that would other wise go up the smoke pipe. And then releasing that heat over a period of time, at a high degree of comfort. Call it a thermal capacitor.
But if one is heating a home with an already highly efficient heat source like electricity or an oil burner, The only thing thermal mass offers is a high area of radiant warmth . When the mass is properly used in this way, comfort can be enhanced at lower air temperatures. And a lot of energy is lost in heated air.

I've heated with a wood stove in the basement for over 30 years. This past year, I put a couple of old cast iron steam radiator units on top of the stove on a angle metal frame. These "thermal mass units" are plumbed to a second CI radiator in the kitchen, and circulates the water within by thermal syphon. The system works GREAT! A bit less heat in the downstairs room with the stove, and a warm spot to set your butt on when one is feeling a bit chilly.

When we rebuild the chimney, which we will need to do some year soon, I plan to run a third flue up from the downstairs stove all the way to the upstairs ceiling level (20 feet of vert or so) then back down to the normal flue port for the downstairs stove. With a sliding gate, this extra pipe will be my "Russian stove". Now how to build in a sleeping bench.....

Oh! Its 33 degrees outside and the indoor air is 62 degrees F. The wood stove is burning hot on a light load, and the world is a wonderful place to be in. If only the sun were streaming in the windows.

PS, There are no common materials that store heat like water does! It's only drawback is that it's a liquid and must be contained ;-)

cheers

 

[dave1949]

I calculate my solar btu input to be about 70K btu per hour on a sunny day. That's equivalent to a decent size wood burner, plus the heat is automagically distributed, not centralized.

320 btu/sqft/hr is the approximate maximum available at sea level. Our glass is clear. Besides the light transmission rating of the glass, I'm sure the amount of light that makes it inside varies with the sun's angle of incidence on the glass causing refraction. As the seasons change, the sq ft of floor space that receives direct light varies greatly also. The depth of sun penetration into a room varies from 0' in June to 13' in December using a 28" overhang at ~45* N. latitude..

Our slab is insulated underneath. I put in 1" styrofoam around the edges to make a thermal break from the walls too. Below the slab, I put a vapor barrier down on the compacted stone bed first, then styrofoam, then a layer of low-e product called Slab Shield that comes in rolls, then the rebar and radiant heat PEX. I think the vapor barrier is important. Earth is not a very good conductor of heat, but moisture in the earth sure is, and our dirt is always moist here. I think that makes it necessary to isolate the slab from any contact with ground moisture.

There are different opinions about what the insulation below a thick slab actually accomplishes. Some say the insulation stabilizes at the surrounding ambient temperature, which for the bottom of a thick slab will be about the earth temperature. My rear walls and floors in rear rooms (that never get direct sunshine) tend to run around 63* in winter with no radiant heat used. That means there isn't a large temperature differential between the slab and earth, which results in low levels of heat transfer between them. Given a low temperature differential, the insulation doesn't have much to do.

One thing is for sure, you cannot heat or cool the earth below your house to any appreciable extent using a home heating/cooling system.

 

[stephan]

OK so in my other house I have Waffle Pod flooring (concrete slab with polystyrene blocks) and ThemalCell walls (again polystyrene but filled with concrete) with double insulation glass. The north facing internal walls are filled with concrete blocks and I have hydronic (water circulated through the floor) heating off a slow combustion in the kitchen. I think the flywheel is correct in that it takes a little more effort to get it up and running but less effort to maintain. Coming into winter, like we are, I will need to heat the slab over a weekend but then just the evening fire maintains the warmth to a shorts and t-shirt comfort level. The slow combustion fireplace has an 18kW wet back that uses thermosyphon to heat water from a 400l hot water system in the roof. This system was solar powered with a single array of 22 tubes when I bought the house but that just did not produce the energy required. The thermal mass is in the floor and the walls and while hard to measure the actual effectiveness the combination works very well.

I suspect the OP friends probably need to try leaving the heat on for an extended period. It maybe that they are baulking at the initial startup cost and not giving the system sufficient time to go into the maintain phase.

 

[powerpace]

I have been in several Thermal Wall designed houses and most were very energy efficient. I also have slept on a huge sun heated rock mass overnight and been comfortable in air temps in the 40's. Native Americans and many other cultures understand how to live with nature and use the Sun and a mass of natural substance. Rock, clay and timber.

Sometimes engineered structures are not efficient because the preferences of the owner are not optimum for the desired results.

 

[CalG]

Anyone desiring high air temperatures as a metric for comfort, will NOT find thermal mass a useful design.

Most all thermal mass systems take advantage of RADIANT heating of the surroundings.

Spaces not "visible" to the radiant mass will very likely feel cold.

Convection might be part of the design, but that usually just ends up overhead.

Life style and expectations are often at odds with energy utilization.

ps Stephan, I like your choice in tractors!
The Lambo runner / Hurlimann Prince are quite handi units! Have you good parts distribution in NSW? Here in the States, the D-F dealer network is iffy at best.

 

[newbury]

You also have to be able to have a living style that works with nature, not against it.

We heat and cook with natural gas and cool with electricity. the house in Virginia is about 30x30, 3 levels with a 18x30 addition on 2 levels.
A BIG winter gas bill is a little over $200, a BIG summer electric bill about $200. Take off about $30 from gas for cooking and $50 from electric for lights, computers, etc. So our heating cooling bills are on the order of less than $1000/year
We've about 40 or 50 1970 era-windows. The wife keeps on wanting to replace them for energy efficiency. Estimates we were given were a possible savings of 10% - or $100/year.

Part of the reason why is because we flex with the temperature. In summer I try to let it climb to no more than 78, in winter it can drop down to 58 at night, 63 during the day.
We don't come in the house and crank on the a/c or heat. As the seasons change we adapt, what felt cool in August (75 degrees) feels HOT by December. What feels warm now (66 degrees, I just checked, it's a warm day outside) will feel frigid by July.

If there is sickness or visitors exceptions are made.

When Carter introduced his energy policy in the winter of '77 our house was already set at 63 and I refused to turn it up.

Another key thing I learned about 35 years ago is to have the wife pregnant over the winter. Helps with both the heat and a/c bill if you time it right. Of course that is only good a couple of times DON"T have them due in September!