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).