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The Great "Thermal Mass" Myth


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#1 jsharris

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Posted 22 January 2015 - 09:12 PM

The false term "thermal mass" come up time and time again on building related forums and discussions, yet as a parameter it has one notable feature - ir does not exist.

There is no such thing as "thermal mass" and never has been.

Mass is a simple physical property, in simple terms it's approximately how much a given volume of something weighs at the surface of the earth. This, in turn, depends on the density of the material.

For example, here are some densities for some common building related materials, in terms of the weight at the Earth's surface for a 1m square cube of the stuff (1m2):

Brick ~ 2000kg/m2

Concrete~ 2400kg/m2

Plaster and plasterboard ~ 2700kg/m2

Water ~ 1000kg/m2

Structural softwood ~ 550kg/m2

Typical hardwood ~ 700kg/m2

Granite ~ 2700kg/m2

On its own the mass of a given volume of material isn't that useful for working out how much heat it would take to either raise the temperature of the stuff, or for it to give off heat as it cools down. What we need to know is the specific heat of the material, expressed as the amount of heat energy (called sensible heat, which can be measured in Joules, J) needed to change the temperature of a certain mass (lets say 1m3 to match the data above) by 1 deg C (or more correctly a deg K, but it's the same thing for this purpose).

So let's list the same materials as above, with the amount of heat energy we need to put into increase the temperature of 1kg of it by 1 deg C:

Brick ~ 840 J/deg C

Concrete ~ 880 J/deg C

Plaster and plasterboard ~ 1080 J/deg C

Water ~ 4200 J/deg C

Wood ~ 1700 J/deg C (This is an average value, as the true range is dependent on variety, with a wide range, from 1200J/degC/kg to around 2300J/deg C/kg)

Granite ~ 790 J/deg C

So, if you want to create a house with the highest "thermal mass" (i.e. Heat capacity per unit mass, if that's a reasonable way of trying to define this unknown term), then here is a list of materials, with the highest heat capacity for 1 kg at the top, and lowest at the bottom:

Water
Wood
Plaster or plasterboard
Concrete
Brick
Granite

You may well spot a few odd things here. The first is that you cannot build a house with water (but you can include water as a heat distribution or storage system). The second is that concrete, brick and stone aren't great materials in terms of storing heat for a given mass.

Surprising, isn't it? Even more so when supposed building professionals keep harping on about the virtues of so-called "thermal mass".

#2 joiner

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Posted 22 January 2015 - 09:26 PM

:) Does it matter, though? Is it nevertheless useful as a short-hand term for expressing an idea that encompasses, albeing simplistically, so much that is difficult for the lay person to get their head around.

"The material from which a building is made is its thermal mass, which absorbs the heat of the sun during the day and slowly releases it when the sun goes down."

No physics there, but not many people would fail to understand what the sentence is saying.

#3 declan52

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Posted 22 January 2015 - 09:27 PM

Where would light weight block sit on that table as I was always told as they are less dense than standard block they won't absorb the heat and steal it from the room making it retain more heat.

#4 joiner

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Posted 22 January 2015 - 09:30 PM

:wacko:

#5 oddball

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Posted 22 January 2015 - 09:39 PM

I would disagree "a false term" everything has a thermal mass. Quite simply it is the ability of a "mass" to store / absorb heat.

#6 jsharris

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Posted 22 January 2015 - 09:45 PM

I don't have the data for the heat capacity of lightweight blocks, but would guess that they are similar to ordinary blocks.

The important thing to note from the above is that there is no rational relationship between mass and heat capacity. Some of the most dense materials in the above list (say granite, at around 2700kg/m3) are actually at the bottom of the list and some of the least dense materials (like wood) are near the top of the list in terms of heat capacity for a given mass.

Logically, therefore, if you want to use the term "thermal mass" in a positive way, with the intention of accentuating the heat storage capacity of any building material in terms of mass, then wood comes out at the top of the list, as having a pretty high heat capacity for a given mass, and concrete and brick comes fairly well down the list.

So, why do people insists on the false statement (and it is very definitely and demonstrably false) that concrete has a "high thermal mass", implying that for a given mass it stores (or absorbs) a greater amount of sensible heat than other building materials?

#7 SteamyTea

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Posted 22 January 2015 - 09:52 PM

Thermal mass is a mixed up term. You have mass, the number of atoms in something, Thermal is really a prefix, so you can have thermal energy, thermal storage, but you can't have thermal watts.
Saying thermal mass is like saying music Beethoven, vacuum Dyson, as long as we don't allow nouns to become verbs.

Jeremy, you forgot the really odd one from your list, air.
SHC of 1kJ/kg, better than granite.

Joiner
It does matter in a way. If you put the material with the highest thermal capacity on the outside, the building will perform different that if it is fully enclosed on the inside. Both will keep the place cool and neither will reduce your energy bill in the UK.

Edited by SteamyTea, 22 January 2015 - 09:55 PM.


#8 archmoco

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Posted 22 January 2015 - 09:54 PM

So my wooden floors in conjunction with the underfloor heating and the log store are better heat stores than concrete or brick

Edited by archmoco, 22 January 2015 - 09:55 PM.


#9 Alphonsox

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Posted 22 January 2015 - 09:58 PM

The way the term "thermal mass" is used seems to have little to do with gravitational (or inertial) mass. The usual question and explanation revolves around the best way to hold the roof up. In which case this is far more of a volume issue. A 1m3 of concrete holds around 2Mj/C, the same volume of soft timber around 1MJ/C

Wood is a better store if measured per KG, worse if measured per M3

Edited by Alphonsox, 22 January 2015 - 09:58 PM.


#10 oddball

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Posted 22 January 2015 - 10:05 PM

Thermal mass is a function of the energy transferred (or latent energy already existing) to the mass as Einstein’s work on relativity demonstrates.

#11 Nickfromwales

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Posted 22 January 2015 - 10:13 PM

I'm all for this informative and impartial breakdown, for one.
Key to understanding and digesting the unknown, is the need for someone to clarify what I'd class as the grey areas. There are a lot of grey areas (unknown terminology) on this forum, as threads digress bloody quickly into the unknown as we're all aware of, and without some stop-offs in between to offer "joe public" a chance to understand the basics, some of these great threads may lose point / interest.
That would be a waste of time and effort on the parts of the members who regularly contribute / guests who are dipping their toes in, unless such gaps / misconceptions are occasionally addressed.
For this place to make sense, even to a certain small brained welsh guy, this kind of thread is invaluable. Otherwise this place might be a cup without a saucer, as the points made won't have as much info / relevance as they could have.
A thread that clarifies, and very clearly, this 'myth', is now an available reference point for all to visit ( should they remain bewildered like I first did ) as they stumble through the many different terms and points, and let's face it these kind of points then won't need to be so heavily reiterated in every likeminded thread.
The main thing is that this one now has an explanation. ;)
Regards, nick.

Edited by joiner, 23 January 2015 - 10:56 AM.


#12 ProDave

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Posted 22 January 2015 - 10:14 PM

Intertesting debate .

I wonder what the joules per deg C is for the high density bricks used in storage heaters?

The thermal mass thing seems to me a measure (real or assumed) at how good a building is at storing heat. i.e turn the heating off and see how long it takes to cool down.

Of course the level of insulation has much more bearing on that than anything else.

#13 Nickfromwales

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Posted 22 January 2015 - 10:21 PM

View PostProDave, on 22 January 2015 - 10:14 PM, said:

Intertesting debate .

The thermal mass thing seems to me a measure (real or assumed) at how good a building is at storing heat. i.e turn the heating off and see how long it takes to cool down.

Of course the level of insulation has much more bearing on that than anything else.
Even that simple breakdown makes a lot more sense tbh. !
:)

#14 jsharris

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Posted 22 January 2015 - 10:28 PM

View Postoddball, on 22 January 2015 - 10:05 PM, said:

Thermal mass is a function of the energy transferred (or latent energy already existing) to the mass as Einstein’s work on relativity demonstrates.

Sorry, but that's not at all how the false parameter is used in relation to buildings, and building thermal control and stabilisation has little or no connection to either general or special relativity, so that reference is just more obfuscation (and, apart from anything else, the velocity element is constant, making relativity a moot point in this case).

Time and time again we hear of "high thermal mass buildings" being made of stone, concrete, concrete block etc.

Let's do some very simple comparisons of building materials and see how much heat energy (sensible heat, in Joules) that each holds.

As a base assumption, lets assume we have 1000kg of each material and the temperature change is 1 deg C (or more precisely 1 K, but it's the same thing in this context).

1000kg of wood will store between 1,200,000 and 2,300,000 J of heat for a 1 deg C temperature change

1000kg of concrete will store around 880,000 J of heat for a 1 deg temperature change.

So, in terms of mass (and lets remember that the adherents of this supposed physical parameter always use the term "thermal mass") then wood is a lot better than concrete at storing heat.

Obviously those that misguidedly use the term "thermal mass" don't actually mean "mass" at all. What they seem to refer to is the heat capacity of the internal structure, which bears no direct relationship to its mass whatsoever. It's this that I'm trying to point out. You can have a relatively high internal structure heat capacity (or specific heat if you prefer) without having, or needing, a high internal structure mass.

It's the use of the term mass that is the red herring, as it misleads people into thinking that adding mass to the internal structure will add heat capacity, when the reality is that it may not do this at all, and they may well be better off by increasing the internal heat capacity (in the form of using materials with a higher specific heat) than they would by being mislead into using materials with a greater mass (but lower specific heat).

Anyway, as Dave rightly says above, insulation (and decrement delay) has a far greater impact than adding internal heat capacity. I suspect that the main reason for this is that most of the usable and effective internal heat capacity come from the plaster/plasterboard, and that anything deeper in the structure doesn't really have a significant effect in a well-insulated house.

The proof of this is in the very long ( as in days) response time of a low mass, but highly insulated, house.

Edited by jsharris, 23 January 2015 - 07:32 AM.
typo


#15 oddball

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Posted 22 January 2015 - 10:53 PM

Its mainly to do with the sun i.e. heating a building structures and materials and in turn what these materials structures do / dissipate this heat. As previously stated Thermal mass is a function of the energy transferred (or latent energy already existing) to the mass as Einstein’s work on relativity demonstrates.

So when the sun is shining which heats up quicker wood or concrete.

This may help those who are interested http://www.reidsteel...n-buildings.htm

Or if you fancy a good read try http://www.arch.ced....ss/mass-sml.pdf

Edited by oddball, 22 January 2015 - 10:56 PM.


#16 SteamyTea

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Posted 22 January 2015 - 11:24 PM

Not really to do with the sun as such, more the energy that it delivers. You can have the same energy delivered by the wind direction changing (bulk mass transport). Energy is energy, don't confuse it with power or temperature.

I am going to support Jeremy, and let him do the sums as I have done literally thousands of them when I studied this area for my BSc. I hate typing so most of it was numbers, about 100 pages of it (oh happy simple days they were with bits of granite and bright light).

One way to calculate the storage capacity of a building to think of it as a homogeneous sphere that has a uniform SHC which is the sum of all the SHC times their mass.
Then change one element and see what happens.
Another way, and one I like to use, is to balance the amount of energy stored/released in the solid components and the gas components. Then see what happens to the gas temperature when you change the solid components. And for a real laugh, plug in the thermal inertia formula and see how the time changes. Time is misunderstood, but it is just a number and needs to be treated as such.

The really tricky thing is the shape of any component parts. A thin wall will perform differently to a thick one, and you will be amazed at how thin a wall has to be before you notice any real difference. And just for a laugh it is not linear, it follows Newton's Law of Cooling.

Ah, Newton, we are dealing with him here, Einstein was dealing with a totally different thing, really massive objects and extremely high velocities (bit on Material World on it tonight, and I know your just teasing Oddball :) )
.

Nick does raise a very serious point. Scientists have worked for thousands of years to classify, clarify and almost agree on physical properties (interesting articular in my comic last week about long-running old experiments). If you want to know what is the current standard that we work to, go to the NPL Kaye and Laby site, it tells you more than you need to know.

Edited by SteamyTea, 22 January 2015 - 11:25 PM.


#17 Nickfromwales

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Posted 22 January 2015 - 11:33 PM

I did what now?

#18 recoveringacademic

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Posted 23 January 2015 - 07:45 AM

OK, I buy it, Jeremy.

The term Thermal Mass, misnomer that it is, does communicate something to the people who use it. Even misapplied terminology works if discussion participants agree to use it.

For each instance of the misapplied term used here on this discussion group, which other term do you suggest we use?

#19 jsharris

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Posted 23 January 2015 - 07:50 AM

The real point that controls how quickly a house heats up from cold is that it is the combination of the rate of heat input to a house and the effective specific heat of the warmed internal structure, and that this is not related in any meaningful way to mass (as in there being a relationship like higher mass = higher specific heat, which the above shows just isn't valid).

The same works in reverse. How quickly a house cools down depends on the rate of heat loss (through the structure and from ventilation loss) and the effective specific heat of the warmed internal structure.

Given the relatively poor specific heat of concrete and brick compared to wood and plasterboard, and given that we know that, in effect, it is only the first ten to twenty millimetres of a wall, floor or ceiling's thickness that have a significant impact over the working temperature range of the inside of a house, I'm at a bit of a loss as to how mass got dragged into building thermal control theory in the first place. So many people now seem to think that adding mass = adding heat capacity that it's in general use.

This may not have mattered when we were primarily designing and building houses that needed a lot of heat to stay warm in winter, but as we shift to building houses that need much less heat to stay comfortable then we need to break away from the false connection with mass, simply because I think it's another old tradition that is holding back modern methods of construction.


Edited to add:

View Postrecoveringacademic, on 23 January 2015 - 07:45 AM, said:

OK, I buy it, Jeremy.

The term Thermal Mass, misnomer that it is, does communicate something to the people who use it. Even misapplied terminology works if discussion participants agree to use it.

For each instance of the misapplied term used here on this discussion group, which other term do you suggest we use?

I would propose that we need to think of the internal heat storage element of house thermal control in terms of effective internal heat capacity, i.e. the amount of sensible heat that the internal structure can store over the limited range of comfortable room temperatures we use.

Edited by jsharris, 23 January 2015 - 07:53 AM.
added a bit in response to RA's post


#20 recoveringacademic

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Posted 23 January 2015 - 08:08 AM

Right then folks from now on it's

effective internal heat capacity

No more of that thermal mass nonsense OK?

Got it?