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The Great "Thermal Mass" Myth


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#21 NeilW

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Posted 23 January 2015 - 09:11 AM

It's not the mass that matters really. It's the volume.

For example a kg of concrete occupies 4/10ths of the space of a kg of water.

So you have to have the heat density related to volume before you can order the materials. So you need to multiply the values in the OP together.

That gives the order as:

Water
Plaster
Granite
Concrete
Brick
Wood

Which is pretty much as you expect.

#22 Alphonsox

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Posted 23 January 2015 - 09:14 AM

View Postjsharris, on 23 January 2015 - 07:50 AM, said:


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.

I applaud the intention but there a danger here of replacing one poorly defined term with another,
How would such a metric be measured and in what units ? It seems difficult to decouple from insulation and decrement delay for a real world application.

#23 SteamyTea

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Posted 23 January 2015 - 09:29 AM

"How would such a metric be measured and in what units ? It seems difficult to decouple from insulation and decrement delay for a real world application."

That is easy, look at the units.
Thermal Conductivity = W/(m.K)
Specific Heat Capacity = J/(kg.K)

Thermal Inertia = Square Root of the Product of Thermal Conductivity and SHC

The time part comes from the W = J/s

I think without knowing the arithmetic and the physics behind it that it is hard to understand. Bit like having something you have never seen described in a foreign language you don't understand very well.
Not meaning to be an academic here, but it all boils down to dimensional analysis, or what goes where and how big is it.

Edited by SteamyTea, 23 January 2015 - 09:30 AM.


#24 jsharris

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Posted 23 January 2015 - 09:33 AM

View PostNeilW, on 23 January 2015 - 09:11 AM, said:

It's not the mass that matters really. It's the volume.

For example a kg of concrete occupies 4/10ths of the space of a kg of water.

So you have to have the heat density related to volume before you can order the materials. So you need to multiply the values in the OP together.

That gives the order as:

Water
Plaster
Granite
Concrete
Brick
Wood

Which is pretty much as you expect.

Exactly!

So, why do so many people refer to "thermal mass" and not "thermal volume"?

I suspect it goes back to the oft-observed comment about massive stone buildings having a more even internal temperature (true, but live in one for a while and you soon find out that this isn't always a blessing!). Somehow a connection was made between mass and temperature stability that now seems to be engrained in the heads of even new house designers.

Terry E's analysis shows that you can have a relatively slow internal temperature response from a low mass building, and our build confirms this practically. Given that we are now building more and more houses with far higher insulation levels, lower ventilation heat loss levels and with glazing that limits solar gain and restricts radiated heat loss then I think we need to wean people away from the idea that they need a lot of mass in a building in order to make it comfortable and temperature stable, as it just isn't true.

Edited by jsharris, 23 January 2015 - 09:33 AM.


#25 Alphonsox

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Posted 23 January 2015 - 10:55 AM

View PostSteamyTea, on 23 January 2015 - 09:29 AM, said:

"How would such a metric be measured and in what units ? It seems difficult to decouple from insulation and decrement delay for a real world application."

That is easy, look at the units.


Thanks for completely missing the point :)
As a number of us have being trying to explain in this thread this is nothing to do with mass. Any new metric that seeks to describe the real world behaviour needs to get away from specific heat capacity (J/(Kg.K) if it is to be of any use. A volumetric heat capacity measure may be more useful (J/m3.K) but still doesn't capture the behaviour of timber frame constructions.

The problem is that people think they understand thermal mass. Hence the constant stream of posts citing this as a reason to build in block rather than timber. The counter-argument at the moment is largely anecdotal and "the plural of anecdote is not data". Weaning people off of the crutch of thermal mass is going to require more than this. An understandable metric that describes the thermal stability of a construction is what's needed.

Edited by joiner, 23 January 2015 - 11:01 AM.
Typos


#26 NeilW

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Posted 23 January 2015 - 11:09 AM

View Postjsharris, on 23 January 2015 - 09:33 AM, said:

Terry E's analysis shows that you can have a relatively slow internal temperature response from a low mass building, and our build confirms this practically. Given that we are now building more and more houses with far higher insulation levels, lower ventilation heat loss levels and with glazing that limits solar gain and restricts radiated heat loss then I think we need to wean people away from the idea that they need a lot of mass in a building in order to make it comfortable and temperature stable, as it just isn't true.

Fully agree. This hanging onto old ways of doing things is very ingrained and difficult to shift. We need to be building houses like we build sheds - assume they are temporary and will have to be taken down, recycled and renewed every few decades.

I've just had a plumber around this morning that insisted I needed to leave one radiator without a TRV on it "to satisfy building regs" even though I have "individual networked radiator controls in each room on the circuit"

#27 wmacleod

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Posted 23 January 2015 - 11:22 AM

View PostNeilW, on 23 January 2015 - 11:09 AM, said:

Fully agree. This hanging onto old ways of doing things is very ingrained and difficult to shift. We need to be building houses like we build sheds - assume they are temporary and will have to be taken down, recycled and renewed every few decades.

Have you bought shares in a building firm?! Upgraded yes, improved yes, but taken down and replaced - no. Designed right and built right there is no need for ripping everything apart every few decades. There are many beautiful and practical buildings around which have been there for hundreds of years providing housing for generations.

#28 SteamyTea

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Posted 23 January 2015 - 11:22 AM

"An understandable metric that describes the thermal stability of a construction is what's needed."
Something like hours/(m2 K) then.

I think there is plenty of evidence about fabric energy storage, generally from the US and written to show that heavyweight construction keeps a place cool. All the same science, just asking a different question.

The other thing is why are we looking towards a 'passive system'. Simple and cheap controls, a switch on a timer, may make life very easy. The doomsday scenario that people dread may never happen.

#29 recoveringacademic

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Posted 23 January 2015 - 12:04 PM

View PostAlphonsox, on 23 January 2015 - 10:55 AM, said:

.......
The problem is that people think they understand thermal mass. ......
Absolutely right , I thought I did understand it when I woke up this morning. But by the time I'd had a cuppa, I didn't (any more).

What's wrong with effective internal heat capacity as a term?
I really 'click' with Steamy's hours/(m2 K).

Edited by recoveringacademic, 23 January 2015 - 12:07 PM.


#30 joiner

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Posted 23 January 2015 - 12:06 PM

Nothing, as long as the person you're saying it to understands what it means. :huh:

#31 recoveringacademic

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Posted 23 January 2015 - 12:17 PM

It's OK Joiner, just thee and me, we'll keep it as our little secret eh?

Edited by recoveringacademic, 23 January 2015 - 12:17 PM.


#32 DamonHD

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Posted 23 January 2015 - 01:02 PM

Living in space-constrained London the metric that would appeal to me is heat capacity (kWh) per unit volume (and per £), with some clarity as to whether this will be a kWh/K value (eg in a water-filed thermal store) or a more abrupt phase-change type of storage eg at nominally constant temperature.

I think of my battery stores in kWh/m^3 and kWh/£ and have found it useful to think about other potential energy stores in the same way, so I have a small Lithium battery in the house and a larger lead-acid outside for my off-grid system for example, because Li is higher kWh/m^3 but lower kWh/£. My lead-acid store has *similar* characteristics to a water tank in *very* broad storage terms for size and weight, given my small house.

Yes I am very clear that the exergy of hot water and electricity are not the same, but I allow for being able to convert back and forth with (eg) heat-pumps. (My TEG in the garden is so far an expensive dud!)

Rgds

Damon

Edited by joiner, 23 January 2015 - 04:10 PM.


#33 NeilW

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Posted 23 January 2015 - 01:34 PM

View Postwmacleod, on 23 January 2015 - 11:22 AM, said:

Have you bought shares in a building firm?! Upgraded yes, improved yes, but taken down and replaced - no. Designed right and built right there is no need for ripping everything apart every few decades.

Yes there is. Jobs and evolution.

The 'old' buildings are costing a fortune and should be demolished - as long as they are to be replaced with something superior (not the current rubbish).

The argument that things should stay the same forever is just not natural. The broad bean plant dies back every year and is replaced with a new one - that may have evolved slightly.

With ever greater automation there is becoming a shortage of things to do - particularly at the manual end. Taking down buildings and replacing them in a sensible recycled manner allows things to be built specifically to the short term purpose rather than trying to gold plate the thing for all eventual uses, or shoehorn a new use into a building that was never designed for it.

A 1960s car is no good on today's roads. Neither is a 1960s power station. Therefore neither is a 1960s house, no matter how much it has been patched up.

And it will be the same in another 50 years.

Pretty much every other society on the planet replaces domestic buildings more regularly than we do. My wife can barely recognise the streets in Japan where she stayed 20 years ago - and that's on the west side where there are no significant earthquakes.

This obsession with old in the UK is one of the things that hold us back.

#34 wmacleod

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Posted 23 January 2015 - 02:11 PM

View PostNeilW, on 23 January 2015 - 01:34 PM, said:

A 1960s car is no good on today's roads. Neither is a 1960s power station. Therefore neither is a 1960s house, no matter how much it has been patched up.

In this greater scheme of evolution, we had the wonders of the 1960s and vertical living. We had garden cities. A lot of "visionary" thinking of get rid of the old. How did that forward thinking end? Nearly everything from this period and on is utter rubbish in terms of design, materials and manufacture! You can get rid of it, and most of the new stuff as well.

However I can look out just now and see dozens of really delightful houses from the late 1800s and early 1900s. Beautifully built and well kept because they were valued and worked well for generations. Well designed, family homes that people cared for. Sure some could do with a little more insulation perhaps now, new heating systems and perhaps a little bit of PV somewhere, but knocking them down would be a crime and in no way financially or ecologically sound. The rubbish housing from that period has long gone, the good stuff remains.

Don't believe that newer is by definition better, it isn't. Read old architecture books and learn from old buildings what we have lost. We still have basic needs to house people, we have been building for a good long time now and have lost far more building knowledge than we realise. How did they build the pyramids?

#35 SteamyTea

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Posted 23 January 2015 - 02:17 PM

Radio 4's Monday comedy play is about an Angel giving the human race 1000 years of development in one century.
http://www.bbc.co.uk...rammes/b04yb2x2

#36 NeilW

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Posted 23 January 2015 - 03:27 PM

View Postwmacleod, on 23 January 2015 - 02:11 PM, said:

Sure some could do with a little more insulation perhaps now,

No, they need demolishing because they are uneconomic to bring up to the standard required when we have an overheating planet.

And yes we need to build better modern houses using more automated techniques, so that they can be put together with no more skill than is required to build a Lego set.

Most of the Victorian terrace housing around me are essentially environmental slums that are way past due for replacement. Much as the nuclear power stations are well past due replacing.

We spend too much time propping up the old, rather than putting in place a proper system for recycling and renewal - largely because new houses are so crap and we can build so few of them.

#37 wmacleod

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Posted 23 January 2015 - 04:11 PM

View Postjsharris, on 22 January 2015 - 10:28 PM, said:

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

I don't want to take this further OT, so to bring it back on topic - Jeremy, can you tell us what your TMP figure is in your SAP calculation? I am interested to see whether SAP methodology tallies with what you are seeing.

#38 SteamyTea

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Posted 23 January 2015 - 04:12 PM

Here is a function that describes my thermal inertia with over 90% accuracy.
Hours = (2E-06x6 - 0.0002x5 + 0.0081x4 - 0.1759x3 + 2.2656x2 - 16.633x + 42082) / 12m
Where x is temperature within the range of 13.4°C to 21.1°C and m = m2

I think I may know why people don't get it :D

#39 joiner

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Posted 23 January 2015 - 04:23 PM

Nick, we do 'get it', we just don't necessarily get the physics.

Subjects like this can sometimes appear to the layman to be saying that aeroplanes don't fly, they just don't fall out of the sky.

#40 temp

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Posted 23 January 2015 - 04:46 PM

View PostSteamyTea, on 23 January 2015 - 02:17 PM, said:

Radio 4's Monday comedy play is about an Angel giving the human race 1000 years of development in one century.
http://www.bbc.co.uk...rammes/b04yb2x2

Isn't that what we had in the last century?