The Great Thermal Mass Myth................

[Construction Channel]

7:50

Shall I tell him?

[SteamyTea]

More worrying is why you are viewing soapstone.

Back in the 1970's I worked with some Australians, they said the best place to hide your money from a Pom was under a cake of soap and when they turned the plane engines off on arrival, the shining carried on!

[Construction Channel]

I subscribe to his channel, don't panic i have got no interest in soap.....stone or otherwise

[TerryE]

Yes but his rampump is amazing

[Jeremy Harris]

6 minutes ago, TerryE said:

Yes but his rampump is amazing

I helped to fix a ram pump around 30 years ago.  It was in a small stream at Bochym Manor, near Mullion, Cornwall. I can't remember the date that was cast on to the case, but it was made around the mid-1800's I think, and supplied water to the house from the stream.  Fixing it was easy, as it was only the failure of the 100-plus year old leather on the clack valve that had stopped it working,  Once stripped, cleaned, fitted with a new valve leather and given a coat of paint it was fitted back in the stream, connected to the original cast iron pipe and supplied water to a fountain.

These things have a quite remarkable pumping capacity, given their simplicity, and astonishing reliability.  I think the only down side is the noise from the valve banging shut every second or two, not something you'd want near to a house.  At Bochym the pump was well down the valley from the house, probably 100m or more away, so the noise couldn't be heard up there.

[Ed Davies]

On 22/05/2016 at 20:09, tonyshouse said:

Would it be possible to define your thermal time constant please?

 

For the conventional definition of time constants used in mathematics, physics, electronics, etc, it comes out as simply the temperature difference between inside and outside divided by the rate of change of inside temperature.

 

E.g., if the inside of the house is at 20 °C and the outside is at 10 °C and the inside temperature drops initially (when, say, the heating is turned off) at 0.2 °C/hour then the time constant is (20 - 10)/0.2 = 50 hours.

 

That's the length of time the temperature would take to drop to the outside temperature if it continued downwards at the same rate as it started. Actually, of course, as the temperature difference decreases the rate of cooling will also decrease so it will curve out (following an exponential curve) getting closer and closer to the outside temperature but never (in a theoretical steady-state) quite reaching it.

 

When the temperature has dropped to 15 °C the rate of cooling should have halved so the time constant is now (15 - 10)/0.1 = 50 hours. Yipee, the time constant is constant.

 

After the time constant it should reach 1/e ~= 0.368 times the original difference. So, after 50 hours our example house should be at 13.68 °C.

Edited November 12, 2018 by Ed Davies

[Cpd]

Nice explanation. Slowly but surely I will get my head around this stuff. 

[ToughButterCup]

In the pursuit  of openness, balance and intellectual rigour I attach a serious and, apparently well-argued paper, explaining the concept of thermal mass. The arguments advanced appear to be properly referenced, but I haven't followed them through yet.

ThermalMassExplained_Feb19.pdf

Accessed April 1st. 2019 

https://www.concretecentre.com/Search-Results.aspx?searchtext=thermal+Mass&searchmode=anyword

 

This URL  links to a download for a publication that explains the concept of Thermal Mass in the context of housing. (you'll need to register - hence comment below)

[MB_Thermal Mass for Housing.pdf : I have uploaded this copy to our server to save you having to register - and be spammed] 

 

In the context of this tread, I find myself wishing I hadn't found this resource : my brain hurts enough coping with the build. (and @Onoff's tiling saga)

This isn't an April Fool's joke. 

Ian

Edited April 1, 2019 by recoveringacademic
To save members having to register to read the document on Thermal Mass and and housing

[ProDave]

First, if they talked of Thermal Capacity, rather than Thermal Mass (a non existant unit) they might get more respect?

 

Who sponsored this paper?  Blue Circle?  It is clearly a reason to use concrete in houses, no more, no less.

 

Now if they concentrated on discussing insulation levels and insulation types, and decrement delay, and discussed windows and thermal gain, they would find all the answers to minimising or reducing overheating, regardless of what material the house is actually built from.

 

And if, in spite of your best efforts you still need some summer cooling, put some PV on the roof and that cooling can be done without burning fossil fuel.

[Jeremy Harris]

Like all other supposed explanations for this mythical physical property, it fails because it doesn't define the units of measurement.

 

As mentioned many times, we can measure and quantify heat capacity, thermal conductivity, decrement delay time, thermal time constant, temperature etc, but no one has yet invented any units to define thermal mass (hardly surprising as it doesn't exist as a physical property).

 

The graph (figure 1) showing the stabilising effect is nothing to do with "thermal mass", it's just a consequence of having sufficient insulation, with a long enough decrement delay, to damp and offset the time taken for the inside of the house to change relative to changes in outside air temperature.  This is aided by having a means of storing enough heat in the internal structure (and then only the first 100mm or so of it), in a layer with a reasonably high thermal conductivity, that can act as a buffer heat store.

 

 

 

[Tennentslager]

Nice paper @recoveringacademic Makes a good case , especially on such a fine day as it is today...well up here anyway ???

Edited April 1, 2019 by Tennentslager
Thought it WAS a1st April special!

[ToughButterCup]

The Poor Bloody Infantry in this discussion deserve better.

 

How - with two apparently well qualified scientists (Competent Person(s) in the lingo)  taking diametrically opposite  positions - can we (the people whose bank balance takes the hit) make up our minds? There will be people - many I suspect - who invest large sums of money on the basis of reading the attached paper(s).

 

In my old jobs (running   Virtual Learning Environments inside three UK Universities) I would be delighted - because I'd see the opportunity to arrange a classic Disputation (defence of a Doctoral thesis) so that students could listen and read and perhaps watch well-regarded scientists put their case.  That's never going to happen here is it? More's the pity.  

 

And it's our bank balances that take the hit

 

25 minutes ago, JSHarris said:

[...]  This is aided by having a means of storing enough heat in the internal structure (and then only the first 100mm or so of it), in a layer with a reasonably high thermal conductivity, that can act as a buffer heat store.

 

Is that ( ' ...that can act as a buffer heat store .... ')   , J, your way of saying Thermal Mass ?

 

Edited April 1, 2019 by recoveringacademic
grammar

[ProDave]

My house is built entirely of timber, plasterboard and insulation. The only concrete / brick is the foundations "hidden" under the insulated suspended timber floor.

 

Yet my house takes very little to heat it, keeps it's temperature very stable, does not heat up or cool down quickly at all, and has a thermal time constant of over 13 hours.

 

It does this (if you believe that paper) without any Thermal Mass.   

 

When they have explained that one, how my house can possibly maintain a comfortable stable temperature without lots of thermal storage capacity, I will start to take what they say seriously.

[Gone West]

31 minutes ago, JSHarris said:

This is aided by having a means of storing enough heat in the internal structure (and then only the first 100mm or so of it), in a layer with a reasonably high thermal conductivity, that can act as a buffer heat store.

 

Is there any data showing what the thermal gradient is or whether it is linear in different materials with differing thermal conductivity and capacity.

[jack]

31 minutes ago, recoveringacademic said:

Is that ( ' ...that can act as a buffer heat store .... ')   , J, your way of saying Thermal Mass

 

I know what you mean Ian. I'd personally prefer it if we got less bogged down complaining about the phrase itself.

 

In my opinion, it would be more helpful to concentrate on the fact that the phrase is currently used as a shortcut to a poorly-defined concept or set of concepts. If there were a clearer definition, then the phrase itself would be perfectly fine. Indeed, since it's actually shorthand for a number of factors, I think it's useful to have a phrase that doesn't refer to any individual factor (okay, "mass" is in the phrase, but I don't have an issue with that given the context).

 

It's a fact that the general public - and even the building trade - is never going to dig into the details of a multi-factorial heat-buffering model. All anyone cares about is whether the house or extension meets building regs. Hence, short of building regs changing, nothing is going to improve on this front - not linguistically and not practically. 

 

 

58 minutes ago, ProDave said:

Now if they concentrated on discussing insulation levels and insulation types, and decrement delay, and discussed windows and thermal gain, they would find all the answers to minimising or reducing overheating, regardless of what material the house is actually built from.

 

To be fair, the document talks repeatedly about the importance of insulation, airtightness and solar gain control over the course of the year. Page 3 even shows decrement delay in graph form (note the time offset of the damped temperature curve).

 

Okay, they don't talk about high-decrement delay insulation, but other than here and a couple of other forums, I've never heard any self-builder or anyone in the building industry discuss the decrement delay of different insulation types. I'm sure that's at least partly because high decrement delay insulation is relatively expensive, takes up more room for a given U-value (compared to rigid insulation), is difficult to explain to the average person, and - perhaps most importantly - isn't compatible with current standard construction methods used by the big builders and most smaller builders. 

 

[dpmiller]

^well said @jack

[Jeremy Harris]

44 minutes ago, recoveringacademic said:

Is that ( ' ...that can act as a buffer heat store .... ')   , J, your way of saying Thermal Mass ?

 

 

Of course it isn't.

 

Heat capacity is the physical property of a material that defines how much sensible heat it can store per unit mass (or volume). 

 

Thermal conductivity determines how well heat flows through any material.

 

Both of these properties can be measured and have defined units.

 

Thermal mass has no units, so how can anyone measure it, or define what is "high" or "low"?

[epsilonGreedy]

Living in a static caravan is the quickest route to comprehending thermal mass, or lack of it. I liked the paper linked to by @recoveringacademicbecause it illustrates how for a given U-value the thermal mass can be varied with different building materials.

 

Surely the combined wisdom of this forum can arrive at a definition of thermal mass. I will start:

 

Quote

The ability for a building structure to stabilize internal temperature, thermal mass can be expressed as the product of the building's insulation level times by the rate at which the building structure can transfer its heat capacity to the internal air space. Good thermal mass cannot be achieved through either high heat-capacity or high insulation alone.

 

And having thought about thermal mass for 15 minutes I propose a more useful additional measure, I introduce "Effective Thermal Mass".

 

Effective Thermal Mass = Sum( Thermal Mass over a 10 hour cooling period ).

[jack]

Just now, JSHarris said:

Thermal mass has no units, so how can anyone measure it, or define what is "high" or "low"?

 

It might be a subtle point, but to me the critical starting point is that "thermal mass" has no agreed definition. Without such a definition, there's no point even considering units imo.

 

If "thermal mass" is used as shorthand for something like "real-world structural thermal buffering capacity", then it potentially does have units - ie, the units that apply to the factors you listed above, although we still need to define how they can be combined in a way that allows comparisons.

 

Unfortunately, many people - including architects and builders - use "thermal mass" to refer to some ill-defined combination of mass and perceived heat capacity. That's the real issue imo.

[Ed Davies]

58 minutes ago, ProDave said:

Yet my house takes very little to heat it, keeps it's temperature very stable, does not heat up or cool down quickly at all, and has a thermal time constant of over 13 hours.

 

13 hours is pretty short. Are you sure that's right? That would mean that if it was left unheated overnight it'd get down to about half the temperature it had relative to outdoors by morning.

Edited April 1, 2019 by Ed Davies

[Jeremy Harris]

57 minutes ago, PeterStarck said:

Is there any data showing what the thermal gradient is or whether it is linear in different materials with differing thermal conductivity and capacity.

 

It can be worked out from first principles if the conditions are known, but one problem is that often the conditions aren't known accurately enough to make modelling that useful.  One way to try and measure it is to look at thermal admittance, which is a measure of the ability of a material to absorb and release heat into the interior of the house.  However, accurately assessing this is compounded by the way that we tend to have most of the internal structure in a house (at least the part that can act as a thermal buffer) made up of layers that have widely varying thermal conductivity and specific heat/heat capacity.

 

Plasterboard is a good example, as there is a large area of it on the inside of a house normally.  It has a specific heat  of around 1.09 kJ/kg.K, so for a skimmed plasterboard wall each m² will store around 10 kJ  of sensible heat for a 1 degree temperature increment.  Taking our house as an example, it has a total wall and ceiling area of around 600m², so the plasterboard and skim will store around 6,000 kJ, or about 1.66 kWh, of sensible heat per degree change in temperature.  The plasterboard/plaster skim contributes a fair bit to the thermal buffering effect

 

The challenge is then to try and work out how well any surface in the house will transfer heat in or out of the house.  Materials with a high thermal conductivity will tend to do this more quickly than materials with low thermal conductivity.  it's not much use having a quantity of high heat capacity/specific heat material in the house, but buried behind layers of material that have a low thermal conductivity.

[epsilonGreedy]

56 minutes ago, ProDave said:

When they have explained that one, how my house can possibly maintain a comfortable stable temperature without lots of thermal storage capacity, I will start to take what they say seriously.

 

 

Could this be explained by the presence of 2.5 mobile bio heaters in your middle sized house that generate enough heat to match the low heat losses of your excellent insulation.

[ProDave]

12 minutes ago, epsilonGreedy said:

Living in a static caravan is the quickest route to comprehending thermal mass, or lack of it. I liked the paper linked to by @recoveringacademicbecause it illustrates how for a given U-value the thermal mass can be varied with different building materials.

 

No living in a static caravan just illustrates how poor a building can be if you only use a VERY small amount of insulation, and that small layer has a fast decrement delay to compound the issue.

 

If you want to do a scientific test for the forum, load your 'van with a load of concrete blocks inside to give it some "thermal mass" and report back what changes you notice (other than even less space to live in)

[epsilonGreedy]

8 minutes ago, ProDave said:

No living in a static caravan just illustrates how poor a building can be if you only use a VERY small amount of insulation, and that small layer has a fast decrement delay to compound the issue.

 

 

Decrement delay is an observed outcome. Thermal Mass is an attempt to create a unit of measure that can predict decrement delay before a build commences.

[ToughButterCup]

1 hour ago, jack said:

[...]

In my opinion, it would be more helpful to concentrate on the fact that the phrase is currently used as a shortcut to a poorly-defined concept or set of concepts. If there were a clearer definition, then the phrase itself would be perfectly fine.

[...]

 

" Thermal Mass mate? Wodja mean? " And then listen carefully

 

That'll do for me. Is it a Unit of measure? (not yet) Is it a mess?  (yes)   Is it widely mis-applied and misunderstood? (yes)

Well don't bloody use it then.

 

39 minutes ago, JSHarris said:

[...]

it's not much use having a quantity of high heat capacity/specific heat material in the house, but buried behind layers of material that have a low thermal conductivity.

 

Exactly.

 

45 minutes ago, jack said:

[...]

Unfortunately, many people - including architects and builders - use "thermal mass" to refer to some ill-defined combination of mass and perceived heat capacity. That's the real issue imo.

 

And sometimes they publish them.