What's more important for comfort U value or 'thermal mass'

[Gone West]

I have an external wall in the kitchen which is around 650mm thick and is of sandstone/rubble construction. I am trying to work out if adding a thin layer of insulated plasterboard, say 25mm PIR, will improve comfort levels. Alternatively if I just plaster skim the wall, will the comfort level be better because of the thermal mass effect.

[joe90]

Don’t let @SteamyTea catch you using “thermal mass” he will get his knickers in a twist 🤣🤣🤣 I don’t know the figures but insulating internally will make the room heat up quicker , thermal mass (sorry) will store heat but needs heating so will absorb heat out of the house but keep heat levels more stable.

[SteamyTea]

4 minutes ago, joe90 said:

thermal mass

Find a derived SI unit for it and I am happy to change my mind.

5 minutes ago, joe90 said:

thermal mass (sorry) will store heat but needs heating so will absorb heat out of the house but keep heat levels more stable

Stable at a lower mean temperature for the same energy input.

Did the calculations on that years ago.

 

Ideally what you want is an airtight box made from insulation.

Then clad it in your favourite coating.

If a brick/block is 100mm wide, that is 100mm less insulation.

[joe90]

1 minute ago, SteamyTea said:

Did the calculations on that years ago.

So what’s the answer to the original question?

2 minutes ago, SteamyTea said:

Ideally what you want is an airtight box made from insulation.

Not what the OP was asking 🤷‍♂️

[IanR]

24 minutes ago, Gone West said:

I have an external wall in the kitchen which is around 650mm thick and is of sandstone/rubble construction. I am trying to work out if adding a thin layer of insulated plasterboard, say 25mm PIR, will improve comfort levels. Alternatively if I just plaster skim the wall, will the comfort level be better because of the thermal mass effect.

But the Thermal Mass is still there, so you're improving U Value (a little) with the same mass, which may/should be an improvement in comfort. The closer the internal face of the wall is to ambient the lower the temp gradient is across the room reducing convection currents/drafts and comfort should be improved. But, will it make enough difference to notice it?

Has the solid wall got a good DPC? A PIR insulation on the inboard face may trap condensation/damp. Would a lime plaster be better if there's a lack of DPC?

Edited January 8 by IanR

[SteamyTea]

5 minutes ago, joe90 said:

So what’s the answer to the original question

Insulate and airtightness.

 

Thermal stability is more to do with ACH than anything else.

Keep them low, and recover the energy, and the temperature will be quite stable enough.

 

[MikeGrahamT21]

40 minutes ago, SteamyTea said:

Find a derived SI unit for it and I am happy to change my mind.

 

 

🤣 enjoy!

[Gone West]

43 minutes ago, SteamyTea said:

Find a derived SI unit for it and I am happy to change my mind.

Everybody knows what it means though.

 

https://www.greenspec.co.uk/building-design/thermal-mass/

 

https://www.designingbuildings.co.uk/wiki/Thermal_mass_in_buildings

 

[joe90]

4 minutes ago, Gone West said:

Everybody knows what it means though.

Except steamy and Jeremy 🤷‍♂️

[Gone West]

53 minutes ago, joe90 said:

I don’t know the figures but insulating internally will make the room heat up quicker , thermal mass (sorry) will store heat but needs heating so will absorb heat out of the house but keep heat levels more stable.

Yes I agree, but what will be more comfortable. We found when we first moved in that it took the best part of a year for the temperature of the fabric of the building to stabilise. @SteamyTea patiently explained about how much energy is required to dry out the old walls. The house felt cold even though the thermometer was reading the right temperature. I'm not sure that covering up the solid wall with a small amount of insulation will help and might make it less comfortable.

[Gone West]

46 minutes ago, IanR said:

But the Thermal Mass is still there, so you're improving U Value (a little) with the same mass, which may/should be an improvement in comfort.

Yes, it's the "may/should" bit, I'm not sure about. If the thermal mass is hidden, am I going to end up with an old wall that becomes cold again.

[joe90]

5 minutes ago, Gone West said:

it's the "may/should" bit, I'm not sure about

 

1 hour ago, SteamyTea said:

Did the calculations on that years ago.

🤷‍♂️

[joe90]

12 minutes ago, Gone West said:

The house felt cold even though the thermometer was reading the right temperature.

Then

55 minutes ago, SteamyTea said:

Thermal stability is more to do with ACH than anything else.

 

[SteamyTea]

12 minutes ago, MikeGrahamT21 said:

Is it bollocks.

Where did you get that from?

 

10 minutes ago, Gone West said:

Everybody knows what it means though.

They think they know what it means.

Trouble is, it gets confused in peoples minds.  What it really is is thermal inertia which is defined as the ‘property of a material that expresses the degree of slowness with which its temperature reaches that of the environment’ (Ng et al. 2011) or the ‘capacity of a material to store heat and to delay its transmission’ (Ferrari).

Thermal Inertia has the units

W √s / (m2 K)

[SteamyTea]

1 minute ago, joe90 said:

55 minutes ago, SteamyTea said:

Thermal stability is more to do with ACH than anything else.

 

I should really have added in 'and glazing area and angles'.

But in modelling you start with the easy bits then add in the complicated bit.

[IanR]

39 minutes ago, Gone West said:

Yes, it's the "may/should" bit, I'm not sure about. If the thermal mass is hidden, am I going to end up with an old wall that becomes cold again.

 

Even without "rising damp", it's a condensation risk. It's a good improvement in U Value 1.5 -> 0.565, but moisture collects between the PIR and Stone wall.

 

 

 

Temp on the left and humidity on the right.

But, the condensation may not be an absolute risk. It shows from 1° and below it's a risk, but it doesn't consider how long the temp needs to be below 1° for it to stabilise.  The 650mm wall has a 23hr decrement delay, so it's got to be down at that temp for over a day before the inner face could drop below dew point.

Edited January 8 by IanR

[ToughButterCup]

5 minutes ago, SteamyTea said:

Is it bollocks.  [Thermal Mass]

....

 

@SteamyTea 's tendency to brevity sometimes needs explanation.  Here's more information on the matter than you could ever need ...

 

 

[Gone West]

1 minute ago, IanR said:

But, the condensation may not be an absolute risk. It shows from 1° and below it's s risk, but it doesn't consider how long the temp needs to be below 1° for the for it to stabilise.  The 650mm wall has a 23hr decrement delay, so it's got to be down at that temp for over a day before the inner face could drop below dew point.

Thanks, that's really interesting. Food for thought.

[MikeGrahamT21]

22 minutes ago, SteamyTea said:

Is it bollocks.

Where did you get that from?

 

They think they know what it means.

Trouble is, it gets confused in peoples minds.  What it really is is thermal inertia which is defined as the ‘property of a material that expresses the degree of slowness with which its temperature reaches that of the environment’ (Ng et al. 2011) or the ‘capacity of a material to store heat and to delay its transmission’ (Ferrari).

Thermal Inertia has the units

W √s / (m2 K)

 

That was good old wikipedia, but theres plenty more to go at. Energy Education Canada?

 

An ideal material for thermal mass will have:

• high heat capacity
• high material density

Heat capacity of a substance is the amount of heat energy required to change the temperature of an object by a given amount. The SI unit for heat capacity is Joule per Kelvin (J/K). The total amount of energy stored by a thermal mass system is proportional to the size of the system or material, therefore specific heat capacity (J/m²K), heat capacity per unit mass, and volumetric heat capacity (J/m³K), heat capacity per unit of volume, are common metrics used to determine a good thermal mass material.

 

the things i've read over the years, people used the term thermal mass, to mean a storage heater effect, the ability the hold hot or cold and then release it back slowly over time, so i would say your version, and this version alike, are all correct and part of the bigger picture. Inertia is talking about the speed at which the transfer happens, and thermal mass (heat capacity) is how much energy a material can hold, you need them both together to understand how it will affect the environment. One which sticks in mind is from my youth, and comparing aluminium and copper heatsinks on CPU's.

Edited January 8 by MikeGrahamT21

[SteamyTea]

1 hour ago, MikeGrahamT21 said:

specific heat capacity (J/m²K)

J/m³K, could be a typing error or getting confused with W √s / (m² K)

1 hour ago, MikeGrahamT21 said:

Inertia is talking about the speed at which the transfer happens

Speed is a scalar, so only has magnitude i.e. m.s^-1 or in this instance J.s^-1, which we know as a watt.

1 hour ago, MikeGrahamT21 said:

thermal mass (heat capacity) is how much energy a material can hold

Kind of, but not really.  As soon as it becomes volumetric (as in fixed in place rather than floating anywhere in space), as opposed to specific, shape becomes important as that affects the exposed surface areas.  It is those surfaces that have a temperature difference. 

It is the thermal conductivity that governs the "velocity" of thermal energy transfer.  Velocity has speed and direction.

It can be modeled as simple harmonic motion (close enough), if the limits are known.

 

The biggest problem is that in the UK (and other islands especially, above/below Latitude 40°), we don't have evenly spaced hours of daylight, or stable weather systems.  This makes it hard to actually use a dynamic model as the energy forcing happens way to rapidly i.e. thermal changes in hours, not days.  The materials do not have enough time to react much.

It is often quoted that if wine is kept in caves in the South of France, the temperature is quite stable, but way below what is comfortable for people i.e. 12°C.

Now you could add some energy into the cavern, but I then think you will agree that you will not like the bill and as soon as you turn the heating off, the air temperature will quickly equalize with the core rock temperature.

 

Even after doing my BSc project in just this area, with the twist that the experiments also included active and passive solar gain, I still find it very hard to actually describe what happens (10k words and ten pages of calculations still only got me a 2.1, but my cataracts were so bad I could not read, so shall put it down to that).

Terminology, and using the correct SI units, becomes quite important in thermodynamics.  The is T and t (temperature and time), K and k (absolute temperature and 1000), e and e (effusivity and Euler's number), m, m² and m³ (distance, area and volume), then the horrible ones kg, h and H (mass, hours and inductance), j and q (flux as a scalar and the flux integral sum) and finally s and S (time and electrical conductance).

And people wonder why I am a pedant about the correct usage.  Get them wrong and formula soon stop making sense.

 

And that is why the universe is shaped like a saddle.

 

Edited January 8 by SteamyTea

[joe90]

That’s all well and good but what’s the bloody answer🤷‍♂️

[SteamyTea]

17 minutes ago, joe90 said:

That’s all well and good but what’s the bloody answer

Insulation is more important for stability.

It is in the units.

W.m^-1K

As it has a W (J.s^-1) in it, that includes the time element, and it is the time that describes the stability.

The heat capacity (J.kg^-1K^-1 or J.m^-3K^-1) just describes the instantaneous amount of energy and has nothing to say about how it moves.

Edited January 8 by SteamyTea

[ProDave]

2 hours ago, Gone West said:

Yes I agree, but what will be more comfortable. We found when we first moved in that it took the best part of a year for the temperature of the fabric of the building to stabilise. @SteamyTea patiently explained about how much energy is required to dry out the old walls. The house felt cold even though the thermometer was reading the right temperature

Are you sure it was not YOU that took a year to get used to the building?

 

I have relatives in an old stone farmhouse and when you first go there you notice the damp smell as you enter and the general damp feeling.  After a week or 2 there, you stop noticing both of those things.

[SteamyTea]

6 minutes ago, ProDave said:

you notice the damp smell as you enter and the general damp feeling.  After a week or 2 there, you stop noticing both of those things.

Is that because the asthma is so bad you are gasping from breath.

[Gone West]

9 minutes ago, ProDave said:

Are you sure it was not YOU that took a year to get used to the building?

No. Now the air temperature has been 23C for a long time the wall surfaces feel warm. We had an old wet door mat out in the unheated boot room and that did smell damp until I dumped it.