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

[Visti]

Specific heat capacity is all well and good, but useless without thermal conductivity from that mass to the rest of the house that also matches envelope losses/gains. 

 

If the conductivity is bad, you'll just have a very hot block of whatever with the rest of the house being unaffected.

 

If the conductivity is super high, it'll dump that heat into the house faster than it can be lost. Nice sauna!

 

Either way, it is unnecessarily complicated way of trying to setup a stable temperature as you've got to dynamically account for both boundaries over time (daily, with annual fluctuations).

 

You're better off with a single boundary problem by focusing on the losses/gains via the external envelope alone. Then it becomes a much simpler problem of matching those losses/gains with some energy input such as via UFH or aircon.

 

Thermal mass sounds good, but only focuses on one property of the material, where in reality thermodynamics has a lot more stuff also going on.

Edited January 1, 2020 by Visti

[epsilonGreedy]

4 hours ago, Visti said:

If the conductivity is super high, it'll dump that heat into the house faster than it can be lost. Nice sauna!

 

 

My own thinking on this subject has changed. Many here think that diurnal temperature stability is a desirable outcome for a new build and I accept it probably indicates low heating bills.

 

I however think a house that drops 4 degrees internally overnight leads to better sleep quality. For example on a recent autumn holiday in a well insulated cottage it was too hot at night and I resorted to turning the heating off at 8pm in the hope it would be pleasantly chilly by bedtime.

[Stewpot]

5 hours ago, Visti said:

Specific heat capacity is all well and good, but useless without thermal conductivity...

 

Who can forget Judith Hann, on Tomorrow's World, picking up a red hot heat-shield tile from the early days of the space shuttle - I don't know what the material was, but it was such a poor thermal conductor that the heat therein didn't get transmitted to her hand.

[SteamyTea]

5 hours ago, Stewpot said:

Judith Hann

The Anni-Frid Lyngstad of science, as opposes to Maggie Philbin, who was the Agnetha Fältskog.

 

[Jeremy Harris]

7 hours ago, epsilonGreedy said:

 

My own thinking on this subject has changed. Many here think that diurnal temperature stability is a desirable outcome for a new build and I accept it probably indicates low heating bills.

 

I however think a house that drops 4 degrees internally overnight leads to better sleep quality. For example on a recent autumn holiday in a well insulated cottage it was too hot at night and I resorted to turning the heating off at 8pm in the hope it would be pleasantly chilly by bedtime.

 

 

The simple answer is to just design the house to give you the specific temperature you want in the bedrooms and the specific temperature you want in the living areas in the house, if these are different.  Ours does this; like you, we don't like warm bedrooms at all.  We have no heating at all in the bedrooms, and they typically sit at around 18 to 19°C for most of the year, whilst the rooms downstairs sit at around 21.5° to 22°C.  I increased the fresh air flow rate from the MVHR to the bedrooms to ensure that they stay a bit cooler than the rest of the house, and increased the thickness of the acoustic insulation in the ground floor ceiling (acoustic insulation also works well as thermal insulation).  This works very well for almost all of the year, except in very hot weather, where after a few days of high outside temperatures the bedrooms do get up to an uncomfortable level (for us) of maybe 22°C or more.

 

I fixed this last summer by adding a small air conditioning unit upstairs, that is very effective at lowering the bedroom temperature in very hot weather.  It doesn't get used a lot, but we feel it's worth it.  Had we been staying in our old house I'd have done the same, as that used to overheat badly in hot weather, and my fix there was to use a portable air con unit.  OK, but noisy, and because the house thermal time constant was short, it needed to be on all the time in hot weather.  The air con we have now only needs to be on for a few hours during the day and the long thermal time constant means the bedroom stays cool overnight and well into the next morning with it turned off.

[epsilonGreedy]

3 hours ago, Jeremy Harris said:

The simple answer is to just design the house to give you the specific temperature you want in the bedrooms and the specific temperature you want in the living areas in the house, if these are different.  Ours does this; like you, we don't like warm bedrooms at all.  We have no heating at all in the bedrooms, and they typically sit at around 18 to 19°C for most of the year, whilst the rooms downstairs sit at around 21.5° to 22°C.

 

 

Ok maybe I have attached too much significance to comments that MVHR systems tend to redistribute heat around a house and reduce natural temperature variations.

 

I my case I am thinking about a 2 story house with only minimal bathroom x 2 heating upstairs and UFH downstairs. The stairs will be open plan to the living room. Wall U value unlikely to be better than 0.2.

[Jeremy Harris]

Any normal MVHR will pretty much always cool the house, except when the air outside is hot.  It just cools the house a lot less than conventional ventilation in cold weather.  It doesn't heat the house, or move heat around to any significant degree^[1], as it always delivers fresh air that is cooler than the air in the house (has to, as the efficiency is rarely greater than about 85% to 90%).

 

 

^[1] The exception is active MVHR, that includes either heating, or an air-to-air heat pump to deliver both heating and cooling, but these tend to be expensive compared to normal, passive, MVHR.

[Tom's Barn]

On 01/04/2019 at 08:56, ProDave said:

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.

Hi Dave

 

this entire thread to me is really interesting and quite remarkable because you (and your house construction) have achieved a thermal time constant that should not really work based on your having no thermal mass. Also, @Jeremy Harris note at the start of this thread outlined the best materials to build a house with in order to help achieve a better themal time constant. Therefore, what approach did you take to constructing the walls and floor that provides you with such a comforable internal environment? I assume in the summer you remain cool again despite having no traditional thermal mass.

 

The main reason for my interest and question is I have built a house from Isotex blocks and I have decided to add a service zone on the walls by fixing 25mm battens. My intention is to fill these voids with rockwool RW5 not becuase i need sound proofing but becuase it has a mass of 100kg m3 and therefore I thought it would add to the thermal buffering.

 

I would be interested in your thoughts. 

 

[TerryE]

@Tom's Barn when you refer to someone by name use the "@" format to let them know and so we know who you are talking about.

 

I did a calc of my thermal mass of the house somewhere and IIRC the biggy by a long way was our "warm slab" which is inside the thermal envelope, then the internal plaster board.  Our external stone skin overlaying the 300mm cellulosic filler plays a big impact on the decrement delay and pretty much eliminating diurnal temperature variation of out external wall temperatures inside the house.  I am not sure that 25mm rockwool is going to make a lot of difference compared to the type of Isotex block as these can vary in U value from around 0.6 down to 0.1 depending on type.

 

IMO, the main issue with any block construction is that sealing the blockwork relies on scrupulous quality control during build, to a level that few construction teams seem to achieve.  The as-built thermal performance is rarely anywhere near the as-designed.

[ProDave]

8 hours ago, Tom's Barn said:

Hi Dave

 

this entire thread to me is really interesting and quite remarkable because you (and your house construction) have achieved a thermal time constant that should not really work based on your having no thermal mass. Also, @Jeremy Harris note at the start of this thread outlined the best materials to build a house with in order to help achieve a better themal time constant. Therefore, what approach did you take to constructing the walls and floor that provides you with such a comforable internal environment? I assume in the summer you remain cool again despite having no traditional thermal mass.

 

The main reason for my interest and question is I have built a house from Isotex blocks and I have decided to add a service zone on the walls by fixing 25mm battens. My intention is to fill these voids with rockwool RW5 not becuase i need sound proofing but becuase it has a mass of 100kg m3 and therefore I thought it would add to the thermal buffering.

 

I would be interested in your thoughts. 

 

The long thermal time constant is down to using insulation with a long decrement delay, generally more dense materials like wood fibre, celulose, mineral wool etc will have a longer decrement delay than lightweight materials like kingspan etc.  There are various on line tools you can use to look up the themal time constant of different wall build ups.  Mine has a time lag of something line 13 hours.  I recall someone modeling a wall on here with the same insulation U value but kingspan type insulation and it had a thermal log of just a few hours.

[scottishjohn]

37 minutes ago, ProDave said:

The long thermal time constant is down to using insulation with a long decrement delay, generally more dense materials like wood fibre, celulose, mineral wool etc will have a longer decrement delay than lightweight materials like kingspan etc.  There are various on line tools you can use to look up the themal time constant of different wall build ups.  Mine has a time lag of something line 13 hours.  I recall someone modeling a wall on here with the same insulation U value but kingspan type insulation and it had a thermal log of just a few hours.

not disagreeing with anything you have said 

but I think  the main reason for use of PIR  is  so widespread  is you can get the value required for Building spec in a much thinner wall.+ therefore cheaper framing costs and more internal space for same footprint

 

[ProDave]

4 minutes ago, scottishjohn said:

not disagreeing with anything you have said 

but I think  the main reason for use of PIR  is  so widespread  is you can get the value required for Building spec in a much thinner wall.+ therefore cheaper framing costs and more internal space for same footprint

 

If you are limited by external size that may be a valid argument, but on a 1/3 acre plot with no size limits you are better choosing the internal footprint you want, then choosing the "best" insulation and what does it matter if the walls are a bit thicker so the external size is bigger?

 

Until I learned about decrement delay I thought "what's the difference" and thought it was all about U value.  I suspect that is all a lot of people still look at and so it is natural to buy the insulation that gives your required U value in the minimum width, but that may not always be the best choice.

[SteamyTea]

34 minutes ago, ProDave said:

The long thermal time constant is down to using insulation with a long decrement delay, generally more dense materials like wood fibre, celulose, mineral wool etc will have a longer decrement delay than lightweight materials like kingspan etc

If you analysis common building materials, you tend to find that the ones that can hold moisture/water tend to have the highest specific heat capacity.  This is not surprising as hydrogen, the major constituent of water, has a SHC of 14304 J/(kg.K).

 

What makes things muddled is that there is a unit clash between energy storage and conductivity.  They are joules and watts respectively.

Joules is just the stored energy, watts are how fast you can release that energy.

The difficult bit is that a house has variable power inputs during the day, some are external, some are internal.  And to make matters more complicated, some of the internal forces are caused by external forcing.

 

Ideally, you want to reduce the conductivity of a components (why we have legislation that makes us do this, can be treated as a lower bound), but you do not want to store too much energy that overheating is a problem.

And to make it even more complicated, as the temperature of the storage mass reached equilibrium with either the internal, or external (and sometimes these are equal) temperatures, no storage or transfer of energy is taking place.

Now when you think about it, how often are the conditions right to store enough energy to last a few hours, when there is only a small change between internal and external temperatures (use the kelvin scale).

If you really want to store energy for later use, you have to go to an active system i.e. pumped solar thermal to an insulated store, PV to a battery.  By doing this, you can then release that stored energy at the correct rate (this is power, and measured in watts).  Relying on the walls of a house to do that is only going to work in a very narrow band regardless of what they are made from.

So after many years of looking at this, I think that good insulation levels have a greater impact that high mass, as they limit the variable inputs and outputs.

Just don't ruin it by putting in windows and doors, live in a cold cave, or a thick polystyrene box, the effects will be the same.

 

[scottishjohn]

once we get orbital manufacturing  where there is a vacuum -- then eps bubbles made in that vacuum  would have a vacuum inside them not air

could be a another leap forward 

 as we all know a vacuum does not transfer heat very well --hence a thermos flask works

 not sure the transport costs would work out 

 maybe a wee project for some student at uni--making eps bubbles in a man made vacuum ?

presuming the eps bubble did not collapse .LOL

[Bitpipe]

3 minutes ago, scottishjohn said:

once we get orbital manufacturing  where there is a vacuum -- then eps bubbles made in that vacuum  would have a vacuum inside them not air

could be a another leap forward 

 as we all know a vacuum does not transfer heat very well --hence a thermos flask works

 not sure the transport costs would work out 

 maybe a wee project for some student at uni--making eps bubbles in a man made vacuum ?

presuming the eps bubble did not collapse .LOL

 

Can't wait to see the delivery charges for that

[SteamyTea]

7 minutes ago, scottishjohn said:

maybe a wee project for some student at uni--making eps bubbles in a man made vacuum

Been done, how vacuum insulation panels are made.

Basically a foaming resin, just that as it foams, there is no gas transfer into the void.

Cheaper VIPs use an evacuated open cell foam, surrounded with an airtight membrane.  These ones are rubbish.

[scottishjohn]

2 minutes ago, SteamyTea said:

Been done, how vacuum insulation panels are made.

Basically a foaming resin, just that as it foams, there is no gas transfer into the void.

Cheaper VIPs use an evacuated open cell foam, surrounded with an airtight membrane.  These ones are rubbish.

then it has not been done correctly ---if not done in a vacuum then it will have air inside --not a vacuum-and open cell foam cannot have a vacuum -as air paases through open cell -same as moisture does

 

[SteamyTea]

Just now, scottishjohn said:

then it has not been done correctly ---if not done in a vacuum then it will have air inside --not a vacuum-and open cell foam cannot have a vacuum -as air paases through open cell -same as moisture does

That is why it is surrounded by an airtight membrane.

I think if you look at most VIPs they are of this design.  The ones in a Sunamp are.

[Carrerahill]

On 20/05/2016 at 16:08, Jeremy Harris said:

The term "thermal mass" comes up time and time again on building related forums and discussions, yet as a parameter it has one notable feature - it does not really 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 ?

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 building professionals keep harping on about the virtues of so-called "thermal mass".

I love this. I love breaking down myths and nonsense created by marketing groups, the poorly informed etc. which then sadly sticks. A bit like Covid. We have some neighbours whos Covid thought process on what they need to do to stay safe is comical.

 

Anyway, I have only read your OP Jeremy (need to get working) so anything said after your post, forgive me if it has been said, but would it be better in fact to try and use the existing term thermal store? 

 

Thermal mass did sound sort of acceptable as long as you don't dive into it, a bit like on the face of it "socially distanced" sounds about right, however, analyse both of them and they are a misnomer. If I was to very quickly give 1 reason why thermal mass sounds wrong I'd immediately say the use of the word mass is simply incorrect here, water, as you mention above, has one of the highest specific heat capacities, therefore a smaller volume of water, by a factor of over 4, would actually give the same "thermal mass" as concrete therefore somewhat muddying the waters regarding "mass".

 

I think it is probably safe to say that a big slab of concrete sitting at 20° could be deemed a thermal store? But, yes, thermal mass just isn't quite accurate.

 

On the socially distanced point that I think is wrong - I think it should be physically distanced. I am being social with all of you right now - we can be sociable without being in the same city or even country! 

Edited February 5, 2021 by Carrerahill

[scottishjohn]

28 minutes ago, SteamyTea said:

That is why it is surrounded by an airtight membrane.

I think if you look at most VIPs they are of this design.  The ones in a Sunamp are.

but if its not a vacumm --then it will not work like e thermos flask-and a real vacuum is the key

[scottishjohn]

1 minute ago, Carrerahill said:

On the socially distanced point that I think is wrong - I think it should be physically distanced. I am being social with all of you right now - we can be sociable without being in the same city or even country! 

exactly 

which is why I keep going back to that little song on the internet --says all you need to know to keep safe "stay the fxxk at home"

don,t need any other complicated and expensive solution -- just stay distant from others -- and why we are only now getting to TOTAL border close down and enforced isolation -- -something a 5year knows is the best and only solution  to stopping it spreading 

[SteamyTea]

@scottishjohn

I am not sure what you are on about, started off about making vacuum panels in space, then getting a PhD student to try it out on Earth.  I pointed out that it has been done, and how it is done, and you seem to be getting defensive about something and going to about thermos flasks and proper vacuums.

[scottishjohn]

3 minutes ago, SteamyTea said:

@scottishjohn

I am not sure what you are on about, started off about making vacuum panels in space, then getting a PhD student to try it out on Earth.  I pointed out that it has been done, and how it is done, and you seem to be getting defensive about something and going to about thermos flasks and proper vacuums.

thats the problem --

 you cannot have anything like a vacuum in a flat panel -- you get vacuums in a sphere or a cylinder as that are the only structure that can suffer the pressure from the outside without distorting or collapsing  at a reasonable wall thickness

VIP panels are a markerting ploy -- not a real high spec vacuum  

 

thats why you fill double glazing with argon -as it works pretty well at a reasonable cost as an insulating gas with no moisture content as air would have   -- but it is under slight positive pressure not a vacuum

 so lets discontinue this as you seem not to understand the problems 

[SteamyTea]

34 minutes ago, scottishjohn said:

understand the problems

Spend a few minutes with Google. No one has been talking about an unsupported panel with a vacuum in it.

1 hour ago, SteamyTea said:

Cheaper VIPs use an evacuated open cell foam, surrounded with an airtight membrane

I shall just do a Jackie Weaver.

Edited February 5, 2021 by SteamyTea

[stubiff]

Scenario - 2 rooms with the same U value, with infrared radiators or some form of radiant heat.

One with walls with some wood fibre boards.

One with walls with something else.

Would the room with wood fibre boards use less energy to maintain a certain room temp, as the heat retained in the boards radiates back into the room when it starts to cool down.

 

Same as when people but a concrete structure around a stove to retain the heat, to heat a house.

 

Thanks.