Timber Frame Pros Cons

[Jeremy Harris]

1 minute ago, Sensus said:

 

You don't need them to define thermal mass.

 

 

Time is a key element when analysing decrement delay (as, indeed, is thermal mass, and the various other factors alluded to above).

 

 

 

 

So far I still haven't seen a valid expression for "thermal mass" given in this thread.   Expressions that I believe were intended to be those for heat capacity have been given, and an expression for heat energy per unit area have been given, but that is all.  That latter seems not to be very useful unless the specific or volumetric heat capacity for the material used for that defined area is also included, together with its thermal conductivity.

 

[SteamyTea]

2 minutes ago, Sensus said:

What is it, specifically, that you think I don't understand?

I don't think that you understand that you have to include all elements to have something useful, you can't pick and choose just one or two.

[Jeremy Harris]

Just now, Sensus said:

 

Whether you choose to call it 'mass heat capacity' or 'thermal mass' really doesn't matter.

 

It's a label, that's all.

 

I understand that it deeply upsets you, but the term 'thermal mass' is well defined and in even more common use.

 

 

I'm not in any way upset.  It is simply a matter of using units consistently, within a well-defined framework, so that any calculation that uses them is repeatable, when undertaken by different people at different times.  Consistency of units is important, which is why mankind has put so much effort into creating very specific definitions for them.

 

Heat capacity can be defined in four ways, and four ways only:

• Non-dimensional heat capacity is energy per unit temperature, J/K
• Volumetric heat capacity is energy per unit temperature for a specified volume of material
• Mass heat capacity is energy per unit temperature for a specified mass of material
• Molar heat capacity is energy per unit temperature for a specified molar quantity

[SteamyTea]

3 minutes ago, Sensus said:

Your argument seems to be analogous to saying that 'mass doesn't exist, because I'm considering density, and it doesn't give me all the information I need'.

Mass is just the count of 'stuff' in an object.  So no way am I saying that mass does not exist.

This is getting absurd, but I am going out as it is sunny.

Next time you are in a meeting and someone mentions the benefits of thermal mass, you will remember all this, and then question if what they are saying makes sense.

Edited September 8, 2019 by SteamyTea

[Jeremy Harris]

5 minutes ago, Sensus said:

 

And thermal mass is perfectly and very simply defined in terms of units of measurement.

 

It's simply the label that is attached to it that you're getting hung up on.

 

 

Not at all.  So far I've seen no valid units given for "thermal mass", nor have I seen any valid expression given by which it might be calculated.

 

All I've seen given has been an expression for non-dimensional heat capacity, together with an unusual expression for heat capacity per unit area.  Neither of these expressions would allow what I believe most might understand to be the effect of "thermal mass", in the context of building thermal behaviour, to be determined or measured.

[SteamyTea]

3 minutes ago, Sensus said:

or acceleration by time and distance

That is speed isn't it.

 

[SteamyTea]

6 minutes ago, Sensus said:

Yes, we know it doesn't fully describe transient thermal response: it doesn't claim to, and it's not intended to.

But that is exactly what people think it does do, why this whole conversation started.  People thinking that mass adds to thermal stability.

Edited September 8, 2019 by SteamyTea

[SteamyTea]

1 minute ago, Sensus said:

You need to use the time twice

Not in your world, it is thermal conductivity that adds the time in.

It is all in the units, ignore them at your peril.

[Nick1c]

I am no physicist or mathematician, but it strikes me that from a practical point of view (having a comfortable house with a reasonably stable temperature) there are an awful lot of variables in this which make accurate predictions very difficult, if not impossible. One of these, which hasn’t been mentioned, is the colour of the materials, which would affect the time element - a black surface would behave differently to a white one. Another is airwash. It has reminded me of a quote from an ex-president of the CBI I used to use in a previous job:

 

‘There is a great danger that because we tend to value things we can accurately measure we risk being precisely wrong rather than roughly right. ‘

 

Rather than dancing on the head of a pin are we not better served by trying to use materials with adequate specific heat capacity and thermal conductivity, adequate being determined empirically rather than modelled to the nth decimal place. 

 

[Jeremy Harris]

15 minutes ago, Sensus said:

 

Then I can only assume that you're in some way related to Nelson, though perhaps with selective blindness in both eyes?

 

Please refer to my first post on the subject on this thread, and to the website that you yourself linked:

SAP and thermal mass

SAP 2009 uses thermal mass in calculating the heating and cooling load of the building.

SAP uses the kappa (k) value to determine thermal mass. 'k' is the measure of the heat capacity per unit area in kJ/m²K of the 'thermally active' part of the construction element:

k = 10^-6 Σ_i p_i c_i d_i

p_i = the density of the layer 'i' in the construction (kg/m³)
c_i = the specific heat capacity of the layer 'i' (J/kg K)
d_i = the thickness of the layer 'i' (mm)

 

 

 

I do wish that you would refrain from making ad hominem attacks in some replies.  I find the references to my supposed disabilities particularly offensive.  So far you have alleged that I'm autistic, have Asberger's Syndrome and have selective blindness.  This is solely a debate about facts, not personalities.

 

I fully accept that SAP misuses terminology, as do some architects and other building professionals.  That does not make such misuse correct though, as no internationally recognised standards body (such as Système international (d'unités)) has either ratified or adopted this measure for "thermal mass".

 

This definition also does not match either of the expressions given in earlier replies, given that the SAP definition of kappa includes specific heat capacity, a linear dimension and density.  Unfortunately it also does what other aspects of SAP does, and simplifies the calculation of the true property it is trying to define in order to help make the SAP assessment process somewhat easier.  As I think we all know, SAP is imprecise, because it both makes some assumptions that are based on standard methods of construction, and because it simplifies many of the calculations in order to make them somewhat easier, or to make it easier for an assessor to obtain, or derive, the base data.

 

[Dreadnaught]

As the dust settles, and for other the benefit of future readers, here are two articles that I found to be a good summary of the matters being discussed here …

1. http://www.greenspec.co.uk/building-design/decrement-delay/
2. http://www.greenspec.co.uk/building-design/thermal-mass/

(Source: here).

[ToughButterCup]

58 minutes ago, Sensus said:

Then I can only assume that you're in some way related to Nelson, though perhaps with selective blindness in both eyes?

[...]

 

Last time ... I'm going to lock this thread if there are anymore comments like that - from anyone.

[Jeremy Harris]

56 minutes ago, Sensus said:

 You might add: and because building construction is imprecise, and external environmental factors and human comfort levels are so variable as to render more complex analysis pointless.

 

See my comments above about inexperienced theoreticians: when you have been in and around the building industry and building design as long as I have, you come to appreciate the truth in @Nick1c's post.

 

You can waste months trying to analyse the thermal performance of a building to the Nth degree, but the outcome of that analysis will be barely, if at all, more accurate than applying relatively rough calculations.

 

Thermal mass is useful in making that rough assessment (the factors it encompasses are also essential in attempting a more accurate analysis, so to deny its existence, by any other name, is absurd).

 

I agree that there is always going to be a degree of imprecision when modelling the thermal performance of a building, just as there is when modelling the performance of anything, in any respect. 

 

However, my point about the use of the term " thermal mass" is primarily to do with both measurement, and hence the units used, and the perceived benefit, in terms of comfort of the occupants, that it may, or may not, help to provide.

 

The most common argument for including "thermal mass" is that it will help to stabilise the internal temperature, so that any rapid step change in external temperature will not be mirrored by a similarly rapid step change in internal temperature.  I believe that this is what many people, perhaps most, would understand as being the thing that "thermal mass" contributes to a building.  Do we agree on this, or not?

 

Assuming, for the moment, that we are in broad agreement with regard to perceived comfort being a key factor for wishing to have sufficient "thermal mass", then we perhaps need to get back to the original point in this thread that created the diversion.  How do different construction methods contribute to internal thermal stability?

 

The definition that SAP uses isn't particularly helpful, as it doesn't directly relate to internal thermal inertia, my guess is that BRE came up with the term as a way to very roughly model the impact of diurnal temperature changes only.  The SAP expression takes no account of the heat capacity of the internal structure, and I think that most of us would be in agreement that this is a key factor in terms of stabilising internal temperature.  I've both modelled the way the internal structure of our house behaves, thermally, and measured it over a period of several years now, and have a pretty good feel for the elements that contribute the most to the measured thermal time constant, or thermal inertia if that term is preferred.

 

Our house is of all timber construction, a twin stud timber frame, clad with larch externally and with a 50mm service void and skim coated plasterboard internally.  The floor is a reinforced concrete slab 100mm thick, with all the floor insulation underneath the concrete.  With an internal/external temperature differential of about 10°C and no heating, either direct or incidental and no occupants, the inside of the house will initially cool at about 1°C per 24 hours.  It behaves in a similar way when the temperature differential is the other way around in hot weather, but solar gain through the glazing contributes a significant amount of incidental heat gain.  I can measure no heat gain through the walls or roof, the internal surfaces just do not increase in temperature to any measurable degree (absolute accuracy of measurement around +/- 0.2°C (corrected), resolution of measurement +/- 0.0625°C).

 

From the data collected by logging the temperature on the internal and external surfaces of the house, every 6 minutes for several years, it's clear that the two internal elements that contribute the most significant temperature stabilising influence are the plasterboard walls, followed by the ground floor.  The plasterboard walls have both a relatively high specific heat capacity and a fairly high thermal conductivity, so heat from inside the house can fairly quickly and easily flow in either direction.  The effective heat capacity of the concrete slab is almost as powerful at helping to stabilise internal temperature, but is hampered slightly by the poorer thermal conductivity of the floor coverings, furniture etc.  This shows in measurements, where the slab usually remains very slightly warmer in its centre than the internal surface of the walls.  The effect is small, perhaps no more than 0.3 to 0.4°C at most, but it is consistent.

 

One other thing of note is that the real temperature stabilisation effect of any material inside the house is limited to a fairly shallow depth.  The surface layer of the ground floor, perhaps no more than the top 50mm or so, has the greatest effect, and the lowest part of the concrete slab, 100mm down, seems to have very little effect.  From this I'd (very broadly) conclude that it is probably better to have relatively a thin, high heat capacity and high thermal conductivity layer of material over as large an area of the inside of the house as possible.  It may well be that doubling up on the thickness of plasterboard, for example, could make a very worthwhile contribution to internal temperature stability, as well as increase sound attenuation between rooms.

[Jeremy Harris]

Just now, Sensus said:

 

Those have been repeatedly linked above... ironically by both myself and JS!

 

As I said right back on page 1, decrement delay is an effect. It's what you get when, as Steamy Tea put it, you include all elements to get something useful.

 

Thermal mass is a cause. It's (an important) one of those things you have to include.

 

Actually, to split the hair before someone beats me to it, it's two things (mass and specific heat capacity) combined, to produce something that is useful of consideration in its own right.

 

 

 

Just to be absolutely clear, I will repeat that specific heat capacity = mass heat capacity.  The two are exactly the same thing, so it is not possible to combine them to create a new term.

 

Mass heat capacity, or specific heat capacity,  are both measured in units of J⋅g⋅K⁻¹

 

I cannot understand how any measurable parameter can be a "cause", it is either a property that exists, and can therefore be measured and assigned units, or it does not exist, in which case it can neither be defined or be assigned units.

 

 

 

 

[Patrick]

?OK, originally this was about TF pros and cons,

 

[SteamyTea]

16 minutes ago, Sensus said:

the internal temperature barely fluctuated over the course of a year, never mind peaking when the sun came out from behind a cloud!

Was it always cold.

Just teasing, I don't give two hoots really.

But just for a giggle, did it have a low glass to wall ratio?

[Jeremy Harris]

2 minutes ago, Sensus said:

 

 As I said back on page one, but just to be clear, specific heat capacity is the amount of the amount of heat energy required to raise the temperature of a substance per unit of mass.

 

Thermal mass combines it with the mass per unit area of a thermal element.

 

In other words, specific heat capacity (or mass heat capacity, if you prefer) tells you how much energy a kilogram of material will absorb. To know how much energy an actual structure or structural element will absorb, you also need to know how many kilograms of that material are in that structure. Thermal mass combines those two factors.

 

 

OK, that corrects the error in the previous comment, and we are now in agreement that specific heat capacity and mass heat capacity are the same thing, and that combining two different expressions for the same property does not create a new property.

 

The area argument still doesn't make any logical sense, though, without including thermal conductivity in the expression.  Taking an extreme example to illustrate this, the specific heat capacity of silica aerogel is around 0.84 kJ/kg⋅K⁻¹  Concrete has a roughly similar specific heat capacity (typically concrete is about 0.88 kJ/kg⋅K⁻¹).

 

So, if we have two structures, one made from silica aerogel insulation and one made from concrete, of equal area, which has the greater "thermal mass"?

 

 

3 minutes ago, Sensus said:

....but then I could have told you that it's the thermal mass that's exposed to the internal surface that plays the biggest role in stabilising temperatures, without several years of datalogging. ?

 

 

How could you have told me this?  The approach that you have previosuly outlined could not possibly have determined this at all, as it ignores key material properties that are essential to the calculation.

 

18 minutes ago, Sensus said:

Relevant to this thread: you have to recognise that a typical timber frame building either has very little thermal mass, or (if brick or stone clad) places most of that toward the outside face, where JS has just - quite rightly - told us that it won't do much good.

 

 

This may be the case for some timber frame properties, but also may not be the case for other timber frame properties, so it's not really a reliable general expression of relative thermal properties.  For example, many of us here have built timber frame houses that use relatively high specific heat capacity insulation.  One popular choice here, and the one I made, is to use blown in cellulose.  The fairly popular twin stud construction method has the depth of the walls, from the inside of the inner skin to the inside of the outer structural skin, completely filled with insulation.  In our case we have a fairly typical wall and roof build up for this type of timber frame, so have 300mm of cellulose in the walls and 400mm of the stuff in the roof (between I beam rafters).  The cellulose has a specific heat capacity of about 1.6 kJ/kg⋅K⁻¹, almost double that of concrete.

 

[SteamyTea]

21 minutes ago, Sensus said:

and you're still left with a relatively low figure.

FFS, go back and look at the figures I posted up, if they are calculated correctly, there is little difference.

If just brick or concrete were the answer, why do you need to add insulation?

I can answer that, but I want you to, then you might start to realise that the mass is not the be all, and end all, of thermal stability.

[SteamyTea]

4 minutes ago, Sensus said:

untangle your panties

I never wear any, the reduced mass of water that is saved by not cleaning them is, just one, of my contributions to the world.

[Jeremy Harris]

1 hour ago, Sensus said:

 

From experience and common sense. When you have these, you don't have to rely quite so much on calculations.

 

 

Sorry, but I spent a bit over half my working life measuring things, and there is no way I could just use my experience as even an extremely crude substitute for calculation, even when I though I knew roughly what the result would be.  In this case (design stage of our house) I needed accurate data to be able to compare one structural configuration with another, in order to be able to make a reasoned decision as to which best suited our requirements, and to inform our choice of heating system, as heating response time is  as critical to comfort as thermal inertia (or the thermal time constant).  Without having hard data this would have been impossible, and we could easily have ended up with an uncomfortable house, or one that failed to meet our performance requirement.

 

1 hour ago, Sensus said:

And this is where the concept of Thermal Mass actually comes in useful as a rule of thumb.

 

Your blown cellulose insulation may have a relatively high specific heat capacity, but it also has a relatively low density, and therefore mass. Multiply the two together, and you're still left with a relatively low figure.

 

Let's do the sums, shall we?

 

Assuming a 0.3m. thickness, and a density of 6.72kg/m³, your cellulose fibre will give a mass of 6.72kg/m²

Taking your figure of 1.6 kJ/kg.K^-1, that gives a thermal mass of 6.72 x 1.6 = 10.75

 

Compare with my limestone marl cottage, which, if we could actually build a 300mm thick wall from the stuff (you can't, it's too weak, and you'd need much thicker to give a reasonable U-value, anyway), would give:

 

1960 kg/m³ x 0.3m; mass = 588kg/m²

Specific heat capacity of limestone ~ 0.806 kJ/kg.K-1

 

Gives a thermal mass of 588 x 0.806 = 473

 

Now consider that for structural reasons, the limestone marl cottage I mentioned had walls approximately twice as thick as the above calculation, so lets say actual thermal mass = 950.... even with its relatively high specific heat capacity, cellulose fibre is barely on the same continent, let alone in the same ballpark.

 

 

First off, the density of cellulose is a lot higher than the figure you have used, plus there are some major arithmetic errors in your calculations.  Each m² of a wall that is 0.3m thick will have a volume of 0.3m³, so using your incorrect figure for cellulose density the mass per unit area would actually be 2.016kg/m².  This is a moot point as the data used is massively in error anyway. 

 

The Warmcell data sheet gives the blown in density for cellulose as being between 30kg/m³ and 48kg/m³, depending on the pressure with which it's blown in and the height within the structure (it will tend to be slightly denser at the base of walls and slightly less dense towards the top, due to gravity).  Let's assume that the mean density is around 40kg/m³ for the sake of this discussion.

 

For 0.3m thickness, and a density of 40kg/m³, cellulose will give a mass of 12kg/m² wall area.  The "thermal mass" (using your definition), ignoring the other elements in the structure, will be ~19.2 kJ/m²

 

SAP includes a limiting material thickness of 100mm, so in reality this comes down to 6.4 kJ/m² for cellulose and 158 kJ/m² for limestone marl, an apparent ratio of 24.7:1 in favour of limestone marl.  Technically there is also another limiting factor that applies in SAP when calculating k, and that is that it is only layers outside the first layer of insulation that are included in the "thermal mass" calculation.  In the case of our cellulose wall, that means that only the external larch cladding and the OSB outer skin should be counted as "thermal mass", not the cellulose insulation layer.  This rather makes a nonsense of the way SAP works, though, as in the case of our construction we have near-zero "thermal mass" as far as SAP is concerned, yet I know from measurements that the house has a very long thermal time constant. 

 

This is similar to the question I posed earlier, "if we have two 1m² squares, that are dimensionally identical, and have roughly the same specific heat capacity, one made from silica aerogel and the other made from concrete, which has the greater "thermal mass"?"

 

However, the real problem with your comparison is that it completely ignores the very significant impact of thermal conductivity.  The rate of heat flow through your limestone marl walls will be around 1.1 W·m^-1·K^-1  That through our cellulose filled walls will be around 0.038 W·m^-1·K^-1 .  This has a massive impact on the temperature stability, as when there is a step change in temperature differential heat will flow through the limestone marl walls around 29 times faster then it will though a similar thickness cellulose filled wall.  This makes it less capable in terms of stabilising the internal temperature than a similar thickness cellulose wall, as the rate of heat loss or gain will be greater.

 

For completeness, we should also be looking very seriously at thermal admittance, or the heat transfer coefficient, as it is a good measure of the way fabric elements absorb and release heat, and this is key to thermal inertia.  It's easy to assess this, from the known physical properties of the materials used, and the behaviour of their junctions, all that is needed are measurements of the heat gained or lost per unit area and the temperature differential across the structure. 

[MikeSharp01]

21 minutes ago, JSHarris said:

thermal admittance, or the heat transfer coefficient, as it is a good measure of the way fabric elements absorb and release heat,

Yes there is a reasobable explaination of this here http://www.greenspec.co.uk/building-design/thermal-mass/

[K78]

Thermal mass is one of the things mentioned on this forum that is easy to understand without all the scientific explanations. It is basically how effective a material is at absorbing and retaining heat.

 

I think most idiots would guess block is better than timber in this regard. 

 

Whether the name is technically correct or not, it makes perfect sense to me. Not worth heated debate imo. 

[Jeremy Harris]

1 minute ago, Sensus said:

 

Yes, you're quite right - basic error on my part.

 

As I'm sure you well know, the density of blown cellulose is variable. Would you care to suggest whatever figure you like, then do both the sums again?

 

 

I did give the variability in density exactly as quoted in the Warmcell data sheet, nothing made up on my part.  The data sheet states that the blown in density varies from 30kg/m³ to 48kg/m³, with some of this variation being due to the blowing pressure and some due to the natural variation there will be from top to bottom of any wall or roof (just because of the effect of gravity tending to compress the lower layers slightly).  I suggested using 40kg/m³, although I know that the pressure setting on the machine was set to deliver at 45 kg/m³.  40kg/m³ seems a reasonable estimate for the likely mean density.

 

 

1 minute ago, Sensus said:

I notice you are being very careful to avoid the actual comparative figures.

 

 

I gave a very clear and unambiguous set of comparative figures, and stated that the ratio (uncorrected for the limiting values in SAP) between the calculation method you used for each material was 24.7 : 1 in favour of limestone marl.  I don't think I could have been more open and honest than that.

 

1 minute ago, Sensus said:

I also notice that you are quoting the methodology used by SAP when it suits you, and dismissing it entirely when it doesn't.

 

 

Not at all, I'm just highlighting that SAP is both inconsistent and a crude measure when applied to this specific parameter, that is all.  It seems clear that the SAP methodology tries to only measure the specific heat capacity of the first 100mm of the outer skin of a structure, or the thickness of the outer structure outside the first layer of insulation,and use that to derive an analogy for what they refer to as "thermal mass".  SAP ignores thermal conductivity, and it is clear from simple physics that thermal conductivity plays a very significant part in helping to regulate the internal temperature of a building.

 

To be fair to BRE, SAP is not intended to be used for building design, it is very much a simplified method that enables an assessment of the approximate performance of a dwelling, for the purposes of compliance with Part L1A.  I think that most people who have compared a SAP assessment result with the real-world performance of a house will have noted that there are often disparities, sometimes fairly large ones, between the two.  It is for this reason that I personally would not rely on SAP to give an accurate assessment, particularly for any house that is slightly unusual in terms of construction.

 

16 minutes ago, Sensus said:

 

That's not the point - we're discussing the existence or otherwise of the factor know as thermal mass, which you have very clearly asserted is a myth, and has no units. Please stick to that point and stop the attempts at distraction in other directions.

 

 

I stick to that view.  "thermal mass" is a myth, from the standpoint of being able to make any meaningful measurement of the effect it may have.  For example, we had an earlier comment in this thread suggesting that adding a large masonry hearth would add "thermal mass".  Under the SAP definition, or your own definition, this would have no effect at all.  Similarly, I noted that the effect of a 100mm thick concrete floor, laid as a passive slab, would have a far more significant stabilising effect than a very much higher "thermal mass" (in SAP terms) than a beam and block floor with insulation laid internally.

 

I'm making no attempt to distract this thread from the original point raised, which was a comparison of different build methods, specifically the thermal performance, as this might relate to thermal inertia, or thermal time constant.  That seems to me to be key to the questions raised by the OP, when comparing block and brick construction to timber frame construction.  It seems very clear that timber frame construction can have a high level of thermal inertia, or a long thermal time constant.  Equally, it seems clear that a block and brick house, with a low decrement delay construction, may well have a much lower thermal inertia, or shorter thermal time constant.

 

Neither of these are universal truths, and it would be just as possible to build a timber frame house with a low level of thermal inertia as it would to build a block and brick house with a low level of thermal inertia.  The point I set out to make originally was that the type of material chosen for the primary structure need not determine the thermal performance.  Both block and brick and timber frame can be used to build houses with a long thermal time constant, that will be equally comfortable.

 

[Jeremy Harris]

6 minutes ago, K78 said:

Thermal mass is one of the things mentioned on this forum that is easy to understand without all the scientific explanations. It is basically how effective a material is at absorbing and retaining heat.

 

I think most idiots would guess block is better than timber in this regard. 

 

Whether the name is technically correct or not, it makes perfect sense to me. Not worth heated debate imo. 

 

 

The snag is this isn't actually that accurate, as has already been shown in the other thread.  It's largely folklore, and widely accepted as being true, when the reality is that it isn't really.  The ability to retain heat is given by the heat capacity, and if we use mass as the measure (as the term used is "thermal mass") then timber holds a lot more heat per unit mass than concrete.  Plasterboard holds a lot of heat per unit mass too, and I've found that it is the largest contributor to stabilising the internal temperature of our house, just because there is a lot of it and it has a large surface area, so it can absorb or release heat out of or into the house relatively easily.

[K78]

1 minute ago, JSHarris said:

 

 

The snag is this isn't actually that accurate, as has already been shown in the other thread.  It's largely folklore, and widely accepted as being true, when the reality is that it isn't really.  The ability to retain heat is given by the heat capacity, and if we use mass as the measure (as the term used is "thermal mass" then timber holds a lot more heat per unit mass than concrete.  Plasterboard holds a lot of heat per unit mass too, and I've found that it is the largest contributor to stabilising the internal temperature of our house, just because there is a lot of it and it has a large surface area, so it can absorb or release heat out of or into the house relatively easily.

 

I don’t pretend to understand this nearly as much as you. 

 

A concrete block house with EWI was always the example I thought of. Surely the concrete blocks retain heat better than a timberframe with EWI? The storage heater comparison always made sense to my simple mind.