Timber Frame Pros Cons

[puntloos]

14 minutes ago, Sensus said:

 

TL:DR - Energy modelling of buildings is complicated. But I think we all know that, already? ?

 

The next question is - how would one 'ensure' this is taken care of by your team.

[Jeremy Harris]

1 hour ago, Sensus said:

 

Thermal Mass as both a term and a concept is only problematic if you choose to make it so.

 

 

Really, it's the fact that the term has no formal definition that is recognised by any international standards body, has no internationally recognised units of measurement and is misleading in the way that it tries to conflate two well-understood terms into something that is only a unusual definition (in general measurement terms) of an already well-defined material property, heat capacity.

 

A quick look at the OED tells us all we need to know about these two words:

 

Quote

thermal

adjective

1.         Relating to heat.

            ‘thermal conductivity’

            ‘the thermal properties of food’

1.1       another term for geothermal

             ‘thermal power stations’

1.2       (of a garment) made of a fabric that provides exceptional insulation to keep the body warm.

‘thermal underwear’

noun

1          An upward current of warm air, used by gliders, balloonists, and birds to gain height.

2          (usually thermals) thermal garment, especially underwear.

 

Origin

Mid 18th century (in the sense ‘relating to hot springs’): from French, from Greek thermē ‘heat’.

 

 

In the context of building science, the first definition "relating to heat" is clearly the one that applies here.

 

Quote

mass

noun

1          A large body of matter with no definite shape.

            ‘the sun broke out from behind a mass of clouds’

1.1       Any of the main portions in a painting or drawing that each have some unity in colour, lighting, or some other quality.

            ‘the masterly distribution of masses’

2          A large number of people or objects crowded together.

‘a mass of cyclists’

2.1       A large amount of material.

             ‘a mass of conflicting evidence’

2.2       (masses) informal A large quantity or amount of something.

‘we get masses of homework’

3          (the mass of) The majority of.

‘the mass of the people think that the problems are caused by government inefficiency’

3.1       (the masses) The ordinary people.

            ‘seaside towns that catered for the masses’

4          [Physics]

[mass noun]    The quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field.

‘stellar objects of intermediate mass’

4.1      (in general use) weight.

            ‘multiply the mass of the payload by the distance travelled’

adjective

[attributive]

             Involving or affecting large numbers of people or things.

            ‘the film has mass appeal’

            ‘a mass exodus of refugees’

verb

            Assemble or cause to assemble into a single body or mass.

            [with object] ‘both countries began massing troops in the region’

            [no object] ‘clouds massed heavily on the horizon’

 

Equally clearly, as we are discussing the physics of buildings, it is the fourth noun definition that applies,  "The quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field."

 

1 hour ago, Sensus said:

It is no more (and arguably a lot less) misleading than just quoting the specific heat capacity of a material, on its own, yet nobody seems to see that as problematic?

 

 

Heat capacity, either specific (i.e. relative to mass) or volumetric (relative to volume) seems to be a much easier term to understand (happy to be corrected if most here find this concept confusing).  Most people would probably be OK with the concept that a mass or volume of something is able to store a given amount of sensible heat (specifically sensible heat here, as it is energy in the form of heat). 

 

In everyday terms, most people would probably be able to accept that a bottle filled with water heated to a particular temperature (above room temperature) would cool down to room temperature more slowly than the same bottle filled with air heated to the same temperature.  Clearly the volumetric (in this case) heat capacity of water is significantly greater than the volumetric heat capacity of air.

 

Using something that is a part of a house as another example, I would imagine that most people might be able to readily accept that a block of concrete of a given volume, heated to a particular temperature, would be able to store more heat than the same size block of foam insulation.

 

Note that both of these examples use volume, rather than mass.

 

I believe that the use of mass, i.e. "The quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field" is really the term that is less than useful when considering the ability of the component parts of a building to store useful amounts of sensible heat.  Leaving aside structural engineering concerns, and alternative meanings for mass (as in the verb, "Assemble or cause to assemble into a single body or mass"), most people will, I believe, consider elements of a building in terms of dimensions, i.e. how big stuff is.  Given this, then volumetric heat capacity is probably a more easily understood term.

 

Most can visualise the size of a wall, or the size of a floor slab, so relating that to the ability of that part of the fabric to store sensible heat seems to me to be more logical.  Taking this one step further, and considering that we know that only a modest thickness^[1] from the surface of most materials has a significant effect in terms of the impact of its heat capacity, then it becomes, perhaps, easier to visualise the likely heat storage capacity, and hence the likely impact on thermal stability, of most elements within the house, or, taking the limited view SAP uses, those elements that make up the external skin of the house, excluding the insulation layer.

 

I would suggest that there are good reasons to think of the effect of elements both within the thermal envelope of the house, and immediately outside it, in terms of volumetric heat capacity, as a first order approximation as to how effective they may be at helping to stabilise temperature^[2].

 

 

¹ BRE have settled on 100mm from the surface for SAP,  my measurements suggest that a 50mm thickness from the surface may be more appropriate for a low energy house, as do the conclusions reached by Jae Cotterell (an architect and director at Passivhaus Homes)

 

² This ignores thermal conductivity, just as the BRE definition of "thermal mass" ignores thermal conductivity.  This is a reasonably valid simplification as long as the insulation layer is ignored (just as it is in the BRE/SAP "thermal mass" expression).

 

 

Quote

The only thing you need to remember is that it can't tell you much about the building's thermal response on its own... that there are numerous other factors and complications to consider.

 

But then the same can be said of those numerous other factors, of course... there isn't a simple 'magic formula' that will tell you everything you know about a building's thermal performance.

 

TL:DR - Energy modelling of buildings is complicated. But I think we all know that, already? ?

 

 

I agree, it is of no use at all in modelling either the thermal inertia, or thermal time constant, of the house.  It cannot provide a meaningful contribution to any of the standard equations for calculating thermal behaviour, such as calculating:

 

• how much heat it takes to raise the temperature of a building to a specific internal temperature for a given heating requirement
• how long the house will take to cool down when the heating is turned off under given external conditions (both temperature and wind speed and direction)
• how quickly the house will heat up under specific conditions (i.e. elevation and azimuth of the sun in clear sky conditions)
• how much cooling may be needed to remove excess heat from solar gain
• what capacity the heating and cooling system may need to be, how effective solar gain reduction or enhancement schemes may be, etc.

 

There is a reason that PHPP^[3] is a pretty complex spreadsheet, and that is because thermal modelling of a house is fairly complex.  It is not impossible to model, though, and as the energy requirements for new houses are gradually reducing, the importance of more detailed performance modelling will increase.  Just from anecdotal comments and observations from members of this forum we are beginning to see that overheating, especially in houses with fairly large areas of glazing that may not have adequate solar gain control, is becoming an issue.  Modelling will help to reduce this, and help house designers gain a better understanding as to how well-insulated and airtight houses behave as our climate gradually warms up.

 

³ PHPP = PassivHaus Planning Package, a spreadsheet model developed by the PassivHaus Institut in Germany, that is recognised as being a powerful house energy and thermal performance modelling tool.

 

 

 

[Jeremy Harris]

1 minute ago, Sensus said:

 

Like I said...

 

That's a partial quote, taken out of context.  This is the complete quote, as the context is important, particularly the last phrase:

 

Quote

Really, it's the fact that the term has no formal definition that is recognised by any international standards body, has no internationally recognised units of measurement and is misleading in the way that it tries to conflate two well-understood terms into something that is only a unusual definition (in general measurement terms) of an already well-defined material property, heat capacity.

 

[Jeremy Harris]

6 minutes ago, Sensus said:

Still like I said. ?

 

So, are you in agreement that the definitions that you've previously given for "thermal mass" are either just the standard definition for non-dimensional heat capacity, sensible heat per kelvin, or an alternative definition expressing heat capacity as sensible heat per unit area?

 

[SteamyTea]

I am still impressed that my simple model that I knocked up this morning is very close.

Makes me wonder if a statistical approach is best.

But then I would say that as I like statistics.

I may have to test it against a smaller dataset of temperatures, the one I used was pretty large, 42,000 datapoints.

I may have to put some test equipment into my Mother's house next time I am up.  Her kitchen is almost the size of my house, and it is made of bricks.

Could be interesting.

[Jeremy Harris]

1 minute ago, SteamyTea said:

I am still impressed that my simple model that I knocked up this morning is very close.

Makes me wonder if a statistical approach is best.

But then I would say that as I like statistics.

I may have to test it against a smaller dataset of temperatures, the one I used was pretty large, 42,000 datapoints.

I may have to put some test equipment into my Mother's house next time I am up.  Her kitchen is almost the size of my house, and it is made of bricks.

Could be interesting.

 

 

Be interesting to see if it is valid for a range of different house types. 

 

Be a lot simpler than some of the behemoth models around, like PHPP, if it is.

[ProDave]

6 minutes ago, JSHarris said:

 

 

Be interesting to see if it is valid for a range of different house types. 

 

Be a lot simpler than some of the behemoth models around, like PHPP, if it is.

 

I find there is nothing wrong with your heat loss spreadsheet and no need for anything more complicated than that. The predictions from that agree closely with reality. 

[SteamyTea]

3 minutes ago, JSHarris said:

Be interesting to see if it is valid for a range of different house types. 

Yes it would.

All it does is take some basic material properties, some basic air change numbers (no idea what MVHR would do to it), then from internal and external temperature distributions differences (which are pretty normal around the mean), pro ratas the amount of energy needed to keep it at a steady temperature (the actual mean internal temperature).

One problem is that where I live, there is relatively little deviation from the mean, not like places like Canada (saw pictures today of where I was a couple of years back, seems to have improved Halifax).

I like this approach as good weather data is easy to get hold off, good building data is not.

[Jeremy Harris]

One thing that does seem noticeable is that the fairly crude model that SAP uses (accepting that modelling thermal performance is not the primary purpose of SAP) is often pretty inaccurate.  For our house SAP significantly overestimates the heating requirement (by a factor of about 2), overestimates the hot water requirement (by about 30%) and massively underestimates the overheating risk (by a factor of about 4).

 

PHPP is pretty damned close, the only significant error is the predicted overheating risk, but a large part of that turned out to be the temperature value that PHPP defaults to as the threshold for overheating, which was a bit higher than I think it should be (but we don't like the house to be much warmer than about 23°C).  IIRC, PHPP default to 25°C as the overheating threshold.

[Dreadnaught]

8 minutes ago, JSHarris said:

PHPP is pretty damned close, the only significant error is the predicted overheating risk, but a large part of that turned out to be the temperature value that PHPP defaults to as the threshold for overheating, which was a bit higher than I think it should be (but we don't like the house to be much warmer than about 23°C).  IIRC, PHPP default to 25°C as the overheating threshold.

 

And you also had your local micro-climate issue, with a particularly sheltered plot location.

 

As standard, PHPP uses local climate data for a broad area. I can't remember whether that is county-by-county or something more granular based on postcode. Either way it can lead to discrepancies from reality.

[Jeremy Harris]

22 minutes ago, Dreadnaught said:

 

And you also had your local micro-climate issue, with a particularly sheltered plot location.

 

As standard, PHPP uses local climate data for a broad area. I can't remember whether that is county-by-county or something more granular based on postcode. Either way it can lead to discrepancies from reality.

 

Yes, that certainly messed up both my model and PHPP a bit.  The data I used for the local climate was taken from the Met Office dataset, which for here is reasonably OK, as we have a monitoring station fairly close by, at Fontmell Magna, and I used their data initially, then modified it by increasing the mean temperatures a bit to reflect the impact of our microclimate a bit better.