Sorry to labour this old chestnut, but it would be interesting to get to the bottom of this.

Does thermal mass in a building really help stabilise and raise the overall internal temperature or is it just an illusion?

I did my BSc Dissertation in a related area, mainly the effects of solar inputs on thermal mass. I wanted to show that you could heat a house with relatively low solar inputs (wintertime in Cornwall) by storing the energy in varying amounts of thermal mass. In the small model (physical) I created it matched the computer model I made. It basically showed that Newton's Law of Cooling holds true (probably why it is a Law and not a wish). I did manage to show that it was possible to store a small amount of excess energy for a short period of time (till the next solar cycle started), but it was a small amount in a highly insulated structure and needed fans to transfer the energy into the thermal mass. So not exactly a passive system.
In hindsight I think I proved nothing but learnt some interesting mathematical modelling and experiment design/testing.

Unfortunately real life is not like a laboratory and a house is not like an expanded polystyrene box with a window and a fan, the sun is obscured by cloud, temperatures vary, the wind blows and if you miss a rare event (a thunderstorm) you cannot go back and repeat the experiment.
But having said that I have over the years since thought about it a lot, and collected real life data. Disaggregating the variables is pretty tricky, but since a bit of prompting on another thread I looked at some of last winters data. I have limited it to variations in internal and ambient temperatures, windspeed and solar gain (the charts are below).
It is a bit hard to compare temperature with windspeed and solar gain as they units are different (though may be able to break the units down into SI ones i.e. J/m^2).

I have lots of ideas as to why it may not work, just as others have lots of reasons as to why it does. I am willing when time and cash permit to create 'Silly Sunday Experiments' to test ideas, but I may need online help in the design and execution.

So I hope that we can get to the bottom of this difficult and complicated problem.

Now some charts:

That's good, any idea what they are using for weather data? Bit hard to tell from my data but seems that windspeed has the biggest affect on temperature. But windspeed is related to wind direction which is related to temperature and solar power.

No, but it is a lot better than it used to be. I will go to the scrapyard and get a big fan one day and make my own, we discussed ways to to it on here a while back. I usually have the windows open a crack though, has to be very cold not to.

Does it not depend on the insulative envelope surrounding it?
Then, comparing two buildings with the same external insulative properties, high and low thermal mass, does it not depend on how good the airflow/change management in the building is?
I am tempted to believe that all other things being optimal, and well-designed, thermal mass sort of equates to standing losses??

Surely you get the average temperature, over a period related to the amount of mass. So a say 2pm on a sunny day it will actually feel cooler, but at 2am the same day it will feel warmer. It enables you to provide the heat input it a manner that suits the source, eg a short sharp efficient burning fire, or daytime parabolic solar gain and use that heat when it isn't being created without need of thermal stores pumps sensors etc.

Recently met a retired building services engineer who told of an experiment done 30 odd years ago comparing the energy use of like for like homes, with and without an entirely freestanding wall blocking the prevailing wind. He reckoned it saved half the energy.. will try to chase him up for some evidence.
He mentioned that in complex commercial buildings internal air pressure is managed very carefully, zone by zone and in that way buildings could be, in effect isolated from the external pressure changes that can wick heat away.

I think the answer depends upon your heat source and how you manage it.

If you're relying on passive solar gain, active solar heating, heat dissipated by electrical appliances and/or overnight Economy 7 then thermal mass is very useful for smoothing out the peaks and troughs.

If you use an oversized gas boiler that is turned on 20 minutes before you get out of bed or arrive home then thermal mass is probably a hindrance.

David

Thanks David
It was looking at that, and soil temperatures that prompted me to consider investigating this again.

One thing that has struck me (several times) is the size of the building, this affects the thermal mass to air mass ratio. even though my house is low mass, the ratio may well be higher i.e. more mass to air mass than a larger high thermal mass one.
Maybe this should be part of the comparison, what do people think?

Yes, I work on 0.9 J.kg^-1.K-1 for brick/stone thermal mass and a density of 1,900 kg.m^-3, wood is 2.5 J.kg^-1.K-1 with a density of 420 kg.m^-3. Do they sound reasonable?
Air changes would have to be taken into account, but as you say, still swamps it (or should do).

Posted By: dicksterAveraging 18-21C throughout the year with me in charge of stove, windows (and wife).

Even (especially) if you think you're in control of your wife the opposite is probably true.

David

Just looking at my raw data and I have a mean temp of 19.7°C
No real control, just do what seems necessary when needed.
I need to filter out some min and max temperature readings (currently 51°C, probably from calibration tests), but the min was 11°C (probably a water temp test).
The Standard Deviation is 6.4°C around the mean of 19.7°C, but if I work to 0 decimals to take account of the 0.5°C accuracy, then 70% of the time I am between 14°C and 26°C as the distribution is pretty even.
There is a bias towards the unheated parts of the house (upstairs), until I add my kitchen temperature data (where I work).

I am a big fan of high thermal mass buildings, assuming daily occupancy. My experience of low thermal mass is of a scandinavian holiday cottage that I have rented a few times, near Nottingham, in early Nov. It's triple glazed throughout, and lightweight construction.
Every morning we turn on the electric heaters as it's too chilly - but by mid afternoon the windows have been opened to cool it down again. Gah !

I think our house has little thermal mass and it does warm up very quickly if the heating is on or the sun is out and shining through the windows.

I find lightweight construction more 'friendly' than heavy with a light and airy feel, but that is probably just personal preference.

Ed

My experience of high thermal mass houses is that they are fine for those buildings that need a large heat source to maintain a given temperature but that very low energy houses such as PH have problems with comfort because getting the house up to temperature is difficult unless you can pump a lot of heat into it quickly. I visited a PH which had high density concrete block walls on the ground floor and timber frame walls on the first floor. The owners complained that it was cold downstairs and warm upstairs. I decided to use a lightweight wall construction with a concrete raft foundation for my PH.

Posted By: PeterStarckHeat rises so you tend to get a natural temperature gradient from the ground floor's sub-floor to the first floor's ceiling

Not in my house, always colder upstairs than down in the winter, but that may be for other reasons (no heating upstairs and more air movement).
But agree that thermal mass would not affect the buoyancy of the internal air.

I think response time is part of the problem of using thermal mass to stabilise the internal temperature. If we lived in the tropics (or close to them) and had little temperature variation and solar input then you can use it effectively. Not the same in the UK with wild swings even by the hour sometimes. Ideally we want to be able to add or subtract the thermal mass (which is what you in effect do with a thermal store connected to your heating system).