What is it all about?

U values determine heat losses so ignoring air leakages light, medium or heavy weight homes would loose the same amount of heat

ah but the heavier they are the more heat they can store and the slower they change temperature.

I like to be able to store a lot of heat for longer periods than days.

I also like the comfortableness of very slow temperature swings which also only fluctuate over periods longer than days.


Basics then:-

Thermal mass is good

Mass relates directly to thermal mass except where water is involved where it thermal mass is 4.2 times higher than its mass.

Decrement delay is a measure of how quickly or slowly a building responds to fluctuations in outside temperatures,

Thermal inertia would seem to be very similar to thermal mass but is a good thing to have lots of too.

Go on then....

Trouble is, as someone that has done the calcs over at 'the other place' has shown, energy is only stored in the first few millimetres of the mass.
I think that Ed also pointed this out. Plasterboard probably gives you all the stabilisation you need.
Then add to that the biggest losses from air changes, even with MVHR and your mass starts to account for very little.

As you quite rightly point out, incidental solar gain is possibly the biggest issue, but that is short lived, so what you gain even on the brightest days mostly goes into the air, with only a slight cooling affect from the mass, very slight. That affect is not given back to you 'during the night' much of it gets diffused into the mass, loosing temperature on the way. With a small temperature difference you don't have the power to drive temperature change.
And as I have pointed out lots of times, we neither have the climate nor the need for this type of passive system.
Bot, bless his heart, posted a link to a wine storage place in France that has probably 10s of thousands of tonnes to stabilise the temperature, but it stabilises at below the ambient, in a different climate to ours.

I posted up some data charts about lofts, if you look at them you will see that my unheated area is pretty stable, and above ambient temperature. And this is in the sort of place you hate, a leaky, 80's built timber frame (though with a little improvement).
Now I am not suggesting that everyone lives in a house that is somewhere between 14°C and 16°C, but any simple heating system can stabilise this at a higher temp, you don't need solid walls with insulation on the outside. What you need is insulation and controlled air changes.

Here is the picture, you can see that sometimes there is a 10°C drop in ambient and a 3°C drop in internal temp, but generally the internal temp is within 1° while the ambient is changing about 5°C. The extreme being the 13th to the 14th of Jan, the ambient dropped 10°C but the house dropped, um, nothing, zilch, 0°C. (edit, that may be a bit misleading as it looks like the temperature dropped 2°C, but I had the window open as I was drying washing).

Posted By: SteamyTeaTrouble is, as someone that has done the calcs over at 'the other place' has shown, energy is only stored in the first few millimetres of the mass.

If this is the case then how does the same person shift the excess solar gain around his property using the UFH with just the pump on but no thermal input?

Stored in the first few mm, depends on time scale.

Clearly it is relavent what temperature the whole of a large piece of thermal mass is.

Posted By: tonyStored in the first few mm, depends on time scale.

Yes, but the longer it is there, the more diffused the energy becomes. Think of it as a drop of water on blotting paper, when it first touches, it has not spread much, give it a few seconds and it spreads out more. So if you think of the heat (energy) as the mass of water, and the diameter of the drop as the temperature, as the drop spreads out, it has less energy to give up to the material it is trying to heat. Eventually it stops spreading, it just does not have the temperature difference to make the molecules move any.

Posted By: tonyClearly it is relavent what temperature the whole of a large piece of thermal mass is.

Yes I think it does. If there is little difference (put it on the kelvin scale and work in fractions), then there is not much difference. Also, and I need to think about this a bit more, the lower down the scale it is, then the less variability there is.
I do need to think about this more to get a better understanding.

We also should stop calling it 'thermal mass'. It is just mass, and different masses have different specific heat capacities.

Posted By: DarylP... also depends where the thermal mass is... internal blockwork walls work as thermal 'stores' very well, as do B/B floors.

Yes, but that is partly because there is more surface area. Surface area has to be taken into account when calculating thermal inertia. This is not immediately apparent from the units, but a sphere will behave very differently from a thin sheet.

Posted By: tonyNeed be careful about confusing heat and temperature.

Yes. Heat is energy, temperature is average speed of molecules.
But I think I got them the right way around.
Could think of it as heat (energy) being the mass and temperature being the speed in the car analogy (I think).

Posted By: SteamyTea

Posted By: tonyClearly it is relavent what temperature the whole of a large piece of thermal mass is.

Yes I think it does. If there is little difference (put it on the kelvin scale and work in fractions), then there is not much difference. Also, and I need to think about this a bit more, the lower down the scale it is, then the less variability there is.
I do need to think about this more to get a better understanding.

Thought about it a bit, and then realised that I had done some work on it recently. The chart below is ground temperatures, not wall temperatures, but as this is about variation and usefulness (I think we tend to forget that something has to be useful), but it shows that there is not hardly any time difference between 10 cm and 100 cm depth in the spring, autumn and winter. Basically if the top is cold the bottom keeps in step. There is a larger difference in the summer with it taking a couple of weeks for the ground at 100 cm to start cooling after the top 10 cm has started.
This shows that it is the top part of the ground that is responding quickly (though rainfall will make a difference here), but it also implies that any 'deep storage' is inaccessible in a passive system.
It is also worth noting that the best temperature reached at 100 cm is just 17°C, so if you were relying on that for heating, then you get a cold house. At a more realistic depth (when compared to wall thickness) of 30 cm and during the coldest time of year, the first 20 weeks, you never get more than a couple of °C difference, and little variation from the other depth readings, either in temperature or delay time.

I think that most of this is the small fractions of overall energy levels that we are dealing with here. We may think that 10 kWh is a lot of energy, and saving it is life changing, but when it comes to changing the temperature of a few tonnes of wall, it accounts for little.
I am not sure if I have explained what I mean very well, but a bit busy at moment.

My thoughts on this issue ( I favour high mass currently) is that high mass flattens out temp swings day/night and with a WBS used on cold cloudy days the heat will soak in the mass and act like a storage radiator for overnight.

Posted By: joe90My thoughts on this issue ( I favour high mass currently) is that high mass flattens out temp swings day/night and with a WBS used on cold cloudy days the heat will soak in the mass and act like a storage radiator for overnight.

Temperatures don't fluctuate much in Passive Houses regardless of how they're built, I built a block Passive House for my sister and a timber frame Passive House for my brother over 8 years ago now, there's no difference in the energy consumption or overheating levels in either house, they both open a window downstairs and a window upstairs if it gets too warm. The block house required a lot more attention with regards the airtightness, the first test couldn't be done until the plastering was finished, by then it was too late to do much about it!

Room Sealed Stoves in Passive Houses can act like "Cold Radiators" in cold weather with cold air dropping down the chimney onto the stove, creating convection currents in the chimney. I've measured stoves at 8 degrees on cold mornings in Passive Houses, the room temperature had dropped to 15 degrees while the adjoining room without a stove was 19 degrees.

Posted By: tonyI call them in house winter cooling systems!

Had not thought of that. The cooking stove and central heating stove are always on tick-over during the day so not a problem but I forgot about the one in the snug which does appear to have dropped the temperature quite considerably. Wonder if a candle in the stove would be sufficient to reverse the cold downdraft.

What a cracking idea to my amazement a candle in the stove really does work.

When you say work, what do you mean? Isn't is promoting a thermal siphon of heat out of the house up the chimney, cooling the whole house!!

Chimney balloon - take it out on days when the stove is being used.

As a point of reference, we have a (sealed) wood burning stove that has its own air supply. Even when it's below -20C outside, there's no sign of any coldness in the stove at all - opening the door does not cause a cold draft. Maybe this is because the air supply is at ground level and the pressure of this is the same as the chimney? Anyway, it's definitely not a in-house winter cooling system! Only time I ever felt anything was when the kitchen extractor was on and then I opened the stove door to light it - I guess he extractor was powerful enough to bring air down the chimney and make the stove hard to light. Switching it off solved the problem, and it was fine when the door was closed again. Of course, this is potentially a risk to cause CO poisoning.

Paul in Montreal.