The debate about low or high thermal mass buildings has been going on for ages, and never been resolved.
So a while back (26 days) I started a little experiment.
I built 3 identical 'houses', put them behind a screen so that the effects of solar gain would not change things and measured the temperature in each one, as well as the ambient.
This small scale experiment may or may not be scalable, but it shows, within the bounds of the accuracy of the equipment (+-0.5°C), that thermal mass at best makes less that 1/2 °C difference. And that may be to do with the water evaporating and condensing (shall think of a way around that).
As we have a few days of sunshine forecast, I have moved the kit so that it at least gets the morning sun and see what happens, shall also run a few more stats tests on it and see if anything interesting comes out.

Posted By: SteamyTeaAnd that may be to do with the water evaporating and condensing (shall think of a way around that).

A closed container? 1l PET bottles handily hold 1kg of water.

Paul in Montreal.

en plus, thermal mass needs to be used correctly, depending on local climate. Building a dwelling with thermal mass and just expecting it to perform well is misjudged. It wants to have insulation on the exterior, solar gain, adequate shading and a way to vent excess heat during the night for cooling. etc etc etc.

I dont think this is disputed or misunderstood.
The difficulty is in modeling this effect so as to be accurately predicted when designing a building. For the moment U values are used, while the combined effects of insulation + thermal mass + thermal capacity are still not widely understood.

Could you explain which aspect of the debate you're trying to illustrate? It's not obvious to me, I'm afraid!

Don't forget it's a cube-law in this kind of 3-D scaling

Paul in Montreal.

Isn't the effect you're looking for the delay between ambient (external) and the internal temperature - ie the thermal flywheel effect.

I've a plot somewhere of a large commercial building built with significant exposed thermal mass internally and you can clearly see on a weekend (low occupancy) the delay in the internal temp lagging the external temp in the day - you can also see the impact of the decrement of daytime solar being released to the building at night.

you can even see the effects of opening the windows at night in the weekdays and not on weekends.

I'm guess what you are measuring in terms of temperature is correct - what you perhaps want to look for is the delays in each "house" - or have I misunderstood and that's what you are doing - if so perhaps a problem with scale

regards

Barney

Should the thermal mass hold it's heat longer as it gets cooler?

Right
As a bit of background, I have been claiming that in the UKs climate regime, thermal mass within the insulated envelope makes no difference to the overall thermal performance of the building and I was unsure if it made any difference to the comfort levels.
So the hypothesis was, does adding thermal mass within the insulated envelope keep the building at a higher mean temperature.
My method was to use 3 identical containers of 5 litres each, place a thermocron in each, leave one empty (No Mass), put 1 kg of granite chippings in another and 1 kg of water in the other, put the lid on to make them airtight. I placed them outside of direct sunlight because at this stage I am not looking at solar gain, just mean temperatures. Left them for 26 days and then looked at the numbers (initial results are above)
The Student T-Test shows that there is no difference in the mean temperatures distribution, they tract the ambient and they are normally distributed.
They are positively skewed (all showing a slightly higher temperature than ambient), this is possibly down to two things, the U-Value (keeping any temp rise in) and the air tightness (not just replacing it with ambient temp air) rather than the thermal mass. Any offset is to do with the accuracy of the measuring equipment (what the Student T-Test shows rather than the correlations between ambient and internal temperatures).

As I have not looked at the variation and temporal aspect of the heating and cooling curves yet, it is the BMC Climbing Festival and sunny, I would hope to see that that they are both delayed by the introduction of thermal mass, but that in itself does not raise the temperature or reduce the energy input. When I have had a few hours to sort that out there should be a clearer picture of what really happens. I am still of the belief that airtightness and a reasonable amount of insulation are the key issues when wishing to reduce energy use and keep a stable temperature rather than internal thermal mass. The reaction times to energy inputs should show that hopefully.

Hope that makes it a bit clearer.

Posted By: SteamyTeaI have been claiming that in the UKs climate regime, thermal mass within the insulated envelope makes no difference to the overall thermal performance of the building ... does adding thermal mass within the insulated envelope keep the building at a higher mean temperature

Given this crude (pure?) use of mass, just sticking it into the room, I'd agree completely - no reason why it should keep the building at higher mean temp or reduce overall energy input. However ...

Posted By: SteamyTeaI was unsure if it made any difference to the comfort levels

That, I'd expect 'yes', or rather temp swings about the mean would be reduced in amplitude (decremented?). They would also be delayed, which cd mean e.g. that internal temp is highest when outside temp drops lowest.

Posted By: fostertom

Posted By: SteamyTeaI was unsure if it made any difference to the comfort levels

That, I'd expect 'yes',

To add my 2p worth I took some measurements of my high thermal mass box - aka my garage/workshop. its about 60 sq.M stone/rubble walls 50cm thick with a 20cm thick concrete ceiling (floor to pitched roof attic storage area) It faces SW and that aspect has 3.5M x 2.4M badly fitting wooden doors and 3.25 sq M of single glazed windows.
Yesterday evening at 20.00
outside temp 18.5 deg.
inside temp 21.8 deg.

this morning at 06.00
outside temp 13.1 deg.
inside temp 20.6 deg.

this afternoon at 13.00
outside temp 29.3 deg.
inside temp 21.8 deg.

this evening at 20.00
outside temp 21 deg.
inside temp 22.6 deg.

From this can be seen with an outside variation of 16.2 deg the internal temp varied only 2 deg. and this is a structure with no added insulation and most certainly not air tight.

So for me I agree with FT thermal mass adds greatly to the comfort level.

ST,

Whilst the experiment is interesting, and will be more valuable when temporal variation is considered, there does seem to be a design problem. If I understand what you have done correctly, these are all small containers which will be closely coupled to the thermal mass of the underlying surface - adding granite chips could be seem as just reducing the 'habitable volume' rather than actually increasing the effective thermal mass. That is, unless the containers are sitting on a reasonable thickness of insulation.

Apologies, obviously didn't make myself clear enough - I was concerned that adding thermal mass to the base, which may already be in contact with another significant thermal mass (the ground) wouldn't necessarily change overall conditions that much - was why I suggested insulating the base.

Posted By: SteamyTea

• CommentAuthorhttp://www.greenbuildingforum.co.uk/newforum/account.php?u=4864" >SteamyTea
• CommentTime2 hours ago

Peter in Hungry
Rather than add insulation, how about making it air tight, my feeling is that would, depending on roof structure/design, stabilise the temperature more, just plotted your data and with only one high temperature reading can't make any use of it.
What kind of temperatures does it go to in the winter, which I know at your altitude is pretty cold.

Making the garage/workshop air tight or adding insulation does not even come close to the 'to do' list I'm afraid. (SWMBO would have kittens at just the thought) The roof structure is standard pitched roof with no insulation but there is 20cm of (reinforced) concrete cast on to the walls between the ground floor and the attic space so no air leaks there and some thermal mass between the two.
The winter temperature typically goes down to -9 or -10 over night warming up to -2 to -3 during the day and this is usually only for 3 to 4 weeks spread over the winter. Last winter we had a cold spell in February where for about 10 days the temp. never went above -12 deg. This caused a problem as there is a bit of residual water between the stopcock and the drain tap and for the first time this froze and split the pipe. The implication being in previous winters it has not got cold enough in there to cause a problem. Needles to say the car won't fit in the garage (too much STUFF) so there is never any residual heat from the engine, although the rear wall is half into the hill so I guess some ground heat comes through the wall and up through the floor.

The buffering effect of thermal mass is not to be underestimated in terms of comfort. I don't need sophisticated heating controls and when I mess up or just deliberately abuse the situation by putting the wood burner on far too late the house doesn't punish me. The 40 deg and less water circulating the rads carries on helping and the drop in temp is negligible. I feel that the temp differences are also evened out a bit in that the 'usual' decrease in air temp over the height of the room are either reduced or the felt temp, via infra-red, doesn't suffer this effect. On the other hand the house is pretty much occupied 24/7 and if it wasn't a lightweight house would save fuel.

Well said, in my workshop/garage is similar and on cold winter mornings I open all the doors to let the cold air outside 'warm' the freezing air inside! I am a fan of TMass and am building our project to maximise (dense block with EWI and passive solar design).