Fundamentals: Masonry walls for thermal mass

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- CommentAuthorEd Davies
- CommentTimeMay 20th 2015
Not to derail another discussion [Â¹] too much, two things I think are worth talking about regarding use of masonry walls for thermal mass:

1) How thick?

In the other discussion:

Posted By: fostertom: â€œIf the blockwork is for thermal mass, then Andy's 100 is all that's needed, and confers the max thermal mass that's possible - any greater thickness (like deemed-to-satisfy 190) adds no effective or active thermal mass, because on the diurnal-cycle basis of solar coming in thro windows daytime then leaving again at night, the temperature waves only have time to penetrate the inside 100 or so of blockwork before the heat flow reverses again.â€

This makes sense in New Mexico where it's sunny almost every day and cold at night. Does it make sense in the UK where weather tends to cycle on a longer timescale than diurnally. I'd think that thicker walls would be better for carrying a bit of heat over dull chilly days (as long as they're suitably isolated from the ground, etc)?

2) External?

Putting the thermal mass up against the external insulation seems a bit nutty to me. Surely it'd be better to build the internal walls from heavy masonry and the external walls from insulated timber keeping them as thermally lightweight as practical. Sort of the reverse of many UK houses.

Obviously there are fire-protection considerations but for a detached house in a quiet area internal masonry would be more useful from a sound-deadening point of view.

[Â¹] http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13358
- CommentAuthorSigaldry
- CommentTimeMay 20th 2015
When I looked into this a while back, when calculating thermal mass for UK buildings, only the first 100mm of the construction is considered (or you stop if you get to the mid point of a construction if sooner, or stop if you reach an insulation layer).

The thermal mass seems to not be helpful from an energy modelling point of view (SAP) assuming a standard occupancy if not combined with being very well insulated, as it appears to make the building less responsive. The Dwelling Emission Rate certainly seems to get poorer.

Bear in mind SAP assumes heating for an hour in the morning (8 to 9a.m.) and then five or six hours in the evening (depending on the time of the year). By the time the next heating period arrives, the background heating level appears to have typically reset (unless very well insulated) so when the heating comes on again, the high thermal mass options first need to heat up the construction (affecting reaction time). As SAP assumes no heating needed after these heat demand periods, the longer tail to heat loss from the higher thermal mass releasing heat doesn't give any reduction in heating demand benefit.

High thermal mass can reduce your evening heating demand when its been very warm during the day and much colder at night by soaking up the solar gains and releasing them later - but this doesn't seem to translate into an energy assessment benefit from experience.

When very well insulated, higher thermal mass constructions started to be more beneficial from modelling - but there still wasn't much if any benefit over low thermal mass.

High thermal mass does seem more beneficial for reducing risk of summer overheating. Not so much for reducing heating demand for the periods of the year when heating is most needed.

Caveat: the above based on modelling and a lot of reading - rather than reality necessarily...
- CommentAuthorGBP-Keith
- CommentTimeMay 20th 2015
You will lose this latest posting unless you do it at the new forum.
- CommentAuthorfostertom
- CommentTimeMay 21st 2015
I won't post now, then - we continue this after the transfer?
- CommentAuthorbot de paille
- CommentTimeMay 21st 2015 edited
Every study I have ever read about thermal mass for the average house has always said that anything beyond 10cm on the internal surfaces will work against you.
- CommentAuthorfostertom
- CommentTimeMay 21st 2015
Posted By: Ed DaviesThis makes sense in New Mexico
Actually, NM is just where thick thermal mass comes into its own. It makes all the difference in the world, whether
a) heat flow is uni-directional (albeit with some reversal), 'from' outside 'to' inside, or
b) heat flow is bi- directional, all input and output being from just one (inside) face - so it's all-reversal.

New Mexico adobe is a) - the outside absorbs solar heat by day into its outer layers (and radiates it away by night) - creating an oscillating temperature-wave whose peaks take a calculated time to propagate through to the interior layers. If judged right, the temperature-wave peak (ex-daytime high temp) travels through basically uni-directionally to reach the interior just when it's cold night outside; the temperature-wave trough (ex-night time low temp) reaches the interior just when it's hot day outside. The thickness can be modular - either just enough to give apx 12hr delay, or to give 36hr delay (tho that will function more like a cave's rock-solid inside-surface temp, unvarying regardless of outdoors).

Temperate-zone houses are b) - they absorb solar heat (if any) thro their windows to land eventually on thermally massive interior surfaces - wall, floor. The temperature-wave peak propagates (at calculated slow speed) from inside into the mass, but in apx 12hrs is followed by a temperature-wave trough, which attracts all the heat back out again, before the temperature-wave peak has travelled more than 100mm max into the mass, during apx 12hrs.
- CommentAuthorEd Davies
- CommentTimeMay 21st 2015 edited
Posted By: Sigaldryif not combined with being very well insulated,
Sorry, assumed that if you were deciding whether to put blocks or timber it was new-build and therefore presumably very well insulated. Should perhaps have made that explicit.

To be honest, I'm not convinced that SAP is that interesting from the point of view of evaluating well insulated buildings. PHPP or similar using something approximating real weather might be better.

Posted By: fostertomThe thickness can be modular - either just enough to give apx 12hr delay, or to give 36hr delayâ€¦
I think you'll have to work very hard to convince me that heat travels in waves like that through a thick wall. But that's not the point of this thread which assumes UK and therefore lots of insulation on the outside, i.e.:

â€¦they absorb solar heat (if any) thro their windows to land eventually on thermally massive interior surfaces - wall, floor.

â€¦but in apx 12hrs is followed by a temperature-wave troughâ€¦
Yeah, but that's my point: British weather doesn't work like that. Particularly in the shoulder months you can get runs of a couple of warm sunny days followed some duller chillier ones. So then would more thickness help?

I don't know but suspect that what FT said in the previous thread is relevant:

Only if deliberate measures are taken to activate greater thicknesses, is greater thickness worthwhile!
I'm imagining a house with very thick internal walls which diverts excess solar (PV or thermal) or other cheap energy into the core of the walls somehow so that they act as a structural â€œnightâ€ storage heater.
- CommentAuthorSteamyTea
- CommentTimeMay 21st 2015
Posted By: Ed DaviesI'm imagining a house with very thick internal walls which diverts excess solar (PV or thermal) or other cheap energy into the core of the walls somehow so that they act as a structural â€œnightâ€ storage heater.
That is exactly what I did my BSc Dissertation in. Since then I have been proving myself wrong.

What I did was mimic an internal stud wall that was filled with stone chippings (was a some plastic pipe with a fan on the top).
There was a clear roof area that collected sunlight (a 500W bulb).
The tests where run with different amounts of mass in the tubes and with either the fans on or off.
With the fan off, all that happened at best was nothing as far as storing energy. With the fan on at the same time as the light was on, warmed air from the roof void was pumped though the stone chippings and some usable energy was stored.

The greater the mass inside the tubes, the more energy was stored, but the mean temperature was lower, this showed that the system worked, but only within the parameters of the local environment (the room started off at 20Â°C and got hotter, but this was corrected for).

Now what you could do is make a stud wall with stone chippings in it, and depending on the amount of excess energy from your collector area you could pump some air in or not (that bit is simple).
Then, you can either leave the wall static and let it radiate a small amount of energy out, or recirculate the living area air though the stone chippings to warm up a bit.
I think it would be a bit of trial and error to get the right amount of mass in, but it is easy to change it. Alternatively you could fill it entirely but in independent zone, then experiment by heating different zones or not.

The down sides are you need big fans and some way to stop insects living in a nice cosy dark wall.

To put this into perspective, my dissertation was about the mathematical modelling, not the idea I was testing.
It would probably be easier and cheaper to fit a HWC, that is powered by PV, then pump the warm water though a fan assisted heat exchanger. Then in the summer you can use the electricity for something else.
- CommentAuthorskyewright
- CommentTimeMay 21st 2015
Posted By: SteamyTeaIt would probably be easier and cheaper to fit a HWC, that is powered by PV, then pump the warm water though a fan assisted heat exchanger. Then in the summer you can use the electricity for something else.

Fill the stud with sand, poured over some underfloor heating cable & the wall becomes a large surface area low temp storage radiator?

Just a crazy thought... (maybe?).
- CommentAuthorEd Davies
- CommentTimeMay 21st 2015
Blowing air through rocks was a bit of a 1970s thing, isn't it? (Probably when you did your BSc, or soon after ) It was found to be not that efficient in the 70s and there's no reason to think it would be any different now.

As Skyewright says, I'm thinking of something more like UFH tipped on its side, just not sand.

If you feel like using masonry to hold the roof up on a detached house then I'm suggesting:

a) keep it out of contact with the external walls so that it doesn't lose heat outwards,

b) it deadens sound transfer within the house,

c) you can use both sides to put heat in and get it out and,

d) as an optional extra you can put heat into the middle â€œartificiallyâ€ which would be a silly thing to do to masonry in an external wall.
- CommentAuthorrenewablejohn
- CommentTimeMay 21st 2015
Well you have just described my house dating back to 1650. Two foot thick stone walls with rubble infill. Now combined with 3G windows to give stable temperature home needing minimum heating or cooling.
- CommentAuthorSteamyTea
- CommentTimeMay 21st 2015
Posted By: Ed DaviesIt was found to be not that efficient in the 70s and there's no reason to think it would be any different now.
Been saying that for years.
But I was looking at the modelling not the practicality of it.
- CommentAuthorEd Davies
- CommentTimeMay 22nd 2015
Interesting RJ. Are your internal walls, rather than external, really rubble fill? The croft cottage I rented for a while was rubble fill but mostly just the external walls - only what was an external wall but became internal when the kitchen extension was added could be counted and that was directly connected to the external wall above. It had a nice stable temperature - pity it was about 8 Â°C.
- CommentAuthorfostertom
- CommentTimeMay 22nd 2015 edited
Posted By: Ed Davies
Posted By: fostertomThe thickness can be modular - either just enough to give apx 12hr delay, or to give 36hr delayâ€¦
I think you'll have to work very hard to convince me that heat travels in waves like that through a thick wall
Not travelling waves, but a 'wavefront' spreading outward from the point of input.

Of course it's continuous, but as a dodgy way to visualise, 12hrs cold input diurnally succeeds hot input succeeds cold input etc, at the input face. So a 'cold' wavefront chases a 'hot' wavefront through the wall and of course the cold wavefront pulls back on (lowers temp of) the heat in front of it, another way of saying the hot wavefront pulls forward on (raises temp of) the cold behind it. So the pattern blurs and evens-out (aka decrement) as it passes inward.

At 12hrs, in the New Mexico scenario, that hot or cold wavefront wd be fairly well defined as it reaches the inside; at 6hrs (phased too soon) it would be sharply defined and wd just add to daytime heat/night time cold inside: at 24hrs (exactly wrong phasing) it would be barely defined but still counter productive; after 36hrs of decrement, all trace of wavefront would have gone and you'd have a cave-like near-constant internal-face temperature.
- CommentAuthorSteamyTea
- CommentTimeMay 22nd 2015
What is the mean and range of temperatures in New Mexico? What is the fractional cloud cover? And how is it relevant to the UK. Have Eskimos ever built stone buildings?
- CommentAuthorbarney
- CommentTimeMay 22nd 2015
It's easy to use UFH in a wall - and we operate UFH both in massive thermal weight and very light weight construction so it can just as readily go into a stud construction as a mass masonry construction.

I worked on the design of a hospital (not UK but not dissimilar climate) that had pipes embedded in the wall (which was heavy, and externally insulated) - we had a pretty low temperature flow and (eventually) it settled down to be almost self regulating. At a much simpler level, my previous abode had electric mat UFH in the bathroom floor and two wall - it was tiled over and formed the shower enclosure actually

Internal thermal mass disconnected from the structure is quite useful - a friend of mine has a house with a concrete core forming the main stairwell/landings etc - as well as concrete floors at ground and first floor. The highly insulated and weather proof envelope sits "around" the structure - so it's a lightweight building with a heavy internal mass exposed to the internal environment.

It seems to work well as an internal temperature moderator, and without doubt is capturing solar energy via glazing and releasing later into the building as it cools in relation to the external temperature. I'm not actually convinced that it saves energy however.

To counter Tom's convenient wavefront, perhaps look at Trombe Wall - to see how energy would flow in and out and have effect on the internal environment, but not on energy consumption - heat will only flow "downhill" unless further energy is added - it's easy to store a lot of energy in a mass of structure - it's another thing entirely to keep it moving "inwards" when it's significantly colder outside than it needs to be inside. The basic science tells us this - otherwise why aren't we all building thermally massive boxes with no heat input required - whan actually all we achieve is moderation of an internal temperature over very defined time periods

Regards

Barney
- CommentAuthorfostertom
- CommentTimeMay 22nd 2015
I didn't say any of this greatly reduces overall fuelled heat demand in a climate/loss-gain-balance that needs nett input.

What it can do is smooth the peaks and troughs, which is nice, and also reduces hardware sizing/sophistication - and greatly stabilise internal temp against diurnal or sudden (leaving a door open) changes.

The only way to do better, and only in a non-continuously occupied building, is to go super-lightweight and allow it to freeze (hence no heat loss) when nobody's there.

Mass also opens the door to all sorts of non-fuelled/renewable/intermittent heat inputs. It's fine if a building uses lots of that but no fuel, even if the actual heat requirement is greater than say an optimised but fuelled PH building.
- CommentAuthoran02ew
- CommentTimeMay 22nd 2015
Doesnt thermal mass also help greatly when people open there doors in winter, the constant movement of families in and out throughout day can lose a lot of heat, wouldnt a large thermal mass act as a buffer.
- CommentAuthorrenewablejohn
- CommentTimeMay 22nd 2015 edited
Posted By: Ed DaviesInteresting RJ. Are your internal walls, rather than external, really rubble fill? The croft cottage I rented for a while was rubble fill but mostly just the external walls - only what was an external wall but became internal when the kitchen extension was added could be counted and that was directly connected to the external wall above. It had a nice stable temperature - pity it was about 8 Â°C.

Only have one original internal wall and thats the same 2 ft wide rubble fill. All other walls are modern stud partitions.
- CommentAuthorEd Davies
- CommentTimeMay 22nd 2015 edited
Posted By: barneyInternal thermal mass disconnected from the structure is quite useful - a friend of mine has a house with a concrete core forming the main stairwell/landings etc - as well as concrete floors at ground and first floor. The highly insulated and weather proof envelope sits "around" the structure - so it's a lightweight building with a heavy internal mass exposed to the internal environment.
Nice example; this is just the sort of thing I had in mind. If, for whatever reason, you're set on using masonry in your build then I'm suggesting that putting it in the middle is better, tonne for tonne, than round the outside.

It seems to work well as an internal temperature moderator, and without doubt is capturing solar energy via glazing and releasing later into the building as it cools in relation to the external temperature. I'm not actually convinced that it saves energy however.
At times of year when you need continuous heating or, if it ever happens in the UK, continuous cooling thermal mass isn't going to save energy. It might help if you have an intermittent heat source (a log boiler run every other day, for example) but that's a separate issue.

But at times of year when there's heating required some of the time but at least some excess warmth available from outside air and sunshine at other times within a day or two or three of the heating requirement it's difficult to understand why thermal mass wouldn't help save energy.

Posted By: fostertomThe only way to do better, and only in a non-continuously occupied building, is to go super-lightweight and allow it to freeze (hence no heat loss) when nobody's there.
Yes, mass in intermittently occupied buildings could be harmful as you have to heat the mass to make the place comfortable then the heat you stored leaks out when it's useless.
- CommentAuthorbarney
- CommentTimeMay 22nd 2015
OK - thermal mass is a moderator only (in my opinion) - if you can load it with a "free" energy source and hold it for re use, then that's not quite the same as "saving energy" - ie the energy input is the same for identical internal/external conditions - you are just getting it from a different source.

If you want to capture solar energy to supplant other energy sources, then much more efficient to grab it with an efficient collector like a PV or solar thermal panel - and then decide to stick it into storage for later or use it at the point of generation - a large chunk of stone in the middle of the house would do just that - but again, that's not exactly what we commonly call "exposed thermal mass"

The office I'm sitting in at the moment has an exposed concrete slab at about 4m AFFL - and it's about 300mm thick (limited columns on the floorplate) - our dominant energy input is for cooling (office full of people driving computers - ceiling to floor glazing behind a brieze soliel) - we have automatic opening sections in the glazing and an atrium - we run exract fans with the windows open (at night) and ore charge the slab with "coolth" - it has a significant impact (downwards) on our cooling energy - we can do the same in reverse if required.

It only works for an office building - if you have to be in a darker, even at this time of year, it's bloody freezing in here after about 9.0pm - because we are flushing out residual heat and getting a pre load of cold into the mass. It's so effective, we have to be careful about dropping the slab to a dew point condition for the following morning.

It isn't ever going to work if we operated 24/7 however

So, unless you can tolerate extreme overheating earlier in the day, you can't get that heat into the slab for use later

As most of this debate is focussed on housing, it's pretty well a non starter to reduce energy consumption - in my experience

I like lots of mass for environmental damping, acoustics and the "feel" of solidity - but I don't actually think it has a significant impact on energy consumption in a continuously occupied building or at least one where you need a reasonable internal environment all the time

Regards

Barney
- CommentAuthortony
- CommentTimeMay 22nd 2015
The nice thing about mass is that it can store free solar energy, the more you have the more you can store

The more you have the more difficult it is for the temperature to change.

This offers the occupant (me) comfortable living conditions

More is better
- CommentAuthorbot de paille
- CommentTimeMay 22nd 2015 edited
Its what you do with it that counts, not the amount.

TM is another ingredient in the mix whose effect on overall performance will depend on how the other elements come into play; insulation, ventilation, climate, solar gain, occupancy use, etc

Start with a building in a temperate climate that has 60cm of solid wall and roof,WITHOUT any windows and you will generally create an internal enviroment that is closest to the seasonal average without having to do much.

Introduce windows and you add solar gain to the internal climate. now its more complicated.

Introduce occupants using equipment and generating heat and moisture, opening and closing windows and blinds, coming and going, and it gets even more complicated.

Now introduce a heating system...

etc etc etc
- CommentAuthorEd Davies
- CommentTimeMay 22nd 2015 edited
Posted By: barneyIf you want to capture solar energy to supplant other energy sources, then much more efficient to grab it with an efficient collector like a PV or solar thermal panel - and then decide to stick it into storage for later or use it at the point of generation - a large chunk of stone in the middle of the house would do just that - but again, that's not exactly what we commonly call "exposed thermal mass"
Absolutely. That's why I pointed to FT's comment about â€œdeliberate measuresâ€ to put heat into the store. If you try to live in the path for heat both in and out of the store you're most likely to have either unacceptably large temperature swings or only trivial flows of heat in or out. Better to just live in the path out of the store (mostly).

Still, I think one should look to more than just the diurnal cycle when looking at the effective thickness of even just a passive store.
- CommentAuthortony
- CommentTimeMay 22nd 2015
Agreed
- CommentAuthordjh
- CommentTimeMay 22nd 2015
Posted By: Ed DaviesStill, I think one should look to more than just the diurnal cycle when looking at the effective thickness of even just a passive store.

I think it's more a question of whether you can justify thermal mass based on diurnal effects in your climate and with your lifestyle or not. If you can - like New Mexico - then it's a no-brainer: add thermal mass. If you can't - like UK - then it's a lot more dodgy. If you want to succeed then you have to think about grey days and January and February, otherwise you're still going to have to size heating systems to deal with those periods. My conclusion was that whilst it's definitely possible, it's not economical, but maybe I'm too pessimistic or conservative or something.
- CommentAuthoran02ew
- CommentTimeMay 22nd 2015
So are you saying thermal mass in the uk is bad? And we should only use timber frame! coming from Somerset here damp is a key concern. And timber frame is only suitable for certain builds.
- CommentAuthorSteamyTea
- CommentTimeMay 22nd 2015
Posted By: an02ewSo are you saying thermal mass in the uk is bad?
No, just a distraction and an irrelevance.
There is generally enough mass with a high enough specific heat capacity in a building to not have to worry about it.
You also need a large enough temperature difference to drive the energy to where you want it. A degree or two is not going to be enough.
There is also a big difference between a passive and an active system.
- CommentAuthortony
- CommentTimeMay 22nd 2015
Heat will always pass from warmer to cooler no matter how small the temperature difference.

In my house I rely on all the heavy weight construction and the ground under the house to keep the temperature stable and to eliminate the need for a heating system

I shade the windows in summer (when we get one) to keep the house from overheating. House remains cool in summer, warm in winter.
- CommentAuthorSteamyTea
- CommentTimeMay 22nd 2015
Posted By: tonyto keep the temperature stable

Posted By: tonyto keep the house from overheating

Don't they contradict each other?

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Fundamentals: Masonry walls for thermal mass