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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    Further to the informative article on the Benefits of Thermal Mass in the latest Green Building Magazine I am interested to know if a solid wall with internal insulation was modelled, as this would separate the thermal mass from the living space and possibly provide a clearer picture of the value/performance of thermal mass. I would be interested in the results for any such modelling.
    • CommentTimeJun 13th 2007
    A solid wall with internal insulation would I expect behave much like a lightweight wall.

    I would like to have seen a comparison with a structure that utilised timber frame externally and heavy weight internal walls as suggested by Paul Teather and Keith in the article on SIPS.
    I believe this gives a sensible compromise between the two sides of the debate.
    Hi Both,

    Thanks for the comments. I did look at a lot of different scenarious but tried to limit the content in the article as more data gets more confusing [especially in the charts]

    If you would like to give me more detail of the construction breakdown you are interested in I will run a quick simulation. I need type of materials; their order and each thickness.
    Mike, thanks for the generous offer.
    OK - from the large selection of solid walled building types available lets go for:
    500mm-600mm stone (outer layer, rubble infill, inner layer all approx. 150mm) with 25mm lime plaster retained to internal face.
    Internal insulation to be woodfibre board (100mm Diffutherm) on plaster dabs with lime or earth plaster skim coat finish (say 12mm)
    This cannot obtain the same u-value as the external insulation in the example in Mike's article (0.2 w/m2K) but achieves approx. 0.35 W/m2K.
    The insulation chosen is compatible, and will have some thermal mass itself, but it is anticipated that this will isolate the thermal mass of the stone wall from the living space(?).
    For the model the same level of air-tightness will have to be assummed.
    If you need more info let me know.
    I look forward to the results!
    Many thanks
    • CommentAuthorMike George
    • CommentTimeSep 2nd 2007 edited
    Richard, sorry its taken so long to compile this. Anyway i've included an internally insulated wall to the original charts as per your requested construction.
    Chart 2
    Chart 3
    • CommentAuthorchuckey
    • CommentTimeSep 4th 2007
    The examination of thermal mass and thermal resistance I believe can be synthesized, using resistors and capacitors. For this note I shall call capacitors "batteries" because they behave (a bit) like flat batteries. First of all the units, electrically Voltage is identical to temperature, they cause current or heat to flow. Heat can be measured as watts/sec or calories, I'll call it heat. Resistance is the same in both thermal and electrical terms. with a constant heat (current) generator, a higher restance causes a higher temperature drop(voltage). Thermal mass (battery) when it has a higher temperature on it (voltage) it causes more heat(current ) to flow, into a given resistance.
    A brick when put out in the sun, does not assume the outside temperature immediately, it warms up. So the heat from the sun is going through a thermal resistance into the thermal mass, like charging a battery. As the brick warms up the temperature differential between it and the outside temperature reduces so less heat is pumped in, and finaly after some period, the brick is all at the ambient temperature. a bit like when the battery is charged, the current falls off. Like wise when your hot brick is placed in the fridge, its heat is then conducted out and its temperature falls. Just like discharging a battery into a load.
    To give physical examples:-
    1. Black metal barrel, filled with water = Huge thermal mass, low thermal resistance.
    2. Mirror polished insulated barrel, filled with water = Huge thermal mass, high thermal resistance.
    3. Lump of blackened copper = Minute thermal mass, low thermal resistance.
    4. Mirror polished insulated lump of copper = minute thermal mass, high thermal resistance.
    When the above are put out in the sun, 3 heats up first, 2 the last, what about 1 and 3? The time to heat would be dependent on Mt X Tr so 2 and 3 could heat up at the same rate, but each item would contain a very different amount of heat energy in it.
    The "internal" temperature of the above items is:-
    T = Tamb ( 1-e ^ - T/ Mt X Rt) where T = the actual time its been in the sun, Rt = thermal resistance. Mt = Thermal mass. when T is large i.e. the item has finished warming up, T = Tambient. Unfortunately we cannot measure how much heat flux is being absorbed by the surface (as we can measure current) and we cannot measure the internal temperature, only the surface temperature. But the analogy holds true.
    The total circuit is then a resistor to represent the surface thermal resistivity, from ambient to the sample, black or shiny (in the above example), then a pair of equal resistors to represent the total thermal resistance of the material. I make them equal because the first alters the "charging time of the material (i.e. when it gets hotter), the second alters the discharge time ( i.e. as the material gets colder) and as far as I know materials are symetrical, i.e. they gain or loose degrees at the same rate if the heat inflow rate and the outflow rate is the same. At the middle point of these resistors is the battery (capacitor) which represent the actual thermal mass. The second of these two resistors is then coupled to the internal ambient via yet another resistor. As an example, a piece of Aluminiumum plate might have a black surface facing the sun and a shiny face to the "inside". It will heat up quickly but will put little heat into a room (high thermal resistance ), or it could be turned around and it would remain cool because the high thermal resistance is now on the outside.
    From the previous equation, if a wall is subject to a step temperature change (sun comes out behind a cloud) in time T (the time constant), the new temperature differential across the wall has increased by 63% of the original step, and after 5 X T seconds the temperature differential is 98% of the actual.
    A knock on effect of the above is that high thermal resistances isolate the building from the ambient temperature and increase the effect of internal waste heat. The side effect of high thermal resistance is that if external, it increases the thermal time constant of the structure. Its like having your walls surrounded with a tea cosy. If the high thermal resistance is internal, the outside walls sit at outside ambient temperature and the internal temperature relient only on the internal heat source, its like wearing a fur coat.
    • CommentAuthorally.gill
    • CommentTimeOct 15th 2007
    can you just clarify in simple terms : i believe the conclusion is that its better to have insulation (thermal resistance) on the outside to allow the internal thermal mass(battery) less influence from outside temperature/solar gain and to be of a more constant level of charge dependant on internal activity.

    Mike if you have time i'd love to see a model showing 800mm rammed earth with 150mm timber framed outer leaf filled with isonat hemp (0.039) or warmcel (0.036) or similar. Durable external finish to stand up to teenagers and lime or earth plaster internal skim.
    Is for a uni project i'm doing.
    • CommentAuthorchuckey
    • CommentTimeOct 15th 2007
    Ally , yes. As some simple examples, suppose you have really big thick walls, as in living in a cave. The walls average out the high temperatures and the low temperatures over a time frame dependent on their mass. In a cave it could be years. So if you add insulation to the inside with a constant room temperature the heat loss through the insulation to the more or less constant wall temperature, will be constant. The other point is that with this arrangement, upping the thermostat inside will result in an immediate increase in internal temperature.
    The other case where the thermal mass is inside the insulation, Is like having a huge block of concrete in your lounge. When the internal temperature drops and the thermostat turns the heating on, the heating has to heat up the air to make you comfortable and heat up the concrete block. The concrete block is now at the more or less at the constant room temperature so it can not even out the maxs and mins of the ambient temperature. Because of the big thermal mass, altering the thermostat might cause the heating to come on and perhaps hours (or days?) later the temperature is correct.
    Given the analogy with a battery, as it accepts energy, the only way to get it out again, is to construct some sort of circuit to do it. So if you imbedded some sort of radiator type devices in the outer skin of the wall, in the sun the wall gets hot, and at night you pump water round the radiators and pick up the heat and transfer it to the inside of the insulation and warm your house. Ooops! I have re-invented solar heating.
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