[GSBN] Modeling or measuring mass effect of interior plaster

Tue May 14 15:38:16 UTC 2013

Finally 'found' the time to read all your great mails on this 'massive' 
subject written in the usual 'light' GSBN manner and would like to share 
a few things that came to mind.

I find this a subject that is simple and complex (to calculate) at the 
same time. So I too use the 'intuative observation' approach (by lack of 
being able to tap my brain intto Luc's).

Laura's question : what we might lose in having wood rather than exposed 
plaster.

My gut-response is that if you perfectly master/regulate the amount of 
calories to enter and leave your building according to the desired 
confort you would not need any mass. But this regulation of calories is 
complicated (and that always seems to mean expensive) so adding mass 
(plaster 'protected' by insulation) allows us to store the excess heat 
or cold and let it flow back when usefull. Ofcourse, plastering also has 
its cost (and drying time). The question is probably which of the 2 
solutions is the cheapest way to go; but one cannot, as we say in France 
: "have the butter and the money" (for selling the butter).

The only SB house with an over-heating problem I visited was one with a 
too big window on the west (north hemisphere ;-) and just one layer of 
gypsum board over the straw (so no thick plaster to absorb the excess 
incoming heat). The choice to apply so little mass on the SB walls, to 
keep the costs down, was finaly 'somewhat regretted' by the owners...

An other question came up : can we have too much mass?
I'd say : easily!!!!!  I'ts exactly what happens in a swimming pool for 
instance. The the skinny 4 year olds like it 'not to cold' when taking 
their first swimming lessons. The floating grandma's like it a bit 
warmer. And so do the babys that come next with the happy new parents... 
No problémo... We just put on the mega heaters and heat up that water 
during the day. But then come the waterpolo guys and girls going in for 
some serious excersise... and (speaking from experience) I can tell you, 
water has a lot of mass and opening all the doors and windows does not 
change a thing. It will take a loooooong while 'til that water cools 
down... Same goes for a building with a lot of mass. They take a long 
time to adapt (to changing needs).

A third question we could ask : "what would be optimum mass?" was asked 
to Earth-shiped Mickeal Reynolds during a conference in Santa Fé. I 
understood he answered that an equivalent volume of mass (earth) 
connected to the space to be 'conditioned' (+ an insulation preventing 
the mass to be to exposed to the rest of the Universe) is what worked 
best for them.
But, not everybody likes the idea of pushing a hill against the shaded 
side of the house... and, like the point I try to make with the pool 
scénario, "constant temperature" does not nescecarily equal "comfort".

I don't remember where I read that heat at 'a normal temperature' will 
not penetrate more then 7cm/3" of earth (as if I would use any other 
plaster ;-) during a day.
Applying the golden 80:20 rule I believe that 5cm/2" is a practical (not 
to complicated/expensive) and very effective tempering diurnal swing.
Having lived 10 years in an earth plastered SB house, with a masonry 
heater, in a mild climate close to Bordeaux I also 'feel' this is true, 
in all 4 seasons.
If someone would want to use less mass, I'd like them to show me how 
they intend to master the flow of calories entering and leaving the 
building better than I did; wich, I admit, is very much possible.

André - swimming in thoughts- de Bouter
France
www.lamaisonenpaille.com

>
> *My reference for calculations has been from Dan Chiras's book, The 
> Solar House. Around pages 100 to 106*, Dan offers information around 
> relationships of types of thermal mass, with good ratios to use for 
> number crunching. I've been referring to these ratios for years, and 
> every home that we've designed and built since then, has worked quite 
> well for our region-specific designs. And I think you are correct to 
> consider the direct vs. indirect gain on these wall surface areas. 
> I've used Dan's ratios, and fudged them a bit, for lit and unlit areas.
> Dan refers to ratios of glass-to-mass areas, and specifies using the 
> ratios of 1:5.5 for sunlit floor areas, and 1:40 for unlit floor areas.
> He specifies 1:8.3 for unlit wall areas, and I've used 1:6 for sunlit 
> wall areas.  That ratio of 1:6 is from my own imagination, so don't 
> blame Dan for that. Blame my "intuitive approach" to engineering.  
> After observing the various passive-solar homes that we've built over 
> the years, the numbers seem to work.
>

Sometimes single-level homes with a collector slab will, at night, tend 
to stratify.  The efficiency of heat transfer can be improved by just a 
slight level change to drive the convection loop. Even one step between 
the bedrooms and living area is enough to make a significant increase in 
air circulation during the night.

  In cooling conditions moisture in the wall will change from gas to 
water, releasing considerable heat per unit of water (more than five 
times that needed to heat the same quantity of water from 0 to 100°C), 
whereas in warming conditions the reverse occurs. In other words, a 
breathable earthen wall will have more temperature moderating effect 
than one which was sealed against passage of moisture. The magnitude of 
this effect would be a function of several variables, but I can imagine 
conditions in which it could be significant. Note that this apart from 
the moderating effect on room humidity, which would also have an effect 
on perceived temperature.

Earth ship

André :
5-7 cm
1st strategy ; regulate the incoming heat (and outgoing heat).
If that is perfect (corresponds with the need/comfort needed, no mass 
would be just fine.
If that 'regulation' is 'imperfect' mass allows us to absorb the

Stone wall

Le 13/05/2013 22:53, Derek Stearns Roff a écrit :
> Robert Riversong provided me with the chart below, which combines all 
> the relevant factors, to come up with a Thermal Mass Index.  Soapstone 
> is the winner by a large margin, so plaster your bales with soapstone. 
>  Saturated sand is next in line, which would make a great plaster. 
>  Marble is good.  Clay isn't great on this list, but it has so many 
> other virtues that it is my first choice.
>
> Derek
>
>
>
>
>
> On May 13, 2013, at 9:17 AM, Van Krieken wrote:
>
>> Thermal mass, like insulation, its a general expression, but in fact 
>> "thermal mass depends on the type of material we use.
>>
>> It is important to know what are the properties and thermal 
>> performances of the materials we want to use, because each of them 
>> have their own thermal characteristics.  Due to their structure and 
>> their mass they manage the heat in different ways:
>>
>> a) Statics: conductivity or thermal capacity. How does the material 
>> reacts to a thermal flow, independently of the reaction time?
>> b) Dynamics: diffusivity and effusivity. At what speed the material 
>> manages the thermal flow?
>>
>> Because the exterior conditions are going to determine the interior 
>> changes, its essential to know how the materials react. Iron and 
>> clay, both thermal mass, react in a very, very different way.
>>
>> 1. The thermal conductivity (lambda) gives us the information 
>> concerning the amount of insulation a material can achieve (air 
>> passage of calories).
>>
>> 2. The thermal capacity, measures its aptitude to stock the heat. 
>> This is the key element to stock the heat in winter, as well as to 
>> absorb the heat in summer. They are not only heavy materials (like 
>> clay or stone, or cement). Straw, a much more light material, has 
>> also a thermal capacity, and therefore thermal mass.
>>
>> 3. The thermal diffusivity is the measure of thermal inertia and it 
>> increases with the conductivity and decreases with the thermal 
>> capacity. In a substance with high thermal diffusivity, heat moves 
>> rapidly through it (m2/hour).
>>
>> 4. The thermal effusivity measures its capacity to exchange its 
>> thermal energy with the environment. The more the effusivity is high, 
>> the more the material absorbs energy without warming up 
>> significantly. In contrary, the more the effusivity is low, the 
>> faster the material warms up.
>>
>> Obviously, the thermal mass importance of a material depends on these 
>> characteristics, but we can help the final result with some 
>> technology. If in a hot climate I do not have a significant 
>> difference of temperature at night, then I can get 11 or 12º C of 
>> fresh air from the soil (foundations), colling the thermal mass; or I 
>> also can run 19ºC water in radiant walls made of clay. The same we 
>> can do on winter, stocking the heat on the clay walls.
>>
>> What is the best material for thermal mass? I do not have a 
>> scientific knowledge to tell it, but I like to think that "clay" -- 
>> this thermally lazy natural and beatifull material -- is the answer.
>>
>> The simple issue -- my karma its to arrive allways to a easy 
>> conclusion... -- it's how  to use it to keep the heat in cold 
>> seasons, and what to do, to cool it in hot seasons. That's it.
>>
>> All the best
>>
>> Jorge Van Krieken
>> Portugal
>>
>
> Derek Roff
> derek at unm.edu <mailto:derek at unm.edu>
>
>
>
>
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