Subject: Re: Strawbale VS Rammed Earth

From: [email-masked] (Nick Pine)
Date: 27 Nov 1996 10:56:40 -0500

Eric Brown  <[email-masked]> wrote:
 
>I was in Houston this past summer, and there were definitely
>periods where it wasn't cooling off at night, so strawbale should
>be more effective [than rammed earth.]

In what sense? It seems to me that strawbales are basically insulation,
without much thermal mass, which may be good for heating a house, but
natural cooling needs thermal mass, inside insulation, eg to ventilate
the house at night and button it up and keep it cool during the day, and
absorb human infrared radiation.

>(I'm freezing in my Austin apt right now, which is concrete although not
>with the thermal mass of thick rammed earth walls).

Concrete has lots of thermal mass. About 24 Btu/F-ft^3 I think, vs water or
steel at 62, or rammed earth at (?). But it seems to me it needs outsulation,
for a long RC time constant, on the order of 200 hours, to make an energy
efficient house. Consider a 16' concrete cube, with 6" solid walls, with R20
insulation all round: RC = R20/(6x256ft^2)x6x256ft^2x12 Btu/ft^2 = 240 hours
or 10 days :-) Heat it up during sunny weather to 75 F, and let it cool off
for a few cloudy days in a row in Austin in January, where the average outdoor
temp is 48.8 F, and the indoor temperature T = 48.8 + (75-48.8)exp(-t/240),
where t is in hours...

68 = 48.8 + (75-48.8)exp(-t/240) when t=-240ln((68-48.8)/(75-48.8))=74 hours,
ie just over 3 cloudy days in a row. Not a bad solar house... It could be
improved by somehow insulating 1 or 2 walls on the inside, and keeping them 
warmer than room temperature, or using water instead of concrete.

>Overhang design would be more important for rammed earth to shield from the
>summer sun and use the winter sun.

I don't think winter sun shining onto a rammed earth wall will do much
to heat a house, without glazing. With glazing, it makes a poor Trombe wall,
and Trombe walls are already poor solar collectors, because they store solar
heat in a thermal mass wall during the day, and let most of that stored heat
leak out through the low-thermal-resistance glazing at night. A Trombe wall is
also a thermal disaster on a cloudy day, since it is such a poor insulator.

Far better to put some glazing over some sort of dark colored insulation,
with an air gap, and let solar-warmed air flow into the thermal mass of the
house during the day, and let the air gap and glazing get cold at night.

>...when you do use a/c or heat, more thermal mass means more energy required.

Seems to me it makes no difference, if the indoor temperature stays the same 
all day. But thermal mass is a big plus for solar heating and natural cooling.
 
Nick

---

From: Will Stewart <[email-masked]>
Date: Wed, 27 Nov 1996 19:39:48 -0500

> absorb human infrared radiation.

But if the temperature doesn't cool during the night, then you have a
huge hot thermal mass; if you chose to turn on the air conditioner, it
would take forever to cool.
Large thermal mass interiors are best suited to climates with daily
temperature swings above and below the comfort level.
 
> >with the thermal mass of thick rammed earth walls).

Concrete is a very poor insulator.

> also a thermal disaster on a cloudy day, since it is such a poor insulator.

Most current Trombe wall designs include retractable insulation for use
at night or on rainy days.  Such insulation places Trombe walls on the
forefront of passive solar techniques.

> house during the day, and let the air gap and glazing get cold at night.

As long as the air doesn't become to hot during the daytime.  If you
were to pass the heated air through baffles in the walls, that would
provide even, subtle changes in the indoor air temperatures.  One
problem with some passively heated solar houses are the uncomfortable
swings in air temperature.  Large amounts of interior thermal mass may
help mitigate this to some degree, though the air temperature would have
to be significantly higher in order to gain any appreciable heat
transfer.

Another solution to this problem is to have more wall insulation,
substantial south facing glass, water walls just inside the windows, and
insulated shades that could be drawn at night.  The window space overall
would be the same as a normal house, with much of the increased south
window space borrowed from the east, west, and north walls.  The water
walls just inside the south windows would directly absorb much of the
solar heat gain, thereby avoiding the overheating of interior air.  The
insulated shades would reduce the heat loss at night, and can be used on
ordinary windows anyway.

> all day.

If that temperature is too high for comfort, then AC is needed to reduce
it.  If the A/C also has to cool the walls in order to reduce convection
and reradiation, then more energy is required.

>But thermal mass is a big plus for solar heating and natural cooling.

In instances, certainly not as a general rule. 
 
-- 
William R. Stewart
http://www.patriot.net/users/wstewart/first.htm
Member American Solar Energy Society
Member Electrical Vehicle Association of America
"The truth will set you free:  - J.C.

---

From: Eric Brown <[email-masked]>
Date: Wed, 27 Nov 1996 22:54:12 -0600

> all day. But thermal mass is a big plus for solar heating and natural cooling.

Nick, what I'm saying is exactly that -- strawbales are basically
insulation.
I don't think you saw what Gary was asking about.  He is concerned about
rammed
earth in the dead of summer, not the optimal conditions of warm
days/cool nights
or cold days/solar heat that you're talking about.  Uninsulated thermal
mass is
a great heat source/heat sink, which can be liability in the wrong
conditions.

Nice formulas tho... :)

Eric

---

From: Ryan Duston Peterson <[email-masked]>
Date: Fri, 29 Nov 1996 01:16:09 -0800

> all day. But thermal mass is a big plus for solar heating and natural cooling.

   Nick,

    your my GOD. I've been reading you for at least 3 months now.
You know anything about everything. I hope someday to be just like you!