re: strawbale vs rammed earth
27 nov 1996
eric brown 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.