re: strawbale houses, superinsulate verses thermal mass
30 dec 1999
robert w. tom wrote:
>>>plaster, adhered directly to the surface of the bales serves... to
>>>create an air barrier so as minimise the possibility of r-value-lowering
>>>convection currents between the cladding and the insulation (straw).
>> you mean it plugs up the ends of the little pipes through the straw and
>> the cracks between the outsides of the little pipes? seems like there
>> wouldn't be much convection through those pipes and cracks if the straw
>> fibers were in the horizontal direction,
>...air movement reduces the effectiveness of still air
>as an insulating medium.
but warm air rises...
>convection happens in the air space between glazing layers if the
>airspace is in excess of 13/16".
so? it happens even in 0.00000001" airspaces, where conduction is more
important. there's an optimum air gap thickness: too small, and there's
lots of heat loss by conduction. too big, and there's lots of heat loss
by convection. but not if vertical airflow is difficult, eg if there are
lots of small horizontal tubes across the gap. and to have heat loss,
we need to move air from one side of the gap to the other. vertical
airflow alone won't do that, eg if there are lots of vertical tubes
from one side of the gap to the other.
>> seems simpler and better to put [the outside cladding] over
>> an air gap over the straw... to make the outer wall surface
>> more vapor-permeable than the inner, altho the swarthmorians
>> did tests and concluded that no vapor barrier was needed.
>vapour diffusion is seldom an issue even with stick-framed construction.
you changed the subject. exactly why do you feel that a bale wall needs
plaster to seal the inside for good thermal performance? a) to prevent
convection which loses heat or b) to prevent the flow of moist air through
cracks and fissures and voids in the bales, with subsequent condensation,
rot, freezing, etc? i don't follow your physical explanation, and the
swarthmore walls had no inside cladding, but they were fine insulators,
with no moist air transport problems, after testing for weeks in the
winter with 100% indoor humidity.
the point's moot anyway, since we are both suggesting a layer of plaster
directly inside the walls. i suggested a thicker one (about 3") to add
thermal mass and strength (with studs as well) to the wall. also, i can
imagine an easier time a) scooping plaster into a 3" cavity between bales
and stucco mesh, then smoothing it over the outside of the stucco mesh
than b) plastering right on the bale wall, with nothing to hold the
plaster up on the wall...
being earthy-crunchy ain't enuf. strawbales need to offer more practical
and economical advantages (eg very low heating bills and indoor pollution,
with reasonable labor costs) to compete with conventional housing.