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re: strawbale houses, superinsulate verses thermal mass
26 dec 1999
ralph davis  wrote:

>didn't catch all of the thread...

neither did i...

>...strawbales are insulation, not mass per se.

us r50 or so, if there's no thick layer of plaster running from one
side of the wall to the other to reduce their thermal resistance. otoh,
a thick layer of plaster inside the house can be desirable thermal mass.
(otoh, water may take less labor.)

real plaster seems to have twice the thermal capacity of drywall, ie
2 btu/f vs 1 per board foot. an 8' r50 strawbale cube with an inch of
real plaster inside would have an rc time constant of

     50ft^2-f-h/btu/(6x64ft^2)x2btu/fx6x64ft^2 = 100 hours
    
(which doesn't depend on the size or shape of the "cube" :-), so
starting at 70 f on a 30 f day, after h hours it cools to temp
t = 30+(70-30)exp(-h/100). ("exp" is "e-to-the-x" on your $9.99
scientific calculator, and "ln" is the natural log, the inverse 
of "e-to-the-x.") if 60 = 30+(70-30)exp(-h/100), h = -100ln(30/40)
= 29 hours. add a few more inches of plaster, a low-thermal-mass
sunspace outside a strawbale wall, and some electrical energy usage,
and you've got a passive solar house.

>to prevent rot here in the semi dry state of new mexico, you must first
>coat the concrete (footer) with tar and then wrap the first layer of bales
>with thick plastic on three sides (top can be open).

swarthmore college's strawbale house in damper pa (a 1500 ft^2 shoebox
attached to a 2-story cylinder, built by students without formal drawings)
has a foamboard footing. the earthy-crunchy me advisor considers this
a good compromise. the house was designed to be dismantleable.

>...as to heating/cooling with mass, you wouldn't want to throw the doors
>and windows open.

you might for cooling, at night. steve baer says "superinsulation is good
for heating, but for cooling, you need thermal mass." thermal mass seems
good for solar heating too, since it keeps the house warm after sunset.

>> even if you don't open the windows at night, most people
>> turn off the heat at night. in that case, the house would
>> drop to about 15 degrees f above the low temp outside.

that "balance point temperature" depends on the insulation and size of
the house and its internal energy usage. how fast the house gets to
that temp depends on its thermal mass, ie its time constant. if our 8'
cube used 300 kwh/month of indoor electricity (10 kwh/day or 1421 btu/h
of power), the indoor temp would eventually drop to 1421xr50/(6x64ft^2)
= 185 (f) degrees above the outdoors :-) adding some windows or air leaks
or reducing the electrical energy usage would lower that balance temp.
adding more 300 btu/h people or 20 watt cats inside would raise it.

a 32' cube with the same electrical usage would eventually settle to
11.6 f above the outdoors if we turned off the heating system but
kept on using electricity inside. 

>> if you have a lot of thermal mass, you will have to spend
>> money to heat up that thermal mass.

well, it gives back all of that heat when you turn off the furnace.
however, as you say, keeping the house warmer at night means more heat
is lost to the outdoors, which doesn't come back, so low-thermal-mass 
houses can be more energy frugal. 

>> a low thermal mass house cools off quickly at night and does
>> not dump heat all night long and is therefore more efficient.
>> i could see in this case thermal mass would raise the heating
>> bills instead of lowering them.

it's hard to make a very low-thermal-mass house with durable materials.
even 1/2" drywall inside r20 insulation makes rc 10 hours, before adding
some furnishings inside, eg a few 55 gallon accent drums.

nick 




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