re: old school buses (aka cheap buildings) SOLAR.KnowledgePublications.com

re: old school buses (aka cheap buildings)
23 mar 1999
>> i know someone that has an old school bus that he insulated
>> on the inside with some sort of sprayed-on foam insulation,
>> and he tells me that he can keep the temperature in there
>> above freezing using just one 100-watt light bulb.
>> (this in grey county, southwestern ontario, canada.)

hmmm...

that may be a rural myth. a 100 watt bulb makes 341 btu per hour.
at 32 f indoors and say, 12 f outdoors, with a maximum thermal
conductance c for the bus, 32 = 12+341/c, so c = 17 btu/h-f...

single pane windows have about 1 btu/h-f per square foot. let's say we
spray foam the entire inside of the bus, except for a 1'x1' patch
of window. that leaves 16 btu/h-f...

now say the bus is 8' square and 32' long, with no air leaks at all.
it would float forever, if driven into a lake...

with 1,279 ft^2 of exterior surface with r-value rv, we need 1279/rv = 16,
so rv = 80. spray foam might have r7 per inch, at best, so that's about
a foot of foam, about 1,200 spray cans...

at -8 f, we'd need 4' foam walls, with this kind of thinking, filling
the bus entirely with foam, except for the space for the light bulb... 

nick