re: how to calc. heat loss down thru timber floor
28 may 1996
phil irvine wrote:
>i am looking for pointers to the method to calculate the heat loss downwards
>thru a suspended timber floor. there seems to be little information about
>downward heat flow where i imagine that the dominant mechanism would be
>radiation since any air space should become stable with the warm air on top.
or maybe there are too many numbers :-)
>here in australia it is quite common to have a house built with the floor
>raised above ground level but not much enclosed meaning that the under floor
>temperature is not much above outside temp. the floor itself is timber planks
>or sheet material supported on timber joists.
table 1 on page 22.1 of the 1993 ashrae handbook of fundamentals, inch-pound
edition, gives us thermal resistances (metric ones would be less by a factor
of 5.68) of surfaces in still air with heat flowing downwards, eg a floor, as
emissivity 0.05 0.20 0.90
r-value 4.55 2.70 0.92
there are 6 talmudic notes that follow, eg "2. no surface has both
an air space resistance value and a surface resistance value."
you might use table 2 on page 22.1 to find the us thermal resistance
of an airspace with downward heatflow as:
emissivity 0.03 0.05 0.20 0.82
air space 3/4" 3.77 3.52 2.38 1.02
1 1/2" 6.27 5.63 3.18 1.14
4" 9.60 8.17 3.86 1.22
these numbers only apply at a mean temperature of 50 f, with a temperature
difference of 30 f... for other mean temperatures and temperature diffs, you
would want to look at some of the other 688 numbers in table 2. there are
another 5 notes following table 2, eg "c. a single resistance value cannot
account for multiple air spaces; each air space requires a separate resistance
calculation that applies only for the established boundary conditions.
resistances of horizontal spaces with heat flow downward are substantially
independent of temperature difference."
table 3 gives emittances of varous surfaces and effective emittances
of air spaces as something like this:
ordinary boards, one surface 0.90
ordinary boards, two facing surfaces 0.82
galvanized steel, white or aluminum-painted paper,
one or two facing surfaces, average 0.20
builder's foil, highly reflective, one surface 0.05
builder's foil, highly reflective, two facing surfaces 0.03
builder's foil, with condensate just visible 0.30
builder's foil, with condensate clearly visible 0.70
so, if the aluminum foil gets moist often, or turns white from corrosion
with moisture, it won't be as good an insulator...
note (a) follows: "these values apply in the 4 to 40 micrometer range
of the electromagnetic spectrum." house-type temperatures...
soft wood has a us r-value of about 1.25/inch. hardwood, 0.91/inch.
seems to me you might well seal up the perimeter of the crawl space, and
put down a vapor barrier if possible, and add a little fan and a humidstat
or a few $12 automatic foundation vents that open when the temp is above
20 c, and close below that.
> phil irvine csiro division of wool technology,ryde, nsw, australia 2112
ever thought of using sheepskins for insulation? treated with salt or
formaldehyde? sealed inside polyethylene pillows? i wonder about their
r-values... my neighbor burns his when he slaughters sheep...
here's an estimate for a wool/poly pillow with an thermal resistance of 2,
under your floor, with 2-sided builder's foil (~15 cents/ft^2 in 4' wide
rolls) under that:
element thermal resistance
4" softwood 5
3/4" airspace below that 1.04
wool/poly pillow 2
3/4" airspace below that 1.04
double sided foil below that
with a 4" airspace below 9.60
total thermal resistance 19.68
i have probably made some mistake here. perhaps someone else could give
a more accurate ashrae exegesis... seems like all these notes and the
700 numbers could be made easier to understand... the irs comes to mind :-)