DIYbanter - View Single Post - Heat conduction from basement to earth/soil below

Klaus Kragelund

wrote:
Klaus Kragelund wrote:

I have a basement in my house. The floor is about 1.5m below
earth/ground level...

Covering the ceiling with foil would give it about US R10, ie 1.76 mK/W,
with E = 0.03 and a Tc = 20 C ceiling temp and a Tf = 14 C floor temp
and a large air gap. This would reduce the radiation from the ceiling
to the floor, es((Tc+273)^4-Tf+273)^4) W/m^2, with s = 5.6697x10^-8
W/m^2-K^4. If the upper foil surface is perfectly clean, with no dust
(you are German, right? :-), this may work even better. The linearized
radiation conductance is 4esTm^3 W/m^2-K, where Tm is the approximate
mean absolute temp.

The floor is not insolated, so in order to save some money on the
heating bill I am considering insolating it with sheets of polystyrene
foam... with some rafters in a mesh to lay the wooden floor on.

The English word is "insulating." InsOlation is sunlight.

My theory is that since the floor is 1.5m below ground level, the
temperature of the soil will never be very cold. Searching the net I
find something about 14degree celcius.

The temperature of the middle of the floor might be about the same as
the yearly average air temperature. The walls and the floor near the walls
might be closer to the average daily outdoor temperature.

So if I have 60square meters of floor heated to room temperature of
20degrees, how do I calculate the heattransfer when I have the data for
the insulation and the concrete floor?

With difficulty :-) The floor surface will probably be cooler than 20 C.
That's good.

Will the earth behave as an ideal giant block that has 6degrees of
tempeature. So the gradient from the room temperature to the earth can
never be higher than 10 degrees (20-14)?

The temperature difference between the room air and the middle of the floor
might be 6 C. Then again, the room air will warm the floor, which has thermal
capacity and resistance to downwards heatflow. Some people estimate soil's
resistance to downward heatflow as US R10, ie 1.76d mK/W. Upward is less,
with evaporation from lower soil layers and condensation above. And moving
water can change this.

Concrete, k = ~1W/mK
Polystyrene, k = 0.03W/mK
Wood, k = 0.14W/mK

Air, k = 0.025 W/mK

Power needed to keep temperature stable: P=KAT/D

Concrete using 60square meters and 30cm thick: P = 1*60*6/0.3. P = 1.2kW

Adding polystyrene: k = 0.042 , P = 0.03*60*6/0.05 = 216W

Not k = 0.03, as above?

The poystyrene is in parallel with the rafters. Assuming the rafters
take up 5% of the floor instead of the polystyrene

P= 0.14*60*0.05*6/0.05 = 50W

So from these calculations it seems I need 250W to keep the room heated

You might cover the ceiling with foil and cover the walls with thin
foil-faced foamboard over spacers and carpet the floor, with no polystyrene.
Each layer of wall foil adds about US R3, plus the bulk resistance of
the foamboard. If there's no vapor barrier under the concrete, you might
put a layer of plastic film under the carpet.

Thankyou for the very good points

The basement is going to be used for office, so it will be heated all
year round.

I should have said I'm situated in Denmark (just north of Germany :-)

Perhaps then my number of 6 degrees gradient is overly optimistic due
to fringe effects near the outer walls. I want as little insulation as
possible since adding too much to the floor will limit the height of
the doors.

Currently no mostiture film is used. I want to add a wooden floor with
a carpet on top of this. The basement is dry, but my intention was to
add ventilation holes in the wooden floor and use no platic film to
allow the construction to"breathe". But adding plastic film might be a
good idea since this also limits heat flow caused by travelling
moisture.

Thanks

Klaus