The answer depends on the surface temperature the humidity the temperature gradient through the atmosphere and what exactly you mean by the temperature of the clear night sky.
Roof radiant heat loss to clear night sky formula.
The general heat loss formula is.
Below is an adjustment for air temperatures.
Otherwise we d be losing heat a lot faster.
Composite wall r values up calculating wall heat loss example problems.
Heat loss from roof 2 750 ft 2 6 100 f days 24 h day 22 ft 2 f h btu 18 30 mmbtu.
Q u a δt or in plain words the heat loss of an area of size a is determined by the u value of the materials and the difference in temperature between inside and out that is the difference in temperature of the two surfaces not the two air temperatures which might not be quite the same.
If made sufficiently reflective to sunlight these materials can also achieve radiative cooling during the day.
As a result the net radiation loss on.
Is p text thermal 1 kc 2 8 78 times 10 13 t 5 852 rh 0 07195 where.
397 9 24 90 25 0 25330 4 0 74 5 67 8 p tk εσ xe 0c radiation cooling time note that in these examples there was a 34 increase in downwelling sky radiation between an overcast 330 4 wm 2 and clear sky 246 6 wm 2.
Total heat loss from the house 13 78 84 91 18 30 116 99 mmbtu in a year or heating requirement is 116 99 million btus per year.
The swinbank formula provides an ad hoc expression for the power radiated by the night sky.
Using 90 humidity the radiation from the sky is predicted to be 168 w m 2 53 27 btu hr ft 2.
The net radiant heat loss from the outer surface of a low sloped roof can be expressed by.
The most common radiative coolers found on buildings are white cool roof paint coatings which have solar reflectances of up to 0 94 and a thermal.
We can also determine the radiative overcast night sky temperature as follows.
Showing cooling potential around the.
Determine cltd for a roof number and b hour of the day.
The radiative night sky temperature is by your reference 3 9 celsius not 3 kelvin.
Roof and wall heat gain cltd method.
7 ru 1 where e emissivity of the outer surface 6 stefan boltzmann constant 7 outer surface temperature t y sky temperature.
The 10 degrees f in my example is about 261 degrees r.
Night sky radiant cooling nsrc has been something of a lost art even though it has been documented and discussed by hvac and solar heating professionals for many decades.
The microwave background radiation of the universe is about 3 kelvin but our atmosphere is much warmer than that.
The cltd tables are based on this condition.
For a clear sky the first term goes away and the formula becomes 8 78e 13xt 5 852xrh 0 07195.
A modified version of this formula from goforth et al.
Consequently they radiate heat into the sky to passively cool buildings at night.
A correction is required for other conditions cltd corrected cltd table 78 tr tm 85.
The roof is thus in radiative exchange with the earth s night sky not with deep space.
