THERMAL INSULATION EXPLAINED
Insulation is that property of any material that obstructs the flow of heat through it. In warm countries, insulation serves to reduce the heat entering the building, and in cold countries, to reduce the heat that escapes.
The thermal conductance (the ability to allow heat to pass through it) of any material, K, is measured in Watts / m2.Kelvin, in metric units, for a given thickness of that material.
This means that if the thermal conductance of a slab of material 10 cm thick is 1 W/m2.K, then 1 Watt of heat will flow across one square meter of that slab if the difference in temperatures on the two sides of the slab is 1 degree Kelvin. One degree Kelvin is exactly the same as one degree Centigrade. If the difference in temperature of the two sides is 10°C, (for example if the temperature outside is 35°C and inside, in an air-conditioned office, it is 25°C), then 10 W of heat will flow across our slab. One watt is equal to one Joule, a unit of energy, per second.
The thermal conductance (the ability to allow heat to pass through it) of any material, K, is measured in Watts / m2.Kelvin, in metric units, for a given thickness of that material.
This means that if the thermal conductance of a slab of material 10 cm thick is 1 W/m2.K, then 1 Watt of heat will flow across one square meter of that slab if the difference in temperatures on the two sides of the slab is 1 degree Kelvin. One degree Kelvin is exactly the same as one degree Centigrade. If the difference in temperature of the two sides is 10°C, (for example if the temperature outside is 35°C and inside, in an air-conditioned office, it is 25°C), then 10 W of heat will flow across our slab. One watt is equal to one Joule, a unit of energy, per second.
r Value, k value and u value explained
The R-value, which is the thermal resistance of any material, is the inverse of the K-value. That is, R = 1 / K.
So if we have a material whose K value is 4 W/m2.K,
its R-value will be 1 / 4 = 0.25 m2.K/W
We must note that so far we have been dealing with European S.I.(metric) units. However, when in the commercial world we speak of R-value, we mean in American units. The Americans use BTUs per hour instead of Watts (a BTU is a British Thermal Unit!), square feet instead of square metres, and degrees Farenheit instead of Centigrade or Kelvin. So their unit for R-value is hr-ft2-°F/Btu.
So if we have a material whose K value is 4 W/m2.K,
its R-value will be 1 / 4 = 0.25 m2.K/W
We must note that so far we have been dealing with European S.I.(metric) units. However, when in the commercial world we speak of R-value, we mean in American units. The Americans use BTUs per hour instead of Watts (a BTU is a British Thermal Unit!), square feet instead of square metres, and degrees Farenheit instead of Centigrade or Kelvin. So their unit for R-value is hr-ft2-°F/Btu.
Now 1 m2.K/W = 5.678 hr-ft2-°F/Btu
So, to convert from the SI R-value to the American R-value, multiply by 5.678.
Let us return to the slab we were previously considering. If we now double the thickness of our slab to 20 cm, the amount of heat entering will halve. This is because the R-value is directly proportional to the thickness of the material. So when we speak of R-value, we are always referring to the R-value of a specific thickness of that material. If we do not mention the thickness, then we mean the R-value for a slab of that material 1 inch thick.
So if we see from a table that the R-value of a brick wall is 0.2, then we should understand that the R-value of a brick wall 1 inch thick is 0.2 hr-ft2-°F/Btu. And we can then calculate that the R-value of a 4 inch thick brick wall is 0.8, and of a 8 inch thick brick wall is 1.6. K-value is also sometimes known as U-value. Also note that the k-value may sometimes be given as a general property of the material, and specified in W/m.K. This means that the value is for a one metre thick slab of material that is one metre square. One should correspondingly reduce this value for a thinner slab
So, to convert from the SI R-value to the American R-value, multiply by 5.678.
Let us return to the slab we were previously considering. If we now double the thickness of our slab to 20 cm, the amount of heat entering will halve. This is because the R-value is directly proportional to the thickness of the material. So when we speak of R-value, we are always referring to the R-value of a specific thickness of that material. If we do not mention the thickness, then we mean the R-value for a slab of that material 1 inch thick.
So if we see from a table that the R-value of a brick wall is 0.2, then we should understand that the R-value of a brick wall 1 inch thick is 0.2 hr-ft2-°F/Btu. And we can then calculate that the R-value of a 4 inch thick brick wall is 0.8, and of a 8 inch thick brick wall is 1.6. K-value is also sometimes known as U-value. Also note that the k-value may sometimes be given as a general property of the material, and specified in W/m.K. This means that the value is for a one metre thick slab of material that is one metre square. One should correspondingly reduce this value for a thinner slab
