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# A crash course in designing an energy-efficient building

Updated: Dec 12, 2019

Energy efficiency is one of the most important aspects of green building design. The following are some basic terms that one needs to comprehend before going any further.

k VALUE / κ VALUE = THERMAL CONDUCTIVITY is the amount of heat which will go across a meter thickness of a square meter of material if the temperature difference between the two ends is 10 Celsius (W/m2 0C). It is a material-specific property at a specified temperature. This property in our context is used to characterize the thermal insulation property of the building materials.

U VALUE/ λ = THERMAL CONDUCTIVITY, but for an assembly of various materials in series. In this context it defines the Thermal conductivity of a roof or wall assembly and thus indicates how easily heat can enter or exit the building (Heat or Cold, respectively).

R VALUE = THERMAL RESISTANCE of a Building wall or Roof assembly. It is calculated as the inverse of U i.e. 1/U = R1+R2+…+Rn =1/ λ1+ 1/λ2+…+1/λn. This is just the reverse of U value and characterizes how the wall or roof assembly of a building resists the influx of Heat or Cold.

µ VALUE = WATER VAPOUR RESISTANCE FACTOR is a property of the building material indicating its resistance to passage of moisture in comparison to air, with µ.d being derived by multiplying µ with material thickness that indicates the equivalent air gap in terms of moisture movement

Degree Days = Product of the temperature Difference (Avg) during a day between ambient and inside temperature. This can be downloaded for most Indian stations from http://www.degreedays.net and then totaled to get the degree days for a year

Dew Point = The temperature at which moisture in ambient air gets saturated and starts condensing out of the air. This is a very important parameter in HVAC / insulation design.

If we can protect our green building against these four routes of heat influx by creating a “moat” (as was done in old castles) we would have completed the thermal engineering aspect of the green building design. The lesser energy that this moat creation takes from cradle to grave, the lesser harm it causes to the environment, and the greener it is.

It may be noted that we have treated convection and air infiltration separately as convection currents can be there even without air infiltration. For example, in an improperly insulated room with a lot of thermal bridging, especially, a draft can be felt even without infiltration because of circulation of cold air

Another important aspect to be taken care of is “Thermal Bridges”, that are areas of significantly higher thermal conductivity (k value) within the insulation layer. In this thermographic image you can see a higher temperature image of the slab beam (Concrete with steel) which can be a big source of heat ingress into your air-conditioned interiors. The image on the right-side is of a roof with insulation sheets laid, and it shows the joining edges of sheets lit up in orange, indicating the thermal bridging of the sheet joints. The formation of these thermal bridges can be avoided by using moulded insulation tiles such as Reliable Kooltiles, or by properly arranging insulation layers such as we do in the case of Koolsheet application .

Once having taken care of the moat (insulation+ design) in a sustainable fashion and avoiding the thermal bridges through design intelligence, we have a satisfactory, sustainable green building design complete.

To make an assessment of the energy we are likely to save or the total HVAC load that the building envelope is likely to contribute to, we would need to assess the U factor of the wall and roof assembly. For this you may use the “U Value Calculator” designed by Reliable Building Solutions, which shall appear as under:

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