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R-value quantifies the thermal resistance i.e. the amount of heat transfer retarded by a material. It is measured as a ratio of difference in temperature across the insulator and the heat transferred per unit area per unit time. Heat transfer around a house takes place in four ways – conduction, convection, radiation and air infiltration. Factors like humidity, wind and temperature changes affect a change in the R-value of a material. These variables create a difference in the air pressure between the exteriors and interiors of the building.
Measurement of R-value:
R-value can be measured based on the direction of the heat transfer i.e. upward or downward. up Rvalue refers to resistance to upward heat transfer (also known as winter R-value) and down R-value refers to resistance to downward heat transfer (also called summer R-value). Natural vented houses have high down values and low up values for the roofs and ceiling during hot, humid weather.
Mathematically it is denoted by the equation R = Difference in temperature (∆T) X Area X Time / Heat Transfer
Two ways in which R-values are stated:
a) System R-value: The insulation value of air spaces and material in accordance with the insulation material at the time of installation during the construction of a building.
b) Product R –value: The R-value of the individual product.
Role of Surface Temperature
R-values of an insulation material are measured in a laboratory. The lab tests are required to analyze the properties of the material’s level of resistance to heat transfer. The difference in surface temperature depends on the mode of heat transfer. In the absence of convection/radiation, the insulation material’s surface temperature should be proportionate to the air temperature. Light, metallic and reflective surfaces which are exposed to radiation help in decreasing the temperature compared to dark, nonmetallic surfaces. It is an important point to consider while construction of buildings as heat transfer occurs in different proportions. In hot and humid season, solar radiation acts as a major source of heat transfer as a result the radiative heat transfer creates warm surfaces. During winters and cooler times, heat transfer takes place by conduction and convection. At low temperatures, fibrous insulation and cellulose insulation play a major part in the construction of buildings.
Spray foam insulation R – Value
R-value is the insulation material’s ability to reduce the rate of heat transfer. Heat loss in an uninsulated aerated space occurs due to changes in the density of air in accordance with the temperature.
R-value depends on the ability of a solid material to resist conductive heat transfer. In porous materials, R-value determines the material’s ability to resist heat transfer caused by radiation and convection.
Insulation thus reduces this process of natural convection and helps in heat transfer conduction. Porous insulations achieve this in a great manner along with minor transfer of radiation.
The material having higher R-value will act as an effective insulation product. Spray foam having R-value of 1.8 – 2 pounds is considered to be the best insulation for homes and other buildings.
Spray foam insulation blocks all the forms of heat transfer – conduction, radiation and convection.