What Does the R-value Mean in Context of Home Insulation?

Meaning of greater R-value
R-value is an important parameter in civil engineering, which governs the insulation capabilities of the walls in our houses, offices, and other buildings. We shall explore the implications and importance of R-value.
Fact
Even though one insulator might be twice as thick as another, if both have the same R-value, their insulating capabilities will be the same.
The weather outside can be as brutal as it may be beautiful. The summer sunshine and clear skies will charm you into venturing outdoors, but frolic around the streets for too long and the heat will roast you like a barbeque chicken. Similarly, the first snow of the season is only enticing until the cold sinks in and your bones start to shiver. The longer you remain outside in winter, the more you risk becoming a human Popsicle. So what is there to protect you against all the mad elements of nature? The four walls of your house.

The walls of our houses are the protective barricades that have been erected to check the advancement and infiltration of both, the hot and the cold armies of weather. However, any barricade is only as strong as its core element, which in this case, is the insulation. The better the insulation used in the walls, the better they will shield you against the weather outside. The R-value is what decides the insulating capabilities of a wall.
What Does the R-value Mean in Home Insulation?
Thermal insulation of a roof
The walls of our homes, offices, and other buildings aren't just simple brick and concrete structures. Take a cross-section of any one of them, and you will find that internally they comprise several different layers, each having its own unique purpose. One of these important layers is the insulation.
The insulation layer is added to stop the transfer of heat from one side of the wall to another. It is made up of a material which has very low thermal conductivity. The insulation layer prevents the outside heat from entering our homes during summertime, while during the winters, it stops the heat within from escaping outside.
However, no insulation is perfect, and some amount of heat transfer is but inevitable. Yet, some insulators perform better than others. Therefore, to measure the performance of different insulators, civil engineers measure their R-value.
The 'R' in R-value stands for thermal resistance. It is basically a measure of a solid material's ability to resist the flow of heat through it. The R-value takes into consideration the convective and radiative heat transfer through various solid materials. However, it doesn't account for the radiative and convective properties of a material's surface. The greater the R-value of an insulator, the higher will be the degree of thermal insulation it will provide.
How to Calculate the R-value of Insulation
The following formula is used to calculate the R-value of an insulation.

R=∆T
QA

This formula indicates that the R-value is calculated as the ratio of the temperature difference across an insulator (∆T) to the heat transferred per unit area and per unit time (QA).

The SI unit of R-value is K·m2/W. To convert it to US units of measurement, the following equality is used.

1 K·m2/W = 5.678263 h·ft2·°F/Btu

Thus, if RSI and RUS represent the SI and the US 'R' values, then,

RUS = RSI × 5.678263337

RSI = RUS × 0.1761101838
U-value of Insulation
The R-value is the thermal resistivity of a given solid material. The inverse of resistivity is conductivity, and in case of insulators, the thermal conductivity is given by something known as its U-value. The following is the formula for U-value.

U=1
R

U=QA
∆T
Factors Affecting R-value in Insulation
Every material used for insulating walls has an associated degree of thermal resistivity. A material having higher thermal resistivity will naturally be more effective. However, in practical applications, many more factors come into play, which can affect the R-value.
R-value and Thickness
The thicker the insulation in a wall is, the more difficult it will be for heat to penetrate through it. Therefore, thick insulators can increase its R-value, making it more effective at insulation heat from being transferred through it.
R-value and Wall Type
The type of wall is an important deciding factor on which the effectiveness of an insulation depends. Even an insulation having a high R-value won't be able to perform when used in certain types of walls. For instance, in walls having large openings, such as windows, doors, etc., the heat is able to travel through these open areas. As such, even if an insulating material having a very high R-value is used, it won't be able check the heat transfer. Also, nearly 12% of a house is made out of wood (frames, studs, etc.), which typically has a low R-value. Hence, the overall R-value of nearly every wall is lowered by this.
R-value and the presence of Gaps
When walls are constructed, proper care is taken to close up all the holes or gaps that may be present. However, the fact remains that, even in such solidly constructed walls, minute gaps tend to be present. Through them, heat gets transferred unchecked, and therefore, the efficiency of even highly rated insulators in such walls is reduced.
R-value and Environmental Effects
Manufacturers of insulation calculate the R-value in ideal laboratory conditions. However, practically, the walls in which these insulators are used are exposed to all different elements, such as rain, humidity, air flow, etc. These can cause the rated R-value of the insulation required in the walls to be higher than the effective R-value.
R-values of Typical Home Insulators
The following are the R-values of typical insulators that are used in homes.

R-value of fiberglass insulation: 0.63 - 0.88 K·m2/W·in or 3.6 - 5 h·ft2·°F/Btu·in

R-value of spray foam insulation: 0.63 K·m2/W·in or 3.6 h·ft2·°F/Btu·in

R-value of slag wool insulation: 0.52 - 0.68 K·m2/W·in or 3 - 3.85 h·ft2·°F/Btu·in

R-value of cellulose insulation: 0.52 - 0.67 K·m2/W·in or 3 - 3.8 h·ft2·°F/Btu·in
Thus, the R-value is an important parameter that can help in determining the effectiveness of an insulation material in resisting the transfer of heat through it. It is important to note that, though the R-value gives you a fair idea about the insulation's capabilities, and is sufficient for most practical considerations, there are several other important parameters which must be taken into account when a high degree of insulation is needed.
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