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

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.

Satyajeet Vispute

Last Updated: Oct 03, 2018

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.

How to Calculate the R-value of Insulation

The following formula is used to calculate the R-value of an insulation.

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 (Q

**R=∆T/Q**_{A}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 (Q

_{A}).^{2}/W. To convert it to US units of measurement, the following equality is used.

1 K·m

^{2}/W = 5.678263 h·ft2·°F/Btu

Thus, if R

_{SI}and R

_{US}represent the SI and the US 'R' values, then,

R

_{US}= R

_{SI}× 5.678263337

R

_{SI}= R

_{US}× 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=Q**_{A }/∆TFactors 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.

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.

**0.63 - 0.88 K·m**

R-value of fiberglass insulation:R-value of fiberglass insulation:

^{2}/W·in**or**3.6 - 5 h·ft2·°F/Btu·in**0.63 K·m**

R-value of spray foam insulation:R-value of spray foam insulation:

^{2}/W·in**or**3.6 h·ft2·°F/Btu·in**R-value of slag wool insulation:**0.52 - 0.68 K·m

^{2}/W·in

**or**3 - 3.85 h·ft2·°F/Btu·in

**0.52 - 0.67 K·m**

R-value of cellulose insulation:

R-value of cellulose insulation:

^{2}/W·in

**or**3 - 3.8 h·ft2·°F/Btu·in