Rvalue is a measure of thermal insulation in a house. Each of your walls, ceilings, and floors has a certain amount of insulation and a corresponding Rvalue. The higher the Rvalue, the better the insulation, and the less heat is lost through that surface. In this post, I will explain 2 ways to calculate Rvalue for a given building assembly.
Calculate theoretical Rvalue from wall structure
If you know how the wall or ceiling is constructed, you can calculate its Rvalue from its component material Rvalues. You can look up the Rvalues of common building materials on the Internet, and the Rvalues add when they are layered in the direction of heat flow. For example, rigid foam insulation is R10, and fiberglass batt is R13. Putting the foam and then fiberglass gives a total of R23. Keep in mind that wall studs have lower R value than fiberglass batt, so those areas have a lower Rvalue. You can get an average Rvalue of a mixed surface by multiplying their respective Uvalue by the percentage of their area, then sum them. Uvalue is the inverse of Rvalue. A material with Rvalue of 10 has a Uvalue of 1/10 = 0.1. For example, a 2x4 studs area has R4, and occupies 10% of the area; the fiberglass batt has R14, and occupies 90% of the area. You can calculate the overall Uvalue as 10%*1/4+90%*1/14=0.0893. Convert back to Rvalue gives 1/0.0893=11.2. Note that the typical fiberglass batt filled walls don’t achieve the R value rated on the fiberglass batt because of the higher thermal conductivity of wood studs; this is also referred to as thermal bridging.
Calculate Rvalue from temperature measurement
Instead of calculating the theoretical, optimal Rvalue of a given wall, you can calculate the actual, empirical Rvalue by measuring some temperatures. This can be done with a handheld infrared thermometer. The best way to use an infrared thermometer is to hold it as close to the surface as possible, and avoid shiny surfaces.
Here's a table that will give you an estimate of wall Rvalues based on outside temperature and the temperature of inside surface of an exterior wall:
Estimated RValue
 
1

2

5

10

15

20

25

30

35

40
 
Outside Temp (F)

40

20.4

10.2

4.1

2.0

1.4

1.0

0.8

0.7

0.6

0.5

30

27.2

13.6

5.4

2.7

1.8

1.4

1.1

0.9

0.8

0.7
 
20

34.0

17.0

6.8

3.4

2.3

1.7

1.4

1.1

1.0

0.9
 
10

40.8

20.4

8.2

4.1

2.7

2.0

1.6

1.4

1.2

1.0
 
0

47.6

23.8

9.5

4.8

3.2

2.4

1.9

1.6

1.4

1.2
 
10

54.4

27.2

10.9

5.4

3.6

2.7

2.2

1.8

1.6

1.4
 
20

61.2

30.6

12.2

6.1

4.1

3.1

2.4

2.0

1.7

1.5
 
30

68.0

34.0

13.6

6.8

4.5

3.4

2.7

2.3

1.9

1.7
 
40

74.8

37.4

15.0

7.5

5.0

3.7

3.0

2.5

2.1

1.9
 
Interior wall T  exterior wall T

This table assumes your interior wall temperature is 70°F, but you can still use it even if your indoor temperature is a little different; the Rvalue will be less accurate if indoor temperature deviates from 70°F. To use the table, calculate the temperature difference between interior wall temperature and the inside temperature of the exterior wall that you want to know the Rvalue of. Look at the first column and choose the row that corresponds to the outside air temperature, look for the temperature difference closest to your measurement, then look up on top row for the estimated wall Rvalue.
Now, if you want to be more precise, calculate the Rvalue directly with the formula R=(T_{h}T_{c})/(T_{a}T_{h})*0.68+0.68, where T_{h} is the interior temperature of an exterior wall, T_{c} is the outside air temperature, and T_{a} is the indoor temperature. The indoor temperature can be measured on an interior wall or door or an object that should be in thermal equilibrium with indoor air. Outside air temperature can be measured on an outside object that’s in thermal equilibrium with outside air, such as a trash can or a deck. When using an infrared thermometer, avoid using it in daylight or measuring shiny objects. Also avoid measuring objects on or near the ground because the ground is often at a different temperature than air.
Note that all this discussion assumes no air leakage, no convection, no radiation, and no condensation. Only thermal conduction is considered here. Typically, air leakage or significant air movement would dominate heat loss so much that conduction becomes meaningless and usually the best thing to do is to eliminate air movement first.
Appendix
I will try to explain how the Rvalue formula is derived. First you need to understand the definitions of Rvalue and Uvalue. Rvalue is the thermal resistance of a material. Uvalue is the thermal conductivity, which is the inverse of Rvalue. 1/Rvalue = Uvalue. For example, an R5 wall has a Uvalue of 1/5=0.2. Uvalue has the unit of BTU per hour per degree F per square foot (BTU/hr/F/sq. ft.). To calculate heat transfer through an R5 wall with 70°F on one side and 60°F on the other side, just multiply its Uvalue by the temperature difference: 0.2*10 = 2 BTU/hr/sq.ft. To calculated the heat loss through a wall 8 feet high and 10 feet long, multiply by the area: 2*8*10 = 160 BTU/hr.
The model for calculating the Rvalue of a wall is that heat is moving from indoor (T_{a}) to inside surface of the exterior wall (T_{h}) through a layer of air film with Rvalue of 0.68, and the same amount of heat is moving from inside surface of exterior wall (T_{h}) to outside (T_{c}) through the wall with Rvalue of wall being the unknown variable. The equation is U_{air}*(T_{a}T_{h}) = U_{wall}*(T_{h}T_{c}). Solve for Rwall by rearranging the equation. In the above formula, you add another 0.68 at the end because the overall Rvalue of the wall includes an interior air film with Rvalue of 0.68. You can leave it out if you want just the Rvalue of the wall material itself.