Wood Frame Exterior Wall R-Value and U-Factor Calculation

What is the R-value and U-factor of a wood frame exterior wall construction assembly under winter conditions?

1/2" cementitious shiplap siding (treat as 5/8" OSB)

Vapor barrier

1.5" expanded polystyrene insulation board

2"x6" framing 24" on center

5/8" drywall

Include interior and exterior air film effects.

Answer:

The R-value of the wood frame exterior wall is approximately 15.79 and the U-factor is approximately 0.0634.

To determine the R-value and U-factor of the wood frame exterior wall, we need to calculate the thermal resistance of each component and consider the air film effects on both the interior and exterior surfaces.

First, let's calculate the thermal resistance of each component:

Cementitious shiplap siding (treated as 5/8" OSB): The R-value of 5/8" OSB is approximately 0.63.

Vapor barrier: Vapor barriers typically have a negligible R-value, so we can assume it to be 0.

Expanded polystyrene insulation board: The R-value of 1.5" expanded polystyrene insulation board is approximately 7.5.

Framing: The R-value of 2"x6" framing is approximately 6.25.

Drywall: The R-value of 5/8" drywall is approximately 0.56.

Next, let's consider the air film effects:

Interior air film: The interior air film has a standard R-value of 0.68.

Exterior air film: The exterior air film has a standard R-value of 0.17.

Now, we can calculate the total R-value:

R-value = R-value of siding + R-value of vapor barrier + R-value of insulation board + R-value of framing + R-value of drywall + R-value of interior air film + R-value of exterior air film

R-value = 0.63 + 0 + 7.5 + 6.25 + 0.56 + 0.68 + 0.17

R-value = 15.79

The U-factor is the reciprocal of the R-value:

U-factor = 1 / R-value

U-factor = 1 / 15.79

U-factor ≈ 0.0634

Therefore, the R-value of the wood frame exterior wall is approximately 15.79 and the U-factor is approximately 0.0634.