Building Science Education - 3-7 - Calculating R-Value for a Wall (Part 1)

Summary

In this episode of the Solar Decathlon building science education series, Paul Tercellini explains how to calculate R-values for a simple wall construction. The video covers the heat transfer equation and the importance of the R-value, or thermal resistance, in building envelopes. It demonstrates how to find the total R-value of a wall by adding the R-values of individual materials like concrete, foam board insulation, and stucco, and includes the effects of surface film coefficients for both interior and exterior surfaces. The video also touches on how wind can affect thermal resistance and concludes with insights on improving insulation by adding more foam board.

Takeaways

• 🔍 The video discusses calculating R-values for building materials, specifically for a wall construction.
• 🏡 The example used is a concrete house with foam board insulation and a stucco finish.
• 🌡️ R-values are essential for understanding heat transfer through the building envelope, and they are the reciprocal of the U-factor.
• 📚 R-values for materials can be found in textbooks, professional journals, or on manufacturer websites.
• 🧱 The wall is composed of four inches of concrete, two inches of foam board insulation, and half an inch of stucco.
• 💨 Surface film coefficients account for the thermal resistance of the stagnant air layer on both the interior and exterior of the wall.
• 🌀 Wind affects the exterior surface film coefficient, reducing the resistance by moving air away from the wall.
• 🔢 The total R-value for the wall is calculated by adding the R-values of each layer, including the interior and exterior film coefficients.
• 🏡 The foam insulation contributes the most to the wall's insulation value, with the concrete having a minimal impact despite its thickness.
• 📈 To improve insulation, adding more foam insulation to the wall can increase the overall R-value.
• 📘 The video concludes by emphasizing that these calculations are models to approximate real-world heat transfer physics in buildings.

Q & A

What is the purpose of calculating R-values for a wall construction?

The purpose of calculating R-values for a wall construction is to determine the thermal resistance of the wall, which is essential for understanding and improving the building's energy efficiency and insulation performance.

What is the relationship between the U-factor and R-value in heat transfer?

The U-factor and R-value are reciprocals of each other in heat transfer. The U-factor measures the rate of heat transfer through a material, while the R-value measures the resistance to heat transfer. A higher R-value indicates better insulation and lower heat transfer.

Where can one typically find the R-value of insulation products?

The R-value of insulation products can typically be found on the packaging or on the product itself, as manufacturers often provide this information directly.

What are the factors that affect the R-value of stucco?

The R-value of stucco can vary based on the exact materials used, the manufacturing process, and the thickness to which it is applied to the wall.

What is the exterior surface film coefficient, and what is its typical value?

The exterior surface film coefficient is a measure of the thermal resistance of the thin layer of stagnant air on the outside of the wall. Its typical value is 0.17 square feet degrees Fahrenheit hour per BTU.

How does wind affect the exterior surface film coefficient?

Wind affects the exterior surface film coefficient by moving the air away from the wall, which reduces the amount of resistance and can lower the thermal resistance value.

What is the interior surface film coefficient, and why is it higher than the exterior's?

The interior surface film coefficient is a measure of the thermal resistance of the thin layer of stagnant air on the inside of the wall. It is typically higher than the exterior's because indoor air tends to settle along the wall, and there is no wind impact to reduce the resistance.

How is the total R-value of a wall calculated?

The total R-value of a wall is calculated by adding the individual R-values of all the layers that make up the wall, including the exterior and interior film coefficients, insulation materials, and any other construction materials.

What is the significance of the R-value of concrete in the context of wall construction?

In the context of wall construction, the R-value of concrete is significant because it indicates the material's resistance to heat transfer. Despite its thickness, concrete is highly conductive and thus has a relatively low R-value, meaning it does not provide much insulation.

How can one increase the insulation value of a wall?

One can increase the insulation value of a wall by adding more insulation material, such as increasing the thickness of foam board insulation, which has a higher R-value and thus provides better thermal resistance.

What are the challenges in calculating R-values for horizontal surfaces like roofs?

Calculating R-values for horizontal surfaces like roofs can be challenging because they are often impacted by factors such as solar gain and outside ventilation, which can complicate the one-dimensional heat transfer models typically used for walls.