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.

Outlines

00:00

🏠 Calculating R-Values for Wall Insulation

In this segment, Paul Tercellini introduces the concept of calculating R-values for a simple wall construction as part of the Solar Decathlon Building Science Education series. The R-value, which is the reciprocal of the U-factor, is essential for determining heat transfer through the building envelope. The total R-value of a wall is calculated by considering the thermal resistance of various materials, including concrete, foam board insulation, and stucco, as well as the surface film of air on both the interior and exterior of the wall. The segment explains the importance of finding the R-value for each material and how to account for the thermal resistance of the air film, which is affected by factors like wind and convection. The exterior film coefficient is given as 0.17, while the interior film coefficient is 0.68. The example provided uses a concrete house with foam board insulation and stucco finish, detailing the calculation of the total R-value by adding the R-values of each layer, including the film coefficients.

05:01

🔍 Detailed Analysis of Wall R-Value Components

This part of the script delves deeper into the specific R-values of the materials used in the example wall. The foam insulation has an R-value of 10, while the concrete, with a thickness of four inches, has an R-value of 0.13 per inch, totaling 0.52 for the wall section. The stucco's R-value is assumed to be 0.2 per inch. By adding these individual R-values, the total R-value for the wall is calculated to be 11.47. The summary points out that despite its thickness, the concrete's resistance is minimal due to its high conductivity, and the film coefficients contribute more to the insulation than the concrete itself. The foam insulation plays a dominant role in the wall's insulation value. The segment also discusses strategies to increase insulation, such as adding more foam, and touches on the application of film coefficients to horizontal surfaces like roofs and ceilings. The episode concludes with a reminder that these calculations are models to approximate real-world heat transfer physics in building design.

Keywords

💡Heat Transfer Equation

The heat transfer equation is a fundamental concept in thermodynamics that describes the rate at which heat energy is transferred through a material. In the context of the video, it is used to calculate the thermal performance of a building envelope, which is crucial for understanding energy efficiency in construction. The equation factors in the U-factor and its reciprocal, the R-value, which are essential for determining heat flow through walls and other building materials.

💡R-value

The R-value, or thermal resistance, is a measure of a material's ability to resist heat flow. It is a critical parameter in the design of energy-efficient buildings. The higher the R-value, the better the insulation. In the video, the host demonstrates how to calculate the total R-value for a wall construction, which is a sum of the R-values of its individual components, including concrete, foam board insulation, and stucco.

💡U-factor

The U-factor is the rate at which a material or construction assembly gains or loses heat. It is the reciprocal of the R-value and is used to evaluate the overall thermal performance of a building component. The video emphasizes the importance of understanding both U-factor and R-value to assess heat transfer through building envelopes.

💡Thermal Resistance

Thermal resistance is the property of a material that resists the flow of heat. It is often measured in R-units. In the video, thermal resistance is discussed in relation to various materials used in wall construction, such as concrete, foam board, and stucco. Understanding thermal resistance is key to improving a building's energy efficiency by reducing heat transfer.

💡Foam Board Insulation

Foam board insulation is a type of insulation material made from polystyrene or polyisocyanurate foam. It is known for its high R-value per inch, making it an effective insulator. In the video, the host calculates the R-value for foam board insulation by multiplying the R-value per inch by the thickness of the board, which is a common practice in construction to determine the total insulation value.

💡Stucco

Stucco is a type of plaster used as a wall finish for buildings. It can provide some thermal resistance, although its R-value varies based on the materials and thickness. In the video, the R-value for stucco is given as 0.2 per inch, which is used in the calculation of the total R-value for the wall assembly.

💡Concrete

Concrete is a building material composed of cement, aggregate, and water. It has thermal mass and can provide some insulation, although its R-value is generally lower than that of insulation materials. The video mentions that for a four-inch poured concrete wall, the R-value is 0.13 per inch, which is used in the total R-value calculation for the wall.

💡Surface Film Coefficient

The surface film coefficient refers to the thermal resistance value of the thin layer of air on the surface of a material, which acts as an insulator. In the video, the exterior surface film coefficient is given as 0.17, and the interior surface film coefficient as 0.68. These coefficients are important in calculating the total R-value of a wall because they account for the insulating effect of the stagnant air layer on both sides of the wall.

💡Convection Coefficient

The convection coefficient is a measure of the efficiency of heat transfer by convection at the surface of a material. In the video, it is mentioned in the context of creating a layer of stagnant air on the inside and outside of the wall, which contributes to the wall's thermal resistance. Wind can affect this coefficient by moving the air and reducing the resistance.

💡One-Dimensional Model

A one-dimensional model is a simplification used in heat transfer calculations where heat flow is assumed to occur in a single direction through a material. The video discusses using this model for calculating R-values in walls and ceilings, acknowledging that it is an approximation that may not account for all real-world complexities, such as those found in attics.

Highlights

Introduction to calculating R-values for building envelope heat transfer

The importance of R-value and U-factor in heat transfer through building materials

R-values of insulation materials can be found on product packaging or manufacturer websites

All materials have an insulation value, including those not typically considered as insulation

Example of a concrete house with foam board insulation and stucco finish

Explanation of how to calculate the total R-value of a wall by adding individual material R-values

Inclusion of surface film of air in R-value calculations due to convection coefficient

The impact of wind on reducing thermal resistance on exterior walls

Standard thermal resistance values for exterior and interior surface film coefficients

Variability in R-value for stucco based on materials and application method

R-value of foam board insulation is approximately R5 per inch

R-value for poured concrete is about 0.13 per inch

Total R-value calculation for the example wall results in 11.47

Observation that concrete, despite its thickness, has low resistance due to high conductivity

Film coefficients have a higher resistance than concrete, emphasizing the importance of insulation

Suggestion to increase insulation by adding more foam board to the wall

Introduction of film coefficients for horizontal surfaces like flat roofs

Discussion on the complexity of calculating R-values for unconditioned attics

Emphasis on the use of models to approximate real-world physics of heat transfer in buildings

Conclusion and invitation to future episodes for more detailed calculations and examples

Transcripts

welcome to the solar decathlon building

science education series

i'm paul tercellini and in this episode

i'll be demonstrating how to calculate r

values for a simple wall construction

going back to the heat transfer equation

we see that to calculate the heat

transfer going through the building

envelope we need the

u factor or its reciprocal the r value

we will devote this episode to finding

that r value

as we look at the r value of insulation

we need to figure out how to calculate

the total r value

of a wall the thermal resistance of

various different materials are

tabulated in

many textbooks or professional journals

or on websites

especially manufacturers websites the

easiest one to find

is often the r-value of the insulation

product itself

because they are often written on the

packaging or

on the product but all materials have

some insulation value

and we need to find the r for these

items as well

so let's think about how a wall is built

we'll use this house as our example it's

a concrete house with

foam board insulation and a stucco

finish on the exterior

to calculate the total r value of the

wall we'll zoom in on a specific

section to see how these different

materials come together

looking from the top down we can see

that this wall consists of

four inches of concrete two inches of

foam board insulation

and about a half an inch of stucco on

the exterior

what we don't see is that there is a

tiny

layer of air on the outside and

on the inside of the wall that is caused

by stagnant air

and the convection coefficient

so if you think about the outside being

cold

and the inside being warm and i have a

little bit of

air that's sitting right against the

wall

it's actually going to get warmed by the

wall a little bit

because heat is flowing from inside to

outside

and as that rises it creates this film

of stagnant air

and just beyond that a little bit of

moving air

this thin layer is also called a surface

film of air

another factor that plays a part with

this is wind

and when wind hits that wall it moves

the air

away from it and reduces the amount of

resistance we have

so experimentally it has been determined

that the thermal resistance value on the

outside

for most normal applications is 0.17

square feet degrees fahrenheit hour per

btu

this is called the exterior surface film

coefficient

as a shortcut we can call it the

exterior film coefficient

likewise on the inside of the wall we

have the same thing

except in this case that inside layer

will get

colder than the indoor temperature and

air tends to fall

down and again it creates this kind of

layer along the surface of the wall

and because we don't have the wind

impact on the interior that resistance

is a little bit higher

it's 0.68 and has the same units as the

exterior film coefficient

we'll call this the interior surface

film coefficient or

interior film coefficient for short

so when we really look at the wall from

the outside to the inside we have the

exterior film coefficient the half inch

of stucco

the two inches of foam board the four

inches of concrete

and then we have the interior film

coefficient

and we need to have our values for each

of these layers

so we said that the r value for the

exterior film coefficient was 0.17

and the interior was 0.68 remember this

accounts for a small amount of stagnant

air

on the inside and outside of the wall

and that stagnant air is an insulator

now for stucco the r value varies based

on the exact materials used

the way it's manufactured and how thick

it gets applied to the wall

for this example we'll assume the r

value for our stucco is 0.2 per inch

so 0.1 with

foam board insulation we can look that

up pretty easily

on the back of a piece of foam board

you'll see that it is usually around r10

or sometimes you'll see that it's

written as something like

r5 per inch and so in order to find the

total r value we would take the

r5 and multiply it by the two inch

thickness of the board

and we see that for the r

value the foam is 10.

for the concrete you'll notice if you

look up

in the table there are lots of different

forms and densities of concrete

for a four inch poured concrete wall the

reference where i looked it up said that

the r value

is 0.13 per inch so

0.52 for our four inch wall

and then as we move through layers of

our wall we can add these

individual r values to get the total r

value for the wall

which is 11.47 now

there's a couple of interesting

observations here one of them

is that the resistance of the concrete

is very very small

and even though it's pretty thick that

concrete is highly conductive

the second observation is that the film

coefficients together have a higher

resistance than the concrete

so the foam insulation is dominating the

insulation value

of this wall

we can use this technique of adding up

the r values for any wall that is made

up of

these consistent layers in this case the

foam is continuous across the entire

wall

the concrete and stucco are continuous

and homogeneous and

we have an inside skin coefficient and

an outside skin coefficient

the strategy perhaps to increase the

insulation of this wall would be to add

two more inches of foam so now i have

r20

for the foam and i could repeat our

calculation

so in this episode we introduced the

concept of effective thermal resistance

of the air surface

on the edge of the wall called a film

coefficient

we gave values for the interior air film

coefficient and

the exterior error film coefficient

based on a vertical wall

another application of this concept is

with

horizontal surfaces such as a flat roof

the interior air coefficient is actually

a little bit less

because the air layer is a little

thinner than it is for the wall

and because the air is then falling from

the ceiling down

towards the floor as it cools the

interior air film coefficient is 0.61

for horizontal surface

such as the ceiling and for the exterior

it's 0.17

however unconditioned addicts become

fairly complicated because they

often are impacted by solar gain and the

amount of outside ventilation that we

have

moving through that attic space that

complexity means that

the simple one-dimensional solutions are

difficult to do

our target here is to help you

understand some of the physics and how

it applies to building design we're

going to see that for well-insulated

buildings the ceiling and roof have a

minimal amount of heat transfer

compared to the walls for the time being

we will consider ceilings and attics

with the one-dimensional model and

remember that these

are all models to approximate real world

physics

of heat transfer

and so that concludes our introduction

to calculating our values for a wall but

stay tuned for more episodes on this

topic

we'll look at stud frame wall

construction as another example and

we'll also take a closer look at the

calculations by demonstrating a

spreadsheet approach

to determining the r values let us know

if you have any questions and

as always thanks for watching

you