Board Foot Calculator For Spray Foam Insulation

If you’re planning to install spray foam insulation yourself, you have to calculate board feet. If you don’t know what a board foot is or aren’t sure how to calculate board feet for your project, you’ve come to the right place.

South Central Services has insulated hundreds of homes in and around Greencastle, PA with spray foam insulation. While we do not recommend DIY spray foam insulation, you may have your heart set on it. In that case, understanding board feet and how deep your insulation needs to be is in your best interest.

By the end of this article, you will know:

• What a board foot is
• Which variables influence board feet
• How to calculate board feet in our climate zone

Don't have time to read right now? Check out everything you need to know at a glance.

First, What Is A Board Foot?

If you already have a solid understanding of board feet, you can skip this section. But if you don’t know what board feet are exactly, only that you need to calculate them, then take a moment to learn about them.

A board foot is like a three-dimensional measurement of a square foot. Square feet only tell you the height and width of a surface, which is two-dimensional. Board feet add the dimension of depth.

Board feet make calculating the cost of spray foam insulation so much simpler. Without board feet, every type of project would have a different cost per square foot. Instead, contractors can give their pricing by the board foot.

So, what does a board foot realistically mean?

Let’s say you want to install spray foam on your attic floor. If your attic floor is 100 square feet, then 1 inch of spray foam would be 100 board feet.

However, an inch of spray foam won’t be enough for your attic floor. If your attic floor really needs 3 inches of spray foam, then 100 square feet is actually 300 board feet.

What Determines Board Feet?

How are you supposed to know how many board feet you need? There are three variables that can change the board feet necessary to meet building codes and provide effective performance.

These three variables are project type, foam type, and climate. Let’s examine each of them in more detail.

1. Project Type Influences Board Feet

Different areas of a home need different depths of insulation. Spray foaming your attic requires more inches of insulation than spray foaming your basement. In our climate zone, these are the depths of insulation needed for standard retrofit spray foam projects.

As you can see, the depth of insulation needed varies by project type. An exterior wall needs a maximum of 4 inches of closed cell spray foam, while a roof can require up to 6 inches.

2. Foam Type Influences Board Feet

As you may have noticed from the chart above, there are two different types of spray foam insulation.

Most DIY spray foam kits are for closed cell spray foam. However, be careful to read thoroughly about the DIY kit you are researching.

If you were to purchase an open cell DIY kit for your basement or crawl space, you would not be able to use that foam. Open cell spray foam and closed cell spray foam have different properties such as yield and vapor barrier potential.

Open cell spray foam is a less dense foam, meaning more inches must be installed. On the other hand, closed cell spray foam is a dense foam. Closed cell often needs only 2 inches of product to create an airtight seal and provide a vapor barrier.

3. Climate Influences Board Feet

You may have also noticed that the chart above is specific to Climate Zone 5.

South Central Services is located in Greencastle, Pennsylvania. We service areas of southern PA, as well as Maryland, Virginia, and West Virginia. What all four of those states have in common is their climate zone.

If we were to service anywhere outside Climate Zone 5, the board feet for our projects would change. While our rates would stay the same, an attic in a different climate zone may need more or less spray foam installed. Climate zone also influences which type of spray foam is best.

Pennsylvania has distinct seasons. In summer and winter, there is a greater disparity between the temperature outside and inside. For some states in other climate zones, the weather is consistent year-round.

The humidity of the climate will also influence the type of foam. More humid climates have to be more concerned with vapor barriers. In contrast, dry climates may not have to worry as much about vapor condensation.

If you live outside of Climate Zone 5 and are determined to install spray foam on your own, thoroughly research spray foam depths and types for your climate zone.

How To Calculate Board Feet In Climate Zone 5

For readers in Climate Zone 5, the following examples will be accurate for your project. We will break down how to calculate board feet for attics, basements, and exterior walls.

Board Foot Calculator For Attic Jobs In Climate Zone 5

In Climate Zone 5, attics can be insulated with open or closed cell spray foam.

• Open cell spray foam in an attic: 7 to 12 inches
• Closed cell spray foam in an attic: 3 to 6 inches

To calculate the board feet for your attic, choose which type of foam you will install. If you change your foam type later on, your calculations will not be accurate.

Next, decide if your attic will be vented or unvented. Unvented attics insulate more surface area, making them more expensive.

After you choose between a vented or unvented attic, you’ll find the square footage of your attic.

For a vented attic, you’ll find the square footage of the floor. For an unvented attic, you’ll calculate the square footage of the ceiling, walls, and gables.

After you’ve totaled your square footage, you’ll multiply the square footage by the number of inches needed for spray foam.

We recommend making this calculation using the average or highest number of inches needed for spray foam. This is both a consideration of code requirements and yield. DIY spray foam kits do not deliver perfect yield, so you will get fewer board feet from your DIY kit than is advertised.

How much would it cost to have a professional insulate your attic? Estimate the price of professional spray foam to compare to your DIY attic costs.

Board Foot Calculator For Basement Jobs In Climate Zone 5

In Climate Zone 5, basement walls are insulated with closed cell and basement ceilings are insulated with open cell.

• Open cell spray foam in a basement ceiling: 5 to 10 inches
• Closed cell spray foam in a basement wall: 2 to 3 inches

To calculate the board feet for your basement spray foam, find the square footage of your basement walls or ceiling.

After you find your square footage, you’ll multiply that number by the inches needed.

For example, basement walls may total to 500 square feet. 3 inches of closed cell means 1,500 board feet.

A basement ceiling may be 250 square feet. But with 10 inches of open cell spray foam, the basement ceiling is 2,500 board feet.

How much would it cost to have a professional insulate your basement? Estimate the price of professional spray foam to compare to your DIY basement costs.

Board Foot Calculator For Exterior Walls In Climate Zone 5

As long as the walls are above-grade, either open cell or closed cell spray foam can be used in exterior walls.

If the walls are below-grade, closed cell spray foam is the only option.

• Open cell spray foam in an above-grade exterior wall: 3.5 to 5.5 inches
• Closed cell spray foam in any exterior wall: 2 to 4 inches

Before you find the square footage of your walls, you first need to determine if your walls are above- or below-grade. This detail will determine if open cell spray foam is an option.

Next, you’ll find the square footage of your walls and multiply that square footage by the inches of spray foam needed.

100 square feet of above-grade exterior wall is 550 board feet with open cell or 400 board feet of closed cell.

Would it be less expensive to hire a professional to insulate your walls? Calculate the cost of professional spray foam and compare it to the price of your DIY spray foam kits.

The Bottom Line About Calculating Board Feet For Spray Foam

As a homeowner, accurately calculating board feet for spray foam can be difficult. The climate you live in, which type of spray foam you’re using, and where the foam is being installed all complicate board feet.

To be successful with a DIY spray foam kit will require additional outside purchases and thorough research. Now that you understand how board feet are calculated, explore building codes in your area to ensure you install enough foam for your project.

If you are interested in DIY spray foam, you need to read these next.

• Discover if DIY spray foam will save you money
• Compare DIY spray foam to professionally-installed foam
• Find out which insulations can be safely installed DIY