You’ve come to the point in building or remodeling your home where you need insulation, and your contractor is recommending flash and batt for your walls.

You feel confident in your insulation knowledge, but you aren’t really sure exactly what flash and batt is or how it works. Don’t worry. We’ll break it down for you in a way that you can understand.

RetroFoam of Michigan has more than 20 years of experience insulating thousands of homes with foam insulation. While we don't offer a combination of spray foam and batt insulation, we understand how this method should work.

In our ongoing efforts to educate homeowners, we have put together this article to explain what flash and batt is, how it works, and its pros and cons.

What is Flash and Batt Insulation?

Flash and batt is spray foam insulation with fiberglass batts laid over the top.

It’s a technique used by some insulation contractors. In this technique, a flash of closed cell spray foam at 1—to 2-inches is applied to create an air seal, and then fiberglass batt insulation is put over it.

It’s typically done in walls.

Why Would a Contractor Recommend Spray Foam and Fiberglass Together?

The theory is that the layer of closed cell spray foam gives you an air seal, and then the fiberglass on top of that adds the R-Value needed to get to the prescriptive code.

With most closed cell spray foam insulations, you need a minimum of 2 inches to create an air seal. There are some manufacturers that require 3 inches for an air seal.

That 2 inches of closed cell is about an R-14 in the walls, and the prescriptive code in Michigan for walls in new construction is R-21. The contractor adds the fiberglass just to hit the prescriptive code in an attempt to cut costs.

Although this method is one way to meet code and cut costs, we will get into problems with this model shortly.

What is R-Value?

You might have heard the term R-Value thrown around a lot when it comes to insulation. Simply put, R-Value measures an insulating material's resistance to heat flow. The higher the R-Value, the better the material is at insulating. Sounds straightforward, right? But here's the kicker: R-Value isn't the only factor you should consider when choosing insulation.

For instance, let's talk about flash and batt insulation. This method combines spray foam and batt insulation to offer a mix of benefits. While the spray foam provides an air barrier, the batt insulation adds extra thermal resistance. This combination can be particularly effective in areas like your attic, but weighing the pros and cons is important.

One of the advantages of flash and batt insulation is that it addresses heat loss through convection (airflow), which can account for about 40 percent of your home's total energy loss. On the flip side, some issues, such as potential gaps, can occur if the insulation is not installed correctly, leading to less effective insulation.

So, while the R-Value is important, remember that insulation is about more than just numbers. Understanding the strengths and weaknesses of options like flash and batt insulation will help you make a more informed decision for your home.

Flash and Batt Insulation Pros and Cons

Flash and batt insulation can be a great hybrid method, but it all hinges on proper installation. Unfortunately, many contractors skimp on the required 2 inches of closed-cell spray foam needed to create an effective air seal. If this step isn't done right, the insulation won't perform as it should.

On the flip side, if your contractor does apply the full 2 inches of closed-cell foam, you might not even need fiberglass to meet code using the performance method. This is because the spray foam alone creates a complete air seal. While the prescriptive code focuses on hitting a specific R-Value—like the R-21 required in Michigan—the performance method allows for flexibility. It lets you pass code requirements by demonstrating that the insulation will perform at or above the required level, even if its R-Value is technically lower.

For example, with 2 inches of closed-cell or 3 inches of open-cell spray foam, you can still meet or exceed the R-21 requirement in Michigan, thanks to the superior performance of the foam in creating an air seal.

So, what are the pros and cons of flash and batt insulation? Let's break it down. On the plus side, it combines the benefits of both spray foam and fiberglass, providing good thermal resistance and an effective air barrier. However, the cons include potential issues with improper installation and the need for careful application to ensure maximum effectiveness.

Flash and Batt Pros

• Flash and batt is less expensive than other insulation options.
• In theory, flash and batt meets prescriptive code for new construction walls.

Flash and Batt Cons

• If less than the required 2- nches of closed cell is installed, an air seal isn’t created, which defeats the whole purpose.
• Fiberglass insulation retains moisture, which can lead to mold and mildew issues in the walls. This will happen if an air seal isn’t created in the wall cavity. The warm air and cold air will meet in the cavity, creating condensation.
• If you create an air seal with 2 inches of closed cell, you don’t need the added fiberglass because the performance method can be used to pass code.
• A new build home shifts and settles, and because closed cell is a rigid foam material, it won’t move with your home. This can lead to the material pulling away from the studs and allowing air into your home.

Flash and Batt Cost

When it comes to insulation, the size of the area you need to cover is a big factor in determining the cost. For a typical 2,000-square-foot home, insulating the wall cavities with flash and batt insulation would cost about one-third to half of what you'd pay for full spray foam insulation.

Saving money is always appealing, but there's a trade-off. You need to consider whether it's worth the potential long-term issues that can occur if the air seal isn't properly achieved. Remember, the spray foam component of flash and batt insulation can pass code requirements on its own because of its performance in creating an effective air barrier.

So, while flash and batt can be a more budget-friendly option upfront, weigh the initial savings against the possible headaches down the road. Ensuring a proper air seal is crucial for your home's energy efficiency and comfort.

Learning More About Foam Insulation

Now that you have read all about this method, you may be deciding the flash and batt insulation problems outweigh the good.

If you’d like to learn more about the benefits of foam insulation on its own, check out the Learning Center on our website.

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Michigan Insulation Code: New Home Insulation Requirements