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What is Flash and Batt Insulation?

flash and batt insulation

What is Flash and Batt Insulation? Blog Feature
Amanda Ringler

By: Amanda Ringler on February 13th, 2018

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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 15 years of experience insulating thousands of homes with foam insulation.

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

What is Flash and Batt Insulation?

Flash and batt is spray foam insulation with fiberglass batts. It’s a technique used by some insulation contractors where a flash of closed cell spray foam at 1- to 2-inches is applied in an effort to create an air seal and then puts fiberglass batt insulation 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 prescriptive code in Michigan for walls in new construction is R-21. The contractor adds the fiberglass just to hit the prescriptive code and in an attempt to cut costs.

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

What is R-Value?

R-Value is the capacity of an insulating material’s resistance to heat flow.

That means the higher the R-Value, the greater the insulating power of the material. While it’s important to understand R-Value, it isn’t always the revered determiner of insulation most people think it is.

Insulation is much more than just a number since heat flows in and out through radiation and convection. Heat loss through convection, or airflow, can account for around 40 percent of total energy loss in your home.

This is an issue if you are only using the prescriptive code for R-Value to choose your insulation.

Pros and Cons of Flash and Batt Insulation

This hybrid method could work if it was installed properly, but many contractors who recommend this install less than the required 2-inches of closed cell to create an air seal.

On the flip side of this, if the contractor applied 2-inches of closed cell, you wouldn’t need the fiberglass to pass code when using the performance method because the spray foam created a total air seal. The prescriptive code focuses on the R-Value needed for the area. For example, R-21 is what is required for Michigan. The performance method helps meet code for insulation materials like foam that creates a total air seal in the home. This means the wall with 2-inches of closed cell or 3-inches of open cell that is less than the prescriptive R-21 will still pass code since it will perform at that level or above.

Here are some of the pros and cons of flash and batt insulation


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


  • If less than the required 2-inches 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, 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

Just like any insulation project, the size of the area to be insulated determines the cost.

The cost to insulate the wall cavities in a 2,000-square-feet home with flash and batt would be between one-third to half that cost of insulating properly with spray foam alone.

Sure, everyone wants to save a few bucks, but you will need to decide if it is worth it considering the potential long-term problems that can arise when an air seal isn’t achieved. It’s also important to note that the amount of spray foam insulation needed for this method passes code alone due to its performance.

Learning More About Foam Insulation

Now that you have read all about flash and batt insulation, you may be deciding it isn’t the best option for your home.

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

Ultimate Foam Insulation Buying Guide for New Build Homes

About Amanda Ringler

Amanda previously has worked as a breaking news and crime reporter, TV news producer, and editor in Flint and Detroit. Throughout her career as a journalist, she has won several awards from The Society of Professional Journalists - Detroit Chapter and the Michigan Press Association. As part of the RetroFoam of Michigan family, Amanda uses her experience as a journalist to write content that will help educate homeowners on the benefits of foam insulation. When Amanda isn’t writing, she’s spending time with her husband and rescued huskies. She also loves knitting, making art, cooking, and hosting dinner and a movie night for friends and family.