What is backer rod? A guide to backer rod.

Posted by Terry Jennings on

What is backer rod? A guide to backer rod.

First we will start with a question we’ve all asked at some point.

What is backer rod?

Before you can apply your chinking or caulking, you have to first apply backer rod. Backer rod is a synthetic foam that is placed behind the chinking in order to ensure that chinking only adheres to the top and bottom of the log joint. Backer rod comes in many different shapes and sizes ranging from 1/4" to 5”. Chinking will eventually free itself from the backer rod and be able to move freely. Backer rod also provides more log protection and insulation. There are many different types of backer rod both for caulking and chinking. The different types of backer rod are:

Grip Strip Backer Rod

Grip strip for chinking and caulking. Guide to backer rod.

Each year more and more people find salvation with grip strip backer rod. Grip strip is a trapezoidal shaped (4 sided) polyethylene foam backer rod that is used for caulking and chinking. Grip strip is different from other round backer rods, as it has flat surfaces.  Most people end up cutting round backer rod to have flat edges, buying grip strip saves this extra step.The flat surfaces on grip strip backer rod brings many benefits to chinking. Here is more information on Grip strip.

The many benefits of Grip Strip Backer Rod are:

  • Compatible with all synthetic chinking and most caulking.
  • Does not require stables, nails or glue on most occasions.
  • Grip strip’s flat surfaces create a better seal and require less chinking.
  • Grip strip is closed cell and does not absorb water.
  • Can be used for exterior or interior surfaces.

How to choose the right grip strip size.

When choosing your grips strip size… choose the size that fits flush with the logs. Grip strip comes in 48” lengths and is available in sizes: 3/4", 1", 1 1/4", 1 1/2", 2", 2 1/2"

Closed Cell Backer Rod

Backer rod for chinking. Closed cell backer rod.  

Closed cell backer rod is a popular option for larger gaps. Closed cell backer rod is a closed cell polyethylene foam that is round and flexible. Closed cell backer rod ensures that your chinking or caulking only adheres to the top and bottom points of the log gap. This allows the chinking to move freely with the logs and provides extra protection. Closed Cell backer rod is great for caulking and smaller spaces. Closed Cell backer rod is a stiffer material and normally is applied using a spray adhesive. For most application cases you will want to use closed cell backer rod as opposed to open-cell backer rod. You can find really good deals on closed cell backer rod here.

The benefits of Closed Cell Backer Rod are:

  • More moisture resistant than open-cell backer rod.
  • Added insulation and log protection.
  • Reduced amount of caulking or chinking needed.
  • Flexible and great for log spaces.

How to choose the right closed-cell backer rod size.

Since closed cell backer rod is stiffer than other methods, when choosing a size, you want to choose the size that will fit snugly into the spaces and leave you with no more than 1/2 inch deep chinking space. Closed cell backer rod comes in long rolls and is great for chinking or caulking smaller spaces. Closed cell backer rod comes in the following sizes: 1/4", 3/8", 1/2", 5/8",3/4",1", 1 1/4",1 1/2", and 2" and various lengths.

Bi-Cellular Backer Rod

Bi-cellular backer rod. What is backer rod? Guide to backer rod.

Bi-cellular backer rod is often the choice for smaller or difficult spaces. Bi-cellular backer rod is water resistant on the outside, but porous on the inside. Bi-cellular backer rod is great because of its flexibility and compressibility. Since bi-cellular backer rod is easily compressed, this makes it great for irregular joints and smaller jobs such as caulking. This is most commonly used for caulking in doors, windows, small wall joints, and other irregular spaces. Here is a great example of bi-cellular backer rod.

The benefits of Bi-Cellular Backer Rod are:

  • More flexible and easier to compress into irregular spaces.
  • Water resistant outer layer.
  • Added insulation and log protection.
  • Great for caulking.
  • Easy to work with.

How to choose the right bi-cellular backer rod size.

Bi-cellular backer rod is softer than other methods. When choosing the size of your bi-cellular backer rod, choose the size that is approximately 25% larger than the joint. You may want to use staples to ensure that backer rod does not pop out. We carry fill rope in the following sizes: 3/8" and 5/8" in 20’ rolls.

Open Cell Backer Rod

Open cell backer rod. Types of backer rod guide

Open cell backer rod should not be used for chinking or anywhere water might settle. What is open cell backer rod? Open cell backer rod is backer rod that is porous on the inside and outside, and very soft. Open cell backer rod is mostly only used on the inside of the cabin where it is dry. Open cell backer rod is also used for smaller joints because it is much more compressible than other backer rods. If you are using open cell backer rod, you should always be careful to avoid any extra moisture exposure as it can contribute to rotting in the logs.

The benefits of Open Cell backer Rod are:

  • More breathable and cures faster.
  • More compressible and great for small difficult joints.
  • Better than using tape on difficult joint locations.
  • Typically does not require glue or staples.

How to choose the right open cell backer rod size.

Open cell backer rod is the most compressible. This means that the best sizing method is to test the different sizes. You might want to start with a size that is 50% larger than the joint.  


If you have questions, feel free to reach out and our experts will be happy to answer your questions. 


  • Thank you for your inquiry! You could use closed cell extruded polystyrene as a backer rod and long as it is a closed cell. This is what needed in the Northeastern sector to repel moisture. The chinking will not adhere to the backer. The backer is mainly used as a filler to help fill the void and also help with insulation. Also Jim you wouldn’t need to place packing tape where chinking meets the backer you could staple in backer or use a product that is called Grip Strip which is shape like a trapezoid foam that fits in the grove snug. You can view this on our website at www.iwoodc.com. . If you have any other questions feel free to call us at I-Wood-Care at 607-674-2591 and we would be happy to help you!



    I-Wood-Care Sherburne, NY 1-800-721-7715 on

  • I was wondering if I could use closed cell extruded polystyrene for a backer rod. I have square timbers with 2 inch gap between timbers. I was hoping I could cut my own out of full sheets? Maybe place packing tape where chinking meets the backer for a no stick surface. Any information would be appreciated. Thanks Jim.

    James Dorf on

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