Expanding Foam Vs. Concrete Fence Post: Which Is Better?

Setting a fence post can be tricky, but it does not have to be. The two most viable options for setting a fence post is using expanding foam or concrete.

Expanding foam is better for setting posts than concrete because it creates a better foundation. Concrete is also solid, but it is harder to apply than expanding foam. The main benefit of expanding foam is that it expands into the ground surrounding and creates a secure base.

The reason that expanding foam is such a great option is because it expands widely filling the surrounding cavities creating a strong seal. Let’s get into the difference between expanding foam and concrete for setting fence posts.

Expanding foam is a product that is meant to expand and harden when it comes into contact with air. Most often found in a canister with a spray nozzle, expanding foam is easy to apply in a variety of situations. Though it is generally used as a form of insulation, it can also be used to support fence posts.

Sika PostFix Fence Post Mix is just one type of expanding foam used for this purpose. It is advertised as a “two component, mix-in-the-bag expanding foam [that supports] non-structural posts such as a fence, mailbox, or sign posts.”

Application of expanding foam is easy, requiring that you just roll it out, mix it, cut, and pour in the hole. Setting in just three minutes, this product is durable and highly resistant to the infiltration of moisture. It also boasts that it will seal and protect the base of your fence posts and can be used in cold climates, without the need for any additives.

Homeowners and builders use expanding foam to set fence posts because of how easy it is to mix and how quickly it sets, usually 5 minutes. Expanding foam is a substance that when mixed and applied will expand and eventually become hard.

It can expand to 15 times more than its original size which makes expanding foam’s looks quite deceiving. There are many forms of expanding foam, but typically it is polyurethane, or latex based.

Several preparations of expanding foam exist, with some that expand widely and others with low expansion. When it comes to setting fence posts, both low and wide expansion expanding foam are effective. Many expanding foams can set within three minutes of application, making it a quick method.

Typically speaking, expanding foam is sold in bags that need to be mixed before applying. It has become extremely common to use expanding foam to set fence posts and mailboxes because of how effective it is.

The key advantage expanding foam has over concrete is within the expansion itself. Because expanding foam expands up to 15 times its size, it goes the extra mile in securing a fence post. The cavity beneath the post gets completely filled by the expanding foam which gives it extra security.

Concrete is used to set fence posts primarily because of its strength and how long lasting it is. Despite expanding foam being a better option for certain reasons, concrete is a great option if you want your posts to be secure.

When you fill a hole with concrete to secure your fence post, you can expect a 20-40 minute wait time before it sets. Ideally, you want the post to have 2 feet of its length in the ground before you secure it with the concrete.

That way, the post will be reinforced by concrete even beneath the surface and be able to withstand high winds. There is some debate as to whether or not the concrete should be mixed before filling a hole for a fence post or not.

For many professionals, however, the clear choice is that you should mix the concrete with water before pouring it into the hole. The alternative would be dumping dry concrete mix into the hole, adding water, and letting the air solidify it.

The reason it is best to mix concrete before pouring is because that way, each layer of the concrete will dry the same and hold the same.

Ultimately, expanding foam is better for setting fence posts than concrete. That is because, with concrete, you can’t always get consistency all the way to the bottom layer even if you mix it well.

With expanding foam, however, it more or less does the work for you. Upon expansion, expanding foam grows and fills up the hole including every nook and cranny within the surrounding soil.

That is in contrast to concrete which can often be solid on top, but rather brittle and inconsistent on the bottom. Even still, concrete is a great choice so long as you mix evenly and thoroughly before you fill the hole, not after.

Here is a metric showing the difference in cost between expanding foam and concrete for equivalent sized holes for the fence post:

Before you can begin the first step, you need to ensure that you have the proper outdoor conditions – warm and dry temperature. You also want to have your hole dug. Refer to the manufacturer’s directions for specifics. However, many will suggest the hole to be eight inches in diameter and one of third of the length of the post should be positioned in the ground.

If your soil allows for it, try to dig two to three feet deep. That way, you will have a few feet of the post in the ground for extra security. Then, make sure that the hole is free of any debris and standing water.

Although the odds of having better results with expanding foam than concrete are higher, there are instances where it may not work. Keep in mind, that when a product fails, it comes down to two possible reasons why: user error or the product simply did not live up to expectations.

Like anything, it’s impossible for everyone to have perfect results with a product. In fact, some online product reviewers have had some pretty undesirable results with using expanding foam to set their fence posts. If this has happened to you, consider the following:

Setting a fence post with concrete is simple, but there are a few variations. No matter what, you need to begin by digging a hole. From there, decide whether or not you want to pre-mix the concrete or mix it in the hole itself.

Pre-mixing the concrete is a safer bet because it will set faster and will be consistent all the way to the bottom. Otherwise, you can pour the concrete mix into the hole, add water, and wait. For the sake of clarity, this instructional will use pre-mixed concrete as an example. It is best to set a fence post with concrete with the help of one other person if possible.

Both concrete and expanding foam are amazing for setting fence posts, but many believe expanding foam to be better. Not only is it slightly easier to work with, but it has the potential to create a more solid foundation for the post, and ultimately the fence.

Its ability to expand in all directions, allows it to adhere to virtually anything in the hole. In fact, expanding foam is just as solid and performs just as well, if not better, than concrete.

Whether you choose to use expanding foam or concrete, it is always good to enlist the help of an extra set of hands. That is especially true when it comes to mixing concrete because it takes some elbow grease, so to speak.

So long as you dig the hole deep enough to allow 2-3 feet of the post to be below the ground, either expanding foam or concrete will do the trick. Remember to never rush the mixing process and to always keep the post secure for several minutes after placing it. Setting fence posts is easy and does not take much effort at all. That is especially true when you are using expanding foam to do so.