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

Upgraded Home Team
by Upgraded Home Team
Thinking about setting your fence and looking for the best option to do so? Choosing between expanding foam and concrete can be tricky, but you cant go wrong with either. Follow along as we compare concrete and expanding foam and see which is the sturdiest option.

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.

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What is Expanding Foam?

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.

Benefits of Expanding Foam

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.

Benefits of Concrete

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.

Which One is Better?

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:

Equivalent FactorsExpanding FoamConcrete
Volume32 oz2x 50lb concrete mix
Cost$17.60$5-$6 x 2
Dimensions of hole8” x 24”8” x 24”
Post Size4 “x 4”4” x 4”

How to Set a Fence Post With Expanding Foam

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.

  • Stick the fence post in the hole
  • Mix the expanding foam by giving the bag a good shake. Keep in mind that you have a very short window to do this (between 20 and 30 seconds) since the product dries very fast.
  • Pour the expanding foam into the hole evenly
  • Apply light pressure to the fence post and keep it in place for between 3 and 5 minutes. Keep a close eye on the post over the next several minutes, ensuring that it stays completely level and does not shift. For best results, consider using a post level.
  • Let go of the fence post and check to see if it is secure.

Possible Problems

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:

  • Was the hole that you dug deep enough? If your fence posts aren’t strongly fixed in the ground, it’s possible that the hole you dug wasn’t deep enough and didn’t allow for the foam to penetrate as far.
  • Maybe the soil surrounding the hole was too moist? Most expanding foam products recommend to remove any standing water from the hole, along with loose soil. Though it does mention much about wet soil, it’s possible that this might be the reason for undesirable results.
  • Air pockets around the post? If the resin isn’t able to adhere in one spot around your fence post, resulting in air pockets, this can greatly contribute to a lack of stability.

It’s important to note that not all outdoor conditions are created equal. While the expanding foam product may work wonders for someone, it could completely fail for someone else. Fortunately, if the expanding foam doesn’t work, it can be easily removed by using a reciprocating saw.

Tips for Best Results with Expanding Foam

Follow these tips to ensure that you have success with using expanding foam for your fence building or repair project:

  • Consult your local building code to find out the frost depth in your area. By using this, you’ll be able to determine how deep your holes should be.
  • Carefully read the directions and data sheet found with the particular expanding foam product that you purchased.
  • Understand the product temperature. For most, the expanding foam must be between 18 and 25 degrees Celsius for at least two hours prior to application. Keep in mind that the average temperature of a garage floor is roughly 14 degree Celsius. Therefore, if your product is stored on the ground, it will be the same temperature. Just four degrees can make a major difference in the success of the expanding foam product.
  • Make sure that you have no standing water in your holes. This cannot be overstated. If you do have standing water, backfill the hole partially with either gravel or soil until the water is no longer visible. Or, simply pump the water out by other means.
  • Strictly adhere to the 20-second mixing time. This is a very short window, allowing little time for error.
  • Do not move the posts during the first 10 seconds after the initial pour. For best results, use a bracing device to keep the post stable inside of the hole as the compound is curing.
  • Be fully prepared to work quickly. Have your safety gear on (gloves and goggles), fence post already placed in the hole, all of your tools nearby, and avoid any possible distractions. Since the product works incredibly fast, you also need to work fast.

So long as you follow the installation steps carefully and listen to the tips outlined above, you can expect great results with expanding foam.

How to Set a Fence Post With Concrete

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.

  • Mix the concrete in the bag or in a tub by adding water and stirring well
  • Fill the hole with 1-2” of gravel at the very bottom for stability and smooth it out
  • Set the fence post inside of the hole
  • Pour the concrete mix into the hole surrounding the post as evenly as possible
  • Use balancing braces to hold the post in place for 5-10 minutes
  • Remove the balancing brace and allow 4 hours of dry time before putting the actual fence itself up

Step 2 calls for adding gravel to the hole. Not all homeowners do that step, however, it is beneficial because it provides an extra layer of surface beneath the concrete more stable than the dirt.

What Did We Learn?

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.

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Related Questions

How long does expanding foam take to dry?

It takes an average of 16 hours for expanding foam to dry, but it is ideal to wait for 24 hours. Use a mist of water or humidity to help make your expanding foam cure more quickly.

How long does it take concrete to dry?

It takes 48 hours for concrete to dry, and it can take 28 days for it to cure. Fresh concrete can withstand rain within if you poured it at least 3 hours before.

How do I stop my fence post from heaving?

Dig a bell-shaped hole around the fence post and fill it with concrete or expanding foam. Expanding foam can prevent heaving effectively because it expands into the surrounding ground and holds more securely than concrete.

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Upgraded Home Team
Upgraded Home Team

We are a team of passionate homeowners, home improvement pros, and DIY enthusiasts who enjoy sharing home improvement, housekeeping, decorating, and more with other homeowners! Whether you're looking for a step-by-step guide on fixing an appliance or the cost of installing a fence, we've here to help.

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