Expanding Foam vs. Concrete for Fence Posts

Expanding Foam vs. Concrete for Fence Posts

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If you are building a new fence, then something that you are going to need to do is to set posts in the ground. Of course, if you want the fence to be as secure as possible, using real fence posts that go two or even three feet down into the ground is something that you should definitely consider.

With that being said, there are various methods of setting these. Two of the most popular methods include using expanding foam and using concrete. What we are here to do today is to figure out which of these two things is better for the job.

Concrete and Expanding Foam: The Basics

Before we start looking at the differences between expanding foam and concrete in terms of setting fence posts, let’s first figure out what both of these materials actually are.

What Is Concrete?

Concrete Concrete is a special type of building material that is composed of various substances including coarse aggregate (small rocks), fine aggregate (sand), chemical additives, and cement (cement is made mostly of lime and volcanic ash).

These dry ingredients, so to speak, are then mixed at specific ratios with water, which then hardens over the course of several hours or days. It is the inclusion of water that causes concrete to harden, due to the fact that water causes the cement to form crystals that bond with each other, as well as the various aggregates in the concrete mixture.

Concrete hardens into an extremely durable material that is ideal for a number of weight-bearing applications including the building of columns, beams, foundations, driveways, sidewalks, slabs, and other such things.

What is Expanding Foam?

Expanding Foam for Fence Posts As the name of it implies, expanding foam expands and hardens when it comes into contact with air. Most types of expanding foam come in a canister with a spray nozzle for easy dispensing. Some may require you to mix them on your own before use. Generally speaking, the stuff that you would use for setting fence posts is a two-component type that you mix yourself, as the spray-on variety might not be strong enough to hold up fence posts.

Remember that expanding foam is generally used to insulate homes from the cold, not for weight-bearing applications, although it can definitely be used for some weight-bearing applications, such as for fence post setting, as long as there is not an overly excessive amount of weight involved.

In case you are wondering how expanding foam works, it generally contains components made of polyurethane, and when this comes into contact with the air, it causes that liquid to expand and then quickly harden.

Similarities of Using Concrete and Expanding Foam for Setting Fence Posts

What you are going to quickly realize here is that there really aren’t any similarities between expanding foam and concrete. Sure, there are a couple of minor similarities, but in the grand scheme of things, they’re completely negligible when compared to the vast differences between them.

Anyways, let’s quickly go over them.

#1: (Generally) You Will Need to Mix Both

Although this is not always the case 100% of the time, generally speaking, you will need to mix both of these products before using them. The mixing process is of course not overly difficult but it still needs to be done properly. With that being said, you may also choose to use expanding foam that sprays out of a canister rather than a product that you need to mix yourself.

#2: Both Are DIY Friendly

If you like working with your hands and you have the ability to follow basic instructions, then setting fence posts with either expanding foam or concrete is something that you can do yourself. When it comes down to it, there is no reason to hire an expensive contractor to do something like this for you when you can do it on your own for much cheaper.

#3: Both Can Hold Up Fence Posts

OK, so we do realize that this is quite a vague similarity, but the fact of the matter is that both expanding foam and concrete are more than strong enough to hold up a fence post. However, with that being said, one is not quite as strong as the other, and moreover, this is where the similarities end. Other than the basic similarities that we have discussed so far, expanding foam and concrete are very different from each other.

Differences Between Using Expanding Foam and Concrete for Fence Posts

Let’s now jump into the differences. After all, these will be the factors swaying you in one direction or the other.

#1: Application Process

Bear with us here because there are different application processes for both concrete and expanding foam fence post setting.

First off, when it comes to concrete, you can either choose to mix your concrete in a wheelbarrow or any other such container and then pour it into the hole or you can also choose to pour the dry concrete mix into the hole and then mix it inside of the hole.

When it comes to expanding foam, there is of course a variety that you mix inside of the bag and then pour into the hole and there is also the variety that you spray out of a canister. If you are going for the easiest possible route, then using expanding foam out of the spray canister is definitely the most user-friendly option. All you have to do is point and shoot.

#2: Strength and Weight-Bearing Capabilities

Now, as mentioned above, expanding foam is not something that is usually used for weight-bearing applications. With that being said, it can bear a bit of weight, more than enough to support a fence post, however, it just can’t bear nearly as much weight as concrete. Concrete is of course designed specifically for weight-bearing applications, such as for the building of foundations, beams, and other such things.

When it comes down to it, concrete is much stronger than expanding foam. If you have a very heavy fence post, it might cause the expanding foam to warp or crack over time.

#3: Wind and Impact Resistance

Another big difference between expanding foam and concrete is that concrete is also a lot more impact and wind-resistant. If you drive into a fence post with your tractor and it has been set with concrete, there’s a chance that it might not get knocked down. However, if you drive into a fence post that has been set with expanding foam, chances are nearly 100% that it is going to get knocked over. This same principle applies to high winds, such as hurricane-force gusts.

When it comes to weight-bearing capabilities, impact resistance, and other such strength-related factors, concrete is definitely the material of choice.

#4: Expansion and Security

Another difference between these two materials is the fact that expanding foam does of course expand sometimes up to 10 or even 15 times its original size, whereas concrete does not expand. The issue here with concrete is that it really won’t fill in all of the small nooks and crannies, and therefore it might not be 100% secure.

Over time, a fencepost set with concrete may start to wobble for this reason. Concrete is also quite heavy, so it can compact the ground below, thus causing the fence post to sink.

This is not a problem that you will experience with expanding foam because it fills in all of the nooks and crannies, and therefore provides your fence posts with an extremely solid base that won’t wobble.

#5: Consistency and Quality

Another difference that you need to consider here is that expanding foam, if you are using the variety that you have to mix, is much easier to mix in a very consistent manner, whereas concrete can be hard to mix consistently. When concrete has inconsistencies in it, it will suffer from a variety of problems, mainly cracking and fragility in those areas that have not been mixed properly.

With all of that being said, this really has more to do with user error than it does with the quality of the product because if you mix concrete properly, you really won’t have this problem.

#6: Moisture Resistance

Although concrete is of course extremely water-resistant, it is not totally waterproof. This is of course a problem when it comes to something that sits in the ground and often sees rainfall. Now, although not all types of expanding foam are totally waterproof, there are definitely those that are.

If you are looking to protect the bottom of your fence post from water damage, then expanding foam is probably the better option to go with. Concrete is waterproof as long as there are no cracks in it, but eventually, the surface of it can absorb water, especially if it has not been sealed.

#7: Weight

A huge convenience about expanding foam is of course that it expands, which means that the original mixture is not very heavy, and there’s not much of it either. On the other hand, concrete is of course super heavy, and this can make the fence setting process a bit difficult. You really need some muscle to work with large volumes of concrete.

#8: Cost

Although the difference here is negligible, expanding foam is going to cost you a little bit more than concrete.

Expanding Foam vs. Concrete for Fence Posts: Which Should You Use?

When it comes down to it, although concrete is stronger due to the fact that it is also much heavier, and because it’s not as water resistant as expanding foam, it’s generally not the better option to go with here. Expanding foam expands to fill in all of those nooks and crannies, it creates a secure base, and it even helps protect the bottom of the fence post from damage. Moreover, expanding foam is also just easier to use.


Both concrete and expanding foam are fine options to go with for fence post setting. There are of course differences between the two, but as long as you are aware of all of the most important ones, you can make an informed decision.

How Does Concrete Compare with Other Materials?

Below, you can find articles that compare concrete with: