Expanding foam is a versatile product, first developed in the 1930s for the aviation industry, and it is safe to say that it has come a long way since then.

Not only is it used for gap-filling and insulation purposes, but closed cell foam technology has moved on so far as to make it a product favored by fence and utility companies for setting posts and poles in the ground.

Is foam really better than concrete? Does it have the long-term durability, strength, or the solid hold in the ground needed to make it a serious contender for your project?

Check out our comparison and then make up your own mind.

Concrete Fence Post: Pros and Cons

Concrete has been used for millennia, and Roman concrete has often been marveled at in terms of its strength and durability. It’s unlikely that the fence at the bottom of your garden will need to last as long as the coliseum at Rome, but it says a lot for the expected durability of concrete.

Advantages Of Concrete Fence Post 

  • Most ready-mix sacks that you buy at the hardware store today come with a guarantee that if mixed right, in the best conditions, then the concrete post hole filler you mix up and use today should last fifty years without breaking down.
  • Ready-mix concrete post filler is fast setting, but slow enough for you to be able to check and reset a post for plumb and level for a minute or two after pouring in and mixing with water.
  • It is a straightforward and easy process. Pour water into the hole around the post, then tip the mixture into the hole. Just make sure you have enough concrete, depending upon the size of the post.
  • Fast-setting concrete claims it is ready to go within two hours, which means you can start assembling your fence or hanging a gate the same day. Many contractors say that a longer delay, say a day or two is optimal before getting into construction of the fence proper, to allow the concrete to fully cure.

Disadvantages of Concrete Fence Post

  • To ensure a good solid foundation for your fence posts it is recommended that up to four bags of concrete post filler are used per post. Some bags weigh from 20 lbs up to 50 lbs each, so that’s a lot of weight to carry around the plot.
  • A box containing the expanding foam materials required to hold two fence posts weighs in at just 8 pounds, which is not only going to take a load off your back, but also off of the vehicle that brought it to site.
  • Concrete eats up a lot of carbon in its manufacture, and don’t be mistaken thinking it’s just rocks and cement, there are many chemical compounds used to bind concrete and set in motion the exothermic (heat producing) reaction required to start the setting and curing process.
  • Wherever you are, concrete will need a water supply, whether it’s on site or you transport it there yourself. This all adds to the time taken to get the job done.
  • Concrete is known to leach chemicals into the surrounding soil over time, causing contamination that is long lasting and impossible to remove.

Expanding Foam Post Filler: Pros and Cons

Expanding foam was never a Roman invention, but it has been around for almost a century and the technology has moved on rapidly. Basically it is a polyol and resin mix that expands to several times its premix volume. Check the manufacturer’s directions on the packaging.

You might be familiar with it as a product used for insulation or for closing the gaps around newly fitted or replacement windows, but post hole filling expanding foam has been developed to have a compressive strength several times higher than regular gap-filling foam.

Advantages of Expanding Foam

  • Expanding foam post hole filler sets incredibly quickly making the post ready to take a fence assembly or a gate after just half an hour. If you are in a hurry, or need to get a repair carried out quickly, then foam will outclass concrete for speedy delivery every time. 
  • As mentioned before, foam filler is incredibly light, if you need 80 lbs of concrete per post for concrete as opposed to 4 lbs of foam, for a dozen posts you are going to have to move half a ton of concrete material before you even start. Replacing a 40 lb sack of concrete with a 2 lb pack of foam will save transportation costs too.
  • Expanding foam post hole filler is greener than concrete. But how? It’s plastic and chemicals, isn’t it? This is true, but it takes less energy to make and transport expanding foam and you won’t require gallons of water to either make it or mix it.
  • Foam is also inert when it has finished its curing process and will not leach chemicals into the surrounding soil, making it a clean, green choice too.
  • Which brings us to another distinct advantage that expanding foam has over concrete. If you are working in a remote area, or indeed any part of a project that might be some distance from, or have restricted access to a water supply, then this is going to be another logistical problem if you are using concrete.
  • No one likes working in sub-zero conditions, but if you have to, because a post is down, or there’s a time limit that has to be observed, then expanding foam will still work. It might be slower than it might be when used within a more optimal temperature range, but it will still react, expand, and fill the hole. Fast 2K foam claims to be usable at up to -20°F (-30°C).
  • Post-Loc has seen its product taken up by utility companies to straighten up and secure 80-foot utility poles. It has been used this way for over 25 years and has been used for up to 10% of the US network. If it can stand up to the effects of nature in this setting then a six-foot fence post should be child’s play.

Disadvantages of Expanding Foam

  • Not all expanding foam hole filling products are created equal. Some work better in damp soils, others are great in dry ground. Secure Set does well all round, while Sika does not like the wet. 
  • It is recommended by all makers that the bottom of a post hole be as dry as possible before use, which might mean a lot of siphoning, or, if you are in an area with a high water table then it’s probably not the right post hole filler for you because you’ll never get that post hole dry.
  • You have to be quick. Once mixed the foam won’t wait, so you have to make sure that the post is well braced and level as there will not be time for adjustment later. If the product starts expanding in the bag it can explode, making quite a mess, so ensure your preparation and timing is spot on.

Expanding Foam vs Concrete Posts: Summary

Both concrete and expanding foam have their advantages and disadvantages. Depending upon site and ground conditions either one will do the job of holding up your fence for decades to come. Foam usually comes with a forty-year guarantee, which will see most fence projects out.

At an average cost of about $8 per post a 2lb pack of expanding foam is about the same price as a 50lb bag of quick setting concrete, so it is much kinder to your back and the back axle of your automobile.

For ease of use, spray foam edges concrete in almost every way. Used correctly it can help save time on any project, and when time is money it’s likely the best choice.


Evan has decades of experience as a project manager for large-scale commercial renovation home-building projects throughout the US. Currently, Evan runs a successful construction management company in Virginia.

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