Using Expanding Foam for Fence Posts: When, Where, and How

1 – Courtesy: Fast2K

If you’re about to install a fence post in your backyard, you’ve probably considered using concrete to fill in your post hole. That’s not a bad idea, as concrete is one of the most common materials used to add durability to fence posts. However, concrete isn’t the only solution. Expanding foam is an amazing alternative that works just as well as concrete. Here’s all you need to know about this foam.

When Should You Use Foam Instead of Concrete?

Skipping concrete and going for expanding foam is a brilliant idea overall. If you’re a DIYer that doesn’t have that much experience working in their backyard, it’s much better if you stay away from concrete. Expanding foam is much more user-friendly and will make the job much easier for novices. Let’s break down the different aspects of expanding foam and way its pros and cons as an alternative to industrial concrete.

  • Foam is Environmentally Friendly

If you’re passionate about using eco-friendly materials for your post holes, concrete should never be your first choice. The Guardian even called concrete the most destructive material on Earth in a report they did in 2019. The cement manufacturing industry is the second biggest emitter of greenhouse gases and contributes to 5% of the annual CO2 production globally. It’s impossible to avoid the production and usage of cement completely. It’s the main component used in the development of houses. However, in situations like these where there are alternatives to use, you should definitely consider them.

Secondly, even if we ignore the (unignorable) impacts of cement manufacturing on the environment, it’s still bad for your backyard. Concrete is a petroleum-based product that will mix with the soil and contaminate your backyard. If there’s groundwater right under your backyard, you’ll end up polluting that water as well. 

If you are passionate about being environmentally friendly, you should definitely avoid using concrete and prefer alternatives wherever possible. 

  • Foam Dries Quicker

Concrete makes setting up fence posts a pretty long job. It doesn’t set properly for several hours and will take days to be completely cured. The worst part is that you’re going to have to set up supporting beams for each fence post to give the concrete room to cure properly. In the meantime, if you mess up ever so slightly, you’ll have a tilt on the fence post, which is a huge no-no and a major hit to the durability of your fence post. 

On the contrary, expanding foam only takes a few minutes to get hard enough to the point that it won’t require support. It takes about 15 minutes to cure completely. 

  • Foam is Compatible with Cold Weather

One of the downsides of concrete is that it requires a specific temperature to set in properly. The ideal temperature for concrete is 50°F – 90°F. This temperature is important because anything too cold or hot is going to affect the structural integrity of the concrete. If you live in particularly cold environments, water inside the concrete could freeze, which will lead to cracks in a few days.

Meanwhile, expanding foam is much less reliant on temperature. Yes, foam likes warmer weather. But, if you’re installing posts in the cold, using foam should be okay as long as you’re storing it in a warm place. 

  • Foam is Easier to Remove

Concrete is more of a permanent solution. If you mix it correctly, concrete could last anywhere from 50-100 years. That’s not really the timeframe you want for your fence posts. So, if there ever comes a time you want to get rid of your fence, you’ll literally have to dig out every single fence manually. That’s extremely tiring and will be quite an extensive job. 

Usually, expanding foam is difficult to remove if you’re using it somewhere around the house and will most likely damage your house. But, since we’re talking about using it to fill posts, it shouldn’t be much of an issue for you.

  • Foam Ensures Long-Term Durability

Concrete might be hard and durable, but it has a tendency to break apart. You’ll notice that small bits and pieces will tear off after a few years. The concrete mix needs the right amount of each material, and if you’re a little heavy on anything, it will affect the quality of the fix. The better the mix, the more durable the concrete and vice versa. But, even the best mix ever will have pieces chipping off every now and then.

On the other hand, expanding foam is much more coherent and will not have pieces breaking off and spreading debris all over your backyard.

  • Foam is More Expensive than Concrete

If you’re looking to keep the cost of your fence low, then you might want to stick to concrete. Expanding foam is much more expensive than concrete. On average, a bag of foam will cost three times the price of a bag of concrete. Foam will definitely put a dent in your wallet. 

  • Foam Poses an Electrical Hazard

You can install expanding foam anywhere except in places where a fire hazard could be a possibility. If you’re building a fence around electrical wiring or an electric box, expanding foam might not be a good idea. Foam uses methylene diphenyl diisocyanate, which is a propellant gas that dispenses the foam and is highly flammable. If this hits a cracked wire or an electric box, it will probably go up in blaze.

Foam vs. Concrete: Which is Better?

There’s no one answer to this question. It’s mostly dependent on what your situation is. In most situations, expanding foam is going to be the better option. However, if you’re looking to keep the budget for your fence low, you should stick to concrete as it’s the cheaper option of the two. 

How to Use Foam – Step by Step Guide

Step 1: Put on Some Protective Gloves

It doesn’t matter how small or big the job is. You should ALWAYS use gloves when handling expanding foam. Once it gets on your skin, it’s going to stay there and won’t remove easily. So, wear a full sleeve shirt and gloves to avoid any accidents.

Step 2: Dig the Hole

The next step is to go ahead and dig the hole. We’re not going to go into detail with this one because the process is the same as when you use concrete. If you’re looking for how to dig post holes, here’s an article that can help you out.

Step 3: Mix the Foam

Now that you have the hole dug and your post set up, you should brew up the mixture of the resin. Expanding foam usually has two packs of resin that react when mixed with each other. One of those resins comes in a large bag with the other packet inside isolated within its own wrapping. You can step on the bag to pop the smaller one inside. This will create the mixture, and the resin will turn into a yellowish color.

Step 4: Pour the Mixture

You should start pouring as soon as you mix the resin. Create a small cut on one corner of the bag with a scissor and start pouring. The quantity may be a bit deceiving, and you might think it’s not enough. However, the name ‘expanding’ foam isn’t just a gimmick. The mixture will expand ten times its original size. 

Step 5: Wait for the Foam to Cure

Once you’re done pouring, all that’s left is to wait for the foam to cure. You’ll need to give the post support for a few minutes until the foam becomes hard enough. Once it sets in, don’t touch the post and give it room to cure. If it’s warmer in your area, the foam will set a lot faster. However, the brand of foam you’re using comes into play as well. 

Where Can you Buy Expanding Foam?

You could go to a Home Depot or your local hardware store, and you should find expanding foam pretty easily. Alternatively, you can also purchase expanding foam from Amazon

The Fast 2K Fence Post Mix from H.B. Fuller is one of the best expanding foams you can buy. Like most types of expanding foam, this one is waterproof to prevent rotting. 

Expanding foam is hard. But, if you’re looking for commercial-grade durability, the Secure Set 20 Post Kit is a great option. Their proprietary formula is super flexible and works with a variety of materials like wood, metal, vinyl, PVC, etc.

Expanding foam is already long-lasting. But, if you want a foam specifically made for durability, Sika Postfix is the right product for you. It’s long-lasting, sets in just 3 minutes, and is super resistant to environmental damage.


With that said, we would highly suggest trying out expanding foam if you’ve never worked with it before. It’s a great, environment-friendly alternative to concrete and will make the job substantially easier for first-timers. Apart from that, I’d also recommend wearing the right kind of outfit and gloves to ensure you don’t end up with foam on your skin.