How To Build A Vermiculite Pizza Oven With A Gym Ball

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When it comes to building a pizza oven, there are a few options to go for – you can use brick, clay, or something alternative like vermiculite. You no longer have to spend huge amounts of money on expensive fire bricks or require masonry skills.

A vermiculite pizza oven is made from concrete composed of the highly insulating mineral vermiculite and a refractory cement suitable for high temperatures. The dome is formed by applying the concrete over a spherical mold, commonly an exercise ball. With a fireproof hearth, tunnel, and chimney, it is a cheap and simple alternative to building a brick oven.

I’ll give you a price, material list, and method to do so here.

This pizza oven is ideal for someone who doesn’t have the time, skill, and money to build a brick oven. It’s extremely cheap, lightweight, and manageable for anyone who is fairly competent at DIY or who is willing to learn a bit.

It is not as traditional as a brick oven, but a vermiculite oven will still reach the same high temperatures needed to cook pizza.

About Vermiculite

Vermiculite is a naturally occurring and inexpensive mineral that is heated intensely to puff up and become a highly efficient insulator. It can withstand high temperatures and will expand and contract with heat, making it less likely to crack.

It can be coarsely ground and mixed with refractory cement to form an insulating refractory concrete which can be a cheap way to create a dome or add layers of insulation.

Price And Where To Get It

Vermiculite is a common building material and is also used in gardening. Therefore you should be able to pick it up from your local builder’s merchant, or gardening store or it is available online.

All the materials in this article should be able to buy easily.

As for price, a 4 cubic foot (100 liters) bag is extremely cheap – and that might do you for your whole build. You can buy the right bag on Amazon here.

As you can see, this build will be much cheaper than a firebrick build, which can take you into the thousands of dollars. With all the other materials, I estimate the vermiculite build to cost you around $200-$300 in total.

If you want to see how much of a price difference between a vermiculite and a standard brick build, then check out my article on how much a wood fired pizza oven costs and see how much you could save.

Is vermiculite safe? In short, yes. This article gives the history of one mine which had traces of asbestos with vermiculite but was shut down in 1990. There are strict industry protocols that now ensure this isn’t present.

Material Shopping List

This material list assumes you will build an oven around 26″/65cm and use an exercise ball as a guide. The links have some examples of the recommended products on Amazon.

  • 4 cubic foot / 100-liter Vermiculite Bag (See on Amazon)
  • 50lb / 25kg Refractory Cement (See on Amazon)
  • 65cm Exercise ball (See on Amazon)
  • Plastic Wrap
  • Wood board for ball and arch supports
  • Cardboard or sheet plastic for tunnel support
  • Ceramic insulation blanket (See on Amazon)
  • 1 inch / 25mm Calcium silicate insulation board (See on Amazon)
  • Chicken wire
  • ~24 Fire bricks for hearth
  • ~16 regular bricks for the entrance
  • Sand
  • Waterproof render

Steps To Build A Vermiculite Pizza Oven

1. Make supports

The first step is to create a support to hold the ball in place to allow you to apply your vermiculite concrete.

You can use anything you want, but a piece of sheet wood is an option that you might have available already. It needs to be able to hold the ball securely while you apply the concrete.

Cut a hole in the board just large enough to fit your ball so it rests firmly. Raise the board up with legs, so that you can place the ball inside and the bottom of the ball touches the floor.

You can use a 65cm ball to make a 26″ oven, or you can scale it up larger.

2. Dome

Now you can make your dome – wrap your ball in plastic wrap before you start.

Make a mix of vermiculite and cement that is 4:1 ratio and add enough water to allow the mixture to form solid shapes. Too much water will mean longer curing and drying later on.

Use gloves to apply the concrete to the ball, building up a solid layer. If you are applying an insulation layer then this layer can be thinner as the additional layers will provide mass.

But if you are not insulating then this layer should be thick, around 4-6 inches. For a thick layer, you will have to do multiple layers, allowing it to dry enough between each layer.

Let the dome dry, remove the exercise ball, and then remove the plastic wrap.

3. Hearth

The hearth will be the base of the build on which the dome will be transferred once dry, so this is the next step. The hearth will be built upon whatever stand you have used.

I have left out the stand construction as that is a generic part of any build but could be made from blocks or bricks, or made from metal.

You need to build the hearth on an insulation layer so that it protects against the material on which the oven stands. It also will retain heat on the oven floor so that this can cook the crispiest pizzas.

You can use a 25mm thick calcium silicate insulation board which can be cut into shape and be used straight away. Other methods would be mixing more vermiculite concrete.

Then the hearth. This is the floor of the oven which you will cook off. It needs to be made of fire brick so that it will withstand high temperatures otherwise it might crumble over time. Use as thick as you can afford here, to retain as much heat as possible.

Some tips when laying the bricks would be to keep them as flat as possible and to lay them in a herringbone pattern. This will hopefully stop your pizza peel from catching on the brick edges when you are transferring a pizza in and out of the oven.

4. Tunnel

Now you have a hearth with a dome on top.

A tunnel needs to be added to allow access to the main chamber but to avoid too much heat escaping. The tunnel needs to be the correct height to promote airflow but also not too high so that too much heat is lost.

There is some math involved here and should be around 64% height of the inner dome. This article has some pictures showing why.

As more concrete will be used to form the tunnel, you need another support structure. You can use anything here that will withstand the weight, such as stiff cardboard or plastic. It just needs to bend enough to allow you to create a solid tunnel support.

The chimney is usually found at the tunnel so should be taken into account when building this step. Any cylindrical tube strong enough to create a gap that is large enough to place your chimney is fine.

More on the chimney in step #5.

5. Entrance

The entrance is an important section as it faces outwards, providing the aesthetics for the front on view. It needs to accommodate a door if you are going to add one, so a recessed step needs to be added so that the door can close tight.

You can leave the entrance as is or else finish it with bricks to give a classic appearance. You can use regular bricks as the entrance doesn’t take direct heat.

Around 16 bricks cut to the correct shape and supported with the wood board will be sufficient.

For a 26-inch oven, a 15-inch entrance width is around the size required.

6. Chimney

This is used to draw smoke out of the top of the oven. As it draws smoke out, fresh air will be pulled from the outside via the tunnel and this will fuel the fire inside.

The chimney should be added at the tunnel, and not the dome. Positioned straight over the dome and too much heat will be lost.

Therefore, it is needed to be dropped into place while you create the tunnel, and should be held in place with some support.

You can get a stainless steel chimney designed for a pizza oven off eBay for relatively cheap.

7. Insulation

We have the basic components of an oven in place, and you could finish here, although I don’t recommend it.

Vermiculite is a good insulator in itself, but more layers can provide better insulation and an overall better quality build. This will keep your oven hotter for longer and will require less wood to maintain the cooking temperature.

A ceramic insulation blanket can be wrapped over the dome once it is dry, and this provides great insulation. It is available online, but can sometimes be slightly expensive.

Next, use chicken wire to fasten the insulation in place and to give a base that will allow the final layer to adhere to.

8. Render

The outside layer needs to be waterproof so that your pizza oven will survive the elements. First, we need to cover the chicken wire with refractory mortar to cover up the insulation.

Then the very final layer. Use a waterproof render of your choice to seal the oven. You can get the final layer to look very smooth to give it a nice aesthetic finish.

You can add some color to your waterproofing layer or alternatively paint the outside once finished.

9. Curing The Oven

When all the steps are completed, you need to cure the oven before it can be taken to the top temperatures. This is because the oven still contains moisture in the concrete and generally hasn’t settled yet.

If you were to take it to extreme temperatures straight away then the moisture could evaporate as steam and crack the oven, destroying all your hard work.

Start with small fires, and build up the temperature and length with each fire. Once you are happy it can withstand each level of heat, then you can reach the higher temperatures and start cooking your pizzas.

Pros And Cons Of A Vermiculite Oven


  • It is super cheap. You couldn’t really get a cheaper material to work with.
  • It is lightweight. The material is porous and so filled with tiny air gaps.
  • It can withstand high temperatures and so is regularly used as an insulation material.
  • As it is porous, the air gaps make it a great insulator to retain heat


  • You can only make an oven as big as the ball you are using.
  • As mentioned earlier, if it was mined before 1990 then it could contain traces of harmful elements. Check your sources.

Frequently Asked Questions About Vermiculite Ovens

Vermiculite Vs Perlite

Vermiculite and perlite are both minerals that are heated to high temperatures so they expand to become porous. This makes them both good insulators and can withstand high temperatures. They are both used for insulation and can be swapped for each other.

Vermiculite And Waterproofness

Vermiculite itself is porous so absorbs water. When it is mixed with cement to form a concrete then this would not be waterproof like regular concrete. A waterproof render on the pizza oven is recommended to make it last.

Vermiculite Oven Cracking

If you build it well then the vermiculite oven should not crack. Vermiculite can contract and expand with high temperatures, and if you use proper refractory cement then this should mitigate the risk.

The water inside the cement can pose a risk so it is advised you cure the oven with increasing fire sizes before maximum temperature.


There you have it, all the information, steps, and materials you need to go and make your own vermiculite oven. They can produce some fantastic looking builds for a fraction of the price.

The videos are worth watching on this page to get some visual cues on what to do.

You should learn more about the different types of ovens so check out my guide on how much a wood fired pizza oven costs where I compare the different types out there.

Vermiculite is just one way to build. Definitely check out my article on 8 ways to build a pizza oven if you are thinking of building your own oven and need some ideas.

Tom Hambly

Tom Hambly is the founder of Crust Kingdom. As a self-taught cook, he has been perfecting making pizzas at home for over a decade. Now he runs this site to help millions of people make pizza every year. About Tom Hambly.

7 thoughts on “How To Build A Vermiculite Pizza Oven With A Gym Ball

  1. hi, Nice project. Wich granulate its good for vermiculite? in my country i have only this.
    Its good?

    1. Hi Gabriel, thanks for your feedback and message. That vermiculite looks good. The best stuff to mix with the concrete is the coarse vermiculite rather than the really fine stuff. At 10-12mm that should be OK.

      Good luck!

  2. Thanks for all of the info in your site. You are the Crust King. Started on my oven this week. Your vermiculite link is sold out. Thoughts on this one?

    1. Haha thank you for the title.

      The description looks good but check the reviews – some are saying it fine and not coarse. You want larger pieces, so be wary of that one perhaps.

      And good luck with your build!

  3. Hi There,
    Your listing of the refractory cement? Do I understand it to mean that I would need two of the 25 lb buckets that are listed on Amazon?

    1. Hi Michael, yes it doesn’t go very far. And if you need extra for the tunnel etc.

      Check this video if you haven’t already – it goes through the materials and mixing

  4. I’ve just built mine using a 6:1 ratio with refractory cement, as I’d seen the same ratio with Portland cement. I can’t find any info relating to how refractory compares to Portland when it comes to the binding qualities. Now I’m a little worried as the concrete seems a bit flaky. I can always go again if I have to but would rather not.

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