Insulation over brick

Hi, wanted to know whether there are any advantages or disadvantages to using vermaculite insulation vs. a ceramic blanket over the top of the bricks. Thanks.

Vermiculite is inexpensive. That’s about the only advantage, and it’s not much of one.

You use vermiculite (or perlite) under the hearth to prevent the concrete becoming a heat sink. It’s a relatively low-stress job because heat rises. Vermiculite can handle that load. It would be a waste of resources to insulate your base with a high-load material like ceramic fiber.

The job above the hearth, in the oven wall, is a very high heat load. Heat rises (if I hadn’t already mentioned that), and it will overwhelm the relatively low R-value of vermiculite. The ceramic fiber blanket is the same material used to insulate kilns and other vessels that are designed to reach a very high temperature. Vermiculite’s insulative properties would be overwhelmed at those temperatures.

So … you’d save about $300. But, you’d have an oven that would maybe achieve the same temperatures as your kitchen range. That seems like a waste of $2,000 and your labor to replicate what you most likely already have.

(Alternatively, you could make a slurry of vermiculite and Portland cement about 3 to 4 feet thick and perhaps come closer to the R-value of ceramic blanket, but that would just look silly.)

Thanks for the assistance, I will definitely take your suggestion. Another question, the bricks I am using are 3 inch deep, so I will not get as good of insulation properties vs. with the thicker brick. They are fire brick, but just not very thick/deep. Any suggestions on how to further insulate the oven? Do you know how well it works to have multiple layers of ceramic blanket?

Kevin can answer this in greater detail, but here’s what I understand:

  • Fire brick’s primary job is to refract heat. It’s the primary surface exposed to direct flames. It will retain heat and release it over a long period of time. It does have good insulation properties, but you want your heat to be released back into the oven cavity, not up and out through the walls. If I understand what you’re saying about the brick you have, it shouldn’t make much of a difference, though you have to account for it dimensionally while you’re building the hearth floor.
  • Ceramic insulating blanket is how you ensure that the heat stays in the oven. It has great performance at the high temperatures the brick will reach, and is what you need to let the fire brick do their job on the walls of the oven.
  • Using a second layer is a possibility, but it depends on the climate where you are building your oven. In Connecticut, where I am, it gets cold enough (and I like pizza year-round enough) that I’ve invested in a second blanket. In more temperate areas the cost wouldn’t be justified because there are so few cold days. Of course, that’s always up to the builder, but you could better invest the funds for that second box in nicer materials for the facing. (Or a nice little refrigerator table setup.)

Hope this helps!

Thanks again for your recommendation. I have purchased the ceramic blanket and will be placing this over the bricks once completed. The next step calls for creating a stucco shell. I live in Ohio (cold winters), so I wonder if it would be valuable to create a 1-2 inch vermiculite shell over the ceramic blanket, followed by stucco. Thoughts?

Vermiculite absorbs water. I’d be concerned that a vermiculite shell will expand and crack the stucco unless you can effectively seal the stucco from all outside moisture. I built a stucco shell (2 separate layers of the fiber-reinforced stucco applied a couple of weeks apart last October) over a single layer of ceramic blanket. That’s worked well for me here in Northeast TN. If you want extra insulation, consider adding another layer of ceramic blanket. Alternatively, if you really want to add a different insulating layer, perlite would be better than vermiculite since it doesn’t absorb water.

1 Like

I wouldn’t. The reason perlite (and here I agree with @Peter on that material versus vermiculite) works underneath is that, again, your firebrick in the floor is going to be refracting heat back up into the oven, and convection is already driving most of that heat up and away.

In the hearth void, you’re providing a stable surface that is reinforced above and below. Above, though, your stucco is subject to that Ohio cold. I am concerned that two structures on top of the fiber blanket (the perlite shell and the reinforced stucco) are going to expand and contract dissimilarly, leading one to fail.

I think, since you’re only building this oven once, that because of your climate you should go the extra step and add another roll of ceramic fiber blanket. It won’t matter in the summer, but it will make some difference in the shoulder seasons, and definitely in those winter days when you just want to fire up and eat like you did last summer. :wink: