I built the base without a void for the insulation layer! What now?

[IggyPK]

I built my base and slab before I decided on Mattone Barile grande. The slab is 5 inches thick 60x65. How do I add an insulating layer between the oven and the slab. There is no recessed void in the slab to accomodate an insulating layer.

[bikerbudmatt]

Welcome to the BrickWood forum!

It’s not an ideal situation, but (believe it or not) it has happened before. It is also fixed easily.

Read this thread to find out what someone in the same boat as you did to recover their build and move forward:

Help needed. Hearth slab poured with no void for insulation

I had the hearth slab poured last week and due to language barriers and frustration, I allowed a straight 5.5” concrete slab to be poured, with me thinking, I can figure out the rest later. Well, now, I can’t. I have a whole lot(200+) of vintage red brick laying around that I plan on using on the hearth , but can’t figure out how to make the void for the insulating material and the sand. My firebrick are the 2.5” thick variety. Could I use 2 levels of red brick with one layer flat and one on e…

If you still have questions after you read this and look at the photos, come back and let us know. There are lots of helpful folks here!

[Drpizza]

I would think you have two choices build up from your 5 inch base or cut into your slab and create an insulating layer. I recall it was 2 inches in the Mattone barile plans. A circular saw with a stone blade might get you a two inch cut depth. Cut a grid pattern and chisel out a two inch void. If you risk cutting you rebar I would think building up two further inches of concrete around the perimeter to create a void is only choice.

[bikerbudmatt]

@Drpizza

If you risk cutting you rebar I would think building up two further inches of concrete around the perimeter to create a void is only choice.

Much easier to build up; it’s going to add two inches to the height, and require either an inner form or the pink polystyrene insulation block as a form. If @IggyPK used the 4000psi or 5000psi concrete for the slabs it’s just not going to be a fun job to dig two inches into that.

And again I hope they take the time to read the earlier thread as this was a topic where we gathered a good knowledge base.

[IggyPK]

Hi Matt. Where in the instructions does it tell you how to make the insulating layer? Could I just buy an insulating board and use that? Forno Bravo makes a 2 inch alumina silicate insulation board.

[bikerbudmatt]

Hi Iggy,

The first part of the instructions for the Insulated Base (steps 1-18) are all about how to build the slab with a space for the insulating layer. Steps 46-50 describe how to mix and pour the insulation into that space once you’ve placed the slabs on top. That section (pages 15 and 16 in the instructions) are what you’d want to look at.

Since you don’t have that space, you could certainly choose to use the silicate insulation board instead of mixing the Portland cement/vermiculite. The advantage of the mix is that it is both very effective and inexpensive. The disadvantage is that it takes a day to dry (I recommend you don’t use Perlite, as it takes several days to cure vs. Vermiculite which sheds water and cures quickly).

Either way, you still need to build the oven slab up to contain both the insulation and your firebrick hearth. Look again at the link above for general insight, and this particular post has the dimensions for the insulation cavity:

Help needed. Hearth slab poured with no void for insulation

You’re right, it’s never spelled out exactly in the instructions for the insulated base. But it’s in steps 5 and 6. (Page 4.) The width is 29 inches. The length is 20 + (10.5 x 2), or 31 inches. That gets centered in the hearth (see page 5). If you look at the CAD drawings there, it should give you a good idea of how to frame your makeshift brick void. Hope that helps!

I hope this gives you enough guidance to move forward, but keep hitting us up here if it does not. We want you to succeed!

[IggyPK]

Hi Matt,

Would it make sense to add cement/vermiculite layer on top of my slab (say 3 inches thick) with dimensions of 60 x 47? Then I could just build the outer brick frame and lay in the hearth floor bricks as per the instructions?

[CMG]

@IggyPK I believe others have used high temperature insulating board instead of the perlite or vermiculite mix. Per @bikerbudmatt, making the mix is cheaper but the board has a higher R value rating. At any rate, first things first, as Matt said the hearth slab needs to be built up so the 2 inch cavity can be created to then place the insulating board or mix into. Good luck and keep us posted!

[bikerbudmatt]

@IggyPK

Would it make sense to add cement/vermiculite layer on top of my slab (say 3 inches thick) with dimensions of 60 x 47? Then I could just build the outer brick frame and lay in the hearth floor bricks as per the instructions?

In theory, sure, but in practice Portland cement mixed with vermiculite is an inexpensive, efficient insulator—and not very strong. The reason it works when you build the slab as designed is that it is contained in a “sling” of high strength concrete reinforced with a rebar grid.

I know it sounds like a pain—more slab work when you’d really like to get started on the “fun” part of this job. But it touches on a truth that builders have discovered: do it right the first time, and if you mess something up (and we all do!), fix it right so you never have to think about it again.

You’ve invested too much time and materials into your build to take a shortcut now. And, you will be thankful 20 years from now when your oven is still standing and turning out delicious pizza.