how many fibre boards?

[L0veless]

in the planning stages of building my first oven, have room for the 42" dia. my question how many insulating boards do you use when they are 24" X 36"?

thank you

[JRPizza]

You just need to find the best layout to minimize wastage and make sure you don't have to buy a bunch extra, if for example you can only get 4 boards in a pack. I used a cad program, so it was easy to find the optimal orientation.

[dakzaag]

In your calculations, remember to add the thickness of the brick. So typically we use a half brick thickness for the wall, your design might be different. Half brick measures 4.5" X 4.5" X 2.5" So in this example, the total oven mass for a 42" oven is 42" + (4.5+4.5)=51". The outside measurement will be 51" using 4.5" wall thickness. Again your design might be different, but you want the support insulation to extend a minimum to the outside edge of the walls.
Personally, I continue the base insulation further out to account for the dome insulation as well which is typically 3-4" (ceramic blanket) if you use a vermicrete or pericrete, you really should double this amount as it is less efficient compared to ceramic blanket.
So now you have added 6-8 more inches to the base insulation area. 57-59 inches. Now of course you have to decide how thick you want your base insulation to be and again 3-4 inches are recommended for the best heat retention. More like 5 inches if you expect to bake bread and all manner of things with residual heat.

So, if I were ordering the base board for a 42" oven, I would order 4 pieces of board for each layer of thickness I wanted. Lets say 4" thick is the desired amount and you order 2" board, it would take 8 pieces to make the base. You would have a little waste, around the edges, but not enough to order one less board, and one corner would actually be a tad short. If you order 1" board, then it would take 16 pieces to get 4" thick base. You can do the math for your situation.

It is not critical to extend the board out past the oven walls, you can bring the blanket down to your support slab as well, it simply depends on how the base lays out and which is more expensive. I have found the board to be the easiest part of the install, asuming everything is flat and level, just lay it down and be done. You might think otherwise.
I attached a pdf of the layout, not sure it is visible or not.


60 inch oven pad.pdf
.