Sealing the calcium silicate boards?

[angelboy]

I've read about people drilling holes in the concrete hearth for any water ingress but can't I just seal around the bottom of the calcium silicate boards with high heat silicon?

I'd seen a video online where the builder puts a layer of aluminium flashing all around the base which is then sealed, but this is after the insulation blanket.

I can't get the aluminium for a decent price so what's the alternative?

Should I be that worried about the boards getting wet if it's sealed?

[david s]

Remember that sealing anything works both ways. If it reduces moisture getting in then it also reduces moisture getting out. I think it's probably better to let the holes in the supporting slab act as vents and chase any moisture out with fire. Water will not enter through holes at the bottom of the oven, unlike holes or cracks around the top. So, it's more logical to prevent water entry at the top, particularly around the flue, but have a means of escape at the bottom. With an igloo style a crack sometimes develops between the outer shell and the supporting slab. This can be a potential water entry if water is allowed to pool in this area. Cracks there should be sealed up. In my build the supporting slab slopes down slightly outboard of the outer shell to prevent water pooling against the base of the outer dome.

[angelboy]

How many holes should I drill and how big?

It's OK to do this retrospectively?

[david s]

Yes, just drill up from the bottom and stop when you feel it break through. Use a pilot hole first (about 8mm) then enlarge it with a 12mm bit. About 3 should be sufficient. Near the floor centre.