How far (laterally) should hearth insulation extend?

[kebwi]

I realize that the hearth insulation should extend to fully encompass the dome walls, all the way to the their outer diameter, but is it necessary to extend the insulation all the way out the door to the most exterior threshold of the arch?

I suppose if it *doesn't*, then there is the question of what replaces it and how to insure that the lowest level of bricks -- dome and arch -- sit on a perfectly level and flat surface, but that is a secondary question, one of structural support. I'm asking if the arch firebricks need to be fully insulated from the bottom for the oven to hold heat properly?

I suppose the same question applies to the arch from the top w.r.t. perlcrete or blanket.

Likewise, how far beyond the diameter *must* the insulation extend? I realize that more insulation always helps, but does it serve a measurably beneficial purpose at any distance beyond the outer firebrick diameter?

I ask both of these questions because, when measuring out chunks of insulating board, I need to decide how far to reach. When pouring perlcrete it doesn't really matter. Going a few more inches, or even filling the entire hearth as per the original Pompeii directions, is all pretty much the same deal, but buying more board is a very different thing. Buying an extra board to cut up and extend the perimeter of the insulation or the arch footprint of the insulation is pretty expensive.

Quick numbers: Pompeii 36" interior is 45" exterior at the outside of the bricks. Pompeii recommends two FB boards, 48"x24", yielding 48" by 48". Thus, a square with 1.5" beyond the dome and extending only 1.5" into the arch, leaving the entire arch uninsulated, in effect. How important is that 1.5" btw. What if it were flush?

Thanks.

[kebwi]

Re: How far (laterally) should hearth insulation extend?

I frequently don't see my posts show up on the new-posts-list after posting them (but sometimes I do, so I know it "should" appear on that list), so I'm bumping this solely for that reason. Sorry if this appears as a duplicate to some people. It is unintended.

[SteveP]

Re: How far (laterally) should hearth insulation extend?

My insulating board only goes to the outer edge of the firebrick all the way out to the outer arch. I laid out my boards, set the fire brick, and once the brick work was complete, I cut the board to be even with the brick and used the remaining insulating board for my door. I believe I used 3 boards that were 3' x 2' x 2". None of the firebrick contacts the concrete hearth. I then put the insulating blanket and perlite/cement layer down to the concrete hearth. I don't guess it would hurt anything to go past the diameter of the dome as I have seen many builders do that here. I have never had any issues with heat loss due to this layout.