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gregarious 04-04-2010 09:44 PM

Another Insulating Remark!
So here is the question- should I fill up the 'house' around my oven with loose perlite? or will air be a better insulator?

I have perlite available at the local Home Depot if I decide to do this but am low on time.

The scenario is this: I have completed a 42 inch pompeii oven with a masonry flue/chimney and the oven is within a concrete block enclosure that continues upward 15 feet to clear the roofline by 2 feet (as per code around here). The oven is thus far insulated with 3 rolls of the Forno Bravo FB blanket. I have just got into the 400F range curing, so I am not sure how well the current insulation will hold heat once all the moisure is out. I have the ability to add perlite though an opening in the enclosure now, but won't in a few days! So I need some advice. After the FB blanket (about 4-6 inches thick by my estimate) is it better to leave the sealed dead airspace, a theoretically poor conductor/good insulator, or add loose perlite to cover the oven more?

kebwi 04-05-2010 12:26 AM

Re: Another Insulating Remark!
Regardless of the blanket, loose perlite will insulate better than "pure" air. Filling the volume with a loosely packed, low-conduction, high-air-cavity substance will make it very difficult for heat to diffuse through the volume: the small air pockets are isolated from one another and the material comprising the fill doesn't transmit heat effectively either. One large air chamber permits the air to "stir" which carries heat away from the inner area to the outer area where it can diffuse through the outer wall and escape.

Aside from that, four to six inches of refractory insulation is pretty darn good. I imagine you won't have too much trouble with or without the perlite. I have about the same amount on my oven (a little less on the sides) and while I admit I could detect some noticeable heat at the top-center of the InsWool (same basic thing as your blanket), in the 140F range, that was before the oven fully dried out. Once it was dry, I imagine it probably lost a lot less heat through the insulation. I am speculating because I never measured it at full blast after drying it out before filling in several more inches of vermiperlcrete.

Oh, and unless your HD sells the large cheap four cubic foot bags of perlite, try to find another supplier. The smaller bags are much more expensive.

SCChris 04-05-2010 07:18 AM

Re: Another Insulating Remark!
I filled my house with rockwool next to the dome and entry brick and fiberglass attic insulation in the corners. These were available to me through local building supply houses. On top I poured vermiculite, so in the end the structure is filled. I have vented at the gable ends.


dmun 04-05-2010 01:35 PM

Re: Another Insulating Remark!

I filled my house with rockwool
Just a word of caution: rockwool is sometimes a name for domestic fiberglass insulation, has organic binders, and will stink in high heat conditions. Make sure it's a refractory product before using it in direct contact with the dome.

SCChris 04-05-2010 01:43 PM

Re: Another Insulating Remark!
The more specific, more proper, name for rockwool is "mineral wool". See Wikipedia for more info.

some highlights.

The heat that the material can withstand is:

Glass wool 230 - 250 C
Stone wool 700 - 850 C aka "mineral wool"
Ceramic fibre wool 1200 C





Breven 04-05-2010 01:58 PM

Re: Another Insulating Remark!
I say add the Perlite. The general consensus around here seems to be that you can't have too much insulation....and I bet it will be hard to go back and add it later if you think heat is escaping. If money isn't an issue...then add it.

gregarious 04-06-2010 06:23 AM

Re: Another Insulating Remark!
Well I decided to go ahead with adding the Perlite and I appreciate the response/advice. Yesterday I cleaned out the expanded Perlite inventories of two local Home Depots, and 'loosely packed' my Honda with 32 cubic feet of it (and, incidently, after turning on the car heater for only 15 minutes today it still feels like a furnace in there :-)) . Today I'll fill up the enclosure. After looking at the loose Perlite and considering the filling operation, I'm reminded of a scene from the movie 'Real Genius'! I wonder how popcorn would insulate...
Anyway, thanks again for prodding me to do what I wanted to anyways!

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