Block off plate

You don’t really want insulation laying on the stove, Roxul around it is fine though.
Stuffing the damper with insulation is helpful, but doesn’t make a positive seal and still leaks air. Also, there’s no way to insure the insulation stays put. The block off plate has a nice finished look and makes a positive air seal while holding the insulation in place.

Overtime gravity will pull the roxal down, plus roxal is pretty fibrous product so there will be a debris / dust element that will always require clean up, think vicious circle, you go to clean around the insert area for regular dust build up with a shop vac, you end up making a larger mess because the roxal dust keeps coming out. Also all insulation is porous, it only slows the air down, so there will be air leakage.

kennyp2339 said:

Overtime gravity will pull the roxal down, plus roxal is pretty fibrous product so there will be a debris / dust element that will always require clean up, think vicious circle, you go to clean around the insert area for regular dust build up with a shop vac, you end up making a larger mess because the roxal dust keeps coming out. Also all insulation is porous, it only slows the air down, so there will be air leakage.

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Most inserts have a convection jacket surrounding the firebox that’s made in a way that the insulation will not become air born. I’ve heard a report or two of this happening, but with most units it’s not a concern.

Ahh yes, but there have been a lot of people that have been buying free standing stoves only to shove them in a masonry fire place

Bushels20 said:

Do any of you have a suggestion on what the best metal to use for the plate is? What gauge/thickness as well?

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depending on how big many get away with 26g sheet metal, buy bigger than the hole (measured at the lintel) because you want to fold the edges downward making a lip to tap con into the masonry

You have to consider the modes of heat transfer in thermodynamics: convection, conduction, and radiation.

To prevent convective heat transfer you want as airtight of a seal as possible, but the roxul alone will likely stop the majority of it.

Conductive heat transfer will also be highly inhibited by just the roxul alone

I think where the block off plate really helps is preventing the radiative heat transfer. The metal is much denser than the insulation and should absorb and reflect back the infrared photons. However, once heated, it can still conduct heat to the surrounding masonry, so you’ll want to minimize how much is in direct contact with the masonry by putting an insulator in between.