Air flow inside-to-outside an attic is quite slow, and permeability into the living space occurs where ceiling lights and home heat/ac vents penetrate the ceiling sheetrock (in a similar way, one can sometimes feel drafts at wall electrical outlets where they penetrate the wall sheetrock - on exterior walls). In some cases it is a pressure driven outside-to-inside flow, with little or no attic air moving to the outside.
Today's contruction codes require all air flow that is intentionally removed from inside a home, to the outside, to be ducted if it passes through an attic space, e.g., bath vents, kitchen range vents, even plumbing vent stacks. One reason is moisture. Both baths and kitchen operation contain high humidity air which can have a negative effect on attic insulation, and pooling of nuisance odors which can migrate back into the home's interior. Air exchange between the attic and the outside is passive in most homes, and even with mechanically assisted attic ventilation, the flow rate is quite low - nominally moving at a speed less than the speed of our breath when we exhale.
Another reason is fire extension limitation - that is, preventing fire spread. Should a fire develop (for example a kitchen range grease fire), the hot gases and potential flames travel through metal ventilation duct work to the outside. Without metal ducting through the attic, the fire could then spread within the attic space. (As a side note - this is also one reason that exhaust and intake are separated by the proverbial 10' rule, to lessen the chance that a fire proceeding out an exhaust duct, does not immmediately get sucked back into the air intake (makeup) and thus back into the interior).
Flamework torches, when operating with the purist of flames, i.e., neutral, produce primarily H2O and CO2 products. While we cannot see it, it is very moisture laden air (H2O) and would increase the attic humidity level and potential for condensation (especially in winter) on the attic insulation and wood structural members. Even in that purist of neutral flames, significant levels of 'bad gases' are still produced, which are best dealt with by dissipation into the outside air, and not allowed to pool in the slow moving attic air.
One should use sealed, metal ductwork in the attic space to connect the interior flamework vent hood/system to the exterior air, and not rely on the attic's air volume or it's air extraction to remove the flamework combustion gases - in the same way that we would not rely on the attic to do so for a kitchen range or a fireplace.
This was a very good question - one where the answer is not readily apparent, and where 'code says no' needs explanation. I hope I did not bore you with too much detail.
Me
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"Education is what remains after we have forgotten what we learned" ~ I forget who said that.
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