This is really only ideal as the icing on the cake for those already having a very air-tight & well-insulated enclosure to dwell in otherwise, with effective HV

This is really only ideal as the icing on the cake for those already having a very air-tight & well-insulated enclosure to dwell in otherwise, with effective HVAC in place to keep up the oxygen level for when the enclosure needs to be kept tightest during outdoor temperature extremes. If everything else can be brought much tighter than average, then adding something like a third window pane can take you the extra mile.