An interesting article that leans heavily on the work of Derek Mitchell, formerly of this parish, who was driven away by numpties who attacked his science with hearsay and prejudice - seems to be a recurring theme on the internet, doesn't it?
The article was dated OCTOBER 21, 2016 so not this month's issue of Bee Culture.
The author of the piece, William Hesbach, says, in relation to sleeves of rigid insulation "For the sleeve’s rectangular dimensions, just measure your boxes and add one inch to both measurements". That gives a half inch gap all the way round the brood box, which in my opinion is a wide enough gap for air currents (convection) to be set up, resulting in heat loss out of the gap at the bottom. He talks about supporting the insulation sleeves -"I install a small shelf of wood that supports the foam and keeps everything at the correct height" but he does not say if this is continuous around the base of his sleeves, thereby sealing the bottom of the gap between the sleeve and the brood box.
My own hive cosies are the dimensioned only 10mm greater than the external dimensions of the brood box and I think 5mm all round the brood box is small enough to prevent heat losses due to convection currents.
CVB