More Basics

The next big factor is the air (or gas) space. Air is a great insulator, but is prone to transfer heat by convection. Remember, hot air rises, so, if there is room for convection currents to develop, heat is transferred from the warm inner pane to the cold outer pane. If the airspace is kept below about 18mm, the viscosity of the air tends to limit this convection, so the insulation improvement to a sealed unit increases with the airgap to around 18mm, after which it falls off. In reality, the curve is very flat from 12mm to 20mm, so the improvement beyond a 12mm airgap is pretty small. 16mm is probably the peak in insulation terms, beyond which any improvement is doubltful or unlikely. To complicate matters further, the failure mode of sealed units is generally via moisture permeation (at molecular level) through the edge sealant, and the wider the airgap, the shorter the life of the DGU.

So sealed unit design, as with almost everything else in life, involves a compromise. In my considered opinion (borne out now by some thirty years of experience) a 12mm gas-gap is a good balance of the factors involved, and for timber windows a 20mm DGU (4/12/4) allows an economy of framing material while still achieving the desired insulation values and long unit life.

On the face of, the 4mm airgap (gas-gap) of Slimline units - with its reduced insulation value - might be offset by a reduced expectation of moisture permeation, and a thus enhanced unit life.
Regrettably, it's not as simple as that.

More next time.