Argon filling helps with the U value of units by so much that people dont realise how important it is. U value changes on DGU when you tilt them into a roof so the Argon becomes even more important on roof units.

As an alternative to Celsius, try Reflex as it has the Edgetech super spacer technology instead of the traditional metal spacer bar. This is the primary seal on the DGU and therefore the Argon is far more likely to stay in situ. Also they offer a 20 year warranty on their units against seal failure, which means you have no chance of condensation inside the cavity of the unit. All manufacturers offer warranties on their seal but they can never be really sure of how good it is on any single unit with the traditional aluminium spacer bar but with the super spacer it provides a permanent seal. You then have the hot melt (black goo) as the secondary seal (which is what normal aluminium DGU use as the primary seal). The warranty may cover this but the hassle factor of getting a failed unit replaced is not really what you want to have to experience.

You should be thinking of a solar control glass for the roof, and not just any old low E glass otherwise you will end up with a Sauna. Ask your installer for promotional literature on the above brands or search on the internet yourself for more info.

Tanner