I am trying to choose between two conservatory suppliers. One uses Pilington K and the other two layers of glass with an inert gas between. which would be the coolest in a south facing conservatory?

The gas is most likely ARGON and should be cheapest. Pilkington K Glass will give you a better level of insulation overall.

However your question is about what keeps you coolest - neither of these options in them selves will be the best to use for that specific purpose. You need to consider some sort of solar control glass - maybe combined with K Glass or argon filled units.
Have a look at:
http://www.conservatoriesonline.com/...l_glass_uk.htm

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http://www.pilkington.com/europe/uk/...architects.htm is a link to some PILKINGTON RELATED FAQs

They are all worth a read in particular FAQ 4

What glass would you recommend for a domestic house or conservatory for South facing applications?

ANSWER The Pilkington Optifloat? Tinted (formerly known as Pilkington Antisun?) range has long been an industry standard for low-to-medium performance glass with the Pilkington Suncool? range suiting high performance needs. The Pilkington Eclipse? range fits between the two.

To maximise the reduction of solar gain, the ideal position for a solar control glass (e.g. Pilkington Optifloat? Tinted) in a double glazing unit is the outer pane. Pilkington K Glass? is not a solar control glass: however, its presence in a double glazing unit can considerably improve its thermal insulation (reduce the U-Value) over conventional units. The ideal position for the Pilkington K Glass? low emissivity coating is on the inner pane facing the cavity. Hence, if the requirements are for reduced solar gain and improved thermal insulation, both types of glass can be combined in one unit. Both Pilkington K Glass? and Pilkington Optifloat? Tinted are available in toughened and laminated form.

Better insulation and no cold spots near windows make Pilkington K Glass? an excellent choice for conservatories, because thanks to a special low emissivity coating on the surface of the glass, it let?s the sun?s rays through, but reflects heat from fires and radiators back into your home - heat which ordinary double gazing allows to escape.

In fact it will typically keep an unheated conservatory much more comfortable for a greater part of the year than ordinary double glazing, allowing the occupier to make the most of the conservatory all year round.

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