Low-e is a spectrally selective coating. With double glazed units placing the coating on plane 3 (counting from the outside) gives you a higher solar heat gain. To make the most of passive solar in the winter placing the low-e on plane 3 is recommended. If a low-e coating is placed upon plane 2 then the solar gain is reduced thus impeding passive solar gains.
Be very careful with west facing glazing as this could easily lead to over heating and discomfort.
(You don't mention the roof of the garden room so I'll say this anyway, avoid glazing if you don't want to cook.)

Mark

Biff, that was a very good explanation -- please could you expand on it a bit?

I do as it happens but I was more meaning in terms of what to do for max solar gain, max comfort, minimum gain etc

My understanding of the coatings is that they let in infra red radiation in but dont let it out again. The effect being in heat loss terms the same as having the curtains drawn in the day.

And the moral of that is make sure the low-e coating is on surface 3, where it should be, and you didn't put the unit in back to front.

("the radiant absorbed component and direct component", Eh? Perhaps if you could be a bit more techie I'd be able to understand that.)

That's right. When you get the units you should find that the manufacturer has put a little paper sticker on one side saying: "This must face in to the room" or some such. You can get an electronic detector that tells you which way round it is. Pilkington K is the commonest brand in England. In France you are probably getting something from St Gobain. There are two different types of low-e coating 'hard' and 'soft'. The hard coat (eg Pilkington K) is slightly cheaper and gives a slightly worse performance. Soft coat (eg Pilkington Optitherm) gives a slightly better performance but is delicate so requires special handling equipment when making up the sealed unit and is therefore more expensive. Not all unit manufacturers have the equipment so it's not so widely available.

Sorry to go on about this but following is a recent e I have received from a former manager of a UK double glazing company:


Double glazed unit panes are numbered 1-4 (1 being the outside pane).
For the radiation effect to take place, the coating needs to be on pane 2 (the inside of the outside pane).
On a summers day, this coating will restrict the heat from the sun passing into the conservatory.
In the Winter, the coating will only absorb 15 - 20 % of the heat in the room (as opposed to some 80% if none coated), the other 80 - 85 % of the heat will reflect back into the room.
(If for instance, the coating is on pane 3, this would trap the suns heat and drive it inwards and could result in breakage due to thermal stress.)
Therefore : With the coating on pane 2, the net result is less overall heat gain. In Winter, whilst there will be less heat gain during the day, there will be less heat lost at night, and in Summer the reverse applies.
In my opinion, this is the best balance in a conservatory. "


So, pane 2 or pane 3, breakage due to thermal stress, its all very confusing. Logic tells me that with the deliberately large overhanging eaves, heat gain through the glass in Summer shouldn't be a problem, but the non opening double glazed roof light might be. So should it be plane 3 for the "vertical" windows, and plane 2 for the roof light.?