I can find very little on the forum about heat mirror glazing. As one supplier explains it, "Heat Mirror is a film suspended inside double insulating glass. It is a three layer system with two independent air spaces, but has the advantage of weighing the same as a double glazing unit. "

So it's like triple glazing, but the middle pane is made not of glass but of a film. Allegedly U values are as good as or better than triple glazing - see http://www.albo.co.uk/what-is-heat-mirror.htm. The advantage for me would be that's it lighter, which is why my architect has suggested it since we're considering large expanses of glass in a roof structure.

Does anyone have experience of this? I can envisage various potential problems. Even if it looks OK to begin with, it would be no good if the suspended film started to sag after a few years.

All I've found here is from three years ago, when mbartosik wrote at http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=527&page=4:

"For anyone reading from elsewhere [than the US], try looking for windows with "Heat Mirror" films. Adding a 3rd layer of glass makes the window heavier which is not typically an issue unless you want impact resistance or lamination for sound deadening. The Heat Mirror films are very light weight and come in various formulations and single and double coated. Alpen make windows with up to 3 films (4 gas spaces) and up to double coated. The more films and coatings the more the tinting effect, so it is important to see an example. I'm ordering mostly with two single sided films (some with two double sided films). The center of glass R value is about 9.5 (US units) for two single sided films"

Oooh dear!

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8288&page=11#Item_24

Try this link: http://www.southwall.com/southwall/Home/Commercial/Products/HeatMirrorInsulatingGlass.html - basically it's a low emissivity film, sort of like triple glazing but without the inner pane glass, just the coating that would be on the glass.

Paul in Montreal.

Looks like they are using Imperial U/R values, not RSI. 2.81 is the conversion factor if I recall correctly (either multiplied or divided depending which way you're going). R20 is equivalent to RSI 7.11 which is U=0.14

[ edit - I screwed up the conversion from R to RSI. Multiply/divide by 5.678. So R20 is RSI3.5 is U=0.28 - sorry for the confusion! ]

Paul in Montreal.

Heat Mirror is a well-established product, and notably recommended by Amory Lovins.

Paul, I don't know which numbers you're looking at but U=0.14 seems fairly remarkable. As far as I know, a glass-HM-glass triple-glazed unit has a similar U-value to an all glass unit. But it's lighter. But they do make units with multiple interior HM layers, so maybe they can get to 0.14. If so, I would check the percentage of visible light transmitted & g-value as well as the U-value.

Posted By: Paul in Montrealan industry-leading U-value of 0.05 (R-value of 20)."

Seems I screwed up my conversion factors!! 5.678 is used to go from R to RSI. So R20 is U=0.28

Still not clear? Is R20 = U 0.05, or U 0.28? Depends what units these U values are in?