Tuesday, 18 November 2008

Cellular Shades

I like the look of these shades (or blinds, as I think both blinds and shades are usually called in British English):


These shades appear to be not only aesthetic, but potentially very useful for making energy-efficient buildings.

This is because, in a low energy building (in a cold climate), the fenestration is usually responsible for a large proportion of the heat loss. For example, in a Passivhaus design, the u-value of the wall will typically be about 0.15 Wm-2K-1, but the u-value of the windows will be about 0.8 Wm-2K-1. This is a much higher rate of heat loss (although admittedly of course the area of the windows will be much smaller than that of the wall). The reason for the greater conductivity is that currently we don't have any way to make building materials both highly transparent and highly insulating.

Because of this problem, it would be very useful when making buildings to have a removable insulation layer that can be placed over the window when transparency is not needed and removed when it is. Unfortunately, this hasn't really proved practical in most cases. Insulated shutters are bulky and not mass-produced, and typically occupants won't use them extensively anyway. This is why cellular shades have such potential; they're a collapsible insulation layer that the occupant can use for privacy exactly like as a normal window covering, a window covering being needed for most windows in any case.

I am particularly impressed by the manufacturer EcoSmart mainly because they offer so many options (including sidetracks, which surely must be vital for decent insulation) and have such an informative website, which also appears to actually have a useable ordering system (although I have not actually used it to buy anything).

I do have one serious criticism of their marketing, however, which is that the r-values they quote for their product are cumulative i.e. they include the r-value of the window as well as that of the shade. One can therefore only guess what the r-value of the shade itself is. In addition, the r-values are based on figures supplied by the manufacturer of the cellular fabric, rather than on independent testing, although I don't think this is actually a big problem.

I emailed EcoSmart to ask about the lack of a definitive r-value for their product, and in their reply they claimed that the r-value of the standard light-filtering shade with sidetracks is about 0.65 m2KW-1. I note that they have not added this claim to their website, however.

If correct, this represents a considerable energy saving, as it would reduce the u-value of a typical double glazed window from, say, 1.5 Wm-2K-1 to 0.75 Wm-2K-1 i.e. the shade would be as insulating as the window itself, and, albeit only when covering the window as at night, would reduce the u-value to Passivhaus levels. Impressive!

One thing that does trouble me about the whole concept is that the shades are not vapour-proof. This suggests that, because they necessarily reduce the temperature of the inner surface of the window, they probably promote condensation at that point, leading in all likelihood to mildew. I don't know whether or not this is a significant issue in practice, though. Condensation forms on double glazed windows in cold conditions even without coverings.

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