Q: Would attaching a 4mm polycarbonate sheet to a single glazed window with magnetic tape (giving an air gap of 3mm) be worthwhile as thermal insulation? Is a 3mm gap too small to be effective?

A: A single glazed window will have a U-value of around 5.4W/m2K. A 4mm thick sheet of polycarbonate (about the same thickness as the glass) will have a U-value of about 4.7W/m2K. Taken together using polycarbonate as secondary glazing will certainly get the overall U-value below 3.5. How much below is where the air gap comes in.

A 12mm gap is typical although some authorities suggest that 16mm is optimum. What is wanted is to get the two panes close enough to minimise air movement but not so close as to allow radiant heat to cross the gap. In this case a 3mm gap will allow radiant heat to be transmitted but there will be very little air movement.

If the magnetic tape provides a good, draught-proof seal then the overall U-value will be in the region of 3.2W/m2K.  A 40% improvement in thermal efficiency over a single glazed window.

So, in short, yes it works.

But all this talk of U-values might be missing the main point. The reader does not say, but if this polycarbonate sheet is going over rattlely sliding sash windows the biggest benefit will be in the draught-proofing qualities. Getting a good, air-tight seal around the window will cut down the heat loss from the room far more than the insulative qualities of the polycarbonate sheet.

Photo by Greeblie

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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