A claim that a window film would "keep in 90% of your home's heat" was likely to mislead consumers the Advertising Standards Authority has found.

The claim appeared in a newspaper ad for Enerlogic Window Films placed by High Performance Window Films Wellington (HPWF). This type of secondary glazing product has been shown to improve the insulation performance of single-pane windows – which have a low insulation value – but it won't "keep in 90% percent of your home's heat" as claimed.

The ASA’s Complaints Board accepted evidence that the window film could provide "a 95.8% improvement" in insulation over windows that had no film. However, the board said this figure couldn't be extrapolated by advertiser HPWF to mean the film was capable of retaining as much as 90 percent of a home's heat. The board ruled that the ad was likely to deceive or mislead consumers and had not been prepared with a due sense of responsibility.

Ads for secondary- and double-glazing have been on the rise in recent years. If you're buying one of these products, ask the supplier for reliable evidence to back up any energy-efficiency or energy-savings claims. If you think the claims are misleading, you can make a complaint to the ASA or to the Commerce Commission.

Our April 2012 report on double-glazing provides info on which types of secondary and double-glazing are likely to perform best.

*This is now a revised ASA decision. The original decision identified MEP Films as the advertiser not HPWF. The revised ASA decision notes that MEP Films did not approve the advertisement placed by HPWF.