Its nearest competitor was a well-fitted Roman blind at just under 30 per cent. While the humble curtain barely got a look in, with even floor-length, heavy, lined curtains only breaking 25 per cent.
"We started out thinking it was important to choose curtains or blinds that created a sort of seal against the wall and window, that stopped air getting to the glass," said Consumer NZ's head of testing Dr Paul Smith.
"But then we also thought that the thickness of the material would help as well. It turned out it is both of those things. And the way we installed them, honeycomb blinds managed to have best of both of those elements."
Curtains that fall to the floor and create a seal against the window are more effective than their shorter counterparts.
Smith says there were two reasons the blinds worked so well. Firstly, they were fitted inside the window frame, with the testers trying to "minimise the gaps down the sides and at the top and the bottom".
"Typically, you get a gap between the blind and the window frame where the cord hangs, whereas these didn't have that. They fitted right up to the edge."
Then there were the pockets of air inside the blinds themselves.
"I don't want to call them a down jacket, but they are almost like a down jacket in that they have these big pockets of air, so they're creating this really good insulation. It's a combination of the two actually: creating a still insulating layer of air, but also fitting really tightly.
"I think from everything we did that's the two bits of advice that we would give to anybody, no matter what type of blind or curtain you've got: make sure they're fitted as tightly as you can to minimise air gaps all around. Then go for as many layers or insulating thicknesses as you possibly can. That will get you the best result I think."
Thermal curtains performed unexpectedly poorly, not even retaining 20 per cent of the heat lost through an uncovered window, although in all cases, floor-length curtains outperformed sill-length ones.
"That was a bit of a surprise actually. Thermal curtains worked, but the thermalness of them didn't - and these are the thermal curtains that have the plastic backing bonded to them. They didn't work as well as we would expect because the plastic backing doesn't actually do anything much to stop cold going through. It stops the air passing through, but then so does an ordinary thick curtain anyway.
"What we found was better was the sort of curtain with a separate liner behind it. Having two layers there actually did that same job of trapping air, and that's what made more of difference. So it was a bit of a surprise, but then when we looked at all the results we could explain why.
Curtains that aren't close to the top and bottom of the window sill were found to make the situation worse, by leaving a channel for warm air to sink into, be chilled by the window panes, and flow down into the room in a continuous, chilly circuit.
"I must admit I was sitting there feeling a little bit smug, because I put those honeycomb blinds all round my house about eight or nine years ago. Only I didn't do it because I thought these are going to be great thermally insulating. It was just I quite liked the look of them and we needed blinds, so we chose those.
"Since we did it, I'd been noticing when you pull them up in the morning you can feel the cold air fall out from behind them and I thought, 'these must be doing something'. So seeing the result, that they actually are, is really pleasing."
Smith says the "key take away" from the report should be that new blinds are an expense, and you're not going to get that money back by changing your blinds.But if you were going to change your window treatments anyway, there are a few things to think about.
"It's about trapping air, basically, it's not about having the thickest single layer of thermally insulated curtain. You can take some really cheap, lined curtains, put them up properly, full length, to the floor, make them wide enough so they create a sort of seal against the wall, and you'll probably get a much better result than a set of expensive curtains that aren't fitted very well."