In the Netherlands, insulated glass is rated by a standardized rating system, classifying it into 3 categories by the u-value:

  • HR : U value between 2.0 and 1.7
  • HR+ : U value between 1.6 and 1.3
  • HR++ : U value less than 1.2

The price difference between these types of insulated glass is quite large. After the glass has been installed, is there any simple way to measure the U value or otherwise check the type? I guess it would be quite easy for a swindling contractor to sell HR++ and install HR without it ever being noticed?

3 Answers 3


Louis it is a really good question. First if you have the space to put the cheaper insulation then you can simply buy more of that to get the same results. If the insulation type is the same but different U value then it only matters what total you are getting.

How to verify this? This has been answered on the site many times - a couple by me. There isn't an exact way but you can get a rough estimate. Remember you are also measuring the installation not just the insulation. If not packed correctly and there are gaps then you will have the same poor performance.

How do you know what a contractor installs? You watch them do it. If the insulation is covered in walls or whatever then there isn't an exact way to tell that they did or if it is the right stuff. You need to be there and take pictures and ask questions. A good contractor won't care if you are doing this.

  • Seems like you misunderstood insulated glazing with fiberglass insulation? Insulated glazing is used for windows, and is not hidden behind walls. It is the transparent part of the window and is made out of two layers of glass with a gap between them, sealed and filled with some invisible transparent gas. The glass should be coated with an (invisible) thin metal coating. The window and all glass parts and layers are openly viewable, however the features are all invisible to the naked eye. The only thing I can verify is two layers of glass with a gap between. How can I verify coatings and gas? Commented Mar 10, 2014 at 19:07

This is hard to do and I agree that there is a big range of cost between basic insulated sealed units and some of the high performance glass units which can be triple the cost. Here in Canada when we install a commercial glass project we are required to provide warranty documentation from the glass supplier which shows the makeup of the sealed units ( glass thickness, low-e coatings and any gas filling details ). This information even provides the invoice numbers for glass provided for the job and can be a great help in matching up replacement glass when breakage or insulated glass failure becomes an issue down the road. I would try to get this documentation from window / glazing contractor. If this is not available there is a measuring tool from CR Laurence that can tell you the glass thicknesses, airspace thickness and what surface the low-e coating is on. It won't tell you the type of low-e glass nor whether there is any argon gas but will provide you with basic information. A professional glazier can often take a look at the colour of the glass from inside and outside to point you in the right glass type direction as well. Good luck!

  • Do you have a link to the tool from CR Laurence? I couldn't find it. Probably it wouldn't be worth the it for just me, but offering measuring as a service it could turn out profitable. Especially if it uncovers some wide scale swindling. I'm really wondering if there's any coating at all on my windows, I don't see any difference between the inside or outside, and no marking. The glass layer on the outside is a fraction thicker than the one on the inside. Here we only get warranty for the window frame, glass is usually covered by a separate insurance. Commented Apr 5, 2014 at 9:50

I finally (9 years later) found an easy way at the end of this video on YouTube (it's in Dutch).

Hold a lighter, match or candle in front of the glass and look at the reflections, with double glass you should see 4 reflections (each surface will reflect a little). If it is HR++ glass then the 2nd or 3rd reflection should have a slightly different color.

Here's an example where it can be clearly seen: https://youtube.com/shorts/a5ULReRtAbA

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