What causes this?

I have done many houses where some of the windows have metallic looking spots or streaks between the panes. I assume this is a problem with the low-E coating. My question is: what actually causes it? Does it come this way from the factory (defective), does it develop over time? Does anyone know what is actually going on with these windows?

I think what you are referring to is a blown seal. It will sometimes look like a metal sheen or little metal dots in certain areas. It almost looks like a light coating of oil (rainbow like) between the panes. Most of the time I see this is when i was cleaning on the exterior (looking into a dark room). Its hard to spot looking from the inside to the outside. Not really sure what does cause it. Ive seen it on new glass mostly. It does seem to spread out more and more over time. Maybe email Dan Fields. He would probably know.


I think its probably a blown seal, once the seal is gone moisture can get in. Once the moisture is inside I believe the low-e coatings have some type of metal component that starts to oxidize.

It is not (imo) a blown seal. I have seen this w/ peach tree windows and it seems to be a precurser to the seals going but can last for years before seeing the condensation buildup. I have seen during this time from year to year, the metalic spots grow in size and become more numerous. Peachtree has been honoring the warranty w/ whatever this defect may be.

It’s a low-e reaction. It’s caused by many factors. One that hasn’t been mentioned yet is fabrication debris. Low-e is a metallic compound that is spatter coated on the surface and when it hits FD it reacts in a variety of ways. The most common is the dreaded fish eye pattern. I have also seen the rainbow discoloration. One of our customers just had a huge (6ft by 6ft) picture window replaced because of several of these spots. He said for the last few years he would see a small 1/4" line of frost at the bottom of the window. The window company attributes it to the desiccant in the IG unit failing. Moisture seems to be the main culprit though.

Tony, I thought fab debris was on the outside and the low-E was inside the IG unit? If that is the case, how can the low-E hit fab debris?

Fabricating debris can be present on any surface (1 thru 4) of an IGU (insulated glass unit.)

While manufacturers prefer to place the (known) fabricating debris on surfaces 1 or 4 so that a pristine surface is available for low-E coating, that is not always the case.

Nice informative post Tony!


Larry explained it very well. The manufacturer is aware of the reactions and so will turn the side w/ the highest potential for FD to the outside to avoid this. If you see this reaction it is because they didn’t keep track of which side they normally would turn in.