The windows at Trillium House are made by Duratherm. Duratherm makes windows from spanish cedar (as well as other woods). It is my understanding that milling western red cedar can be unhealthy for the people inside the shop, so spanish cedar is used. It has a similar characteristics as western red cedar without the environmental issues. Western red cedar is commonly used for siding.

Is is treated with a stain and a polyurethane over it?

Duratherm offers a range of woods and finishes for their products. We spanish cedar with a 4-coat "cedar" stain finish color. They use Sikkens stains for the finishes. Poly was not used.