PVC Foam Board Buildings

[nicebutdim]

Here's the datasheet http://www.bostik.co.uk/diy/product/evo-stik/Impact/6/technicalData safe for pvc. No mention of waterproof though but I have used it outdoors before with no issues.
open the 'impact' section part way down.

 

[LGB-Sid]

Had a rummage around in the Garage as I knew I had some UPVC plumbers cement somewhere, tried it today you get a bit more wriggle time than you do with Supa Glue which is what I was after. It's defiantly waterproof as it's used for sticking Plastic drain pipes. It doesn't melt the foam board and after about 30 mins if you try to pull it apart the foam board tears, not the joint giving way so it differently sticks

 

[400Parker]

Apologies for coming to this a bit late. I use PlasWeld or equivalent (Polyweld?). It's a liquid plastic solvent. The only thing that's important is to make sure the two edges you are gluing are flat so that there is a good capillary action to 'suck up' the solvent.
Nice windows and door you are using!
Steve