Polystyrene insulation in bottom half of roof only - why?

[--tom]

Noticed roofers inserting polystyrene sheets in rafter bays only at the bottom half of the roof. Halfway up they ran what looked like Eave plastic vent strip across the rafters.
Any ideas why they were doing this?
I also noticed them inserting green plastic snakes of what seemed to be some kind of infill at verges . What and why?

 

[cmother1]

Did you try asking them??

 

[Nige F]

I keep seeing those snakes--coming out my computer

 

[Nige F]

I`ve had a lie down now.. .The "snake" is probably to stop birds etc. getting into the roof...An eaves filler. The polystyrene is possibly to do with the enormous amount of insulation -10 inches of fibreglass- that goes in roofs now. Then the air has to circulate above this.

 

[--tom]

Nigel, thanks for the info. however i dont think that i explained myself well enough. The polystyrene filled the rafter bays up to half way up the roof, and then the strip of black, plastic ,eaves venting was fixed - presumably venting something.
there was no venting, that i could see, at the eaves.
When i'm next past the site i'll look to see how it was finished.

 

[Nige F]

Ah, probably a room in the roof section, with skielings (spell)sort of a chalet. up to the end of the polysyrene then normal roof void above. maybe a discreet vent @ the eaves