In article , Franz Heymann notfranz.h
writes

I do indeed have a point which you clearly have not appreciated. The
desirable polythene is *not*used because it blocks UV, as you
originally implied. All polythene always block UV light. The
desirability of the UVI/EVA polythenes lies in the fact that they are
reasonably protected against UV damage to themselves, as numerous
posters have now pointed out.

So the material is correct for the purpose, rather than as you
suggested. The plants receive reduced ultraviolet light compared with
outdoor ones, but then that is also the case in a glazed greenhouse,
whether horticultural or window glass is used. Polycarbonate sheeting,
sometimes double glazed, is now being used for conservatories etc., does
anyone know what light transmission that has?

Suppliers of UVI/EVA polythene sheeting suggest a five year life span in
polytunnel use. Quite often the sheet will need replacing earlier than
that due to structural damage, but we find that by the time a sheet has
been in place for more than two or three years, it is becoming opaque.
It also collects airborne dust, bird droppings, algae etc, which we
clean off, but we cannot do anything about the opacity. In fact, growth
and health of plants in the polytunnel does not seem to be affected by
this extra light barrier, but we prefer to change the sheet before the
five years of it has become badly fogged.
--
Alan & Joan Gould - North Lincs.