"David" wrote in message
...
Janet,
Some polycarbonate roofing sheet has uv filtration

No polycarbonate of the thickness encountered in horticultural
applications lets through any UV worth talking about. It is a very
expensive business to make transparent objects which will transmit a
substantial amount of UV. Fused quartz is the sheapest one. So, for
any horticultural purposes, you can forget about UV light penetrating
into the interior of the greenhouse or tunnel, whatever they may be
made of. Anybody who waffles of UV filtration in polycarbonate
roofing sheet is therefore performing a con trick.

and reflective
qualities to cut down the amount of light, so watch which one you go
for.

For a transparent material which does not have a surface bloomed to
the accuracies of typical light wavelengths, there is a one-to-one
correspondence between the reflection coefficient and the refractive
index. All transparent polycarbonates have the same reflection
coefficient to within a gew percent. They wiol therefore all have
thsubstantially the same reflection coefficients. How then does the
manufacturer achieve the reflective qualities of which you speak?
Oncd again, I suspect a rip-off.


Another factor is cost, polycarb is more expensive than glass.
Cutting to
size is easy as it can be cut with any decent saw. To fix, rather
than
screwing, on the assumption that you have a wooden greenhouse,
either use
glazing bars (from the same people that sell polycarb) or make some
plastic
(or aluminium) clips with a single screw hole long enough to support
the
sheet across the join (50mm x 20mm say). The sheets are incredibly
light and
strong so won't require much support. Ensure that you seal both ends
using
tape, otherwise you will find all sorts of insects getting into the
channels,

Yes. The sealing of the open ends is very important also to prevent
the ingress of water, which would encourage algal growth inside the
cells of the double walls.

Franz