Appropriate lid of cold water tank

When we moved into our house a couple of months ago, there was no lid on the domestic cold water tank in the loft. I cunningly fashioned one from a sheet of exterior ply, but now find, only two months later, that there is a horible layer of green mold on the underside of the wood. Can anybody suggest a more suitable material? [The tank looks to be fibreglass, and so I'm fairly sure that a 'standard' plastic lid will not fit]
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mike Hall wrote

The mould is due to condensation forming on the underside. This wouldn't happen if the lid was of rigid insulation board. 20 years ago I made a lagging set (including a lid) for my F/G tank using 50mm polystyrene board held together with 4 nails pushed in. It's still OK and there's no mould, except particles of the polystyrene sometimes fall in the water so I wrapped the lid in a bin liner. If I was doing it today I would use Celotex or Kingspan.
Peter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Hey Mike, fancy meeting you here in the repository of all diy knowledge.
You get lots of condensation on the underside of the lid, ideal mould growing conditions especially on something absorbent like wood. A nice smooth piece of plastic would be better, got any conservatory roof offcuts or anything like that?
--
Tim Mitchell

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
mike snipped-for-privacy@peppertree-broadcast.co.uk (Mike Hall) wrote in message

A sheet of acrylic double-glazing material from a window supplier? Clean up your plywood sheet and apply a coat of waterproof polyurathane?
hth David
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.