Plywood Strength

I am planing to build a vacuum insulated " hot" water tank using FRP covered half inch plywood with a half inch space between walls.
Does anyone know how many square inches of unsuported area I can have at 25-27 in. Hg ? Supports will cause heat bridging which I want to avoid, naturally.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.