Robert Brenchly's comment about having at least a 3" solid board at the front of the screened area, in order to avoid bees becoming confused and going under the screen, is a useful piece of information and perhaps explains why the screened bottom board shown at beesource .com is shown as 4". Perhaps the 3/4" space above the screen is also important since a previous screened bottom frame which I made with a 1/2" space caused the bees to build brace comb down from the frames, although this frame was simply inserted between the deep and the existing solid floor (top bee space Smith hives). On the top ventilation versus insulated top cover debate I would probably favour ventilated tops but with the proviso that the ventilation should be at the sides rather than in the middle to prevent chimney drafts. It is common in the west of Scotland (a damp area) to insert a matchstick under each corner of the top cover in winter prior to replacing the roof. The absorbent top covers still used by many beekeepers in the UK on WBC hives (a type of twin walled bottom beespace hive) is usually placed directly on top of the frames and probably works by "wicking" the moisture right up through the top into the upper open ventilated space within the outer box. Water vapour as well as being colourless and much lighter than air is also "thin" and would escape through textile material easily.You can only see water when it recondenses into water droplets - as your breath on a cold day. Just as well water vapour is lighter than air or we'd have the clouds continually round our ankles. Alan Riach Bathgate,Edinburgh