This section is from the book "The Gardener's Monthly And Horticulturist V26", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
F. B. Smith, Danville, 111., asks: "Which is the best way to glaze a greenhouse? Should the glass be butted up, or should it be lapped?"
[ Some attempts have been made to glaze by butting the edge of one sheet of glass against another - dipping the edge first in white lead - and it is the perfection of glazing. All the light possible is generally needed in greenhouses, and it is desirable to retain all the heat possible in cold weather, and this is all aided by this system of glazing. But even paradise had its attendant evils, and so has this system of glazing. For instance, if a pane gets broken it is very difficult to fit in another so that there will be no leakage. Another trouble is, that when a house is very tightly glazed the moisture freezes on the glass, and when the sun comes out there is a shower of snow on the plants, which is not pleasant and often injurious. This moisture escapes through lapped glass, and, although there is some loss of heat, cultivators generally prefer to lose the heat than have the snow.
On the whole, lapped glass is preferred. But laps should be as narrow as possible. If the laps be broad, dirt settles under, and this just so much shades the house. If we have twelve-foot sash, six-inch glass and half-inch laps, when the laps get dirty it is equal to one-twelfth less glass. Again, in wide laps the moisture condenses, and the water is held and then freezes, the expansion cracking the glass through the middle. In many greenhouses the broken glass from cracking through the ice under the broad laps is enormous. - Ed. G. M].