Subject: Re: greenhouse windows

From: [email-masked] (george p swanton)
Date: 2 Apr 1996 19:05:51 -0500

In article <[email-masked]>,
Carl Steen <[email-masked]> wrote:
>
>Does anyone know of inexpensive ways to make =double= paned
>greenhouse glass panels that do not trap condensation?
>We intend to build them of wood frames and thought that
>sealing them in a very dry environment and sealing them
>well in the framing process would help.
>Perhaps there is some type of caulking that is better than
>other types for preventing condensation?

 Wood is porous and will absorb and release moisure continuously
 in an effort to reach eqilibrium moisture content. If the 
 ambient relative humidity is high and the moisture content of
 the wood is low, mositure will be absorbed. If the air is 
 relatively dry, moisture will pass from the wood to the surrounding
 air.  If you were to seal glass to wood in a very dry environment, 
 moisture will inevitably pass through the wood from the outside
 air to the air gap.

 Also, a large pane of glass will expand and contract a fair amount
 with changing temperature. Maintaining an airtight seal will not
 be easy. Independant of any attempts to prevent condensation, be 
 sure to provide for this expansion and contraction or your windows
 WILL break.

 I am not expert in this field but I believe prefab units are both
 hermetically sealed and include a dessicant band to avoid condensation.

 A low tech approach might be to provide for ventilation to allow 
 moisture to escape. Many frames have 'weep holes' to allow condensed
 water to escape and prevent rot. Small vent holes could probably 
 serve without significant thermal penalties. 
 
 More expert parties are encouraged to substantiate or refute at will.
 gps