Condensation is Appearing on the Glass Wall of my Wine Cellar

Welcome to the club. You wanted pretty; you got pretty. But, like most things in life, beauty comes with a negative. If you own a wine cellar with at least one glass wall, the question is not if condensation will begin dripping down the glass, but when. You are not a victim. It is simple physics. Specifically, it is a condition centered on an obscure principle of water called dew-point. When the air outside the cellar reaches a certain temperature combined with a specific relative humidity, the atmosphere is no longer capable of holding water vapor. The water vapor condenses and forms water droplets on the glass.

This principle is demonstrated regularly when you drive a car. When a film begins to coat the inside of the windshield, you automatically reach for the defroster. Three things occur simultaneously. First a flow of air is directed against the inside of the windshield. Secondly, the air is heated (even on days when you are using the air-conditioning). Third, the recycle-interior-air switch is defeated, bringing in outside air to complete the action. The cloudiness immediately begins to dissipate.

Question: Rick, I have insulated glass in my cellar walls. I was told that it would never exhibit condensation.
Answer: Never is a difficult condition to prove. Any type of glass will exhibit condensation if the conditions are right…even insulated glass. This winter when the temperature plunges below freezing, with humidity in the teens and the home heating set to rock-and-roll, check-out the area along the bottom of your bedroom windows. (House windows are usually double insulated and sometimes triple paned.) If your home humidifier is working correctly, you will find a one-inch border of condensation along the bottom edge of all the windows in your house. Insulated glass resists the formation of condensate better than single-pane; but both will show it.

Question: The glass doors on my cellar have air gaps between them and also air gaps around the hinges. If I manage to seal these perfectly, can I avoid condensation?
Answer: Nope. First, no one can seal glass doors completely in a wine cellar. Vinyl seals, (the only types available), always stretch and flex over time. Moreover, there has to be some space between the hinge and the glass for the pin-hinges to work. And even if you did completely seal the space, condensation would still appear. Condensation is not caused by air gaps. Think about your car windshield. Sealed perfectly? You bet. If it wasn’t car washes would be a disaster. And yet, condensation still appears. It’s the same idea with your bedroom windows. Most are sealed
brilliantly but condensation occurs.

Below are several charts I completed. The first shows a temperature range between 55 degrees and 60 degrees with a constant relative humidity of 60%. Note the third line (dew-point). The next four lines demonstrate what happens when humidity is raised from 60% to 65% and then 70%.

Conclusion: given a constant humidity, as temperature rises, so does the dew point. As humidity
rises in a stable temperature, the dew-point rises.

Now check out the next three charts below. Note the temperatures range from constant 74 deg, 72 deg,
and 70 deg. In each chart the humidity rises in 5% intervals from 40% – 65%. Now, compare the
dew-points highlighted and consider the interior-glass, surface-temperature of your glass-walled
wine cellar. The highlighted temperatures are possible wine cellar temperatures. (For this
example, I considered wine cellar temperatures to range from 54 deg to 58 deg.)

With an outside air temp of 74 deg and a relative humidity of 55%, any glass surface inside the wine cellar that is 57 deg will show condensation. At a temperature of 72 deg and a 55% relative humidity that glass-box wine cellar kept at 55 deg may have condensation on the glass. Note also that as temperatures retreat it takes a higher percentage of relative humidity to make the water vapor in a wine cellar turn into condensation. (This only makes sense. Hotter air can hold more moisture.)

So, what can I do? My wine cellar glass is dripping water onto the floor.

You have the same three approaches that the auto engineers used when designing a defroster system. Any one of these three or all three combined may solve the problem. (I use the term ‘may’ because each cellar is unique and each ambient environment is even more specialized. My parents ran their furnace at 78 deg constantly in the winter and 81 deg when they were cold. My house is a constant 67 deg in the winter and 63 deg at bedtime. My parents would not survive my home and a wine cellar in either house would face a myriad of different challenges.)

I suggest the following:
1) Begin by placing a box fan inside the cellar. Turn it on high and angle it so that it blows across the glass, not directly on it. Condensation has a hard time forming on glass in a cellar where the air is moving. If this approach does not work over time, try redirecting the flow slightly.

2) Raise the temperature of the cellar. I understand that this suggestion is an anathema to all those who believe that a constant 55 deg must be maintained at all cost. Remember, the aging of wine is a chemical reaction. It’s just physics. Raising the temperature by two-degrees is neither going to ‘hurt’ the wine nor be perceived in the taste of the wine at opening.

3) For those of you with a humidifier in your wine cellar systems, turn it off. The cooling unit will gently reduce the humidity during standard operation. It may be enough to move the dew-point.

4) Do nothing. The weather will change. Humidity will lower and the temperature will rise and fall. Open one of the better bottles, pull up a chair, look at the wine cellar, and consider this statement. When I was a kid I never thought I would ever own a wine cellar. Now I own one that I not only enjoy but right now am relishing in its beauty. Life is good.

My best, Rick
Owner, Chicago Wine Cellar Expert Inc.