Vapor Barriers in Wine Rooms

DEAR TIM: I read your comments in AsktheBuilder about it being absolutely necessary to install a vapor barrier on the warm side of the walls and ceiling of a wine room. I am building a new home with a wine room and there is some disagreement about the need for a vapor barrier. We are using green board walls and mildew resistant paint on the inside. The temperature and humidity control will be a Breezaire.

Please explain why the vapor barrier is so important. The walls of the wine room are inside walls, and not exposed to the exterior of the house. Also, I live in Fresno California, in case climate is an issue. Thanks for your help. Terry L.

DEAR TERRY: There are several dynamics happening in wine rooms. First, the temperature of the space is supposed to be 55-58F. This cool temperature can create condensation on the backsides of the wine room walls *if* they come into contact with humid air whose dew point is at or above that temperature. Here where I live there are many summer days where the dew point is above 57F. So the challenge is to isolate the cool temperature from the humid air that can find its way to that surface. A vapor retarder on the warm side of that wall system will work just as it does inside my own home's exterior walls.

Then you have the issue of the elevated humidity. The wine rooms love relative humidity levels of 55 - 75% relative humidity. That is frighteningly high humidity. If that water vapor escapes from the wine room somehow, it can condense on cooler surfaces in a heartbeat. There are climates where that is possible. Granted, as that air hits warmer air, the water vapor is accepted by the warm air which can hold the water vapor in the vapor state. The trouble is, you want to be sure the water vapor does not condense. So why not take that out of the equation? Why not block the movement of the humid air so it can't get out of the wine room?

Fresno as you well know is in the Central Valley of California. If memory serves me right, your winters are very mild and summers are hot and dry. This being the case, your humid air will constantly be trying to leave the wine cooler to get to the warm air. This would make your humidifier work that much harder to keep the humidity levels high in the cooler. If this hypothesis is correct, it would be to your benefit to trap the humidity in the wine cooler just the same as you are trying to insulate the walls to make it easier to keep the wine room temperature constant.