Latest Discussions : HVAC

zippyt

04:46AM | 08/13/04
Member Since: 08/10/04
1 lifetime posts
I need advice solving a condensation problem on the inside of my garage wall.

The Structure:

The garage is bordered on two sides by heated living space and is insulated but not actively heated. It maintains a moderate temperature year round. The back wall has a direct southern exposure and its construction is 2X6 with fiberglass insulation and 6mil plastic vapor barrier on the inside.

The problem:

The problem is that during sunny days in the late fall and especially early spring, condensation forms on the stud side of the vapor barrier on the back wall. This happens because cold overnight temps cool the garage down and then the sun heats the outside of the wall driving the warmer moist air inward. The moisture is enough to discolor the studs and insulation and I believe it will cause damage over time. I can not simply remove the vapor barrier inside because during the winter I will experience the opposite effect and rot the exterior.

Solution?:

The only possible solution I have found so far is to pull the plastic and fiberglass out and spray the inside of the exterior sheathing with at least one inch of expanding foam insulation, then replace the fiberglass and leave the plastic off. I read that this would both act as a vapor barrier and provide enough thermal isolation to prevent condensation. However, this info was from a foam manufacturer and could not be verified.

Any thoughts, advice or expertise would be greatly appreciated.


Anonymous

12:10PM | 08/28/04
Although possible, air doesn't necessarily "move" inward due to temperature differentials. Moisture however, will always migrate toward the more dry area. This is refered to as "vapor pressure".

The temperature differential in your wall doesn't "drive" the moisture across as moisture is present in the entire wall cavity. However, it does create a condition at the inside surface where condensation occurs at the plastic. Condensation occurs on the cooler surfaces where the dew point temperature is met by the temperature of the plastic (or any other material).

The moisture level in your wall seems to be excessive. Look for a source of moisture other than the ambient air. Is the siding preventing rain and irrigation water from entering? Do you have a roof leak, water pipe leak, or does your dryer vent leak moist air into the wall? If not, we must assume that the moisture level is a normal contribution from the surrounding air.

A definite problem is that solid plastic sheet should never be used as a barrier in walls. Moisture must be given a way to escape. Solid plastic traps the moisture and then creates conditions for condensation. The term "vapor barrier" is incorrect for walls and has, in recent years, been more properly refered to as "vapor retarder". Moisture, at some level, will always be present inside a wall. Therefore, a "vapor retarder" is installed which allows moisture to migrate but prevents passage of liquid water and infiltration (moving air). The vapor retarder is installed on the side of the wall from which the moisture originates. Since your wall is already erected, I suggest that you remove the plastic and leave the interior wall exposed. If the exterior siding is ever removed, install a vapor retarder such as TYVEK under the new siding.

bravey

12:13PM | 08/28/04
Member Since: 06/23/04
161 lifetime posts
Although possible, air doesn't necessarily "move" inward due to temperature differentials. Moisture however, will always migrate toward the more dry area. This is refered to as "vapor pressure".

The temperature differential in your wall doesn't "drive" the moisture across as moisture is present in the entire wall cavity. However, it does create a condition at the inside surface where condensation occurs at the plastic. Condensation occurs on the cooler surfaces where the dew point temperature is met by the temperature of the plastic (or any other material).

The moisture level in your wall seems to be excessive. Look for a source of moisture other than the ambient air. Is the siding preventing rain and irrigation water from entering? Do you have a roof leak, water pipe leak, or does your dryer vent leak moist air into the wall? If not, we must assume that the moisture level is a normal contribution from the surrounding air.

A definite problem is that solid plastic sheet should never be used as a barrier in walls. Moisture must be given a way to escape. Solid plastic traps the moisture and then creates conditions for condensation. The term "vapor barrier" is incorrect for walls and has, in recent years, been more properly refered to as "vapor retarder". Moisture, at some level, will always be present inside a wall. Therefore, a "vapor retarder" is installed which allows moisture to migrate but prevents passage of liquid water and infiltration (moving air). The vapor retarder is installed on the side of the wall from which the moisture originates. Since your wall is already erected, I suggest that you remove the plastic and leave the interior wall exposed. If the exterior siding is ever removed, install a vapor retarder such as TYVEK under the new siding.


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