What is the water vapor journey when walls dry inward?
lmosakow
| Posted in General Questions on
For walls designed to dry inward, where does the water vapor go? It presumably moves through your wall assuming it’s sufficiently permeable. Then it gets into your conditioned space, and then what? I assume then it has to get out of the house and not stay there, but how if you’re not running a dehumidifier? Why is a vapor retarder important in this process?
I don’t fully understand this water vapor journey. How would you explain this (like Feynman would)? Thanks!
-Luke
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Replies
Hello Luke,
Great question! Let's break down the journey of water vapor in walls designed to dry inward, and the role of a vapor retarder, in a simplified manner:
The Journey of Water Vapor
Ingress: Water vapor enters the wall assembly from the outside. This can happen through diffusion (water vapor molecules moving through materials) or air leaks (where vapor is carried by moving air).
Through the Wall: Assuming the wall is designed to be sufficiently permeable, the water vapor moves inward, towards the interior of the house. The materials used in the construction of the wall (like certain types of insulation, sheathing, and drywall) allow this vapor to pass through them because they have a high vapor permeability.
Into the Conditioned Space: Once the vapor reaches the interior, it mixes with the indoor air. Here's where good building design comes into play. Homes are not hermetically sealed boxes; they breathe to a certain extent, allowing some exchange of indoor and outdoor air.
Managing the Water Vapor
Natural Ventilation: In many cases, natural air exchange through windows, doors, and other unintentional leaks can help manage the moisture levels inside a house.
HVAC Systems: Many heating, ventilation, and air conditioning systems can help manage indoor humidity levels. Even without a dehumidifier, air conditioning systems can remove some moisture from the air as a byproduct of cooling.
Dehumidifiers and Exhaust Fans: In more humid climates or during certain times of the year, mechanical means like dehumidifiers or exhaust fans in bathrooms and kitchens can help actively remove excess moisture from the indoor air.
The Role of a Vapor Retarder
Balance: The vapor retarder helps to slow down the rate at which water vapor enters the wall from the inside during colder months. This is important because you want to avoid having too much moisture moving into the wall assembly where it could condense into liquid water if it hits a cold spot. This is especially critical in colder climates where the exterior is much colder than the interior for extended periods.
Why It's Important: Without a vapor retarder, there could be too much vapor moving into the wall assembly too quickly, increasing the risk of condensation within the wall. This could lead to moisture problems like mold or rot. The vapor retarder helps manage this risk by slowing down the diffusion of vapor into the wall, not stopping it entirely, thus supporting a balance between drying and wetting.
Simplified Analogy
Imagine your house is like a person wearing a breathable jacket (the walls with vapor permeable materials) on a misty day. The jacket allows some of the mist (water vapor) to get in but not too much, thanks to a layer that repels water to some degree (the vapor retarder). Once inside, the body (the house's HVAC and natural ventilation) manages the moisture by letting it evaporate back into the air around through breathing (air exchange).
In essence, the journey of water vapor through a wall designed to dry inward is about balance. You want to allow the wall to get rid of moisture that gets in, while also controlling how much moisture can enter from both sides, to maintain a healthy, comfortable indoor environment.