Most builders understand that condensation can form when warm, moist air encounters a cold surface. Condensation is bad, and builders want to avoid it. There’s a solution, though: According to building scientists, we can prevent condensation problems in walls by determining a wall’s temperature profile and performing a dew-point calculation. This calculation may require the use of a psychrometric chart.
A few brave souls, striving to educate themselves, may consult a copy of ASHRAE Fundamentals to learn more about dew-point equations (see Image 1). That’s what I did — briefly, before I decided to close the book and put it back on the shelf.
To wade through this thicket, I’ll attempt to answer a few questions:
Building scientists sometimes talk about a wall’s “temperature profile” or “temperature gradient.” The idea is to estimate the temperature of different wall components, assuming certain indoor and outdoor conditions.
For example, consider the wall of a house on a cold winter day. If it is 72°F indoors and 0°F outdoors, the siding temperature will be close to 0°F, while the drywall temperature will be close to 72°F. The other wall components will be at temperatures ranging between these two extremes.
If we draw a cross-section of a wall, we can calculate the theoretical temperature of any point within the wall. However, since these temperature profiles usually fail to account for air leakage, they are usually inaccurate. Moreover, they represent a theoretical one-dimensional model; since the real world has three dimensions, this model has limited value.
Builders or designers perform dew-point calculations to determine whether a certain component of a wall, ceiling, or roof — in most cases, the sheathing — will stay warm enough during the winter to avoid condensation problems.
To answer this question, we need to know the indoor relative humidity…