On Green Building Advisor, readers regularly ask questions about room-to-room temperature imbalances — the type of imbalances that may occur when a home has a point-source heater like a ductless minisplit or a wood stove. Here’s a typical question: “I’d like to install a bathroom fan to move air from a warm room to a cool room. Will this approach be enough to equalize the temperatures between the two rooms?”
Scott Gibson did a good job summarizing one such discussion in his Q&A Spotlight article, “Can Bathroom Fans Be Used to Distribute Heat?” But there’s more to say on the issue. In this article, I hope to thoroughly address this topic, once and for all.
We can approach this question by breaking it down into parts:
How much heat is lost through the exterior walls and windows of any particular room on a cold night? To answer the question for a specific room, we need to know the area of the exterior walls, the area of the windows, and the relevant R-values (or U-factors).
The exterior wall area for a typical bedroom might range from about 80 square feet (for a small bedroom with one exterior wall) to 220 square feet (for a large bedroom with two exterior walls).
The R-value of an exterior wall might range from R-16 (for a poorly insulated wall) to R-40 (for a superinsulated wall). To determine U-factor, we use the formula U=1/R. So an R-16 wall has a U-factor of about 0.062, while an R-40 wall has a U-factor of 0.025.
The window area for a typical bedroom might range from 6 square feet (for a bedroom with one small window) to 45 square feet (for a bedroom with three large windows).
The U-factor of a window might range from 0.40 (for an older window)…