Published on March 26, 2022

What happens when a building’s design makes it difficult or impossible to vent a bath fan to the exterior? Read this guide for tips on how to improve ventilation issues in your bathroom, especially if your home’s design makes it difficult to install proper ventilation.

In many houses, bathroom ventilation doesn’t present problems. A fan or fan/light combination is installed in the bathroom ceiling, then vented through the roof or through a soffit or sidewall.

With the flip of a switch, odors and excess moisture are easily vented outside. But what happens when a building’s design makes it difficult or impossible to vent a bath fan to the exterior?

As it turns out, there are many situations that can make it difficult to install a bathroom fan that vents to the exterior. Solving this problem calls for some construction creativity, smart product selection, and the installation skills of an experienced HVAC contractor.

Dealing with Bathroom Ventilation and Building Code

Before delving into solutions to difficult bathroom ventilation problems, it’s helpful to understand some basic history and code requirements. Even before indoor plumbing, folks understood that outhouses needed ventilation.

When bathrooms moved indoors, ventilation was required not just to remove odors, but also to exhaust excess moisture. We all know how much moisture can be produced by taking a hot shower—just think about the fogged mirrors and the condensation that forms on windows and walls, especially when it’s cold outside.

Today, the building code that applies in most municipalities calls for bathrooms to be vented by means of an exhaust fan or an operable window. The window ventilation option is a minimum standard that can’t be viewed as effective or reliable. Depending on someone to open a window to vent excess moisture is a dubious proposition—especially in cold weather. A vent fan will always exhaust moisture more effectively.

Signs of Bathroom Ventilation Problems

Houses built (and renovated) today are more airtight, and more highly insulated than those built in the past. The mandate to “build tight and insulate right” provides some important benefits: a higher level of interior comfort (especially during temperature extremes), savings on fuel and electricity, and lower levels of carbon emissions.

But tight construction creates a much greater potential for indoor air pollution. Mold spores are among the most hazardous indoor air pollutants, and mold is a direct result of excess moisture. Mold is a serious health issue, causing a wide range of respiratory ailments and allergic reactions. It will also damage and destroy common building materials like wood and gypsum board.

Mold in a bathroom can be caused by a plumbing leak, but splotchy stains on wall or ceiling surfaces usually indicate inadequate ventilation—too much moisture in bathroom air. This can also cause mold to form on wood or insulation in an attic space above a bathroom. Even if you can’t see mold, the unpleasant smell is a telltale sign of its presence.

While there are other reasons to solve bathroom ventilation problems—the inconvenience of foggy mirrors, for example— preventing mold is the most compelling reason by far.

Basic Guidelines for Bath Fans

Before delving into difficult bathroom ventilation problems and how to solve them, let’s go over some basic details that apply to bathroom fans. Understanding these elements will help you make the right decisions about bathroom ventilation, whether straightforward or complicated.

Buy the right fan

Bath fans are sized according to the volume of air they can move, measured in cubic feet per minute, or cfm. The rule of thumb is that you need 1 cfm for every square foot of floor area in your bathroom. It’s smart to err on the high side, however—especially in a bathroom that gets heavy use or one with a high ceiling. Better fans are engineered to run quieter than low-priced versions. Look for a sone rating of around 1, as opposed to 3 or 4.

Consider special features

A bath fan that includes a light will eliminate the need to install a separate ceiling fixture. A fan with variable speed control enables you to get extra exhaust power when you’re generating extra humidity—like when taking a long shower or using a jetted tub.

Original Article: How to Improve Bathroom Ventilation Problems

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