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Bathroom Insulation and Ventilation

Bathroom Insulation and Ventilation Remodeling Concerns

A lot of client questions that come up during a bathroom remodel, but one question that never seems to make the discussions is bathroom insulation and ventilation. Sure people want a bath fan and their walls and ceilings insulated but do they REALLY know what the contractor is scoping out for their home?

Basco Shower Door

I have seen many situations where homeowners have spent thousands of dollars in a bathroom remodel with no idea of what is being installed in their home relative to bathroom insulation and ventilation

Before you start your remodel make sure to check out our bathroom remodeling series of articles and videos, they were designed to set you on a straight path for getting the job done right! This article will focus on best practices for bathroom remodeling insulation and ventilation issues


No matter where you install it, insulation is a wise investment. Proper bathroom insulation will help keep the room a comfortable temperature year-round.

A good insulating system includes a combination of products and construction techniques that protect your home from outside hot or cold temperatures, protect it against air leaks, and control moisture.

Installing insulation that offers excellent moisture-control properties will minimize the possibility of vapor collecting under the insulation. For years we accomplished this by installing a 6 mil plastic vapor retarder under the wallboard and over the stud walls and fiberglass insulation.

Fiberglass insulation with 6 mil plastic vapor barrier

Vapor Barriers

I get a lot of questions about moisture in the bathroom, specifically rooms with showers. While bathroom moisture vapor in the air can be transferred through walls and ceilings, the real issue is when moisture vapor becomes trapped in walls, resulting in mold and mildew growth.  Mold is unhealthy and can damage your home and present a potential health concerns.

So how do we enjoy the hot steamy showers and avoid mold problems?  The answer is a combination of using the proper insulation combined with excellent ventilation.

Let’s talk about vapor barriers. The level of vapor control required on the interior side of framed walls with fiberglass, rockwool, or cellulose insulation is determined by local building codes, which are based off the Department of Energy’s climate zones for construction.

In my neck of the woods we used fiberglass or Cellulose insulation and a plastic vapor barrier for years, and this method still works well. Lately I’ve been using spray polyurethane foam (SPF), which, at least in North America, is fast becoming one of the more popular options for new and remodeling insulation methods. I should note that it is the MOST expensive option available.

Spray Foam Insulation

What is SPF?

Spray polyurethane foam is a heat-activated polymer that is sprayed into place, turns foamy, expands and then eventually hardens in place. The best part of SPF is that it fill all gaps and crevices that fibourus insulation cannot reach, this sealing off air leaks in the process.

Closed-cell and Open-Cell Spray Foam

These are the two kinds of spray-foam insulation.

Open-cell SPF is the lighter, less dense option; it is the cheaper of the two but has less insulating power (or a lower R-value). Open cell is not a good option to use in wet or moist areas as it can act like a sponge and actually retain moisture.

Closed-cell SPF is denser and more expensive, it provides a more rigid support to certain structures, actually teasing things together.  Closed-cell SPF is less permeable, and can be used as a water vapor barrier.

Installing Bathroom Vent Hood – Seal around pipe

Air Leakage

Air leaks are areas of warm air leaking into your home during the summer and out of your home during the winter. Air leaks typically occur in the same spots on most houses. Spray foam has the potential to tackle air leakages better than many other insulation available.

Spray foam fills cavities and blocks small holes that create air leaks in the buildings building envelope and as a result, is an effective vapor barrier and air leak sealer.

Installing A Bathroom Vent

Now that we have discussed insulation, lets talk ventilation, since the two go hand in hand. The most important factor in a bathroom shower fan is its ability to remove moisture. A properly installed bathroom fan will remove moisture and decrease the humidity in your house.

High humidity damages wall and structure materials as well as causes the growth of mold. Mold can negatively affect your health.

When bathroom humidity builds up, warm moisture-laden air moves toward cooler wall surfaces, where it condenses back into water. Moist air can also work its way into a wall cavity and condense inside the wall. Both situations can result in mold, odors, and sometimes structural damage.

Moisture will also peel off wallpaper or paint. Seeing or smelling , mold and mildew is a good indicator that you have a high moisture problem. The only effective way to prevent mold is to attempt to keep indoor moisture levels low.  A properly vented bathroom shower ventilation fan can avoid this problem.

Choosing The Best Exhaust Fan

Choose the quietest, most energy-efficient fan that will accomplish what you need. There are a ton of quality brands like Nutone, Panasonic, Ventech, and Broan.

Most fan labels have Home Ventilating Institute (HVI) ratings so you can compare noise and energy efficiency.

When looking at the fans consider how much noise they make. Bathroom exhaust fans measure noise in sones. Many bathroom fans come as a 4 to 6 sone fan. Instead, look for a fan that is close to or at a 1.5 sone rating, which is a quiet fan.

Tip: choose a fan with a ball bearing motor for quality and longevity.

Determining Your Rooms Cubic Feet Per Minute [CFM’s]

Before purchasing your fan, you need to make sure it can handle your bathroom needs.  That means you matching the fan CFMs to the size of your bathroom. To do this you need to determine the Cubic Feet Per Minute [CFM] of the room and buy a fan to match that.

Determining the minimum CFM of your bathroom is easy, simply multiply the rooms width x Length x height x 0.13.

Example: [W x L x H x 0.13 – minimum CFM.]

Determine Air Exchange Per Hour [ACH’s]

Air exchanges per hour means an air flow rate sufficient to remove the air volume in a given room at a specified number of times each hour.

Your bathroom exhaust fan should be sized properly to have completely change out the air in the bathroom 11-15 changes in one hour. This is needed in order to remove the moisture while showering and to clear the air after showering.

Fan Location

Locate your bathroom fan to ensure maximum most air extraction. Steam rises so the shower ceiling is always a good spot for a shower fan.

Air removed from a bathroom should be replaced with dry air drawn from an adjacent room or hallway. Replacement air or “make-up air,” is the amount of air that needs to enter the bathroom in order for the fan to work effectively. This air typically enters under a doorway or window.

I typically locate the fan in the ceiling of the shower or right outside the shower, in the ceiling.

Determining Fan Duct Length – How Long?

Bath fan ducting can negatively affect fan performance. Uninsulated, undersized, or droopy flex ducting, excessive length or elbows all affect and can restrict the fans rated airflow. In order to keep the fan effective try to layout your installation with minimal turns.   One thing that people do not realize is that  ninety degree elbows in your duct run is like adding 10-15 feet of air flow resistance.

The best duct pipe to use is solid smooth aluminum. Solid duct pipe has a better airflow due to its low-resistance (smooth) exhaust ducting.

When installing, seal the joints, install seams facing upward, and insulate sections that run through any unheated crawl spaces or attic spaces.

Tip: All fans come with instructions that instruct you how far a run your duct work can be – be sure to follow these duct run recommendations or get a higher CFM fan.


Fan Timer Switch

Run the Fan Longer!

A bathroom fan collects the bathroom’s moist air and immediately expels it outdoors. Using a fan while showering but immediately turn it off when done, does not resolve bathroom humidity issues.

The BEST advice I can give you to keep moisture down in a bathroom is to run your fan longer.  The fan must also run long enough to exhaust the moisture.

Fan Controls

It is recommended that a bath fan runs during, as well as,20 minutes after using the shower. One way to achieve this is through the use of a fan timer.

Installing a timer switch will ensure that the fan has time to work by exhausting the room or moist air that is floating in the air after the shower is complete.

Installing Exterior Vent Hood

Vent to the Exterior

Your bathroom fan MUST vent to the exterior of the house via plastic or metal vent cap, with a backdraft damper.  Most vents exit an exterior wall but can also vent out roofs and soffits.

Exterior Bath Vent Hood


Choosing the best exhaust fan system is an important part of your bathroom remodel. A quality bathroom fan, properly installed will remove moisture and odors from your bathroom which will  improve your indoor air quality.

Remember to take the time to thoroughly understand your bathroom ventilation and insulation needs and make sure that you scope them out in your next remodeling project.

Bath Remodel – American Standard Plumbing Fixtures

Bathroom Remodeling Series – Insulation and Ventilation – Video 5


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