Why Bathroom Fan is Letting in Cold Air (hot air in the summer)

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Having a proper exhaust system in your bathroom is a vital part of its design. Without it, there are potential problems, like moisture buildup, odor, fume, & chemical blockage, and mold growth, that can all cause havoc if left unchecked.

A key part of an exhaust fan’s design is that it has a backdraft flap that enables proper ventilation without air leaking back into the home when the fan is not running – if you are dealing with this issue, here are some solutions along with some guidelines on other common problems to ensure your fan is living up to its full potential.

The Bathroom fan is letting in cold air because the backdraft damper is missing or leaking. As a result, outdoor air is being drafted back into the bathroom instead of being sealed off by the damper. In order to create a proper seal, a spring-loaded rubber-sealed backdraft damper should be used.

Although this article is focused on why cold air leaking from the vent, we’ll also be talking about how you can stop it, why hot air might be coming through, why your fan might be blowing air, and what you might have to do if you already have a backdraft damper.

Additionally, we’ll give you some tips on the installation process and finally how you can spot best prevent future problems with your exhaust fan.

How to Stop Cold Air Coming Through Bathroom Fan

Cold air drafting from the bathroom vent is not only annoying, it will significantly increase your energy bill. Additionally vent that leaks cold air into the home, will also let wasps and other insects into the home.

During hot summer days when the AC is running a leaking/missing damper will let hot air in from the outside of the house. So the solution is the same for that as well.

Install a Quality Back-draft Damper To Prevent Cold/Hot Drafts

The most effective way to prevent cold air from coming into your bathroom is to install a back-draft damper. Some models may refer to something called an exhaust fan draft blocker. Please note that this is the same thing as a back-draft damper.

Everywhere on this site I recommend the AC infinity dampers (amazon link). They have amazing track record for quality dampers that open easily and that will not leak.

AC Infinity 4" Backdraft Damper, One-Way Airflow Ducting Insert with Spring-Loaded Folding Blades for 4” Ducting in Range Hoods and Bathrooms Fans

Also, keep in mind that you’ll want to avoid gravity-operated dampers since they are very noisy when it’s windy outside and they tend to leak when not installed at the correct angle.

Here is an helpful guide if you would like to learn more about ventilation dampers.

Air Seal Bathroom Exhaust Fan

Cold/hot air can also leak from the attic from between the bathroom fan housing and drywall. This is why it is very important to use caulk(amazon link) to air seal the connection.

The leak will be most noticeable when another exhaust fan is running in the house. Such as a range hood.

The range hood exhaust large amount of air out of the house. This means the same amount of air must be replaced. It will be drawn in from inlet vents and any other openings to the outdoors. A gap between the bathroom fan housing and drywall is an excellent place for the air to seep into the room causing uncomfortable cold drafts.

Sealing the fan/ceiling connection is also an effective way to prevent moist bathroom air from causing any problems in the attic. Excess moisture in the attic will condensate on cold surfaces and eventually cause mold and other moisture related problems.

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Hot Air Coming Through Bathroom Exhaust Fan

While hot air leaking from the bathroom fan is not as uncomfortable as cold, it is still a problem and should be addressed since it will increase your energy bill.

When the home is cooled during summertime and the outdoor temperature is considerably warmer than the indoor temperature hot air can blow from the bathroom fan when it is turned off. When this happens the backdraft damper is leaking or completely missing.

Luckily it is quite an easy problem to fix. Simpy install a back-draft damper and enjoy a draft free bathroom fan!

Do bathroom fans bring in outside air?

Bathroom fans exhaust air out of the bathroom and the same amount of air will be drawn back into the house via inlet vents and cracks. Note that this is an indirect process and serves to remove stale air while replacing it with fresh air. This process is also what removes odors and moisture from the bathroom.

Installing Your Back-draft Damper

Regardless of which model you pick up, it should come with a fairly standard set of directions that can help you through the installation process with great accuracy. However, you can use this section as a very brief (and less thorough) guide on important aspects of the installation process and as a way to keep track of some important things during installation. A link to a comprehensive installation guide will be provided at the end.

This is an instruction for the external damper. I would however suggest installing a secondary internal damper as well.


You’ll want to measure out how far your damper should be away from the fan discharge. As a rule of thumb, this should be about half the diameter of the fan.

Install without Racking

The damper should not be racked and needs to be installed in a square orientation without being twisted or bended.

Prevent Frame Distortion

During the installation process, the frame for the backdraft damper could potentially become distorted. Luckily, there is an easy way to prevent this. Simply use shims between the frame and the duct opening. These shims prevent the fasteners from altering the frame’s shape as it is braced in place.

Cycle All Dampers

Upon installation, one last important piece to ensure correct operation is to cycle the system to stop the damper’s linkages, axles, and blades from binding.

Inspect Overall Orientation

Finally, when you’re installing the damper vertically, you’ll know you’ve got the right orientation if the blades are spinning horizontally. Just make sure to double check this before testing it out – there can be some negative ramifications if you don’t.

Full Installation Guide

If you’re still having issues or having a hard time completing your installation, you can head here if you have absolutely any other questions or concerns. This guide from the NCA has everything you might need regarding backdraft damper installation that gets into the technical side of the process. There are even different considerations for single or multi section setups.

Taking Preventative Measures

If you’ve made it this far, then hopefully you’ve made it through the entire installation process without having any issues or you’ve successfully replaced your backdraft damper. In any case, that’s cause for congratulations. Looking forward, it might be helpful to know what you can look out for to prevent other major problems you might have with your exhaust fan.

Be on the Lookout for Moisture Buildup

Seeing moisture in your bathroom isn’t necessarily a bad thing. If you just took a hot shower or have the windows open a bit, you probably don’t have any problem. If you come back hours later and there’s still condensation building up, this can be a clear indicator that something’s not quite right. Noticing this early enough can save you a world of trouble regarding mold buildup.

Listen Carefully

Hearing dripping is just as bad, especially during the winter months. Being attuned with the sounds around your fan/bathroom throughout the year can help you identify problems much faster than waiting for results. As soon as you hear dripping where you aren’t supposed to, take action to redo the insulation promptly. It’ll save a lot of headaches later on.

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