The Range Hood Exhaust Should Not Be an Air Intake

Home Inspector with Charles Buell Inspections Inc.

Modern tight houses can easily become depressurized when exhaust fans are turned on. What this means is there is no place for the air to come into the home to replace the air that is trying to leave.

If there are gaps around door weather-stripping, or gaps around window sashes or similar locations, the air will come into the home at these locations. Sometimes even chimneys might be the path for this air. If we operate and exhaust fan in a bathroom the house becomes depressurized, or an area of “lower pressure.” Areas of higher pressure will tend to make balance with areas of lower pressure, so the air outside the building literally “pushes” its way into the area of lower pressure.

Most houses are not tight enough for the air to not find its way in somewhere, and general infiltration was once allowed to be the source of this air replacement.

This small condo unit was too tight for general infiltration to be the source of make-up air, as was evidenced by its finding a path through the range hood exhaust.

Most range hoods have a back-draft damper in them, but there should also be one in the cap at the exterior of the building as well. You can see in this picture there is no damper—but there is a screen.

Exhaust fan vent termination with no back-draft damper

Exhaust fan vent termination with no back-draft damper


With two bathroom exhaust fans and the laundry exhaust fan running, the purple/violet colors of the thermal image of the chase and microwave/hood shows cold air cooling the chase and the area around the microwave.











The screen at the exterior cap location did hold a tissue paper to show that indeed air was pushing its way through the microwave/hood.

So, let’s say we “fix” the cap at the exterior with a proper back-draft damper. Where will replacement air come from? General infiltration may still be adequate, it is just easier coming from where it is now. If it is not adequate, the functionality of the exhaust fans will be reduced. In other words, they will make noise but not exhaust enough air from the room. It is like turning a 100-cfm fan into a 50-cfm fan.

For exhaust fans to do their job, replacement air is necessary and is required by modern codes when houses get to a certain point of air-tightness. This one may be at that point, even though it is an older home in that respect.

Some “positive” means of allowing exterior air to enter the home may be indicated if exhaust fans do not function properly after the exterior cap is repaired and its back-draft damper installed.


By Charles Buell, Real Estate Inspections in Seattle

Posted by

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Comments (17)

Debb Janes
Nature As Neighbors - Camas, WA
Put My Love of Nature At Work for You

You are one handy home inspector Charles. We are lucky to have your expertise in Active Rain. Thanks for sharing. Always fascinating to look at the thermal images in reports.  D  

Jan 13, 2018 12:06 PM
Charles Buell

Thanks, Debb.  I see you went painlessly by me

Jan 13, 2018 04:23 PM
Debb Janes

You'll always be ahead of me Charles. ;)  D

Jan 13, 2018 04:40 PM
Kathy Streib
Cypress, TX
Home Stager/Redesign

Charles- I've always said that this community always benefits from the home inspectors that we have here. Your example is just why buyers need an experienced home inspector such as yourself. 

Jan 13, 2018 04:08 PM
Charles Buell

Thanks Kathy.

Jan 13, 2018 04:23 PM
Kathy Streib
Cypress, TX
Home Stager/Redesign


Jan 13, 2018 05:18 PM
Charles Buell

Thanks Kathy---I must have timed that just right!

Jan 13, 2018 05:19 PM
William Feela
Realtor, Whispering Pines Realty 651-674-5999 No.

Thisis one issue many people eo not realize is that air goes out it has to be replaced.

Jan 13, 2018 05:20 PM
Charles Buell

William, before we started tightening houses up we did not have to think about this too much---now it has become critical to think about it and provide for it.

Jan 13, 2018 05:26 PM
Barbara Todaro
RE/MAX Executive Realty - Happily Retired - Franklin, MA
Previously Affiliated with The Todaro Team

Good morning, Charles Buell even homes need to breathe.... 

Jan 14, 2018 04:32 AM
Charles Buell

Yes, Barbara they do sort of.

Jan 14, 2018 06:55 AM
Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

On a one-year inspection last week the lady had a towel over the down-draft exhaust on the cook top.  Cold air was pouring in.  I simply took a photo of the exhaust cover outdoors where the builder never bothered to install the flap and problem solved!

Air was blowing up out of the grill!

Jan 14, 2018 04:50 AM
Charles Buell

It ain't rocket science as they say.

Jan 14, 2018 06:56 AM
Dorie Dillard Austin TX
Coldwell Banker Realty ~ 512.750.6899 - Austin, TX
NW Austin ~ Canyon Creek and Spicewood/Balcones

Good morning Charles Buell,

Always fascinating to look at the thermal images in reports and the interpretation of what they mean. This community benefits from the home inspectors that we have really provide value..thank you.

Jan 14, 2018 08:07 AM
Nick Vandekar, 610-203-4543
Long & Foster Real Estate Inc 610-225-7400 - Devon, PA
Selling the Main Line & Chester County

We notice around our older windows and replacement windows cold air making its way in sometimes. However, is it better to have an air tight house or one that breathes?

Jan 14, 2018 10:36 AM
Charles Buell

I consider it best practice to have the house as air tight as practical and then take control over when and where air comes into the home.  But yes, changing the air is essential and even required in modern construction.


Jan 14, 2018 10:47 AM
Sam Shueh
(408) 425-1601 - San Jose, CA
mba, cdpe, reopro, pe

Every leak source should be avoided to keep the house warmer. Fireplace is a major leak source. It warms Santa's butt.


Jan 14, 2018 12:03 PM
Charles Buell

Tons of air can find its way down the chimney and into the home if pressure differentials are right.

Jan 15, 2018 09:13 AM
Paul S. Henderson, REALTOR®, CRS
Fathom Realty Washington LLC - Tacoma, WA
South Puget Sound Washington Agent/Broker!

I still learn a boat load every time I read your blog post. Oh by the way; happy 2018 Charles Buell 

Jan 14, 2018 02:50 PM
Charles Buell

Hey, nice to see you Paul, and happy New Year to you as well.

Jan 15, 2018 09:13 AM
Marte Cliff
Marte Cliff Copywriting - Priest River, ID
Your real estate writer

I remember the years when building inspectors were rabidly enforcing the Northwest Energy Code. For a few years, homes were not allowed to breathe, so some homes became hotbeds of mold.

Jan 14, 2018 02:56 PM
Charles Buell

Marte, while houses were sealed up too tight, that was not the fault of the codes so much as builders not understanding the codes.  Maintaining air changes in the home dates back to 1991, however admittedly---tons of houses are done wrong.

Jan 15, 2018 09:15 AM
Fred Griffin Florida Real Estate
Fred Griffin Real Estate - Tallahassee, FL
Licensed Florida Real Estate Broker

  People talk about this technology and that technology, but the thermal image scanners are one of the greatest inventions ever!   

Jan 14, 2018 04:27 PM
Charles Buell

Cannot imagine inspecting without one Fred

Jan 15, 2018 09:16 AM
Gita Bantwal
RE/MAX Centre Realtors - Warwick, PA
REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel

I learned a lot from your post. Thank you for sharing 

Jan 15, 2018 02:05 AM
Charles Buell

You are welcome Gita

Jan 15, 2018 09:16 AM
Gary L. Waters Broker Associate, Bucci Realty
Bucci Realty, Inc. - Melbourne, FL
Eighteen Years Experience in Brevard County

Interesting information. I have never considered the exhaust fan as an issue. I guess I always assumed it was such a simple process and design that implementation was a non issue!

Jan 15, 2018 04:59 AM
Charles Buell

It is amazing the number of places air can find its way into the home when the house is under negative pressure.

Jan 15, 2018 09:17 AM
Gene Mundt, IL/WI Mortgage Originator - FHA/VA/Conv/Jumbo/Portfolio/Refi
NMLS #216987, IL Lic. 031.0006220, WI Licensed. APMC NMLS #175656 - New Lenox, IL
708.921.6331 - 40+ yrs experience

Definitely a timely post for me personally, Charles Buell ... as we are thinking of exchanging a vent over the stove for a microwave/vent.  Much to know and consider that I had no knowledge of prior to reading your post ... thank you!


Jan 15, 2018 11:22 AM
Charles Buell

Of course this could happen with any type of kitchen exhaust fan and is not a problem with the vent or actually even the missing flap at the exterior.  In this case, it was more about the insufficent air intake for air replacement.  It was pulling air from the easiest place.

Jan 15, 2018 01:31 PM
Carolyn Roland- In Delaware and S. Chester County PA
Independent architectural histor'n - Wilmington, DE
Carolyn Roland, GRI, CRS

This is one defect that is hard to find in the historic homes I handle! We have plenty of air!

Jan 15, 2018 08:26 PM
Charles Buell

No doubt plenty of easier ways for the air to get in :)

Jan 16, 2018 07:33 AM
John Wiley
Fort Myers, FL
Lee County, FL, ECO Broker, GRI, SRES,GREEN,PSA

Great information you have shared.

Not only are you a great inspector, you also share remedies.

Jan 16, 2018 07:16 AM
Charles Buell

Unfortunately there is a trend in my industry to stay away from ideas about repairs

Jan 16, 2018 07:33 AM