Reuben's Home Inspection Blog


How To Fix a Moldy Attic

If you see black stuff covering or growing on the roof sheathing in your attic, it's probably mold.  It's never fun to find out that your house has mold, but the fix for mold in the attic is usually quite straightforward, and doesn't involve the services of any 'mold remediation experts'.

There are a lot of folks in the real estate industry, including home inspectors, who get very excited when the word 'mold' comes up.  I actually sat through a seminar where we were told to never even use the word mold, because "you never know what something is without testing it."  I couldn't help pointing out that by that logic, we should no longer call houses 'wood framed' houses unless we test the material to verify that it's actually wood.

Mold in attic

What causes mold in an attic

When mold grows in an attic, it's caused by a moisture problem.  Period.  Some moisture sources are obvious and have a huge impact - bathroom exhaust fans, kitchen exhaust fans, clothes dryers... they all pump moisture out of the house, and they should never be vented in to the attic.   Everyone knows that, right?

Bath fans venting in to attic

The less obvious moisture sources are attic bypasses; air leaks that allow relatively warm, moist air from the house to get in to the attic.  To reduce the amount of moisture getting in to an attic, the attic air leaks need to be sealed off.  Covering these air leaks with traditional insulation doesn't stop the air movement; there needs to be some type of air barrier installed, such as foam insulation or caulk.  These attic bypasses show up quite clearly with an infrared camera, but any well-trained insulation contractor will already know exactly where to look for these.

Photos of Attic Bypasses / Attic Air Leaks

I've accumulated about a bazilli0n photos of attic bypasses over the years, so included below are a few of the most common places you'll find attic bypasses in just about every Minnesota house built before 1991.  In all of the photos below, I've pulled the insulation away to reveal the bypasses; you won't find any of these in your attic without moving insulation around.  For the record, home inspection standards of practice don't require the home inspector to move insulation... so don't be surprised if your home inspector didn't mention any of this stuff.

Any time small cables or wires pass through the top plates of walls in to the attic, the holes for the wires should be sealed up.  When they're not sealed, they leak air.  Do you see how some of the insulation in the photo below has been darkened?  It's not mold.  This is the result of years and years of air leaking through the insulation; the insulation acts like a filter, and traps all the dust in the air, turning it black.  Even though these tiny holes don't seem like a big deal, having them all over your attic can have a huge effect.  Sealing these gaps can be easily accomplished with a can of foam insulation.

attic bypasses around wires

Larger penetrations in to the attic, such as plumbing vents, also need to be sealed.  Again, foam in a can works well.

attic bypass around plumbing vent

The first place I always check for attic air leaks is around the furnace vent; if there is air leaking in to the attic around this chaseway, there will surely be air leaking everywhere else.  In this first photo, the darkened insulation is a dead giveaway that there is a lot of air leaking through here.  Foam insulation wouldn't be an appropriate repair for this location - the vent is supposed to have at least one inch of clearance to anything combustible.  In the photo below, installing a small block of wood to fill the gap at the bottom (maintaining a one inch clearance to the vent) and then sealing the entire assembly with high-temperature caulk would be a good fix.

Attic bypass around furnace vent with fire stop

Here's another furnace vent - this one is completely missing the sheet-metal collar.  I could see right down in to the walls after pulling the insulation away.  This is very common.

Attic bypass around furnace vent

Here's a huge bypass around a furnace vent; the chaseway that leads down to the basement was large enough for a person to fit through, and it was basically wide open at the attic; they just had a piece of fiberglass insulation covering the top.  You can clearly see the basement ceiling from inside the attic.

Huge Attic bypass around furnace vent

The space around masonry chimneys will often be a source of air leakage.  If the chimney is being used for a wood burning fireplace, there needs to be a two-inch gap to combustible materials.  Sealing these air leaks will require the use of high temperature caulk and sheet metal.

attic bypass around chimney

As I mentioned in my blog about evil recessed lights, they can be a huge source of attic air leakage.  Notice all the light pouring through this fixture; the light equates to a lot of air leakage.

attic bypass around recessed light

When additions are put on, the transitions between the new and old portions of the house are often sources of air leakage.

attic bypass at addition

Whole-house attic fans can be a major source of air leakage in to attics.  These are fans that are designed to be run in the summer only.  The photo below shows light leaking through the attic fan, which means a lot of air is leaking through as well.

Attic bypass around whole-house fan

Probably the largest attic bypass that I commonly find is the one above the stairway to the basement on old ramblers.  All of the wall cavities are wide open to this space, and nothing above it is sealed off.  This is a bypass that's large enough to fit a small family in to.  Other areas where ceilings drop down in older houses will be areas to look for bypasses - especially over bath tubs and kitchen soffits.

attic bypass over stairway

Finally, the space below the knee walls in old one-and-one-half story houses can be a major source of air leakage.  The way to repair this is to have solid blocking installed underneath every joist cavity, and have it made completely airtight.

attic bypass in knee wall

This is certainly not an all-inclusive list of the places to find attic air leaks, but it should be enough to help you understand what an attic bypass is.  This is how air leaks in to the attic.  When all of these air leaks are perfectly sealed and a vapor barrier is properly installed in the attic, the attic interior will stay dry.

What if the air leaks can't all be sealed?  Sealing off all of the attic air leaks is nearly impossible to do without completely removing all of the existing insulation. I've actually recommended doing this many times, but before starting with such a drastic measure, it's a good idea to check the obvious stuff in the house first.  If the house is too humid, fix it.  A large percentage of attic moisture problems are caused by whole house humidifiers.  This fix for this situation is a no-brainer; turn the humidifier off.

Does adding more ventilation fix the problem?  Attic ventilation is required by the building code, but it's not a magic elixir.  Air moving through an attic will help to remove moisture, but this isn't the cure for a moldy attic.  I've heard many people conclude that an attic mold or moisture problem was caused by lack of ventilation, but that's a little like saying a hangover was caused by not taking enough Advil before going to bed.  Adding more ventilation to an attic is just a way of dealing with the symptoms, and it doesn't always work.

So what needs to be done about the existing mold?  It depends on who you ask.  If you talk to someone who has a financial interest in cleaning up the mold, they'll tell you it needs remediation.  If you ask someone who could get sued for not recommending professional remediation, odds are they'll tell you to have professional remediation.  Other people might argue that once the air leaks between the house and attic have been sealed off, it won't matter what's in the attic.  As long as the roof sheathing isn't rotted, I tend to agree.

For the record, sealing off attic bypasses is also one of the most effective ways of preventing ice dams and preventing frost accumulation in the attic.  The Minnesota Department of Commerce has an excellent guide to finding and repairing attic bypasses, which gives more detailed information on how to find all of these bypasses and how to fix them.

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 28 commentsReuben Saltzman • March 20 2012 03:05AM


Thanks for the honest assessment of treating mold. I think even in the south this is a big issue. 

Posted by Frank Rubi, (Frank Rubi Real Estate, LLC) over 6 years ago

This is very useful information. I agree how to address it really depends on who you call to ask, & those looking for a remediation jobs are usually very costly.

Posted by Joan Congilose, Marlboro, Manalapan, Freehold NJ Homes (Century 21 Action Plus Realty ) over 6 years ago

Excellent stuff Reubs!  Suggested of course, as are so many home inspector posts.

We have to be careful not to say something is mold simply because even if it is we are not "experts" and could/might be exposed as saying something out of line! It's silly, but true.  Simply recommend the other "professional."

People don't consider bypasses!  And people especially don't consider the whole-house humidifier!  I tell my clients that all the time.  It's good to see that somebody has my back!

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Thank you for the information. I will not give people any advice on how to take care of mold problems for the reasons you mentioned.

Posted by Gita Bantwal, REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel (RE/MAX Centre Realtors) over 6 years ago

Very informative post. .I'm very appreciative of AR having these opportunities to learn about our chosen careers

Posted by Fernando Herboso - Broker for Maxus Realty Group, 301-246-0001 Serving Maryland, DC and Northern VA (Maxus Realty Group - Broker 301-246-0001) over 6 years ago

Reuben, by far the best post I have read...attics are such a big culprit of our home inspection and deals that not knowing can cause a big panic and the deals fall through. As a parent and a home owner it is a good advise to check the space that is forgotten and lost. Again thanks for putting this information together!!!

Posted by Ritu Desai, Virginia Realtor-Fairfax/Loudoun/PW-703-625-4949 (Samson Properties) over 6 years ago

I just had a moldy attic. The first thing I tell clients is the air in the attic does not communicate with the conditioned spaces. I have heard my inspectors recommend mold testing, remediation when finding mold on the sheathing. What they often fail to do is educate the buyer on why it may be occurring. Testing is completely unnecessary, the type of mold or fungi is inconsequential. Remediation may be needed, but in my experience, not often. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 6 years ago

This is excellent information. Many just think mold is found in dark basements but it can happend anywhere.

Posted by Edward Gilmartin (CRE) over 6 years ago

Reuben, a lot of good information in this post. Never thought about this until you pointed it out. Thanks.

Posted by Michael Setunsky, Your Commercial Real Estate Link to Northern VA over 6 years ago

Reuben, well done. Thanks for all the photos and the descriptions of some of the common causes and preventive measures that can be done to prevent mold in attics. 

Posted by Adrian Willanger, Profit from my two decades of experience (206 909-7536 over 6 years ago

If there ever was proof required that this is the man to hire for an inspection, this post would be exhibit A

Posted by Richie Alan Naggar, agent & author (people first...then business Ran Right Realty ) over 6 years ago

Great information! I never thought to look at wire and plumbing bypasses - and the pictures really help. Very informative. Thanks for sharing!

Posted by Donna Bosze, Ask Me Why I'm Different! (Realty Executives Top Producers) over 6 years ago

Reuben -- Thanks for the exteremely informative lesson. I love your point about vents and dryers being vented into the attic. 'Everyone knows that, right?'  Wrong.....  

Posted by Barbara Altieri, REALTOR-Fairfield County CT Homes/Condos For Sale (RealtyQuest/Kinard Realty Group, Fairfield and New Haven County CT Real Estate) over 6 years ago


Good information Mold is such a scary word for some buyers I can understand the instructor saying not to call it mold. 

Posted by Evelyn Kennedy, Alameda, Real Estate, Alameda, CA (Alain Pinel Realtors) over 6 years ago

Reuben -- I am noticing a bit of said dark stuff -- it is showing up right above the attic access (which has a tight fitting board -- but it is just to the side of the Washer and Dryer in the Garage, so I am guessing all that moisture from them running, is probably causing part of this problem.  Thanks for the insight.

Posted by Steven Cook (No Longer Processing Mortgages.) over 6 years ago

Reuben, good post as always.  There is another factor that often gets overlooked and that is that "black" discoloration is sometimes not mold at all but merely moisture interacting with the natural tannins in wood that turn the wood black.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Excellent post Reuben, I, too, feel that we often get too excited when we find mold.  Many times it can easily be remediated and controlled.

Posted by Chris Smith, South Simcoe, Caledon, King, Orangeville Real Esta (Re/Max Chay Realty Inc., Brokerage) over 6 years ago

Great post to reblog Ruben! Thanks... 

Posted by Erica Ramus, MRE, Schuylkill County PA Real Estate (Erica Ramus - Ramus Realty Group - Pottsville, PA ) over 6 years ago

Ruben:  you put together a very informative post.  The pictures are an excellent learning tool.  We just had a home inspection where there was mold in the attic due to improper ventilation.

Posted by Bonnie Vaughan, CNE SFR - Buyers/Sellers - Lackawanna & Surroundin over 6 years ago

Frank - thanks, I know it's a big deal everywhere :)

Joan - and it's a huge concern for buyers as well.

Jay - thanks!  People spend a lot of time chasing after a cure for the symptoms.  I typically never write mold in my reports, mostly because I don't want the buyer's lender to freak out.

Gita - it certainly is a contentious topic.

Fernando - thanks for reading.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 6 years ago

Ritu - it's especially important to peek up there during cold weather.

James - I completely agree.  Who cares what it is!  You have a moisture problem... fix it.  The end.

Edward - I probably find just as many moisture problems in attics as I do in basements.

Michael - thanks.

Adrian - my pleasure.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 6 years ago

Richie - you're too kind, thanks.

Donna - all those little holes really add up.

Barbara - good point ;).  With the amount I see, you're right... plenty don't know.

Evelyn - if the inspector calls it mold and it's not, that would certainly be a bad thing.

Steven - glad to help.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 6 years ago

Charles - good point.  I know you recently wrote a similar post about black stuff in attics, and the photos you had in your post certainly didn't look like mold.

Chris - exactly.  Fix the moisture.  Thanks for re-blog!

Erica - thank you!

Bonnie - I'm sure you said that tongue-in-cheek, right?  :)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 6 years ago

Thios post is very helpful..  I think folks don't know where to turn when the inspector notest there is "suspicious microbial growth"... what??? are your saying the M word?

Posted by Joan Whitebook, Consumer Focused Real Estate Services (BHG The Masiello Group) over 6 years ago

Hi Reuben..Congratulations on your Feature...Very Detailed post, Bathroom fans are the Main problems here with only being vented into an attic or the connection to the outside coming apart.


Posted by Fred Carver Personal Real Estate Corporation, Accredited Real Estate Consultant (RE/MAX Camosun Victoria BC Real Estate) over 6 years ago

Reuben, Excellent post and right on. Mold/Mildew/Black stain due to tannis are all indication of other issues concerning moisture that need to be addressed. 


Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 6 years ago


This is an absolutely excellent post. I'm going to come back and read it carefully because I propose to re-blog it.


Posted by Brian Madigan, LL.B., Broker (RE/MAX West Realty Inc., Brokerage (Toronto)) over 6 years ago

Wow ... what an amazing amount of good info .. and photos .. you have made available here, Reuben.  I'm like Brian above ... going to read it again, digest ... and then re-blog in the future.

Thanks for taking the time to offer this here on the Rain ... very valuable ...


Posted by Gene Mundt, IL/WI Mortgage Originator - FHA/VA/Conv/Jumbo/Portfolio/Refi, 708.921.6331 - 40+ yrs experience (NMLS #216987, IL Lic. 031.0006220, WI Licensed. APMC NMLS #175656) over 6 years ago