How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manufactured and modular homes

Is mold contagious? Can mold spread to my home if there is a nearby house with mold?

Monday, June 18, 2018

There are literally millions of mold spores floating around in the air all the time and everywhere. Indoor and outdoor air both contain enough spores for a mold colony to begin growing within two days under the right conditions. So mold is definitely contagious.

     An air filtration using a HEPA filter can be used to reduce the level of mold spores in indoor air. A UV-light in an air conditioning air handler is another alternative that, while useful for killing bacteria in the air, is only minimally effective killing mold spores. Even with a HEPA filter in place, air infiltration in a home will allow a steady flow of spores into your home. A “mold house” near you will put more mold spores in the air and, if the breeze is blowing your way, may temporarily elevate the mold spore count in the outdoor air around your home. But, even without a mold-infested house nearby, there are already more than enough spores floating around to start  mold growth inside your home.

    The rare exception occurs when you share a common wall with a neighbor that has an ongoing moisture intrusion problem, such as with a townhouse or condominium. We came across an example of this a few days ago, where a adjoining townhouse garage had a long-term roof failure (see photo below) and mold was coming through the shared wall between the two garages. A plumbing leak in a multistory condo can also spread mold into the walls below, especially if it is a slow leak with no drips or puddling evidence. But both these examples are based on moisture or high humidity from an adjacent property migrating to your home.

    So the key to eliminating the possibility of mold growth in your home is not related to a neighbor’s mold problems. Mold needs three things to begin growing: an organic food source, warm temperature and moisture. Take away any one of those three requirements and mold won’t grow, but the last one that is by far the easiest to control. Keeping the relative humidity of your indoor air below 60% is your single, best preventative for mold, according to the U.S. Environmental Protection Agency (EPA). They further recommend taking it down to 30% to 50% for assured mold prevention.        If your hygrometer has a 68% reading like the one above, it’s time to dehumidify the air. Roof leaks, plumbing leaks, and any other moisture intrusion in the home should be dried-out quickly when discovered, too.

     A central air conditioning system reduces humidity as it cools the air when the system is working properly, and a dehumidifier is another way to remove air moisture. Some new a/c thermostats also monitor indoor humidity and can be set to maintain a specific humidity level. Another alternative is a humidistat, which activates the a/c system as necessary to maintain a desired humidity level. 

    But if you want a cheap way to monitor your indoor humidity, we suggest getting an analog hygrometer. The one shown below costs less than $5 on Amazon, and can also be purchased at many hardware and home improvement stores. A brass case and fancy design costs more, but the $5 one works just fine.

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Here’s links to a collection of our other blog posts about MOLD, LEAD AND OTHER CONTAMINANTS:

Should I buy a house with mold?  

Why do new homes have more moisture and mold problems than older houses?

Can infrared thermal imaging find mold behind a wall? 

What is the right humidity level in a mobile home?

Who can clean up mold found during a home inspection in Florida?

How do I look for and find mold in my mobile home? 

How can I tell if there is asbestos in a house?

How can I prevent mold in my Florida winter home when I'm gone for the summer? 

Should I use bleach to clean up mold? 

What are those powdery white areas on my brick walls?

• What should I do if mold is found during a home inspection?

   Visit our MOLD, LEAD AND OTHER CONTAMINATES page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

Water Heaters

Water Heater Age

"What Are The

Signs Of..."

Septic Tank Systems

Structure and Rooms

Plumbing Pipes

Termites, Wood Rot

& Pests



When It First

Became Code

"Should I Buy A..."

Park Model Homes


Shingle Roofs




Wind Mitigation

Roof and Attic

"Does A Home


Pool and Spa

"What Is The Difference Between..."




Concrete and

Concrete Block

Metal Roofs


Modular Homes

Rain Gutters

Mold, Lead & Other Contaminants


Older and

Historic Houses

Crawl Spaces

Mobile-Manufactured Homes

Building Permits

Life Expectancy

Clay Soil





Exterior Walls

& Structures


Common Problems

HUD-Code for

Mobile Homes

Garages and Carports

Flat (Low Slope) Roofs

Electrical Panels

Sprinkler Systems

Electrical Receptacle Outlets

4-Point Inspections

Hurricane Resistance

Frequently Asked Questions (FAQ)

Home Inspection

Heating and Air Conditioning

Building Codes

Fireplaces and Chimneys

Inspector Licensing

& Standards

Energy Efficiency

Washers and Dryers



Doors and Windows



Electrical Wiring

Click Below  

for Links

to Collections

of Blog Posts

by Subject

Plumbing Drains

and Traps


Smoke & CO Alarms

Aging in Place

Top 5 results given instantly.

Click on magnifying glass

for all search results.






Air Conditioner & Furnace Age/Size


Electrical Switches





Water Intrusion

Electrical - Old

and Obsolete


Foundation Certifications

Tiny Houses

About McGarry and Madsen



Buying a home in North/Central Florida? Check our price for a  team inspection by two FL-licensed contractors and inspectors. Over 8,500 inspections completed in 20+ years. In a hurry? We will get it done for you.

Moisture Problems

Crawl Spaces