How much ventilation is required for the under-floor crawl space of a home?

Thursday, June 21, 2018

Homes with an elevated floor and an under-floor crawl space are required to have ventilation openings of at least 1 square foot for every 150 square feet of floor area, and a minimum of one ventilation opening within 3 feet of each corner (to avoid dead air space at corners). Here’s how it is stated in the International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC):

    The math is easier to calculate if you use a slightly higher standard of 1 square inch of ventilation for each square foot of floor area (1 square foot = 144 square inches).

    Because homeowners often don’t recognize the importance of crawlspace ventilation for a Florida home, sometimes they get obstructed or covered up over the years. If you look closely at the photo at the top of the page, you can see that leaves have blown into the recesses of the screened decor blocks used for ventilation, which significantly reduces the available opening. Also, mulching of the foundation plantings over the years can raise the soil level over low vents, like the photo below. 

    The building code allows four exceptions to the ventilation requirement:

  1. No ventilation is required if you cover the ground in crawl space with a vapor retarder barrier which extends up the stem wall and is taped at overlapped seams and along the wall.
  2. Mechanical exhaust ventilation rated at 1 cubic foot per minute for each 50 square feet of crawl space.
  3. Conditioned air supplied from the home a/c system, delivered at rate of 1 cubic foot per minute for each 50 square feet.
  4. Plenum in crawl space (existing homes only). A plenum is  collection area for supply air to the home.

    The most common ventilation defects we find in homes with an under-floor crawl space are lack of vents at some corners, followed by vents that have been filled in or covered over. Usually the square area of the vents is sufficient.

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

To learn more about exterior walls and structures, see these other blog posts:

What is the average lifespan of a house foundation?

What causes vertical cracks in fiber cement siding planks?

What causes raised white lines of residue on a block wall that are crusty and crumbling? 

What is the difference between soil subsidence, heave, creep, and settlement? 

How much ventilation is required for the under-floor crawl space of a home? 

 What causes stair-step cracks in a block or brick wall?

What causes a horizontal crack in a block or brick wall? 

How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?

What causes the surface of old bricks to erode away into sandy powder? 

What are the pros and cons of concrete block versus wood frame construction?

Should I buy a house with a crawl space? 

Why is my stucco cracking?

There's cracks running along the home's concrete tie beam. What's wrong? 

What would cause long horizontal lines of brick mortar to fall out?

How do I recognize serious structural problems in a house?

What is engineered wood siding?

Should I buy a house that has had foundation repair? 

What is a "continuous load path”?

Should I buy a house with asbestos siding?   

How can I tell if cracks in the garage floor are a problem or not? 

What do you look for when inspecting vinyl siding?

Why is housewrap installed on exterior walls under the siding? 

How do I recognize serious structural problems in a house?

Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

Should I buy a house with structural problems? 

What are those powdery white areas on my brick walls?

What causes cracks in the walls and floors of a house?

How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

What are the common problems of different types of house foundations? 

• What are the warning signs of a dangerous deck?

How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

        Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes






Air Conditioner & Furnace Age/Size




Aging in Place


Click Below  

for Links

to Collections

of Blog Posts

by Subject


Doors and Windows


Energy Efficiency

Fireplaces and Chimneys

Heating and Air Conditioning

Home Inspection

Hurricane Resistance

Electrical Receptacle Outlets

Electrical Panels

Garages and Carports

Common Problems

Exterior Walls & Structures



Life Expectancy

Mobile/Manufactured Homes

Older and

Historic Houses

Mold, Lead & Other Contaminants

Modular Homes

Metal Roofs



Pool and Spa

Roof and Attic




"Should I Buy A..."


Termites, Wood Rot

& Pests

Structure and Rooms


Water Heaters

Water Heater Age

Septic Tank Systems

Plumbing Pipes


When It First

Became Code

Park Model Homes

Shingle Roofs


Wind Mitigation Form

"Does A Home


"What Is The Difference Between..."


Concrete and

Concrete Block


Rain Gutters


Crawl Spaces

Building Permits

Clay Soil




HUD-Code for

Mobile Homes

Flat (Low Slope) Roofs

Sprinkler Systems

4-Point Inspections

Frequently Asked Questions (FAQ)

Building Codes

Inspector Licensing

& Standards

Washers and Dryers



Electrical Wiring

Plumbing Drains

and Traps

Smoke & CO Alarms

Top 5 results given instantly.

Click on magnifying glass

for all search results.



Electrical Switches


Water Intrusion

Electrical - Old

and Obsolete

Foundation Certifications

Tiny Houses

About Us