Crawlspace and Porch Ventilation

In Wisconsin we don’t have as many crawlspaces as the southern states do but we encounter them regularly.  Many times these areas are not ventilated or sealed properly. So today I’d like to discuss crawlspace insulation and ventilation.  The ventilation rules for crawlspaces are also used for covered porches and many loan products require the areas under these types of porches to be ventilated to qualify for certain financing.


Unheated or cooled crawlspaces must be ventilated to help control moisture and humidity levels.  When inadequate sealing or ventilation exists a condition conducive to mold growth generally exists.  Extensive mold growth will impact indoor air quality as well as can cause structural damage to framing and sheathing members.  Controlling humidity and any moisture intrusion is paramount to avoiding these conditions.

The amount of ventilation required varies by construction practices.  For crawl space ventilation there are a few rules/variations.  The minimum vent area is required to be no less than 1 square foot per 150 square feet of the crawl space floor area if there is not a adequate vapor barrier installed.  If a proper vapor barrier (generally 6 mil polyethylene sheeting) is installed, this ventilation area can be reduced to 1/1500 of the floor area.  The vents must be located within 3 feet of each corner.  For a rectangular building this is sufficient but if there is a projection of greater than 8 feet an additional vent is required within that area not further than 3 feet from the corner.  The vents must also be free from obstruction, obviously.  Many times we find them beneath low decks.  This does not provide adequate airflow.


That’s a lot of numbers, I know.  But it is important to monitor these areas because it can have a large impact on the comfort of the home.  If the crawlspace is excessively humid it may make the first floor uncomfortable.  There needs to be a balance between temperature and humidity to provide a comfortable living space.


For crawlspaces that are open to the main basement, insulating the walls with no venting is acceptable as long as the space is conditioned.  In most cases the area is just open to the main basement allowing heated air circulate in the area.  In these circumstances the walls of the crawlspace should be insulated and sealed to the vapor barrier.  In most cases rigid board insulation is a good product.  With a stone foundation I often see either fiberglass or sometimes spray foam.  I’m not a big fan of spraying the whole foundation wall since this makes any maintenance to the stone or mortar joints more difficult.  A minimum of R11 is  recommended for the walls which can be accomplished with 2 inch thick insulation in most cases.  The joints should also be sealed both at the ends and the vapor barrier to prevent any moisture vapor or radon gas from entering the area.


Porches and closed decks

The same rules apply to porches and decks.  There are a lot of old homes with large front porches that are constructed with piers in the corners and have wood framing.  Many times homeowners will close off the area beneath to keep rodents and critters from nesting in that area.  This creates an area that will be very humid and will likely result in deterioration of the wood members.  Adequate ventilation will prevent this.  The use of Lattice is an acceptable alternative to a solid wood or metal panel.  I had one home a few years ago that installed stone walls in that area, I wrote it as a defect and it was never vented.  They may have a moisture issue down the road.

So to summarize, controlling the air temperature and maybe more importantly the humidity in the area beneath the living area can have a significant effect on the comfort of the living area, as a matter of fact, I am dealing with this very issue in my new home.

I hope this post was informative. As always, if you have any home related questions, please send them to me via the email address below.  I will always help as much as I can.



Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection and Residential Thermal Imaging.  He also manages several rental properties through Zuehlke Properties, LLC. He can be reached by email at or by calling the office at 608-931-7485.

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