The required 6-mil polyethylene (or other approved) vapor retarder sheet under a concrete floor slab is necessary for the living spaces in a home because it keeps moisture from rising through the slab in wet weather and saturated soil. But the International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC 506.2.3) do not require it in the following areas, which are essentially all unheated areas that are not likely to be enclosed for living space in the future.

    The exception number 1 refers to garages that are “accessory buildings,” which means that garages that share a common wall with the home are not exempt from the vapor retarder requiremt. Carports and patio floor slabs that were installed without a vapor retarder barrier tend to sweat and become slippery occasionally when they are enclosed later into living space. To read more about it, see our blog post Why does my concrete floor slab sweat and get slippery? 

    Also, see our blog post Why are there score line grooves in the concrete floor of the garage?

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Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• Why is the grout cracking and coming loose at my floor tile?

• What are the building code requirements for notching and boring holes in a wall stud? 

• What causes dark or light "ghost" lines on ceilings and walls?

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code? 

• Why is my garage ceiling sagging? 

• How can I identify what kind of wood flooring I am looking at?

• What is the minimum ceiling height for rooms in a house? 

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

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