Vapor Barrier on Concrete Porch


Old 12-04-05, 05:21 AM
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Vapor Barrier on Concrete Porch

Local Inspectors have informed me that I must put down a vapor barrier
on a solid concrete porch that I am going to enclose. Per their instructions,
I must put down 6 mil VB on the concrete.

My structure is going to be 2" joists (11 feet long) pressure treated lumber
supporting OSB, tar paper, then White Oak hardwoods. The concrete structure is soliid to the ground and I can not tell whether it was pour on a vapor barrier. My guess is no. My hope was to put a VB around the wall base.

I am concerned about this recommendation for a couple reasons.

1) I wanted to use liquid nails along with powered actuated concrete nail
gun to secure the joists/shims to the concrete. The VB makes the use
of liquid nails undoable

2) The VB on the concrete will create a moisture trap between it and the
OSB as OSB can act as a VB?

3) Are the alternative to a plastic sheet? Some sort of sealing spray perhaps?

4) The 2" space an VB make me wondered if I should leave this floor unisulated? 2" is not alot of circulation space.

5) Does the Inspectors suggestion the only course of action that passes


I am in Charlotte NC. Warm Humid Summers, Short Mild Winters.
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Old 12-04-05, 06:51 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873
Capillary Break

The plastic sheeting over the concrete is actually a moisture barrier. The difference in pressures between the moisture in the ground, concrete and the porch will cause the moisture in the ground to be drawn to the porch. This is known as Capillary Suction. So go with what your city officials mandate.

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