OK to forego vapor barrier under concrete slab if pressure-treated sill plate is used along with sill gasket?

Question

Is it OK to forego installing a vapor barrier beneath a concrete slab (garden shed foundation) under the following set of circumstances?

there will never be any flooring material placed on the slab and the only concern is the possibility of moisture damage to the sill over time because the soil is frequently sodden and very slow to dry after a heavy rain
an EPDM sill gasket will be installed under "MCA" pressure-treated sill plate lumber

Ok in what sense? Can you leave it out? Of course you can. Is it a good idea? Probably not. Why would you leave it out anyway, the cost savings are minimal and you really don't know all the possible uses you might have in the future. If this were my project, I'd include the vapor barrier. — jwh20, May 27, 2021 at 13:28
OK in the sense of the issue described in the question, that is, will foregoing the vapor barrier under those conditions lead to sill rot even if the wood is MCA treated and I use an EPDM sill gasket? The reason I ask about foregoing the vapor barrier relates to grounding the shed using a concrete-encased electrode, which requires the concrete to be in direct contact with the soil. — mr blint, May 27, 2021 at 14:06
Anything you place on the floor may become damp underneath. That may be ok, depending on how you intend to use it. If you put cardboard boxes, anything made of wood, garden furniture, etc directly on the floor you may get dampness underneath the objects. In fact, it may be best to use flooring that can tolerate this. But in that case you may as well use the barrier. — jay613, May 27, 2021 at 15:45

P2000 · Accepted Answer · 2021-05-27 14:24:35Z

2

Our garden shed and greenhouse are built with pressure treated plates over gasket on concrete slab without vapour barrier.

The soil underneath was compacted well in order to avoid contamination during pour.

This seems to match your proposal.

It rains a lot here, and this set-up was specified by an experienced carpenter.

answered May 27, 2021 at 14:24

P2000

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I would agree there is no real need for a vapor barrier on a non living space. (And I live in Oregon) If the shed was going to be humidity controlled maybe a vapor barrier could save some dehumidification other than that save the $
– Ed Beal
May 27, 2021 at 15:05
And "above-ground" rated pressure-treated is adequate?
– mr blint
May 27, 2021 at 15:08
Yes. Because you have the sill guard.
– P2000
May 27, 2021 at 16:39

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