Do I need a vapor barrier over plywood?

Your plywood would most likely be on a sleeper system (floating wood planks on concrete). Having a vapor barrier below the sleepers is ideal as moisture will not get to the plywood. Regarding on-grade applications, if their is a basement underneath, then you will not need a vapor barrier on top of the plywood.

blog.goforfloors.com - Do you need moisture barrier above, on or below ground for my flooring?

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Almost all flooring installation instructions require to use a vapor barrier or moisture barrier below their flooring. The reason why is they don’t want moisture to be getting to their product and helping you to have a long lasting flooring. When moisture gets to laminate or any type of wood base material, water can damage the flooring over time.

Vapor Barrier Blocks All Moisture Issues

A 6-mil vapor barrier is thick enough to not allow any moisture to pass through. So do you really need vapor barrier and can you get by with it or without it. Here’s are some guidelines to consider:

Subfloor Types (Concrete vs Plywood)

Concrete Subfloor – Concrete does and will emit moisture. It just depends on how much moisture it emits. Concrete is porous in nature and will soak up any moisture that is in the ground and show up on the top of your concrete. So we recommend vapor barrier for all flooring installation that have a concrete subfloor.

So here is the guideline:

Above grade – No

On or Below Grade – Yes

Plywood Subfloor – Opposite of concrete, plywood doesn’t emit moisture however it will soak up moisture from surrounding sources. So when do you really need vapor barrier for plywood?

On-Grade & Below Grade

If you are below grade, you will most likely have a concrete structure underneath. Your plywood would most likely be on a sleeper system (floating wood planks on concrete). Having a vapor barrier below the sleepers is ideal as moisture will not get to the plywood. Regarding on-grade applications, if their is a basement underneath, then you will not need a vapor barrier on top of the plywood. If there isn’t a basement but a crawl space underneath, you would want to verify if the crawl space has a vapor barrier originally installed. You do not want to add a vapor barrier on top of the plywood subfloor if the crawl space doesn’t. The reason why is if moisture is being blocked on top of the plywood, your plywood could over time begin to rot. This would lead to higher expenses in the future.

Above Grade – No

On-Grade – Basement Underneath – No

On-Grade – No Basement with Crawl Space – Yes – vapor barrier installed in crawl space

Below Grade – Yes – Install on top of concrete

If you still aren’t sure, putting down a 6-mil vapor barrier is the best option as it will only benefit you from not damaging your new floor.

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How far can OSB span?

They are manufactured with Span Ratings of 16, 20, 24, 32 and 48 o.c.

apawood-europe.org - Span Rating on OSB panels - APA

Span Rating on APA Trademarked OSB Panels

The Span Rating denotes the maximum permitted centre to centre spacing of supports. It is particularly important for Rated Sheathing and Rated Sturd-I-Floor when the long panel dimension or strength axis is across two or more supports. See technical specifications for Sheathing, Rated Sturd-I-Floor and Structural 1.

What span ratings mean

The trademark gives this as a fraction (in inches), such as 32/16. The number on left denotes the maximum centre to centre spacing of supports (in inches) when the panel is used for roof sheathing (sarking). Number on right denotes the maximum centre to centre spacing when the panel is used for subflooring. Sheathing panels with a roof span rating of 24 or greater may be used vertically or horizontally as wall sheathing over studs at 24 inch centres. Those with roof Span Ratings of less than 24 inches may be used vertically or horizontally over studs spaced at 16 inches on centre. with a roof span rating of 24 or greater may be used vertically or horizontally as wall sheathing over studs at 24 inch centres. Those with roof Span Ratings of less than 24 inches may be used vertically or horizontally over studs spaced at 16 inches on centre. Panels denoted as Wall-16 or Wall-24 have been specifically manufactured for use as wall sheathing over studs spaced at the designated distance apart. APA Rated Sturd-I-Floor panels are specifically for single layer flooring applications where a solid face is required. They are manufactured with Span Ratings of 16, 20, 24, 32 and 48 o.c.