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Pad or no pad between 2F wood subfloor and laminate flooring?

Michael Bluejay | Posted in General Questions on

I’m installing laminate flooring over a plywood subfloor in a second story room.  The laminate comes with a foam pad attached to the underside (which can be easily removed).  I’m concerned that the pad will block moisture from rising and cause mold growth on the subfloor layer.  I Googled about this but advice is mixed on whether or not to use a vapor barrier in this situation.  The manufacturer’s instructions say to use a vapor barrier over concrete, but they’re silent on whether a vapor barrier over wood on the 2nd story is good or bad.  So my choices are:

(1) Use the laminate with the padding attached.

(2) Rip off the padding and install over a breathable pad, like Insulayment.

What say the experts?

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Michael,

    All the air inside your house is close to the same temperature and humidity. The moisture does not rise, and for it to condense or accumulate it needs a cold surface, like it can find if it makes its way into exterior walls, floors open to below, or roofs. There are all sorts of surfaces inside a house that are technically vapour barriers. The counters, mirrors, chairs, furniture, plates, glasses, etc. Unless they form part of a building assembly between the interior and exterior they don't cause issues.

  2. Michael Bluejay | | #2

    Thank you, that makes sense. On the other hand, I'm pretty sure I read that Martin said that the problem with cathedral ceilings is that moist air goes through ceiling penetrations like can lights, then condenses and then rots the sheating. So isn't moisture rising in that case?

    1. Expert Member
      Malcolm Taylor | | #3

      The air in a house typically moves upward due to the stack effect, and can carry moisture with it into the roof it it isn't air-sealed. In the absence of air-movement, the moisture in your house is not rising.

      1. Michael Bluejay | | #4

        Okay, so it seems like these are the scenarios:

        (1) The padding isn't an effective air-seal, so air rises through the 1F ceiling penetrations, through the subfloor, around the padding perimeters, and up through the 2F ceiling penetrations. There's no risk to the 2F subfloor because moist air doesn't get trapped and there's no big temperature difference for condensation.

        (2) The padding is an effective air seal, so air doesn't rise up through the 1F ceiling penetrations and into the 2F living space. With no air movement, the 2F subfloor is safe.

        Do I have that right?

        1. Steve Knapp CZ 3A Georgia | | #5

          I suspect the padding is present to facilitate installation over uneven surfaces and mitigate sound. Laminate can be really noisy, so keep that in mind.

        2. Expert Member
          Malcolm Taylor | | #6

          Michael,

          Sort of, but you are over-thinking this. Pressure imbalances mean you may have air moving in all sorts of directions in a house, although the stack effect tends to dominate. No one thinks about the routes the air travels within the house, or what materials may block it, because the air has no effect. It only matters when it hits an exterior wall or roof where the moisture carried by it may may condense.

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