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Should I use Closed Cell foam on the underside of my hardwood floor?

7VgT6dt48i | Posted in Green Products and Materials on

Renovating an old house.
I am going with closed cell on the attic roof (rafters are only 2×4 so open cell not an option).
The same guy thinks I should use closed cell (just 1″) under the floor of my house. Of course others say I should use open cell.

My hardwood floor is White Oak from the 20’s….
I live in Austin, TX, so some humidity.
There is nothing under it except the crawl space (about 2.5 feet).
So the 1″ of closed cell would spray right up onto the hardwood directly from the underside…..

I like the idea of 1″ of this because I get a good deal from him doing it all at once (40% discount on the floor after the attic).
And I like the idea of it “solidifying” my floor.
I like how closed would not allow moisture up, but don’t like how it would not allow leaks down.
I also like how it is just 1″ vs 3″ of open cell (allows for more clearance and more joists to run wires along).

Is it bad for the floor?
Is it better to open cell?

Thanks! I think I’m ready to go with this, figured I might as well double check though.

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  1. SLSTech | | #1

    Open cell = sponge
    Closed cell = barrier

    As for the leaks - either which product can cause issues if you do not catch it & clean it up properly (including a dehumidifier if required)

    The only real issue, besides making sure you get a full inch everywhere at minimum (I would rather go with 1 lift about 1.5 - 2 thick), is if there is any gaps in the floor it will seep into them & you are locking it together (if you had plywood underlayment - no issue)

    I would never go with OC where moisture can be an issue, unless they apply a retarder to it which includes the joists - CC should be ok if the floor has equal moisture throughout & no major gaps (gaps also apply to OC)

  2. user-659915 | | #2

    Spraying directly onto the underside of your finish floor sounds risky whether it's closed or open cell. Can your insulation installer can show you an installed track record of this technique with at least a 3-5 year history? He's not going to have to live with the long-term consequences if your 80-yr-old floor goes into shock from a drastic change in its installation conditions with the spray foam inhibiting its seasonal movements, you will. You can get the same airsealing benefits from rigid foam board cut to fit between the joists and foamed in place, probably for less cost and with the benefit of leaving your floorboards free to move.

  3. GBA Editor
    Martin Holladay | | #3

    I think the installation of closed-cell spray foam on the underside of your hardwood flooring will work, but it has several disadvantages:

    1. The spray foam is a glue, and this application will make it very difficult to remove the flooring at the end of its life. Of course, since your builder forgot to install a subfloor, it's a substandard installation anyway, so maybe that doesn't matter.

    2. I looks like Austin is on the border between climate zones 2 and 3. According to the 2006 IRC, the minimum requirement for floor insulation is R-13 in Zone 2 and R-19 in Zone 3. That means you need somewhere between 2 and 3 inches of foam to meet minimum code requirements -- not 1 inch.

  4. homedesign | | #4

    Call me a Conservative ;-)
    I agree with James Morgan that the foam may spell trouble for the floor.
    Most likely these are the narrow tongue & groove boards with LOTS of Joints designed to "hide" movement.
    I am concerned that the foam MIGHT defeat the design intent of the floor system.

  5. Foamer | | #5

    Why not leave the floor alone and create a conditioned crawlspace instead? Install a proper vapor barrier to keep ground humidity and gasses out, then close any vents and insulate the foundation and rim joist with a couple of inches of closed cell foam. Your crawlspace will be dry (wood likes dry), your floor will be warm and it will probably cost you less than spraying the floor.

    P.S. After several instances of ballooning vapor barriers, we now install a hose between the underside of the barrier and the outdoors to allow the pressure to equalize.

  6. DanBuilderResnet | | #6

    We used closed cell spray foam under the floor in a house in VA. The area is prone to flooding during hurricanes and, having experienced one, the homeowner wanted insulation that would be impervious to being wet. During construction, a plumber was working in the kitchen and disconnected the dishwasher while the water was shut off and then left it disconnected by accident and turned the water back on as he left. The result was an entire floor of ruined hardwood. We had to drill holes through the floor to let the water out and then pull up the flooring and run dehumidifiers till the plywood underneath dried out. If you do it, put a floor drain in under the dishwasher.

  7. wjrobinson | | #7

    My vote is to do as Torsten outlined for the same reasoning.

  8. 7VgT6dt48i | | #8

    Thx everyone for helping me choose NOT to do closed cell, at least at this point. I love my floor and don't know if I want the risk. It did seem pretty cheap though compared to the below options ($1000 for 900 sq ft). But the downsides are scary.

    The conditioned space thing is something I hadn't even considered.
    Its either that, or "rigid foam board cut to fit between the joists and foamed in place".

    This article seems to sum up the conditioned space thing right?

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