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How do I vent this floor sleeper assembly? Or do I need to? Help!

user-6108177 | Posted in GBA Pro Help on

Redo of old commercial alley space.. 2 story brick bldg turned into sfd

Floor assembly:
1. built on existing slab on grade that is about 1″ above grade in an alley
2. existing slab had some cracks in it and some plumbing with short plumbing trenches in it. 6/23/15
3. 6 mil poly layer
4. 2×6 pt sleepers joists at 16″ oc solid blocked about 4′ oc 6/25/15
5. 3″ closed cell foam sprayed in joist bays 6/30/15
6. 3/4″ t&g osb floor sheathing 1st week of July/15
7. 5″ wide 3/4 solid red oak flooring on flooring paper. 8/12/15
8. Client comment about cupping 10/15 or 11/15

Oak flooring is buckling with moisture as high as 13 on top of some boards and higher on sheathing and sleepers. Some mold found below flooring paper on top of osb floor sheathing. (We did have some water from brick cleaning during construction that was mopped up and we put fans on it. This happened after the sleepers were installed but before we insulated and put the sheathing down. June/July 2015 in Washington DC)

With the flooring paper on top of the sheathing and the 6 mil poly below the sleepers is this assembly too tight? I thought about drilling 3″ diameter holes in the sheathing over the sleepers about every 4 ‘ to allow circulation between the bays. I could possibly add a supply into and return from the joists to the existing hvac system or an added dehumidifier system.

Advice, please! Thanks.

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  1. user-2310254 | | #1

    Is this space conditioned? If so, what is the system and how is it programmed? Are the sleepers pressure treated?

  2. GBA Editor
    Martin Holladay | | #2

    It's hard to understand your description. Clearly, if your oak flooring is buckling, you have a moisture problem.

    I assume that this new building is dried in. Is the space conditioned? Are your operating an air conditioner? Did you heat the building over the winter?

    What is the relative humidity of the indoor air?

    Where was the oak flooring stored? What was the RH of the oak flooring when it was installed?

    Did anyone test the OSB subfloor to determine its RH before the oak flooring was installed?

  3. user-6108177 | | #3

    yes, conditioned with 90 plus gas heat and electric ac with a nest tstat. I do not think it comes on much. I will find out more. The sleepers are pressure treated.

  4. user-6108177 | | #4

    New building is dried in. Space is conditioned. Operating heat and ac but I don't think it came on much this winter. When I looked at this in May the RH was 70% which we have now gotten down to 35 or 40%. As for the storing of the floor I do not remember specifically how long it was on site, typically we get about 4 days. I do not remember what the subfloor tested. Typically it is tested. Just do not remember.

  5. GBA Editor
    Martin Holladay | | #5

    When installing hardwood flooring, it's absolutely essential to be sure that the flooring has acclimated to the average moisture content expected in your climate zone, and to be sure that the subfloor is dry and the indoor air in the building is dry (or in the expected range at which the building will be maintained).

    For more information, here are some links:

  6. GBA Editor
    Martin Holladay | | #6

    Here's a link to a relevant Fine Homebuilding article: 11 Wood-Flooring Problems, and Their Solutions.

  7. user-6108177 | | #7

    Thank you, Martin. I will look at all three articles.

  8. user-6108177 | | #8

    Thank you for the articles.
    At this point I am concerned how to make sure my floor assembly reduces humidity and can maintain an acceptable level. Do you have any thoughts on this?

  9. GBA Editor
    Martin Holladay | | #9

    In you have installed a layer of polyethylene above your slab, the moisture is not coming from the slab. The moisture must have been inside your building when you installed your hardwood flooring.

  10. user-6108177 | | #10

    Martin, I think you are likely correct. At this point how do you suggest I get and keep the humidity down in the floor assembly? Any ideas?

  11. GBA Editor
    Martin Holladay | | #11

    The usual method is with a rented industrial dehumidifier. If you are patient, an ordinary air conditioning system will also work -- or heating the place during the winter.

    If you are in a real hurry, you can operate all three types of equipment at once: a furnace, an air conditioner, and a dehumidifier. Your energy bills will be sky high, but you will dry out your building in a hurry.

  12. user-6108177 | | #12

    We have run a couple of LGR dehumidifiers for a couple of weeks now as well as a desiccant type for a week as well as the AC. The inside air RH is reduced but I am concerned we also remove the under the floor humidity right away so we can replace speedily the oak floors and get my client their house back. Would it create a problem to drill 3" diameter holes in the subfloor as I am thinking or is there a better way to ventilate the floor assembly? Other ideas?

  13. GBA Editor
    Martin Holladay | | #13

    You might want to hire a company that specializes in restoring flooded properties. I'm not sure whether holes will help. Do you suspect that there is standing water in the floor assembly?

    You need to buy several hygrometers (to monitor the RH of the indoor air) and a moisture meter (to monitor the moisture content of the subfloor). Make sure that you know how to use the moisture meter properly -- if you don't, you may misinterpret the readings.

  14. user-6108177 | | #14

    Do you guys have any recommendations for any building scientists in the DC area?
    Thanks very much,

  15. GBA Editor
    Martin Holladay | | #15

    I would start by calling up the folks at Building Science Corp. in Massachusetts:

    If they don't want to travel down to the DC area, they may be able to recommend someone who can.

  16. user-6108177 | | #16

    Thank you, Martin.

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