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Community and Q&A

Wet basement OSB sheathing

Neilc1 | Posted in Energy Efficiency and Durability on

I want to do my walk out basement over, but the area sitting on top of my foundation which is 6″ studs with OSB sheathing gets cold. If I use batting insulation it drips wet, even with humidity at about 50.

It’s new construction, I should mention.

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  1. Neilc1 | | #1

    I forgot to mention the ouside walls are only typed wrapping and vino siding,cheap builder,I also live in massachussetts(patriots rule)the basement walls I have mention are half walls and after reading some posts it seems like i need to just seal the cavitys up with drywall

  2. charlie_sullivan | | #2

    Yes, you need to seal the cavities, and drywall is a good choice.

  3. Neilc1 | | #3

    Should I put the 6" batting back in then seal up with Sheetrock and do I have to paint the Sheetrock as their will another framed wall in front of it as this is a half foundation wall and half studded,also when I make my new full sized studded walls along both full concrete basement walls and half walls should I just use the foam against the walls.sorry for the long post

  4. Neilc1 | | #4

    Reading more posts.I totally confused now, they say add foam to outside of house so I am suppose to take siding down .NOT just want to know how to close up bays to keep from condensating and giving me reoccurring mold.if I use just 6"batting it traps moisture on osb then results in mold so if I put new batting in and Sheetrock over I would be good to go before istud and add 1/12foam on Against my basement walls with studs up. Again my new house does not have any outside foam just use and wrapped with Tyler.thanks getting more confused

  5. Neilc1 | | #5

    I did forget to mention I have to run 2dehumidifiers to keep rh down was at 46rh yesterday now 59rh with dehumidifies off overnight

  6. Neilc1 | | #6

    Have heat and ac being put in next week to heat in winter and dry things out and ac keeper cool in summer as it is a walkout basement

  7. charlie_sullivan | | #7

    Sorry you are not getting good answers here faster. Two things I'd suggest:
    1) Specify what climate you are in. Climate zone from the map on the main Q&A page.
    2) Write in complete sentences with capitalization and punctuation. I don't mean to be snarky about it, but I thought it might be helpful to be honest with you about that fact that it is seeming to me like it would be a lot of work to figure out what you are saying.

  8. Neilc1 | | #8

    sorry, live in climate 5(massachusetts).My house was just built in this past October and it is a 2 bedroom ranch with a full basement.I have a full basement with mostly concrete walls,except around 2 area'swere I have a sliding door leading to the outside and a double hung window.The construction area around both the window and sliding door and one other area have concrete from 1foot high to about 4feet high ,the sill sits on the top of the concrete and I have OSB sheathing that are studded off on those area's.The outside of the house is just vinyl sided and probably just wrapped in a tyvec material(no exterior foam).
    My problem is if I use 6" fiberglass insulation(faced) it traps moisture on the OSB in some of the bays,the humidity is high aroundm 65rh if I don't run a dehumidifier which brings the rh to about 52rh with a average temperature of about 27 degrees.
    i want to finish the basement,so my plan is to use 1/12 thick xps insulation along the foundation walls and framing will be studded with the xps along the walls and then fill the cavities in with more probably 4" batting insulation.
    My problem is getting back to the partial foundation walls(half concrete wall with the top studded) I have a condensation problem in those bay area's,the foundation walls themselves are still drying out which reminds me as of right now their is only partial heat that keeps the temperature in the basement around 48-52 degrees..How do I handle those bays before building studded walls ? Hope htis is better actually wrote this on my laptop instead IPAD it has spell check that I believe alters my words..Thanks Neil

  9. Neilc1 | | #9

    picture of basement during house being built but shows general idea of half walls

  10. user-1072251 | | #10

    new homes contain a LOT of moisture - in the concrete, drywall, paint etc, and it takes months to dry out if you do not use a dehumidifier. So I'd suggest waiting until you have some months of warm weather to let that moisture dissipate. Also, it's unclear to me where the moisture is coming from: are you installing FG batts in the wood walls covered with poly? (not a good idea). I assume you know that one should never place FG on the interior of concrete without a thick layer of spray or sheet foam against the concrete; moisture will penetrate concrete and condense on batts and poly.

    Finally, you really should insulate all of your concrete walls - ideally with closed cell spray foam or 2" polyisocyanurate - both for moisture control, and for insulation*. 8" of concrete has an R value equal to single thickness glass - in other words, very low, so you will lose a lot of heat right there.
    * any foam must be covered by a fire barrier.

  11. Neilc1 | | #11

    The plan is to use sheet foam on all full foundation walls,on the wood walls I just tried faced fiberglass batting by itself but immediately created condensation behind it .thanks for the advise,my plan now is to wait till I have my heating ducts installed in the basement then put faced fiberglass in and Sheetrock right away to create a airtight seal

  12. Neilc1 | | #12

    As you stated though if I don't run my dehumidifier it gets up over 60 rh,must be all the residual water in the building materials as you explained so I should I wait till I can get the high humidity down or will the foam covering up the walls bring it down

  13. GBA Editor
    Martin Holladay | | #13

    Your house has lots of construction moisture. Most of it is probably coming from the curing concrete. This is normal. It can take up to one year for construction moisture to dissipate.

    Your indoor relative humidity is very high. There are three ways to lower it; you can use one, two, or three methods at once:
    1. Operate a ventilation fan during cold weather (for example, a bathroom exhaust fan or an HRV).

    2. Turn up the heat. (Use a heating appliance that does not introduce moisture into your house.)

    3. Crank up the dehumidifier. If necessary, rent a commercial dehumidifier.

    When you get the indoor RH down to a reasonable level, you can insulate your stud bays. Installing drywall on the interior side of your studs will reduce the amount of moisture that can reach your sheathing.

  14. Neilc1 | | #14

    what should rh be before closing in wall cavity.I would think around 40rh but with 2dehumidifiers going the best i can get is 52 and even then they aren't removing water just running

  15. GBA Editor
    Martin Holladay | | #15

    If your dehumidifiers "aren't removing water, just running," then they aren't working.

    Do these dehumidifiers drain into a bucket or into a drain pipe?

  16. Neilc1 | | #16

    into their containers.I have them set for dryest setting to

  17. GBA Editor
    Martin Holladay | | #17

    If there is no water in the buckets, either:
    (a) Your indoor air is bone dry, or
    (b) Your dehumidifiers aren't working.

  18. Neilc1 | | #18

    feels dry in basement,also starting to hear some what i think is wood drying out too much.i just can't get rh guage to drop below 52 rh so i think i will go ahead and fill my bays with fiberglass batting . then sheetrock the bays to airtight them. by the way temp is around 50degrees till i get the heat in next week then hopefully not even use the dehumidifiers

  19. GBA Editor
    Martin Holladay | | #19

    It's possible that your hygrometer is giving a faulty RH reading.

  20. Neilc1 | | #20

    i don't think so it reads close to what the display on one of my dehumidifiers say

  21. Neilc1 | | #21

    hygrometer is now sitting on my kitchen table and is going down,after 10 minutes its down to 50,i will check it when i come back from the gym. I think its cause i don't have enough heat in the basement and the walls aren't sealed up yet

  22. Expert Member
    Dana Dorsett | | #22

    An interior RH of 50% @ 27F (is it REALLY below freezing in the basement?) isn't a problem if you have an air barrier (not just a kraft facer stapled to the studs) on the interior side.

    If that was a typo and it's 50%RH @ 72F, that's a fairly excessive number, and you shouldn't close it in or insulate until the temps warm up.

    If it's 52%RH @ 50F that's not nearly as bad, with a dew point of about 33F, so any time the sheathing is above 33F it will be drying rather than taking on more moisture.

    The problem with insulating without air-sealing is that even very minor air convection getting in behind the batt facers moves a lot of moisture to the now cold sheathing. If you already soaked the sheathing with condensation it really needs to dry out some before insulating and air-sealing it.

    In the picture it looks like you have daylight coming through the OSB seams, which really needs to be dealt with. While you have it open, take the time to paint 1.5" either side of the seams with a quality latex primer, and when it dries seal it with housewrap tape, reinforced by painting over the edges with duct mastic for a solid long-term seal. Caulk the studs & plates to the OSB too.

    To meet IURC 2012 code-min for climate zone 5 (all of MA) the foundation insulation in the basement has to be a minimum of R15 continuous insulation, or R19 between studs:

    Bob Irving's recommendation for 2" is almost there, but still a bit shy. It's going to take at least 2.5" of polyiso from the slab level, on up to covering the edge of the foundation sill. You may opt to end it there and cover it with wainscot for visual effect of that step-back, or keep on going up to the joists above as Bob recommends.

    In MA there are multiple vendors of reclaimed roofing foam, even a couple of fairly big ones (one in Auburn/Oxford , the other in Framingham ), which can take the financial sting out of that much foam, since it's typically 25-35% of the cost of virgin stock. If you use polyiso, stop the foam an inch off the slab to keep the cut edge from wicking ground moisture. If XPS or EPS that's not a problem, but with EPS it would take 4" of foam to meet code, 3" with XPS.

  23. Neilc1 | | #23

    thankyou.Neil I will wait till heat is installed next week and let it dry out until I get the rh down before insulating.

  24. wjrobinson | | #24

    The builder has a permit to build a home, with all the specifications listed in the approved plans and the builder will finish the home and guarantee the work for six years. The builder will not build a home with moisture problems, etc.

    If that is not the case... what the heck is the case?

  25. Neilc1 | | #25

    my builder is a new build should of been built with different exterior materials instead of wham bam products.

  26. Neilc1 | | #26

    Just filled basement bays that have the wood sheathing in between the studs with 2" foam green board,with foam spray installation to fill any voids then put up 1/2 " Sheetrock now going to use a mold resistant primer,what do you think?

  27. GBA Editor
    Martin Holladay | | #27

    I think that you haven't installed much R-value. If you used XPS, that's only R-10 -- below minimum code requirements for walls in most climate zones.

    For more information on the insulation method you chose, see Cut-and-Cobble Insulation.

  28. Neilc1 | | #28

    I am numbing now already taped the Sheetrock except one section I left as an experiment to check and see that now that the 2" foam is up and sealed if I take the experimtal piece down during different time and humidity periods if I could fill in the remaining 4" of void since I have 6" studding

  29. Expert Member
    Dana Dorsett | | #29

    MA code is under IECC 2012. Code min foundation insulation is R15 continuous foam, not thermally bridged by studs, or R19 cavity fill if thermally bridged by studs. For above-grade it's R13+ R5 continuous or R20 cavity fill-only.

    With 2" of XPS against the sheathing in a 2x6 cavity you could have still gotten there with R11-R13 batts filling the rest of the volume, and it would be sufficient for dew-point control at the foam/fiber boundary using only latex paint on the gypsum as the vapor retarder.

    But if you've already closed it in with sheet rock, you'll need to blow some cellulose or fiberglass in there, and it's better to do it BEFORE you paint the wall. Doing it with cellulose and a box-store rental blower (usually free for a day with a minimum purchase) cut a 2.5" hole about 8-12" from both the top & bottom of each stud bay using a hole saw and save the plugs. Stuff a rag in the top hole and start blowing in the bottom hole, angling the hose upward a bit, and keep going until the blower stalls. Then go to the top hole and blow down until it stalls, then angle it up and see if it'll pack in a little bit more. Fix the holes with the sheet rock plugs & caulk, reinforced with wallboard tape. The density will only be about 2.5lbs per cubic foot, but with the 2" XPS on the exterior it won't get enough seasonal moisture cycling to cause rapid settling.

    You only need R7-R8 on the outside of 3.5" of cellulose for dew-point control in MA (zone 5), and since you have R10 you won't have any issues with moisture accumulating in the cellulose.

    In the picture it looks like you have bare concrete on the lower half. To meet code there you can pick up some used 3" roofing polyiso on the cheap from places like Nationwide Foam (Framingham) or Green Insulation Group (Worcester) and glue it to the wall with foam-board construction adhesive, and glue the sheet-rock to the foam. Use 4-4.5" bugle-heads to through-screw the top of the sheet rock to the foundation sill to keep it from separating, should the glue between the concrete & foam fail over time.

  30. Neilc1 | | #30

    Cheaper to take Sheetrock down,I guess I will pull sheet rock and add the batting I already have from earlier install when dealing with the batting getting wet.

  31. user-1072251 | | #31

    Dana was correct; my answer about insulating the basement wall with 2" foam was incomplete. We've used 2" of closed cell spray foam against concrete in combination with an interior insulated stud wall. We've also used two layers of 2"polyiso as a stand-alone insulation in an unfinished basement. With this method the exposed layer must be fire rated.

  32. Neilc1 | | #32

    Must of cost some big bucks. If I leave the void in the wall cavities with just the 2" foam, will it cause a moisture problem in the bays with the 1/2" Sheetrock primed with a mildew-resistant primer? I will live with the R-value loss on those section of walls (maybe).

  33. GBA Editor
    Martin Holladay | | #33

    If you decide to live with R-10 walls, you probably won't have any moisture problems. You'll just have energy bills that are higher than necessary. Dana's suggestion -- filling the stud bays with cellulose -- was a good one, and it really isn't that much work.

  34. Expert Member
    Dana Dorsett | | #34

    Blowing cellulose would be both easier and cheaper than ripping out the sheet rock and starting over.

    All-rigid foam solutions to the foundation insulation can be as cheap or cheaper than studwalls with batts if you used reclaimed roofing foam from the aforementioned vendors.

  35. Neilc1 | | #35

    this should besy but i get posts i dont understand,this my plan now 2"xps against foundation then stud walls then add 4" batting yhen sheetrock prime and paint.Does that sound okay for my basement

  36. GBA Editor
    Martin Holladay | | #36

    If (in your latest post) you are talking about a concrete foundation wall, your suggested insulation method will work. For more information, see How to Insulate a Basement Wall.

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