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Thread: Basement XPS insulation gaps

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    DIY Junior Member drx006's Avatar
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    Default Basement XPS insulation gaps

    Hi I have these panels of XPS covered with OSB. The OSB is shiplaped and is a very tight fit. However the xps behind it has 1/8th gaps at the seams. Is it necessary to fill these seams if the OSB is tightly sealed over the seams. Here are some pictures.Name:  Barricadegap2.jpg
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  2. #2
    Test, Don't Guess! cacher_chick's Avatar
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    Because the XPS is acting as both insulation and a vapor retarder, it should be sealed with spray foam and tape at all joints.

    Open seams such as that in your picture will greatly reduce any improvement which the XPS would otherwise provide.

    Ideally, all your shiplap joints should be offset as to not align with the joints in the XPS.

  3. #3
    In the trades Dana's Avatar
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    Laying a bead of low-expansion foam on the edge before butting up the next panel when installing panels on foundations & masonry walls works a heckuva lot easier than trying to fill it in later. The can-foam would air-seal it and fill in the gaps in R-value. But the 1/8" gaps don't mean much in the way of total moisture transfer or heat loss (the area of an 8' long 1/8" gap is only 12 square inches, after all), and half-inch OSB is semi-vapor retardent by itself, so even those 12 square inchs are only ~3-5 perms. Moisture transfer via vapor diffusion will be slowed signficantly by the OSB, but it has to be air tight to limit air-transported convection- making the seams of the OSB air-tight is more important than filling the foam gap.

    Don't use tape on the OSB seams- it would fail (and quickly) unless you first painted 2" either side of the seam with a high-quality latex-acrylic primer to act as a bonding surface for the tape. It's quicker, easier and more reliable to use fiber-reinforced duct mastic slathered on ~1/8" thick extending 3/4" or more either side of the seam. At the top of the foam where it's accessible it's worth sealing with can-foam to prevent convection from any potential leakage.

    BTW: Is this an above-grade wall or a foundation? If above grade, is the OSB on the exterior or the interior, and what is the total wall stackup from interior to exterior?

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    DIY Junior Member drx006's Avatar
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    Thanks for you help! It is below grade on an interior walkout blockwall foundation that is eight feet tall. Foundation is concrete block (8 inch) with full bricks mortared on the exterior. The panels are 2 inch of XPS and about 1/2 inch of osb with shiplaped edge on the osb. It is actually all already on the walls and I was posting to see if it would be beneficial to take down the panels and seal the seams with the spray foam or just make the osb airtight. I'm worried about the 1/8 " gap letting moisture vapor through to the OSB and creating a mold issue. I don't know enough about this stuff and convection currents to know if that is a realistic concern or not.
    Last edited by drx006; 06-09-2012 at 01:11 PM. Reason: Adding additional information.

  5. #5
    In the trades Dana's Avatar
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    The only material susceptible to moisture in the assembly is the OSB itself, which is essentially on the interior of the thermal & moisture boundary. So as long as you don't put a highly vapor retardent material up as a finish (such as foil/vinyl wallpaper or heavy alkyd paints), the moisture content of the OSB tracks the average humidity in the room.

    The above-grade bricks produce a very high moisture drive when the sun hits it, but if there's an air gap between the brick and the concrete masonry unit (CMU) blocks it wouldn't likely create an issue. If the CMU cores are hollow, same thing- the air gap creates a partial moisture barrier. At 2" XPS is fairly vapor retardent- more vapor retardent than the OSB, so even if the CMU were saturated, the OSB would still be able to pass the moisture into the room without loading up and getting moldy. But if the gaps become an air infilration point it's theoretically possible that enough humid summertime air moisture could come through and condense on the exterior side of the OSB in a cool air-conditioned space, but air sealing the seams with mastic would be enough.

    If you're going to finish the interior with wallboard, stagger the seams of the wallboard with that of the OSB, and use only latex/acrylic-latex paints as an interior finish. Wallboard is fairly permeable to water vapor, and standard latex paints are semi-permeable, 5-10x more permeable than the XPS, which is what you want in order to keep both the OSB and wallboard facers dry and mold-free.

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    DIY Junior Member drx006's Avatar
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    Thanks. I was planning on using 4x8 sheets of 1/4 inch paneling. Would that be ok if I staggered the seams over the mastic sealed seams of the OSB.

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    In the trades Dana's Avatar
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    Staggering the seams is BETTER than OK, as it further improves air-tightness.

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    DIY Junior Member drx006's Avatar
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    Is there a recommended mastic for OSB? Should I use fiber reinforcement tape with the mastic?

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    In the trades Dana's Avatar
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    Any tub of fiber-reinforced duct mastic from a box store would do. For this application tape buys you nothing over what the fiber mixed into the goop already provides. It holds up for decades under the thermal cycling and vibration of metal ducts, and will handle normal seasonal moisture cycling of the OSB, but not necessarily a full-on flood. It bonds better than most paints or caulking ever could.

    The key is to find the word "fiber" on the label, which will protect better against cracking over time.

    http://www.amazon.com/RCD-Gallon-Mas...9536942&sr=8-2

    http://www.amazon.com/13X95-RCD-11-M...536942&sr=8-13

    http://www.amconservationgroup.com/c...forced+Mastic+

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    DIY Junior Member drx006's Avatar
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    Can I use the mastic to seal the tops and bottoms of the XPS as well instead of using sprayfoam?

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    In the trades Dana's Avatar
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    The can-foam would adhere better to the XPS, has similar vapor-retardency as XPS, and expands for a sure-seal, but sure, globbing mastic in there will work.

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    DIY Junior Member drx006's Avatar
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    Thanks for all your help.

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    DIY Junior Member drx006's Avatar
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    One more question. If I decided to take down some of the panels to fill the gaps could something like this work? http://www.emseal.com/products/Archi...Backerseal.htm Or maybe cutting strips of 2 inch thick XPS to stuff in the gaps and then covering it with the fiber mastic before putting the OSB back up.. I'm trying to avoid using alot of sprayfoam as I read it has some toxic things in it like isocyanates.

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    DIY Senior Member dlarrivee's Avatar
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    Isocyanates are only used in 2 component urethane foams...

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    DIY Junior Member drx006's Avatar
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    PRODUCT NAME : GREAT STUFF (TM) PRO Window and Door Gun Foam Sealant 20 OZ HC
    MATERIAL TYPE : One component system
    ISSUE DATE : 07/19/2007
    REVISION DATE : 05/09/2007
    2) COMPOSITION/INFORMATION ON INGREDIENTS
    Ingredient CAS Number %
    Prepolymer of MDI and mixture 40-70, 60-100%
    Polyether polyol
    Polymethylene polyphenyl Isocyanate 9016-87-9 5-10,10-30%
    containing approx. 40-50% MDI
    (4,4'methylene bisphenyl isocyanate)
    CAS# 101-68-8
    Liquified Petroleum Mixture mixture 10-30%
    containing Isobutane (CAS#75-28-5),
    propane (CAS# 74-98-6) and
    dimethyl ether (CAS# 115-10-6)

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