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Community and Q&A

XPS and faced fiberglass

Matt Duffey | Posted in General Questions on

Hi all,

I am currently finishing my basement (6 feet below grade with 3 feet above grade), cylinder block walls. My contractor has installed XPS foam board 1″ behind his 2×4 wall and hasn’t taped the seams. He also is putting faced fiberglass in the cavities. I have painted the interior with water proof paint, 2 coats prior to the work starting.

cylinder block > water proof pain > 1″ XPS, > fiberglass > drywall

I know these are both issues that should be addressed, but given that the drywall is almost up, how bad of shape is this in and should I be concerned or will this be sufficient given the depth below grade?

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  1. Expert Member
    Dana Dorsett | | #1

    While it's clearly better to make both the foam and the drywall layers air tight, as long as drywall area is air tight it won't be all that risky. (That is, as long as there isn't a history of water leakage through the block wall.)

    If the facers on the batts are foil, and adjacent to the drywall it's a problem- ground moisture that gets by the XPS can't dry toward the interior. If they are asphalted kraft paper it's not a problem.

    With only R5 on the exterior of the studwall, if you are in US climate zone 6 (or higher) there is at least some risk of wintertime moisture accumulation from interior moisture drives, but there isn't much you can do about it other than keep the relative humidity in the basement below 30% in the winter.

  2. Matt Duffey | | #2

    There is some minimal leakage - very faint trace of dried water that will run down a few spots on the cylinder wall - I have water proof painted the entire surface with 2 heavy coats.

    Given the minor (normal) leakage in a basement this old with cylinder, am I still good?

    The fiberglass is not foil, it is a brow paper facing the interior - is that OK?

    I also have a drain tile system if that is beneficial or compensating for any remaining risk with the way this is installed/insulated.

  3. Matt Duffey | | #3

    So the foil faced batts are a problem, but is the kraft faced insulation also considered a double vapor barrier when used with XPS board? Not all cavities are insulated yet, so I can have him use unfazed fiberglass for the remained and we can poke holes in the currently installed kraft faced batts. Would this be better?

  4. GBA Editor
    Martin Holladay | | #4

    If water has sometimes trickled through your block wall in the past, it's a little risky to insulate, unless you have solved the water entry problem. Clearly, the footing drains aren't working in that area.

    I vote for removing the kraft facing from the fiberglass if possible.

    It's hard to predict if you'll have problems; it depends on whether there is water entry in the future.

    For more information, see these two articles:

    Fixing a Wet Basement

    How to Insulate a Basement Wall

  5. Expert Member
    Dana Dorsett | | #5

    I'm not quite sure what you mean by "cylinder wall"? I had presumed this was a CMU (concrete masonry unit) block wall, sometimes called "cinder block", from back the the days when such blocks were commonly made on-site using coal cinder as part of the mix. Is that what you have?

    Painting the interior of CMU wall with water proof paints isn't usually a permanent fix for bulk water, even if it slows down vapor diffusion. But the fix is to manage bulk water on the exterior anyway, or with a dimple-mat vapor barrier between the foam layer and the CMU, along with a drain under the slab along the full perimeter. That sort of fix is usually only for chronically leaky foundations.

    There is no definition of "normal" leakage. But if the wall showed signs of heavy efflorescence (or blistering peeling paint, if painted) well above slab level or visible liquid water comes through the wall occasionally, it's probably going to need some bulk water management from the exterior side (where it might have been managed on the interior side with dimple mat & drain, if the wall wasn't already up.)

    The paper facers on the batts are not a problem. When the humidity in the studwall reaches mold-growth levels, the facer becomes vapor open, and the moisture will dry into the basement. The combined vapor retardency of your waterproof paint and the 1" foam is much tighter than a kraft facer plus interior latex paint, and won't trap moisture inside the studwall.

  6. Matt Duffey | | #6

    Thanks Dana. Yes, concrete masonry wall.

    Agree that the waterproof paint isn't a perm fix, but it was better than not doing it (in my mind).

    There is only a few spots of blistering paint (very dry flaky and crumbly paint) - maybe 3 or 4 spots no bigger than my hand.

    Good to know about the paper facers - so they are designed to allow vapor to the inside to dry?

    So I am in relatively good shape with the current implementation? Should I also run a dehumidifier full time in the basement to keep levels below 40%?

  7. Expert Member
    Dana Dorsett | | #7

    Kraft facers are designed to be "smart" vapor retarders. When the proximate humidity is low they are fairly vapor retardent and limit the diffusion of water vapor into or out of the assembly. But when the proximate air is humid they become much more vapor open, and allow moisture to pass.

    In the winter when the exterior above-grade portion of the wall is cold, the humidity of the entrained air in the batt roughly follows the temperature of the cold side of the batt, making it drier, which makes the kraft facer more vapor retardent. But if the humidity in the batts reaches high levels the kraft facer becomes more vapor open than standard latex paint- the assembly can dry toward the interior.

    There is no need to keep basement air humidity as low as 40% RH. In most houses holding the line at 55- 60% is enough to keep the "musty basement smell" at bay.

    If the crumbling paint areas are near the floor you're probably going to be just fine. If it was knee high or higher it's worth looking into exterior side measures. At a minimum grading so that surface water drains away from the foundation is a good start. Digging down a couple of feet on the exterior of the foundation to install a French Drain sloped to a daylight outflow or remote dry well, backfilled with 3/4" screenings wrapped in landscaping fabric can relieve bulk water. It requires some thinking and planning.

  8. Matt Duffey | | #8

    I attached a few pics showing the foam boards and how they were installed loosely on against the block wall with Fb batts in between the studs.

    The majority of the walls are below grade, so do you think the risk of moisture moving either way through the XPS boards will be a concern?

    I also spray foamed the rim joists, so that will hopefully help with the in-wall air flow from the above grade exterior.

    Lastly, you mentioned to keep the basement RH below 30% in the winter, but then said there is no need to keep it below ~50. What is the ideal environment for winter vs summer?

  9. GBA Editor
    Martin Holladay | | #9

    The photo of the framing near the window is scary -- you need to perform air sealing work between the framing and the foundation near the window. Either spray foam or rigid foam with spray foam at the seams might work here.

    The fiberglass batts are also installed sloppily -- wrinkled, wavy, and bowed.

    Everybody needs to slow down, pay attention to details, and do a more careful job.

  10. Expert Member
    Dana Dorsett | | #10

    What Martin said about the workmanship issues! That's some of the crappies batt installation quality I've seen in quite awhile, and if there's a half-inch gap from the stud edges to the foam you have both the issue of the long vertical air column and the prospect of free lateral air movement, since R13s are manufactured at a loft of 3.5", not 4", and won't fully fill the cavity.

    The wintertime vs. summertime humidity limits are all about the average temperatures of the moisture susceptible elements inside the wall, and what constitutes a "human healthy" humidity level:

    A: The dew point of 70F 30% RH air is 37F. In a zone 6 climate with only R5-ish foam the interior surface temperature of the foam on the above grade section of the wall will be below 37F for weeks at a time, which means some amount of moisture will build up inside the wall, and in the studs. If it's 40% RH @ 70F in the basement the dew point of the basement air is 45F, and the surface of the foam will be below 45F for MONTHS, collecting quite a bit of moisture.

    B: In summer the surface temp of the foam isn't an issue, but 50% RH @ 75F is on the high end of the human healthy range. Above that dust mites can bloom, and above 60% mold and skin/lung fungus risks go up.

    So, when the mean hourly outdoor temperatures are in the 30s (eg the daily high of 41F, low of 23, average temp of 32F), it's safe to let the basement humidity rise above 30%, because mean temp at the foam & studs will be above 37F. When mean hourly temp is below 20F there will be some amount of moisture accumulating on the above-grade portion of the wall. With an average outdoor temp of 20F and an indoor temp of 70F will average about 34F, which is a bit below the 37F dew point of 70F 30% RH air. That makes the vapor pressure of the air in studwall is lower than the indoor air, and water vapor will diffuse through the wallboard to the inside of the cavity. As long as the wall is air tight and the wallboard is painted, it won't collect enough to drip liquid water even over a few weeks, but it will accumulate some. Then as average temperatures increase. But if the interior dew points are in the 40s F or higher all winter long the batts can become wet, with a high risk of mold taking hold on the studs.

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