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Community and Q&A

Paper-faced fiberglass insulation in basement

Stripes7 | Posted in General Questions on

I have read that using fiberglass in a basement is a bad idea because if it touches the concrete wall it will get wet, but I have also read that spacing the studs away from the concrete so the fiberglass does not touch the wall leaving an air gap solves this issue.  I have a job coming up that will require stick framing a basement exterior wall with 2×4 16″ on center and insulating it with R13 Fiberglass.  If I use kraft faced insulation, and keep my 2×4 wall at least 1/2″ away from the foundation wall, will I be okay?  This is in Zone 5.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ralph.

    It depends who you ask. Some builders are okay with fiberglass and other fibrous insulation in basement stud cavities, others think it is always a bad idea. That's the simplified version. If you are going to install fiberglass, I would recommend first installing rigid foam insulation against the wall (instead of having an air space). This will offer some thermal, air, and vapor control. Tape the seams and air seal the perimeter of the sheets. This article will do a better job of describing the work than I can do here: How to Insulate a Basement Wall.

  2. Expert Member
    NICK KEENAN | | #2

    I'll try to summarize the issue. The situation you have to worry about is when the dew point on one side of the wall is higher than the temperature on the other, ie one side is relatively warm and wet. If you have an insulated wall assembly under those condition, there will be a point within that is at the dew point of the warmer air. If air is allowed to infiltrate you will get condensation at that point. If it is allowed to accumulate you will have problems with mold and rot.

    There are two solutions. One is to use an impervious insulation, or at least enough of it so that the condensation point is covered during the greatest temperature difference. The other is to use fluffy insulation, but put a vapor barrier on the warm and wet side of the wall and no barrier on the cold and dry side of the wall. Moist air is kept out of the insulation, and if it gets in it has a chance to dry on the cold side. When heat travels from hot to cold it tends to drive the moisture with it which will dry the wall. Obviously weather changes over the course of the year and the same side isn't always warmer but in most placed there is one side that is warmer most of the year. So in cold climates you put the vapor barrier on the inside and dry to the outside and in hot climates you put the barrier on the outside and dry to the inside.

    There are two problems with basements. The first is that the underground side of the wall tends to be both colder and wetter than the interior, so you don't have the opportunity for heat-driven drying. And if you've properly sealed the wall against moisture there's nowhere for the moisture to go anywhere. The second is that if part of the wall is below ground and part above you can have different conditions in different parts of the wall. For these reasons I don't recommend any sort of absorbent insulation below ground.

    All of this assumes that you don't have liquid water intrusion. If you don't have a handle on that it's game over anyway regardless of how the wall is insulated.

  3. Expert Member
    Dana Dorsett | | #3

    >"If I use kraft faced insulation, and keep my 2×4 wall at least 1/2″ away from the foundation wall, will I be okay? This is in Zone 5.

    Not okay, at a number of levels.

    1: Fiberglass needs air barriers on all 6 sides of the batt to perform at it's rated R-value. With a half-inch cavity between the fiberglass & foundation it will lose performance to convection through the batt into the cavity. A sheet of housewrap would be a good enough air barrier, and is sufficiently vapor open to moisture it won't accumulate condensation, but it's still not okay, because:

    2: A 2x4/R13 studwall does't meet IRC code minimum performance in zone 5, even if it DID have something blocking air transfer between the cavity and fluff. IRC calls out 2x6/R19 or R15 continuous insulation, or U0.065 (=R15.4 "whole wall" ). See TABLE N1102.1.2, and/or TABLE N1102.1.4, including the fine print regarding mass walls (since the above-grade portion of the wall is the same as a mass wall).

    Those performance levels can be met with 3/4" foil faced polyiso (R5) trapped to the foundation with a 2x4/R13 studwall. The facers on the polyiso need to be taped, but form a powerful moisture barrier against ground water, and the R5 is sufficient for dew point control at the foam/fiber boundary for R13 cavity fill on the above grade portion of the wall, preventing wintertime moisture accumulation. A kraft facer is a "smart" vapor retarder, and it's fine to use kraft faced goods (and paper faced wallboard) as long as the flooding risk is low.

    Since both polyiso and the bottom plate of a studwall can wick moisture if resting on a damp slab, install an inch of EPS under both, leaving a ~half-inch gap between the EPS (not polyiso) and the foundation as a drain space for any incidental liquid moisture seepage.

  4. Stripes7 | | #4

    So this basement is buried so that only about 6" of foundation is exposed. The rim joist is going to be insulated with foam board and spray foam to seal around that, that means just 6" of foundation is considered "above-grade" for that R19 requirement. As long as I use kraft faced insulation and the whole wall is mudded & taped, is there an issue with mold?

    The basement wall on the exterior has the black tar coating to prevent excess moisture from ever permeating, and the top 6" will always dry to the outdoors if the inside has the kraft vapor barrier, no?

    The labor and material cost to use any form of foam board *before* the stud wall with insulation will blow their budget out of the water. We're going from a $600 insulation budget with $400 in labor to now a $1500 Insulation allowance and a $1000 labor allowance... just took the price and multiplied it by 2.5

    I thought there were many articles talking about the details needed to install fiberglass in the basement for an affordable insulated system. The studs will have pressure treated bottom plates on foam sill-sealer. The concrete wall has various imperfections in terms of old forms being used and things not being dead straight, so installing anything directly on the wall will still need an air gap between that and the stud wall so that the drywall does not show those flaws in the wall. The 1/2" gap seemed to be the end all solution for making the wall nice and straight.

    1. Expert Member
      Dana Dorsett | | #6

      >"... just 6" of foundation is considered "above-grade" for that R19 requirement."

      What R19 requirement? The R19 cavity insulation requirement?

      The IRC calls out 2x6/R19 as one option for the basement walls, but it applies to the ENTIRE wall (from the slab on up), not just the "above-grade" portion. Footnote c under TABLE 1102.1.2 explains it:

      "c. 15/19" means R-15 continuous insulation on the interior or exterior of the home or R-19 cavity insulation at the interior of the basement wall. "15/19" shall be permitted to be met with R-13 cavity insulation on the interior of the basement wall plus R-5 continuous insulation on the interior or exterior of the home. "10/13" means R-10 continuous insulation on the interior or exterior of the home or R-13 cavity insulation at the interior of the basement wall."

      So your primary options are:

      1. R15 continuous insulation (the most robust)

      2. R5 continuous insulation + 2x4 / R13 studwall (very moisture-robust for zone 5, a bit less for zones 6 & higher)

      3. 2x6 / R19 (potentially risky in any climate zone.)

      So, 3/4" polyiso between the studwall & foundation will work (budget ~50 cents per square foot). It's fine if making the studwall straight to flatten the wall results in some gaps at the stud edge & foam in places, as long as the gap isn't much deeper than 1/4"- most R13s can be "fluffed to an additional 1/4" and still have pretty good contact and a compression fit. If there are places where the gaps are deeper than that, compressing an R19 into those bays works. An R19 is just a pre-fluffed, super-fluffed R13, performing at R13 when compressed to 3.5" in a 2x4 stud bay.

      If you're on the warm edge of zone 5 and don't care about meeting code performance, half-inch foil faced polyiso (R3) and kraft faced R13s is still pretty moisture-safe.

      1. Stripes7 | | #7

        So it sounds like the most *cost* effective allowable option will have to be foil faced poly iso in 1" thick pieces. Menards sells 1" foil faced on BOTH sides, apparently silver on one and white on the other. Will this work okay? I know you mentioned 3/4" thick but their 3/4" thick Poly ISO only claims to be R-4.4 while the 1" is R-6. Then I'll do the 2x4 studs and fiberglass (faced? Unfaced sucks to install...)

        They do not want to give up anymore room space than they have to, so something like 2" of foam and then a stud wall would be a bit extreme.

      2. Stripes7 | | #8

        Can you confirm that the 1" is needed since .75" is R4-4?

  5. Expert Member
    AKOS TOTH | | #5

    Around me (zone 5) there are lot of basements insulated with just batts as you suggest. Usually they work, but occasionaly they end up moldy.

    Dana's suggestion is an assembly that will ALWAYS work.

    If I had to choose insulation materials and levels (not taking into account code), I would install rigid insulation on the walls and skip the batts in the studs.

    From energy use point of view, with a basement mostly bellow ground, 1.5" to 2" of rigid insulation will get you most of your energy savings.

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