Foil/kraft or kraft faced fiberglass batts

Discussion in 'HVAC Heating & Cooling' started by TJanak, Jan 4, 2012.

  1. TJanak

    TJanak New Member

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    After kitchen rewiring and some minor plumbing work I am ready to insulate. The whole house, kitchen included, has what appears to be a foil/scrim/kraft type facing on fiberglass Johns Manville batts. The exterior sheathing is black cellulosic fiberboard and then brick veneer. No exterior vapor barrier from what I can tell. I need to put insulation back in the kitchen and was going to go back with what was there but it looks like the foil faced insulations are more of a commercial item. Not only that, it seems from the little reading I have done that for my location the foil as a vapor barrier is on the wrong side of the wall, and should be outside the sheathing if anything.

    So do I try to get the foil/kraft in the kitchen so it's like the rest of the house, or just go back with regular kraft? Or something else entirely?

    Thanks!
  2. hj

    hj Moderator & Master Plumber Staff Member

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    The vapor barrier goes between the heated area and the insulation. On the "outside" of the sheathing it would trap the vapor in the insulation.
  3. Dana

    Dana In the trades

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    The asphalt loaded fiberboard is an exterior vapor retarder, which may have (barely) saved you from condensation in the wall cavities if the fg had been installed with the foil on the interior side, which would TRAP moisture in the wall. For a brick clad wall south TX you should probably skip vapor retarders/barriers altogether if they're not located on the outside the structural sheathing.

    Fiberglass is somewhat translucent to infra-red radiation, making it less effective than rock wool or cellulose in a cooling dominated climate where the sun exposed wall temps can easily exceed 130F on a hot summer day. If you MUST use fiberglass batts, use high-density "cathedral ceiling" batts (R15, for 2x4 construction, and yes, they're more expensive) and trim it very carefully for zero gaps & compressions. While you have the wall open it's also worth using acoustic sealant or other high-quality caulk, or a low-expansion can-foam to air-seal the studs to the sheathing, and even the stud-plates to the subfloor, and any electrical/plumbing pentrations etc. before insulating. Sprayed or blown fiber would generally outperform batts since it fills all spaces leaving no room for convection or bypass currents, but unless you have free access to an insulation blower it's probably not worth it for small projects. Be sure to split the batts over lateral wiring or plumbing rather than stuffing it behind, leaving a big compression.

    Cotton batting is more comfortable to handle, and is somewhat protective during condensation conditions (outdoor dew points in the high 70s or low 80s, with an air-conditioned 75F interior) since it adsorbs considerable moisture without losing R value, releasing it as conditions change. It's somewhat like cellulose in that regard. But it's expensive and sometimes hard to source. Unfaced rock will (Roxul, etc) is more available, doesn't transfer radiant heat the way fiberglass can at the temperature extremes but has major itch-factor going on while handling.

    Kraft facers are also ~0.4 perm vapor retarders, and should be avoided on the interior, even though it makes installation a bit easier. Most of the moisture drives will be from the exterior in a brick-clad house in a cooling dominated climate, and putting a 0.4 perm layer on the interior inhibits the ability of the air conditioning to dry any moisture than finds it's way into the wall cavity.

    Air seal the new wall board with foam/caulk at any plumbing or electrical penetrations to limit air-transfer into the cavity and infiltration/exfiltration, limiting moisture transfer in the process. And avoid highly vapor retardent interior finishes such as foil or vinyl wallpapers, or even oil paints. (Latex or latex-acrylic paint is fine.)

    [edited to add] Some of the black fiberboard sheathing is fairly permeable (I looked some up after making my perhaps errant opening assertion). Is there any asphalt felt layers in there as well? Either way, it's still best to allow the AC to dry the cavity toward the interior.
    Last edited: Jan 4, 2012
  4. Dana

    Dana In the trades

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    That's only true in a fairly cold heating dominated climate, (US zones 5 and up, and parts of zone 4) where the sheathing stagnates at temps below the conditioned space air dew point for weeks on end.) In the hot & steamy south the vapor drives are almost entirely on the exterior. On brick clad structures even in cold climates like Manitoba or norther Ontario/Quebec putting a strong vapor barrier like foil or poly on the interior results in summertime condensation in the cavities, particularly on sun exposed walls in air-conditioned buildings, where the temps rise and drive dew/rain moisture out of the brick at very intense rates. (Problematic in Canada, where code pretty much requires poly vapor barriers on interiors.)

    In south TX outdoor dew points are pretty extreme during the peak air conditioning season, and air leakage or vapor diffusion through the exterior can and will create condensing conditions on the interior wallboard surface at times, unless the interior side is somewhat vapor-permeable (latex paint) allowing the air conditioning do dry out the assembly and there is some sort of vapor retarder or vapor barrier on the exterior to limit the rate at which the moisture gets in. Wintertime condensation conditions are all but unheard of there, and short in duration, and simple latex paint is a sufficient vapor retarder for the interior for any type of construction in that climate.

    In Cave Creek AZ it's better to have no vapor barriers under 1 perm on standard stick built with non-reservoir siding, but with adobe, stucco or brick cladding on wood sheathed stick built it's best to put the stronger vapor retarder on the exterior as well. Dew points rarely top 70F in the summer, and wintertime temps below 40F are short in duration- hours or days, not weeks & months.

    You can look up the dew point & temperature history of most locations on Weatherspark.com for a quick read, if you like. If the mean January temp is above ~40F interior vapor retarders only impede drying, and serve no protective purpose. In Cave Creek you're looking at mean temps in the 50s. In San Antonio it's about 50F, and in Corpus Christi it's about 55F. But in Corpus Christi the mid-summer dew point hang in the low 80s a LOT, making exterior vapor retarders a MUST for air conditioned buildings, whereas in Cave Creek those high dew point events are rare, and you can get away with interior side kraft facers, even if it's less than ideal.
  5. TJanak

    TJanak New Member

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    154
    Location:
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    Dana, thanks for the reply.

    So unfaced high density "cathedral ceiling" batts. Will do.

    You mention air-sealing the studs to the sheathing. Is the purpose to seal where two sheets of fiberboard meet on a stud to block air flow? Is the Dow Great Stuff Gaps and Cracks a low expansion foam?

    Also, I would then seal around any electrical boxes, etc. after I put up the drywall?



    I always read your posts with great interest because you do a good job of explaining the reason/science behind a recommended method. Thanks!
  6. Dana

    Dana In the trades

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    Yes- air seal seams wherever they exist to limit air flow & thus moisture (and heat) transfer. There's no such thing as "too tight" for a wall cavity (or whole house, for that matter- not that it's easy to retrofit a whole house into super-tightness.)

    Great Stuff Gaps & Cracks is a high-expansion version but it may make bump big enough to create convective bypass channels in an a batt installation. You can use it, but you may have to trim it a bit if some bigger blobs appear as is cures. In the Great Stuff line the "Windows & Doors" version is a more flexible minimally-expanding foam, and better for sealing tight cracks without creating a large expanded bead.

    You can use either version to seal around electrical boxes and wire & plumbing penetrations etc. The "Fireblock" version is essentially the same as the "Gaps & Cracks", but is bright orange to make life easy for the code inspectors on assemblies that require air-tight fireblocking, not worth paying extra for for the DIYer.
  7. TJanak

    TJanak New Member

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    Location:
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    No tar paper from what I can tell. I replaced several feet of bad soffet last fall and didn't see any tar paper nailed to the sheathing when I looked down in the cavity.


    Getting a little off topic, my house was build in the mid 60's but had a new metal roof put on about 12 years ago or so according to the previous owner. It has some type of foil radiant barrier that is about 1/8" thick (the thickness comes from the plastic foam/air bubble/whatever backing.) I have 1x4's running across the rafters on about 4" centers because the previous roof was cedar shingle. So the radiant barrier is laid on top of the 1x4's (foil side up) and the metal roof on top of that.

    Does the radiant barrier really do me any good? Seems I would get a lot of conduction through being right against the tin. And it's hotter than hell up there in the summer anyways. Maybe I need more ventilation...
  8. Dana

    Dana In the trades

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    In order for radiant barrier to have any benefit requires an air gap. Foil sandwiched between two other materials has no ability to reflect infra-red heat. With an inch of air between the foil facer and a hotter surface on the top side it'll deliver the equivalent of ~R6 when there's a 30F temperature difference (somewhat less at lower delta-Ts, somewhat higher at higher delta-Ts.) If it's a foil suspended horizontally with an inch or more of air gap on both sides (RB between the rafters in a vented attic, for instance) the R-equivalent @ 30F delta, hot side up is closer to R10. With cold side up (winter conditions) it's less than half that. The effect increases somewhat with bigger air gaps, but shrinks rapidly once the gap is 3/4" or lower, and most vendors (smartly, from a marketing point of view) never provide ASTM test data for gaps less than 3/4" where the performance is abominably low.

    For a typical sample of how various types of single & multi-layer RB products fare at different gaps & orientations see this or this.

    As long as there's at least on air gap facing a low-emissivity surface there's some benefit, but it's hard to measure.

    More ventilation won't dramatically change the attic temp if the insulation is at the attic floor unless you power-vent it, but if you power-vent it it will usually draw more conditioned space air into the attic, sucking more outdoor air into the conditioned space and raise the AC costs (it's usually better to just naturally draft it, even if the attic stays hot). Along the gulf coast a vented attic is far more prone to moisture issues (condensation & mold along the joist tops) than an air tight sealed attic. Sealing the attic brings the dew point of the attic air down to that of the conditioned space. Even if it's a few degrees hotter up there as a result, the lower infiltration drive can lower the overall AC load by lowering the latent load in the house. If you have ducts up there, it's important that they're well sealed if it's a vented attic (less so, in a sealed attic), not just insulated, since the air handler itself will generate infiltration drives if the ducts are leaking.

    Most of the time in a '60s vintage house adding blown rock wool or cellulose insulation is more effective (and more cost-effective) than adding RB, even with ducts in the attic. If yours is a typical R19 batts between the joists situation, doubling that or more (up to R50) with blown higher density fiber that burys the joist tops by 3" or more is usually a good mid to long-term investment compared to what's in your 401K and a better investment than any radaint-barrier product. (None of it has payback if you're planning to flip the place in short years though.) The best time to consider RB options is if you live in a place thats hotter than hell (south TX, check :) ), and you have ducts & air handlers in the attic, and you don't have sufficient space to bulk up the floor insulation to over R40. Code min in TX is R30, and cost effective always in a 25 year present-value analysis, but from a comfort point of view 1.5x code min is usually worth it, and usually worth it financially as well if you use goods that are opaque to infra-red (blown cellulose or rock wool) and can cover the joist tops/truss chords (for lower thermal bridging at the framing.)

    To compare the economics of RB vs. bringing it up to code min (if it isn't already) with or without ducts in the attic,see the chart on page 5 and the tables on p.6. In the Austin simulation if you started out with uninsulated ducts above the insulation and R19 batts between the joists the economics of adding both insulation + RB are quite compelling, but if the ducts are insulated & sealed (or not in the attic) RB is worth maybe $15-40 year if you're at R30, $110/yr if you're only at R19.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    FWIW, I added a radiant barrier foil film to the underside of my roof rafters (a good air gap on both sides). Prior to this, at the end of a long hot summer day, it had heat soaked the blown-in cellulose such that the ceiling was hot and radiated heat a good portion of the evening into the night. After installing the radiant barrier, the ceiling was the same temp as an interior wall. In the winter-time, my roof will have snow accumulation on it for 4-5 days after all the other units in my condo complex are completely free of snow. This also limits the ice damming since the snow tends to sublimate rather than melt. It isn't a substitute for conventional insulation, but it can help, when installed properly. Probably a better solution is a metal roof that has an air gap, such as a wooden shake simulation, where it has a nice air gap rather than say standing seam stuff which is in contact with the sheathing over nearly all of its surface area. Then, the wooden decking doesn't get heat soaked because a lot of the heat is radiated away before it gets there in the first place.
  10. Dana

    Dana In the trades

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    Standing seam on purlins with cross ventilation works OK- better than a vented attic, but might have issues in hurricane country (like South Texas).

    The thermal mass of cellulose does affect the peak sensible load by delaying it, but it also reduces the peak & average loads. In TX there's an argument for concrete roofing (that has even MORE thermal mass), but that's a whole 'nuther can o' worms to pick through.

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