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Decrease in R-value of insulation

Tyr | Posted in Energy, Heating & Insulation on December 12, 2008 08:43am

Anybody heard about the decrease in the R-value of ceiling insulation?  Check out the study below from Oak Ridge.

“There is a problem with loose-fill fiberglass attic insulation is cold climates. It appears that, as attic temperature drops below a certain point, air begins to circulate into and within the insulation, forming “convective loops” that increase heat loss and decrease the effective R-value. At very cold temperatures (-20F), the R-value may decrease by up to 50%.”

<!—-><!—-> <!—-><!—-> <!—->

In full-scale attic tests at <!—-><!—-><!—->Oak Ridge<!—-><!—-> national Laboratory, the R-value of 6 inches of cubed loose-fill attic insulation progressively fell as the attic air temperature dropped. At -18 F, the R-value measured only R-9. The problem seems to occur with any low-density, loose-fill fibrous insulation.

<!—->  <!—->

Nisson, J.D. Ned, JLC, “Attic Insulation Problems In Cold Climates” March 1992, pp 42-43<!—-> <!—->

 

Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden…. Roman Poet Phaedrus 15BC–50AD


Edited 12/12/2008 12:46 am by Tyr

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  1. joeh | Dec 12, 2008 09:11am | #1

    Fiberglass batts in walls perform the same trick.

    It's old news, but good to bring it up again for the benefit of those who are still spending thier money on FG.

    Joe H

  2. Tyr | Dec 12, 2008 09:38am | #2

    So it applies to "loose fill" only?  Not batts?  Cellulose (fire resistant) is better?  I pretty much use poly-iso in thicknesses of 2".   The guy who sent me this today also claims that a sheet of plastic vapor barrier has a value of R-1.

    So I asked--If I have a 10foot room and hang 5mil plastic every foot, floor to ceiling and wall to wall--tight.  What R value do I have by the time I get to the inside wall.

    His answer was 9 (nine sheets of plastic).  I have always been of the opinion that dead air space has an R value (ala storm windows, triple glazed primes, etc. and that too much space allowed air movement, circulation, convection, whatever so he emailed me that Oak Ridge study.

    Probably much ado about nothing.    Thor (Tyr's brother)

    Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden.... Roman Poet Phaedrus 15BC–50AD
    1. DickRussell | Dec 12, 2008 04:32pm | #4

      Air at zero F is about 15% heavier than air at 70, so a layer of heavy air over a layer of lighter air is basically unstable. Having the particles of material that comprise the insulation tighter together makes the insulation layer more resistant to the convective flow that wants to happen to tip over the heavy-light arrangement.In that same paper from ORNL, they compare the same ceiling configuration with loose cellulose as the insulation, piled up to the same nominal R value as for the chopped FG. In that case, not only did the overall R value of the ceiling not drop as the air above it got colder, it actually went up about 10%. I've never seen any explanation for that. In any case, it has been found that adding a few inches of cellulose over a layer of loose FG not only adds to the overall R value of the assembly, it stops the convective loops from happening.There have been discussions here and over at JLC as to whether or not convective flow occurs within vertical walls filled with FG batts. Some say it should, even more easily than in a horizontal ceiling, while others claim that measurements on walls prove otherwise. Convective flow driven by air density differences will be suppressed more as the insulation particles are more tightly packed. I guess, then, it's a matter of whether the density of R19 or R21 FG batt insulation is high enough to do the trick.Edited 12/15/2008 9:50 am ET by DickRussell First edit was: [edit 12/15: make that 13%]
      Subsequent edit: Leave the density ratio as it was. I recalculated the density ratio as ratio of absolute temperatures, got 0.87, which means only that air at 70 F is 13% lighter relative to air at zero. But as originally stated, as heavy/light, the ratio is 1/0.87 or 1.15, so I was right the first time.

      Edited 12/15/2008 10:09 am ET by DickRussell

      1. Tyr | Dec 13, 2008 12:58am | #5

        Interesting.  I had always thought that ceiling insulation (whatever is was) had much more of an impact than wall insulation.  I have come across walls that have had holes drilled in the gypboard every bay and loosefill insulation blown in--always wondered if it was worth the expense.  Never had a HO have it done and then patch every hole, tape, texture and paint to match the existing wall.

        I never recommended it to any of my clients but never talked it down either.  Know of a reputable site that might have good info on the subject?  Thanks, TyrThings are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden.... Roman Poet Phaedrus 15BC–50AD

        1. davidmeiland | Dec 13, 2008 01:53am | #6

          >>Know of a reputable site that might have good info on the subject?

          There are a lot of insulation discussions on the JLC Building Science forum.

          1. Tyr | Dec 13, 2008 02:36am | #7

            David--Thanks, I usually spend more time there than here--but not recently. TyrThings are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden.... Roman Poet Phaedrus 15BC–50AD

        2. Riversong | Dec 17, 2008 07:52pm | #24

          "I had always thought that ceiling insulation (whatever is was) had much more of an impact than wall insulation." 

          Since heat, by conduction, moves equally in all directions, the insulation value of a ceiling makes mo more difference than the insulation value of any other element of the thermal envelope.

          However, since warm air rises (not heat!) because its less dense and hence more buoyant, there is often a higher delta-T (temperature difference inside to out) at the ceiling. Heat loss is directly proportional to delta-T, so there will be greater conductive heat loss through most ceilings (in addition to far greater convective heat losses due to the stack effect).

          A very tight and well-insulated house will have very little heat stratification and hence little difference in delta-T at floors, walls, and ceilings.

          The other reason that ceiling insulation has been given higher priority is that it's easier and more cost-effective to add insulation to a flat ceiling/attic floor than to increase the thickness of walls. There are also far fewer interruptions to a ceiling compared to a wall (doors and windows).

           

          Riversong HouseWright

          Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

          Edited 12/17/2008 8:25 pm ET by Riversong

          1. User avater
            jonblakemore | Dec 18, 2008 01:52am | #27

            I'm not trying to be picky, just making sure I understand.

            "Since heat, by conduction, moves equally in all directions, the insulation value of a ceiling makes more more difference than the insulation value of any other element of the thermal envelope."

            Did you mean to say that "the insulation value of a ceiling makes NO more difference than the insulation value..."? 

            Jon Blakemore RappahannockINC.com Fredericksburg, VA

          2. Riversong | Dec 18, 2008 04:25am | #28

            Yup. 

            Riversong HouseWright

            Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

      2. FHB Editor
        JFink | Dec 15, 2008 05:32pm | #8

        Excellent explanation, Dick. Thanks for that.Justin Fink - FHB Editorial

        1. renosteinke | Dec 16, 2008 01:06am | #9

          While we're on the topic .... does insulation inherently lose it's value over time?

          The insulation in older attics -whether fiberglass or cellulose - seems thinner, less resilient than new insulation. Is it worth it to periodically -say, every 20 years - replace the insulation with new? Just add more, or remove the old first?

          1. DanH | Dec 16, 2008 01:11am | #10

            Most insulation will compact over time, but there's little advantage to be had in replacing it. Just add more over the top of the existing, if you feel more is needed.Getting dirty won't affect the insulation value to any significant extent.Insulation in older attics mostly seems thinner because it is -- less insulation was used in the olden days, and often the insulation used was less dense.
            The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

          2. joeh | Dec 16, 2008 02:41am | #11

            Getting dirty won't affect the insulation value to any significant extent.

            But dirty FG indicates air flow through it.

            That's warm air you paid to heat leaving along with some dirt & dust that eventually turns the FG dark.

            Joe H

          3. DanH | Dec 16, 2008 02:49am | #12

            Insulation in an attic will get dirty from dust in the air. Doesn't require airflow through the insulation.
            The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

          4. User avater
            rjw | Dec 17, 2008 03:32am | #20

            I think you're both right - if the dirt is just on top, it's one thing; if the dirts is along an edge or thru a section, it's the other.

            "Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."

            Howard Thurman

          5. DanH | Dec 17, 2008 04:18am | #21

            Yeah, but even if the dirt is due to airflow and "filtering", the insulation value is not significantly compromised by the dirt -- it's the airflow that's compromising things.
            The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

          6. User avater
            rjw | Dec 17, 2008 09:34pm | #25

            well, I have seen a few fg furnace filters that were so dirty they were impeding air flow <G>

            "Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."

            Howard Thurman

          7. DanH | Dec 17, 2008 11:05pm | #26

            Yeah, there's a chance that dirt in the FG could actually IMPROVE it's insulation value.But the basic answer to the original question is that, for normal FG or cellulose insulation, there is no need to remove old insulation before installing more, and simply being dirty does not reduce the effectiveness of the insulation.
            The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

          8. brucet9 | Dec 16, 2008 02:55am | #13

            "But dirty FG indicates air flow through it.That's warm air you paid to heat leaving along with some dirt & dust that eventually turns the FG dark."Not necessarily. Attic vents can let in quite a lot of airborne dust which settles out on the insulation.BruceT

          9. BHT | Dec 17, 2008 01:41am | #16

            "But dirty FG indicates air flow through it.That's warm air you paid to heat leaving along with some dirt & dust that eventually turns the FG dark."Not necessarily. Attic vents can let in quite a lot of airborne dust which settles out on the insulation."You are talking about two different things.A layer of dust lying on top of the attic fiberglass probably "fell" out of the air and is no big deal.Black areas IN the fiberglass show air movement -- the fiberglass has been acting as a filter. In our attic, anywhere you see black fiberglass it's pretty easy to find a spot underneath it where air is leaking from the house.

          10. renosteinke | Dec 16, 2008 07:04am | #14

            Simply adding more can introduce another issue ... the previous vapor barrier needs to be removed. If batts were used, replacing them might be the easiest course.

            I;m not sure piling unfaced insulation atop the old vapor barrier is a good idea.

          11. DanH | Dec 16, 2008 07:09am | #15

            If you're dealing with attic insulation, and not in the far south, the vapor barrier (if any) will be on the bottom and won't create a problem.
            The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

      3. User avater
        BillHartmann | Dec 17, 2008 01:55am | #17

        A while back I found some reports, IIRC, from Oak Ridge, that the weren't signficant loses from vertical currents in stud walls..
        William the Geezer, the sequel to Billy the Kid - Shoe

        1. DanH | Dec 17, 2008 01:58am | #18

          I think where you get major losses in FG stud walls is where there's some air leakage. Then the air leakage and the FG "ventilation" effect feed on each other, dramatically reducing insulation effectiveness.
          The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel

      4. User avater
        BillHartmann | Dec 17, 2008 02:07am | #19

        http://forums.taunton.com/tp-breaktime/messages?msg=95096.74http://forums.taunton.com/tp-breaktime/messages?msg=95096.72.
        William the Geezer, the sequel to Billy the Kid - Shoe

    2. Riversong | Dec 17, 2008 07:45pm | #23

      "The guy who sent me this today also claims that a sheet of plastic vapor barrier has a value of R-1"

      Poly sheating has virtually no R-value, that is no resistance to conductive heat flow.

      Dead air spaces can have considerable R-value, depending on depth, orientation (horizontal, vertical, or sloped), direction of heat flow (up, down, lateral), and emissivity of adjacent surfaces (e.g. wood, vs paper drywall backing, vs foil).

      See attached graphic. 

      Riversong HouseWright

      Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

  3. DanH | Dec 12, 2008 03:03pm | #3

    Yeah, that's fiberglass. Doesn't matter much if it's batts or loose, so long as it's uncovered. Cellulose is much better, since it doesn't allow nearly as much airflow.

    The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
  4. Riversong | Dec 17, 2008 07:30pm | #22

    "Anybody heard about the decrease in the R-value of ceiling insulation?"

    There are a number of problematic issues with fiberglass, including:

    • low density
    • high permeance to air
    • typically poor installation (compression, voids, side-stapling)
    • high fire spread
    • low sound attenuation

    On top of the R-value diminuation due to convection you refer to, fiberglass anywhere in the thermal envelope will lose R-value as the temperature either falls below or rises above room temperature (the hot-box temperature at which it's tested). So, when insulating capacity is most needed - cold and hot ambient temperatures - fiberglass performance decreases.

    Another Oak Ridge study demonstrated that fiberglass batts in walls have an in-place (tested) R-value that's 11% less than nominal when perfectly installed (outer sheathing removed before installation) and 28% less when typically installed. Even before installation, the batts had 8% less R-value than advertised.

     

     

     
    Riversong HouseWright
    Design *  * Build *  * Renovate *  * Consult
    Solar & Super-Insulated Healthy Homes

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