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Blown or fiberglass insulation. Which?

WillieWonka | Posted in Energy, Heating & Insulation on October 1, 2003 09:10am

I am almost SURE that this topic has been discussed before, and if so forgive me for bringing it up again but I couldn’t find where this has been discussed.

Which is better/best, blown or fiberglass insulation, especially in an attic area?

I’ve always been a fiber die-hard, but I don’t want to be closed minded. In my bathroom remodel I need to sister the ceiling joists (which also act as tie joists for the roof) so as to make the ceiling joist bays deeper so that I can get in R30 which I will then cover with plywood to make a floor for the attic for storage. Since my ceiling joists are approx 24 o.c (the opening is 21 3/4, just enough to tuck fiber up inside the bay) I’ll reduce the joist bay width another 1 1/2″ and it’s gonna mean having to stat to cut the fiber and it’s a PITA to cut narrow strips of fiber that thick. This ceiling also has 6 recessed IC cans and a bath fan of course.

So, with the need to sister and avoid cutting fiber, I thought blown might be better. But…I’d like to know the pros/cons. Fiber is also harder to cut to a nice fit around recessed cans, also, so again I was wondering about blown as the best option??Also, if blown is used, should there not be a plastic vapor barrier on the underside of the joists?

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  1. csnow | Oct 01, 2003 10:02pm | #1

    This is a topic of endless heated debate.

    Unless folks are too tired to argue, you will see what I mean...

    IMHO, the cellulose is superior in every way. 

    You mentioned all of the fitment problems, so it would clearly be a time saver as well.

    The most important thing is not the choice of insulation, but paying great attention to sealing every air leak into the attic.  In most structures, the bulk of heat loss is via convection, not radiation.  Convective loss is also a prime source of moisture problems and ice dams.

    The vapor barrier would likely cause more problems than it would solve.  Just get all the air leaks, and it will be ok (debated).

    1. brownbagg | Oct 01, 2003 10:44pm | #2

      I installed blown cell in mine,  really like it. do not use a vapor barrier on ceiling it will damage the sheetrock on the ceiling

      1. BUSTER | Oct 12, 2003 02:53am | #12

        I "brownbagg"...please educate me..why would a vapor berrier damage the gyp. ?

        Thanks. 

  2. billyg83440 | Oct 02, 2003 12:41am | #3

    I'm an amatuer, but there's zero question in my mind: Blown CELLULOSE insullation is superior for attics.

    Much easier & faster installation then fiberglass batts, plus no itch factor. I even think you get a higher R value per inch, but not totally sure of that.

    As already stated, sealing any and all ceiling holes is paramount to having your attic work properlly, as is adequate ventalation.

    I would only put blown fiberglass in the house of someone I didn't like, and if someone else installed it.<G>

  3. jps | Oct 02, 2003 03:59am | #4

    Speaking of blown fiberglass - I just popped my head into my attic for the first time and that is what I found.  I had never seen the stuff before.  Since I just moved in, I have a couple projects planned for the near future.  Are there any hints someone can tell me before I find myself knee-deep in fiber?

    My first project is a bathroom fan.  I read all the notes, and found some great info, but no one ever addresses how they manage the insulation around it.

    (sorry if this is a bit off topic...)

    Thanks.

    JP

    1. User avater
      Mongo | Oct 04, 2003 09:13am | #6

      JP,

      If you don't have any flooring up there, throw down a couple of walking boards across the joists so you don't accidentally pop a foot through the ceiling below.

      Blown insulation is easy, just clear it from where you need access. When done, push it back.

      Standard precaution when working with blown FG...mask, goggles, gloves, etc.

      Install the fan, make sure you have a good seal where it penetrates the ceiling. From above use canned foam to tighten it up if neccessary.

      If you want to be a real hero, the best thing you can do it to clear out areas of the insulation and seal any penetrations in the attic floor with canned foam. You want to eliminate any holes where air can pass from the conditioned living space below, up and into the unconditioned attic. The materials, a few cans of foam, are inexpensive. Your labor is free. But all the digging around in the insulation will be a pain. A dust pan, used as a shovel,  helps when moving blown insulation.

      If you have can lights and insulation is around them make sure that the cans are IC-rated, meaning "insulation contact rated". If they are IC-rated, you can pile the FG up and around the can. If not, then the cans are old and likely leak air into the attic. You can either replace them (an easy and not-too expensive proposal) or you can build a box and box them in. The box makes them air-tight and keep the insulation away from the non-rated can.

      If you ever plan on adding more insulation, consider blown cells. They can go right over the blown FG and will minimize attic-to-house radiant gain in the summertime.

      1. jps | Oct 05, 2003 08:05am | #7

        Thanks Mongo -

        I took your advice and I am using some of that foam while I am working up there.  I read on the other board to look for the 'dirty' insulation as a sign of an air leak. It seems that the worst of it comes in where the drywall meets the wall.

        My only concern is that I seem to be packing down some of the insulation - is that going to be a problem?  I am doing my best to avoid it, but moving it around seems to un-fluff it a bit.  I was thinking I could buy one or two batts of insulation to put down in one place, so I could use the extra blown fiber to thicken up some of the places that I have mashed.  You mention blown cell - but I assume that isn't something I can just use to patch a hole.

        JP

        1. User avater
          Mongo | Oct 06, 2003 07:50pm | #8

          You can certainly do what you're describing...reloacating the existing blown-in and replacing the left-behind void with batts.

          You could add additional cells by hand. They are sold in bales or bags at lumberyards and most box stores. Break open the bag, fluff it up a bit, and toss it where you need it.

          If you want to add a lot more blown-in, some yards (and even box stores) will let you borrow a blower if you buy a minimum amount of insulation. Others will charge a fee ($40-$70 a day) if you don't buy a minimum number of bags.

          Good job with the air sealing.

        2. csnow | Oct 08, 2003 04:47pm | #9

          Packing down the insulation is not a problem.  In fact, the denser it is packed, the better.  No need to "fluff", just supplement it later if there are voids.

          The whole "fluff" factor is widely misunderstood.  With FG, the R value PER INCH INCREASES with density. 

          Example: If you stuff 2 batts in a given wall cavity that could have been filled by one, you will not get 2x the R value (each batt will be 'derated'), but the NET R value for the cavity will increase significantly.  That is why you see FG manufacturers marketing denser batts now, as if they have invented something 'new'.

          But again, R value is far less important than sealing those air leaks.

          Hey, you have discovered one good thing about fiberglass.  Since it is such a good filter, air leaks are easier to find!  The dirtier the FG is, the bigger the air leak...

          1. User avater
            Mongo | Oct 08, 2003 05:55pm | #10

            Cells, when packed in a bale by the manufacturer, are pretty well compacted. I do think that cells, taken straight out of a bale or bag, should be fluffed a bit if they are going to be placed by hand.

          2. SledBC | Oct 12, 2003 12:31am | #11

            The cellulose we've blown for years has 2 coverage measurements written on the bag, one for blown and a smaller one for hand placed.

            I've been in attics with blown fiberglass insulation where half of the insulation has been wind blown across the attic over time blocking off half of the eave venting and leaving minimal, if no insulation on the other side of the attic. Add that to the fact that a well ventilated attic renders the top couple inches of fiberglass insulation less effective, while cellulose settles and forms a protective crust on top....plus the itch and lung factor....and i think its a no brainer.

  4. User avater
    Mongo | Oct 04, 2003 09:01am | #5

    After six+ years this forum is finally starting to shape up.<g>  Four straight "yea" votes for cells? Unthinkable in the past.

    Another vote for blown cells.

    Densely,

    Mongo

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