How much insulation above block off plate?

lumbering on Posted By lumbering on, Jan 31, 2013 at 10:31 PM

  1. lumbering on

    lumbering on
    Feeling the Heat

    Dec 7, 2012
    New York
    I figure I'm loosing a lot of my heat up the 100 year old masonry fireplace chimney my flue liner sits in, so I've decided to install a block-off plate.

    The flue liner itself is not insulated, in case this matters.

    How many inches of Rockwool insulation should I insist on above the block off plate?

    Thanks in advance.
  2. KaptJaq

    Minister of Fire

    Jan 31, 2011
    Long Island, NY
    Is it an internal or external chimney? An external masonry chimney would benefit from insulation the full length. Is the cap sealed at the top or is hot air able to escape? The block-off plate stops hot air from entering the void around the liner. With the block-off plate you have to make sure the air that is heated by the flue in that void does not escape out the top. If it does your flue will be hard to heat and may cause draft &/or creosote problems.

    I usually stuff as much mineral fiber as high as I can loosely above the block-off. I use a length of broom handle or something similar to push the insulation as high as possible and keep building up the layer until it fills down to the block-off's level. I do the same from the top before I put the cap on, building up to the cap's level. This slows the loss of heat from the top.

    flhpi likes this.
  3. WoodpileOCD

    Minister of Fire

    Jan 19, 2011
    Central NC
    When I installed my BO plate, I stuffed as much as I could up the flue and compacted it with a a piece of board then stuffed some more in. I did the same from the top. I don't have an insulated liner either and so far it drafts great and so far no appreciable cresote buildup.
  4. lumbering on

    lumbering on
    Feeling the Heat

    Dec 7, 2012
    New York
    I'm not sure if it's capped at the top. I guess I should ensure that it is.
    It's internal on the first floor but external on the second and third floor. It's a very tall chimney.
    The house is old and was vacant for three years and poorly maintained before that. The inspector said the chimney itself was not in great shape and would need work unless we used a full liner. But the liner is not insulated.

    Is filling the entire chimney with insulation the best way to go?
  5. kksalm


    Dec 21, 2007
    Kenai Alaska
    My chimney probably rises 15 feet or so above my Hearthstone Clydsedale insert. My stainless steel liner fits snugly into my block off plate. At the top of my chimney there is another block off plate snuggly attached to the liner and cap. I feel there is no need for insulation. Insulation would inhibit warming the well made mass of stone, bricks, mortar, that makes up the chimney and adds heat to my second story living space. Air is a wonderful insulator, goose down's efficiencies are all about air.
    At least that's what I think. Cheers!
  6. Jon1270

    Minister of Fire

    Aug 25, 2012
    Pittsburgh, PA
    Hmm, my liner system has a ~1/4 - 1/2" gap between the liner itself and the plate across the top of the chimney, all the way around, sheltered by the storm collar. I don't know precisely why, but one installer I talked to told me this was required so I assumed they were all like this.

    Air is a good insulator when it's trapped in very small spaces or thin layers (e.g. goose down or fiberglass or double-pane windows) so it can't convect and move around, carrying heat with it. In relatively open spaces like the gap between a chimney and liner, it's a terrible insulator.

    To the OP, I think you probably should've insulated the liner since the upper part is in an exterior chimney. Not doing so means you could have more creosote buildup to deal with, so be vigilant about maintenance.

    I didn't insulate when I installed my insert this year, and am also losing a lot of heat up the chimney. I will be pulling it all apart and correcting this before next year. With a freestanding stove you're probably losing less heat than I am, but it still might be worth correcting.
  7. sailor61

    Burning Hunk

    Nov 28, 2006
    Warwick, RI
    I think the answer to the original question much as you can pack into the space available. Don;t be shy about working it up along the liner and also consider working it down as much as possible from the top. BUT make sure there is a cap plate on top of the flue first. Roxul and other mineral wools are not water absorbent but water is still apt to congregate in the batting, increase it's weight a bit and lead to it falling out of the position you have worked hard to get it into. It sounds as if filling the chimney with the poured in cement product might be the best bet eventually BUT, in the meantime, Roxul is pretty cheap (40 bucks for a big bale), easy to work with and totally reversible. It may not be the perfect solution to your install problem but it will boost performance until you can get a better solution in place if you decide it isn't answering all of the problem.
    WoodpileOCD and KaptJaq like this.

Share This Page