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What is "mill board"? Local lumberyard never heard of it...

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by brider, Jul 31, 2008.

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  1. brider

    brider Member

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    New Haven, CT
    I thought I asked all these questions before, but I can't find anything:

    My Harmon Oakwood manual says the stove requires k=.84 which "is equal to typical .75-inch millboard".

    Call to the local lumberyard, they have never heard of it. I thought it was a fairly common substrate for tile, or special wall covering, etc.

    There is a link I found on this site for mail-order Micore 300, but it's in 1/2" thick 20-inch widths, so I'd need 2 layers, and with my hearth pad size, I'd need to either scab together patches of 20" pieces, or buy way excess. Rather use the millboard in 1 layer.

    Any info on the stuff?

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  2. Jay777

    Jay777 New Member

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    I thought it wasn't made with paper at all.. more like drywall made out of asbestos? (though I'm sure it's not asbestos any more..)
  3. begreen

    begreen Mooderator Staff Member

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    Yes, these days it's made out of other minerals or ceramic.
  4. seaken

    seaken Minister of Fire

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    brider,

    on behalf of our entire industry I apologize for the confusion surrounding the k-factor issues often raised in owners manuals. It's an extremely confusing subject and no manufacturer's manual I have ever read makes it any clearer, and I've been doing this for twenty years!

    Anyway, suffice to say that all that you need to worry about is a k-factor equivalent to 0.84, or better. You do not need to worry about the actual thickness you end up with. If you use the 1/2" Micore you will meet the requirement since Micore has a k-factor of 0.43. This gives you an R-value of over 1.16. Assuming your mortar and stone have an R-value of at least 0.03 you are good to go since you need an R-value of 1.19 (which is equal to k-factor of 0.84). You could add a piece of sheet metal if you are unsure your mortar and stone has at least R-value of 0.03.

    Good luck,

    Sean
  5. brider

    brider Member

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    Loc:
    New Haven, CT
    Sean, thanks for the follow-up.

    Since my last post, I re-educated myself on the K/R-factor characteristic, and realized, as you've said, that I only need 1/2" of Micore 300 to meet my needs. My previous conclusion that I needed 2 layers of 1/2" was admittedly wrong, and I hope anyone who read my post and followed my assumptions continued to read and is corrected!

    Thanks,

    brider
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