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Polyslab vs concrete to suppress vibration?

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  1. #1
    Apprentice (new member)
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    Nov 2011
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    Default Polyslab vs concrete to suppress vibration?

    The external unit for my refrigerated A/C sits on a couple of galvanized channels shaped like inverted 'V's, which in turn sit on a flat ribbed metal roof. So there's not much to absorb the vibration, and at certain speeds a loud hum is transmitted into the room underneath.

    I plan to replace the 'V' channels with a heavy rectangular base, to spread the load and absorb the vibration. I'll also be adding waffle mat or rubber mounts between the unit and the base.

    The base can be concrete (which would be heavy), or Polyslab which would be a lot lighter. However I understand that the Polyslab is formulated to absorb vibration, so despite its lower weight it might not be too bad. (It's actually my first preference, because of the easier installation and reduced risk of cracking.)

    But before I lay down the cash, I'm wondering if anyone here can advise on how the two materials compare?

  2. #2
    Apprentice (new member)
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    Default

    Hi there. We have had a big 18kw ducted unit installed on our roof for more than 6 years now on a Polyslab and haven't had any issues. Originally we were concerned the Poly wouldn't stand up to the harsh QLD sun as it is right in the middle of our roof with no protection. I was up there the other day and no worries at all. Actually both our AC and our Hot Water System are installed on Polyslabs both on the roof.

    The AC unit is screwed down to the Polyslab with stainless screws and some adhesive and they have placed a thin 8mm rubber pad between the unit and the Polyslab. The Polyslab is then screwed straight into the ribs of the metal roofing. We haven't had any noise issues at all and would highly recommend it.

  3. #3
    Apprentice (new member)
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    Default

    Thanks for the reply. I bought a Polyslab recently, and was surprised that it wasn't heavier, but at least it's easy to fit. Also I didn't relish the thought of lugging a large concrete block onto the roof - probably bit of an overkill!

    It sounds like the 'rubber pad' under your unit is waffle matting - is that right? The experts all say it's good, so I bought some to put under the feet as well. Am hoping to do the job soon.

    Good to hear that your unit is quiet, without extraordinary soundproofing measures. That gives me a lot more confidence. Thanks again.

  4. #4
    Golden Member manofaus's Avatar
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    Oct 2007
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    muswellbrook
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    Default

    have mounted a/c's and gensets to mobile offices with these things:
    5/16 RUBBER MUFFLER MOUNTS | eBay
    dunno just an idea
    sorry for the ramble.

  5. #5
    Apprentice (new member)
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    Nov 2011
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    Default

    Hi manofaus, sorry for the delay (I haven't been on the forum for a while). Good suggestion about the rubber mounts - low cost as well.

    As luck would have it, a few days after your post, the installers came to work on my system (long story). In the process, they lifted the indoor unit off the ceiling joists, and suspended it inside the roof using steel straps. They also re-installed the coolant pipes. Man, was the unit quieter after that! There must have been more noise from those things than I realised!

    There's still a slight hum, but you hardly notice it. So I'll probably just slide the Polyslab underneath, when I can be bothered. Might add the rubber mounts as well - will see how things go.

    So once again, thanks for the help guys )

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