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  1. #1
    Join Date
    Mar 2014
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    1

    Default Should I remove fiberglass insulation after upgrading to foam?

    While remodeling our 1986 2 story we encountered a situation where we would need to re-insulate parts of our ceiling. Because we have the perception that open cell spray foam would be better than the blow-in pink fiberglass that was original to the house, we decided to foam under the roof deck in the attic. This would leave insulation below between the ceiling joists and foam above. Our vents on each ends would be covered and sealed creating an "envelope" in the attic. It was suggested we remove the original insulaiton. the reason given was " it will mold". It seems to me that if moisture were to be able to reach the attic space, it would or could mold whatever it happened to gather upon. Should I remove the old insulation and for what reason.

  2. #2
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    6,598

    Default Re: Should I remove fiberglass insulation after upgrading to foam?

    If you lose the good air flow, you'll get mold.

  3. #3
    Join Date
    Jan 2014
    Location
    NH
    Posts
    136

    Default Re: Should I remove fiberglass insulation after upgrading to foam?

    Quote Originally Posted by ron@rileygeo.com View Post
    While remodeling our 1986 2 story we encountered a situation where we would need to re-insulate parts of our ceiling. Because we have the perception that open cell spray foam would be better than the blow-in pink fiberglass that was original to the house, we decided to foam under the roof deck in the attic. This would leave insulation below between the ceiling joists and foam above. Our vents on each ends would be covered and sealed creating an "envelope" in the attic. It was suggested we remove the original insulaiton. the reason given was " it will mold". It seems to me that if moisture were to be able to reach the attic space, it would or could mold whatever it happened to gather upon. Should I remove the old insulation and for what reason.

    Hi, Was a plastic wrap or plastic vapor barrier installed in the walls? The moisture inside the house will try to escape, if it can't escape through the walls then it will try to escape through the attic. Thanks

  4. #4
    Join Date
    Feb 2009
    Posts
    3

    Default Re: Should I remove fiberglass insulation after upgrading to foam?

    Since the open cell is vapor permeable, the mold causing moisture I would think would keep moving up through the foam and out into the roof sheathing and the outside air... My icynene contractor mentioned nothing about the several layers of mineral wool and fiberglass insulation in my attic.

  5. #5
    Join Date
    Aug 2007
    Location
    Portland, Oregon, formerly of Chicago
    Posts
    1,794

    Default Re: Should I remove fiberglass insulation after upgrading to foam?

    Seems to me that if the attic is to be brought into the house envelope, it should be brought into the house envelope. That is, it should be ventilated via the house heating/cooling system. Mold will not occur if relatively dry air is being circulated. If the attic is temperate with the house, there would be no need for insulation on the back side of the ceiling.

    Personally, I always thought it would be better to spray a good coating of foam on the backside of the ceiling, making an absolute air / moisture barrier from the house envelope. For the sake of cost, additional insulation could than be blown ontop of it. The attic would remain vented according to time tested standards.

    My one concern with the above system is what would happen if the roof developed a leak? A lot of water could accumulate before you would become aware of it.

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