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Thread: Duct Insulation

  1. #1

    Duct Insulation

    I am in the middle of a renovation for our HVAC system at the Jersey Shore and need advice. Our 1st floor ducts run through the unconditioned crawlspace. Conditioning the crawlspace is difficult because we have groundwater and surface water periodically entering the crawlspace.
    As of today, the trunkline is wrapped in bubble wrap with a small airspace (accomplished using 3" wide bands of bubble wrap every 2' as spacers) and the bare metal branchlines are to be encased in sprayfoam. All ducts are rigid. I am considering sprayfoaming the trunkline.

    The only question I have is if we sprayfoam over the bubble wrap will there be any negative ramifications either structurally or in r-value? The sprayfoam is 130 degrees at application. Both the HVAC and insulation contractor don't have a definitive answer. I will remove all bubble wrap if necessary but would prefer not too.

  2. #2
    Join Date
    May 2004
    south louisiana
    the air space for a radiant barrier...which is what foil bubble foil wrap a minimum of 3/4"
    air space. using 3" wide bands of bubble wrap doesn't achieve this minimum space.
    that said..foil bubble foil doesn't provide much insulating value, contrary to what
    sales hype tells you.

    here is a comprehensive article about this product:

    spray foam doesn't do a good job of sealing duct leakage.
    I know the foam company will disagree, but I have tested for duct
    leakage on foam insulated ductwork..and it does not seal as they believe.
    as it goes on quickly & expands, it does not get into the small gaps like
    using a paint brush to apply mastic does.
    it isn't an approved ACCA duct insulating/sealing product.
    if you've ever used great stuff in a can, and gone back a couple of months
    later, you'll see that foam shrinks.

    you'd be much better off to mastic seal and use duct wrap. foil faced scrim kraft paper
    with R-8 insulation properly installed, stapled & taped with fsk tape.

    there really aren't shortcuts that work for long, or we would be using them.
    routing the water from under the house & installing a vapor barrier on the
    ground is the solution for the standing water under the house. shortcuts to do it properly.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  3. #3
    Join Date
    Jul 2001
    Bubble wrap is crap, pro's don't use it. To get R-8 takes double wrap with spacers. No one will do it correct to gain the r value. Use r-8 duct wrap and follow instruction printed on cover.


  4. #4
    The ducts are mastic sealed. I want the foam for insulation value. I believe spraying it over bubble wrap may be a mistake because it will not allow the foam to adhere directly to the duct. The water issue is impossible to rectify since the water appears to come up from the soil during heavy rains. I am concerned about the water pushing up on the vapor barrier from below. All water from the exterior of the house is pitched away properly. Also, during floods, the floodvents allow water into the crawlspace. In the course of 3 years we have had flood water in the crawlspace 5 times (including Sandy). I completely understand the need for a conditioned crawlspace, but in my case isn't it better to allow the water to do what it wants and just protect the underneath of the house?
    Our crawlspace is also lower in grade than the outside of the house, thus creating a bathtub. The EPA detail for crawlspaces in flood plains requires raising the grade inside the crawlspace to allow water to exit the flood vents after a flooding event and utilizing a vapor barrier. Here again, I am concerned that adding sand/soil to the crawlspace will not stop groundwater infiltration because the hydrostatic pressure may just push the water up through the new sand/soil and underneath the vapor barrier. Is that accurate?
    Pitching water to a sump pump may work but I have nowhere to pump the water too. The nearest storm sewer in the street floods at high tide everyday. Water table in the yard averages 15-18" deep (if you dig an 18" hole it will fill with water).
    Any thoughts?

  5. #5
    Quote Originally Posted by HVAC/Stud View Post
    Bubble wrap is crap, pro's don't use it. To get R-8 takes double wrap with spacers. No one will do it correct to gain the r value. Use r-8 duct wrap and follow instruction printed on cover.

    Understood. Unfortunately, the humdity and potential flood waters in the crawlspace will destroy r-8 duct wrap. I spent days removing saturated batt insulation from the joist bays and removing saturated flex duct after Sandy. That is why we went to 100% rigid ducts, mastic sealed. The joist bay insulation will be replaced by spray foam. I thought it would be a good idea to also sprayfoam the ducts for protection and insulation value. Will sprayfoam on the ducts over bubble wrap be an acceptable application?
    PS - the sprayfoam is closed cell of course.

  6. #6
    Join Date
    Jul 2001
    Sorry for you flood problem. I'd find a good hvac company who can help.
    Good luck,


  7. #7
    Join Date
    Oct 2010
    Anderson, South Carolina, United States
    As long as the water stays beneath the vapor barrier it won't affect the crawlspace if its encapsulated properly. A sump pump and water level alarm can be installed on top of the vapor barrier in the event that water did get on top side of VB. crawl vb needs to be pitched towards sump using sand dirt etc to get proper pitch. Spray foam or ridged board insulation on foundation walls with vb continuing up to floor joists, be sure to leave inspection band at top for termite inspection per pest control company's mandate to guarantee warranty of no termites.

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