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Currently there is R19 faced insulation installed between the floor joists. I am adding R30 unfaced perpendicular to the currently installed R19. I have two questions:

1) There is an HVAC unit in the attic. Can I cover the duct work (which is already insulated with foil insulation) with the unfaced R30? My concern would be potential condensation in the summer time with the air conditioner running.

2) In some areas of the attic, there is plywood that is nailed down to the floor joists. But, there is the R19 faced underneath it. Can I just add the unfaced R30 on top of the plywood?

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  • Use unfaced. You do not want a double vapor barrier. It will trap moisture which you don't want. Wet insulation is useless
    – user46796
    Dec 18, 2015 at 21:57
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    The Kraft facers on fiberglass batts are not vapor barriers; they are vapor retarders; There will be no problem, even though you are correct that unfazed batts are better here.
    – iLikeDirt
    Dec 18, 2015 at 22:19

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You will not create a problem of moisture in or on the ductwork. The duct work is insulated to keep the condensate from forming on the outside of the pipes and dripping off from the temperature differences in the attic in the summertime with AC running. So more would help. What would be bad is if the duct work is flex duct and gets mashed under the weight of the added insulation on top of it. Don't underestimate what will happen once you cover it up and can't see have may have happened to it.

About the existing decking, I feel storage is valuable in any part of the home. To cover it up and lose what may be valuable space, at least in my mind seems a shame. But if you are not using it, and don't plan on it, then there really needs to be no concern.. If that is the case then insulation added anywhere in a continuous layer is a benefit. I would if it were me, pull it up and set 2X10 material on its edge and screw it to the joists so the new insulation could run tightly between the 2X material, then add plywood over that, ripping it 2 ft. wide if need be to get it through the access.

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  • Thanks for the reply, Jack. I won't use the space for storage - mainly because it is a pull-down style attic access and not a walk-up attic. I don't quite understand what you are saying after your comments about ripping up the plywood. Are you saying to rip it up, add the additional insulation and then add a plywood decking (with standoffs) on top of that, thus providing storage space?
    – Todd
    Nov 1, 2014 at 22:52
  • Sorry about the way I put it. It looks like you got what I meant though. I was referring to pulling up the floor that was in there t build a raised system with the insulation under it. The ripping I was referring to was taking 4X8 plywood and ripping it to 2X8 sheets to get it into the attic
    – Jack
    Nov 2, 2014 at 0:50
  • Thanks. I appreciate the follow-up. One last question. Since I'm not using it for storage, can I just put the unfaced insulation over the plywood? There is faced insulation under the plywood. It sounds like you are saying that is okay. I just want to confirm. And again, thanks for the replies. I really appreciate it. best. Todd.
    – Todd
    Nov 2, 2014 at 13:36
  • Yes in my opinion, insulation is insulation, and it will work as it should when properly put in place. Some may beg to differ about putting it over an existing wood surface, and the purist in me really would not want to do that. But putting a continuous layer, even over plywood or other flooring surface you may apply it over, will work as it should when properly installed.
    – Jack
    Nov 2, 2014 at 14:32

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