How to properly insulate bf Laying hardwood over extension

Nick Migliaccio
by Nick Migliaccio
I just purchased a new home. First off, the house had some significant termite damage. Next, we realized after tearing out carpets that the hardwood oak floor in the dining room stopped at what was the original front wall. Apparently the previous owner extended the dining room 6 feet out (by 13 feet wide) over a slate and concrete covered stoop or porch. They have the floor joists (2 x 10s) sitting vertically right on the slate / concrete. The subfloor (which was termite damaged) has been ripped out. I want to insulate and properly moisture proof before laying new sub-floor and hardwood to continue the dining room floor.
What would you suggest?
Can i lay rigid insulation down directly on the slate / concrete? Home Depot rep suggested caulking that to the joists to seal moisture down... Then laying wall/attic fluffy insulation on top of that and then the subfloor on top.

I also bought vapor barrier that looks like black contractor bag material to lay on top of the joists under the ply wood subfloor for extra vapor protection.
q how to properly insulate bf laying hardwood over extension
  3 answers
  • Shore grandmom Shore grandmom on Jul 10, 2018
    If you want to repair this right, this is what I would do if it were my home. First off, please don't get your information from an employee from a big box store. There are contractors that will come out and give you an estimate and while they're there, you could ask questions. Second, I see that you didn't use pressure treated joists. You have to get metal flashing under the joists. No kind of wood, except pressure treated should ever be put directly on concrete or stone. Third, I think you're wasting your time and money with the rigid insulation and the fiberglass insulation. Rigid insulation is made to be installed on the under side of the plywood floor not on the ground. Attic insulation could get wet from moisture and grow mold. Instead, I would use spray foam insulation because of the concrete under the floor. After it's sprayed on and expanded, you can use a big knife and shave it down. It will be more money, but it will be money well spent. Fourth, the vapor barrier is fine and you should be good from there to put in your finished flooring. Sorry for the not so good news. Good luck in your new home.

    • Shore grandmom Shore grandmom on Jul 11, 2018

      You really want the metal flashing under the joists. And with the spray foam insulation, the vapor barrier isn't really necessary, I just said to put it down because you already have it. If you can return it, I would. Then you can use put that money towards the spray foam insulation.

  • William William on Jul 11, 2018
    The vapor barrier needs to be on the concrete not on top of the joists. No wood or insulation should come in contact with the concrete. If you can get metal flashing under the joists like Shore grandmom suggests do it. Then put down the vapor barrier. If you can squeeze the vapor barrier under the joist the better than flashing. Then you can put down unfaced fiberglass batt insulation in the cavities. Glue and screw the plywood underlayment on top of the joists. Then go with your flooring. Vapor barrier on top of the joists would trap any moisture from the concrete and cause more problems with the joists.

  • Jan Marie Jan Marie on Aug 05, 2018

    Home Depot associates are not always the most reliable when it comes to correct info. I agree with everything that the previous commenters have suggested. Spray foam insulation is great because it fills every bit of the cavity unlike batt insulation. The only thing that I would add is to make sure all debris and damaged wood is removed to insure that no termites are left behind. Good luck with your remaining project.