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Using rockwool under porch

jam1030 | Posted in General Questions on

I am converting a covered screened porch to a a conditioned space by putting in windows and adding insulation. The current floor is treated deck boards but will cover that with ¾ t&g plywood prior to adding finished flooring. 

My question is what would be best way to insulate floor? It is raised about 3-4 feet above ground with 2×8 joists. I was thinking of using r15 rockwool but was wondering about vapor barrier and protecting insulation for pests. 

Also was wondering about using the rockwool in combination with foam board.   

I live in south Alabama. Any ideas would be greatly appreciated. 

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  1. Expert Member
    Dana Dorsett | | #1

    R13 between the joists is IRC code minimum for US climate zone 2, but a a continuous 2" layer of polyiso applied to the joist bottoms would do about as well, and would be a good vapor barrier to keep the joists & subfloor from getting moldy if you're air-conditioning that room. Air tightness is more critical than R-value, and it's pretty easy to tape the seams of foil faced polyiso. The rim joist would need a minimum of R13 on the interior, which could be cut'n' cobbled polyiso cap-nailed in place than taped.

    It may be cheaper & easier (and certainly more critter-retardent) to install a full fill of low density R25 fiberglass in the joist bays, with a continuous layer of OSB or CDX on the underside of the joists detailed as an air barrier. (That would be almost 2x code-mininum!) Plywood and OSB are both "smart" vapor retarders, and would offer better drying capacity than foil faced foam board or a true vapor barrier. True vapor barriers are not really necessary in a zone 2A ( which most of the gulf coast) climate.

    1. jam1030 | | #2

      Thank you for the reply. I do live on the gulf coast. I even thought of raising the existing floor up approximately 3” so the room floor level would match the rest of the house and to get the floor more level as it is now a little out of level. But that would only leave 3” between existing deck boards and new subfloor which only accommodate some foam board. I would cut foam board to fit tightly and use foam gun to seal contact with joist. Plus, it would be quite a bit of work.
      That would only give me around R 15.
      Your idea of just using fiberglass and covering the entire bottom with osb sounds like a better option. What did you mean about osb “detailed as an air barrier “?

      1. MattJF | | #5

        If you want to raise the floor, 2” polyiso laid continuously, taped, sealed the edges and topped with 3/4 t&g plywood would work well.

    2. jam1030 | | #4

      How would using R15 rockwool pushed all the way up to bottom side of floor decking and then use osb on bottom of floor joists to protect rockwool from critters?
      Since my joists are 2x10, this would leave about a 5.75” air gap between bottom of rockwool and osb. Will that cause a problem? I am in zone 2a

      1. Expert Member
        Dana Dorsett | | #7

        With 2 x 10s (9.25" deep) I'd be inclined to sheath the bottom with OSB and blow it full of cellulose rather than leaving a 5.75 inch "critter condo" space. I've met some squirrels willing to chew through 3/4" sheathing before to nest in empty framing cavities, but maybe they're nicer where you live.

        That would be more than twice the code minimum R, but the borate fire retardents in the cellulose would also mitigate against carpenter and termite infestation. The last squirrel that chewed into a bag of cellulose in the garage didn't persist in that endeavor. While cellulose is not fully rodent-retardent, they seem to prefer low density fiberglass.

        But to answer the actual "Will that cause a problem" question, unless you're air conditioning to some ridiculously low temperature, air tight OSB sheathing would be a sufficient exterior side vapor retarder, with low/no risk of making the subfloor punky with air-permeable R15 batts snugged up to the subfloor, even with the big air gap between the OSB and rock wool.

        But if you're keeping it 65F indoors in summer (and there are some that do) it could be a problem, but here again a full fill of cellulose would "share" and redistribute the moisture burden if it's fully in contact with the wood above.

        1. jam1030 | | #9

          Thanks Dana for your reply. How is the cellulose blown to fill cavities? Holes drilled into osb or subfloor? Holes are then filled with what? Not familiar with this procedure. Is this something a homeowner can do?

  2. jam1030 | | #3

    Another thought......what about if I used the R15 rockwool pushed all the way up to subfloor and use 1” of foam board on the bottom of joists and then osb over the foam board to protect foam board?
    The rockwool is 3.5” thick and joist are 7.25” leaving a void of 3.75”. Is that a problem?

    1. Expert Member
      Dana Dorsett | | #8

      I'm not sure what that foam board is really buying you here (?). There are much cheaper vapor retarders, and cheaper insulation than 1" foam board. Typical southern framing species run R1.2-R1.4 per inch so filling the cavities with fluff (even low density cheap stuff) does more for the total thermal performance than 1" of foam board. The 3.75"of remaining joist depth x 1.2/inch= R4.5 about the same improvement as 1" of EPS foam would add for the framing fraction, but even worst-in-class R3/inch fluff would add more than R11 to the rest of the floor area.

      Cheap low-density R38 fiberglass batts would deliver more than R30 when compressed (by about 3") into a 2x10 bay, and when compressed would on whole be comparable to or possibly more air retardent than an R15 rock wool batt (due to being nearly 3x as thick), though not as air retardent as a cellulose fill.

      Cellulose would be by far the greenest solution too:

  3. lookloan | | #6

    Usually besides insulating, the Rockwool resists burning. I would put foam board under sub-floor and spray foam around the perimeter. I would then add the Rock Wool and more important, nail tight wire mesh or fencing on the open bottom to prevent pests from bugs to squirrels from getting in. Maybe 1/16 mesh or 1/8 square holes in the mesh.

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