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Replacing batt insulation in tall crawlspace

nwphil | Posted in General Questions on

new here and just an avid DIY home homeowner
In the process of installing new hardwood floors, I will have to reinforce the floor from below, because rising existing with 1/2″ plywood will cause some clearances issues.
With that said, the crawlspace is 6 feet high, has poly sheeting covering the ground, and foundation vents on three sides – one side joins the garage and and a slab floor room ( this is a split level home, two levels stacked and a mid level, that provides the crawlspace). After buying the house over a decade ago ( it’s from early 90’s), I applied rigid foam and sealant on the wall joists, and drylok around the foundation walls after sealing a few hairline cracks.
There are no condensation, mold or any flooding issues
The batt insulation is in good shape, but has I have to removed in order to install the in-between joists reinforcement; so this  would be a good time to consider a better alternative.
The house is in zone 6
I am thinking too, in applying rigid foam sheeting on the walls, but most important to me right now, is what kind of alternatives would be suitable, if any at this point, to the non-paper faced batt?
There is no piping running thus the joists: only some electric , phone and cable wiring, running mostly on the joists and ofcourse the flexible ducting.
Floor joist are 2×6, @ 16″ O.C. on a +/-30×20 space (four column supports)
Thanks for the help

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  1. Expert Member
    Akos | | #1

    I would figure out if you want your crawlspace either conditioned or unconditioned. Sound like you have something in between.

    A good read is this:

    If you don't have radon issues, conditioned crawlspace with perimeter insulation (no insulation under the floor) is the most energy efficient and provides the warmest floor.

    1. nwphil | | #2

      Hi Akos - Thanks for the reply. Unfortunately, we are in an area prone to radon - got a test done years ago, and we were in the lowest level of recommended ventilation - hence installing a return vent in the lowest room, and keeping the crawl space vented at all times (vents open)
      A secondary issue of sorts, is that I used the "crawlspace" for long storage too - do have smoke/CO2 detectors installed and maintained along with fire extinguisher
      It never get too cold down there neither too warm to cause any issues on stored away wall paint for instance.
      Maybe what I have it's not the best, but seems to be working - however, as I am going to have to remove or place it aside all that insulation, I figure it would be a good time to upgrade.

      1. Expert Member
        Akos | | #3

        You can treat it as an exposed floor and insulate accordingly:

        There are many threads on here about floors over vented crawlspace that will have a lot of details as well such as:

        Keep in mind that you might need a thermal barrier over the foam or use a rated foam such as Thermax.

  2. Eric S | | #4

    You can install a radon system into a crawlspace with radon. I asked my radon mitigation guy who did the basement, how to prep the crawlspace during conversion to un-vented in a way that would allow putting in a radon mitigation system if needed. For my situation, here's what we determined:

    Crawl is dry with gravel bed. I installed dimple mat (overlapped as needed) over the gravel bed ( I then taped the seams and penetrations. Then make a penetration the size of a 5 gallon bucket in the dimple mat in the corner closest to where I would need to run the radon pipe out the rim joist, if we need the system). Next I will install the wall and floor polyethylene sheeting ( including over top of the 5 gallon bucket turned upside down which is now taped into the penetration in the corner. Then insulate the walls with polyiso and do the hvac supply and return (or exhaust, if you choose to handle it that way). Once the crawl space is done, we'll run a long term radon test in multiple parts of the house to check for radon build up post-conversion to unvented crawlspace. If the radon reaches an unacceptable level in a living space, we will mitigate by cutting a hole in the poly sheeting, running a pipe from the top of the upside down 5 gallon bucket to the rim joist and out to the continuously running vent fan which exhausts to the roof line. The new radon pipe will have its penetration in the poly sheeting sealed with tape/mastic.

    Eric S

    1. nwphil | | #5

      Thanks Eric - quite a setup you have going. How deep/thick is your gravel bed?

  3. Eric S | | #6

    Thanks Phil. Like I said, I overdo things, to my own detriment most of the time. I didn't put down the gravel or the clear plastic sheeting below it; it might have been there from original construction in 1977. The gravel is probably 3-4 inches deep over top of the plastic sheeting. Since I doubted the air-seal integrity of the old plastic sheeting, I put my dimple matting right over top of the gravel and will have my finish poly sheeting over top of that -- I'm still working on the whole thing as we speak. Dana Dorsett, Bill, Yupster and others on this forum generously offered great advice as I was planning this and the basement insulation plan.

    Eric S

    1. nwphil | | #7

      Thanks again Eric.
      I always consider minimum requirements as barely acceptable, so do understand well your point.,
      well, I have bare soil just with 6mil poly, and it would be a heck of job bringing in 4" or gravel - not to mention that would make crawlspace a helmet required
      Thinking of insulating the wall with rigid foam, but as most of batt insulation will have to be removed in order to get the floor reinforcement, I am not looking forward to push it back in and lay new twine again....
      rigid board sheeting over the joists is also pretty much out, as I have to keep access to wiring.
      Anyway, good luck with your project.

      1. Eric S | | #8

        Yeah, definitely build from where you start at. I would pull down and don't replace the joist bay batt insulation. Get it out of there for good. Lay down dimple mat (taped at seams) on the dirt floor (bumps down), then lay heavy duty poly sheeting on walls and floor (taped/mastic at seams), then poly iso on the walls fastened through the poly sheeting. Air seal the rim joist with caulk (ge silicone 2 is what I'm using), then cut & cobble in pieces of polyiso to fit with beveled edges. Spray foam the edges. Provide HVAC supply sized appropriately for the area and placed in a way that air washes across the crawlspace from supply to exit. Exit can be continuous exhaust fan in rim joist or transfer grille to living spaces above or astride the crawlspace. I'm not sure, but I imagine it can also be a legit return duct to the main system. Make sure to plan an exit for the supplied air, to meet code and to keep from forcing air into places that you can't control.

        I am only halfway through the project and already notice a difference in air quality and temp & relative humidity.

        Best suggestion is if you're going to put in lighting, do it at the beginning of the project. I've been working with a headlamp and flashlight the whole time, when I should have just done the lighting first.

        Eric S

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