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Q&A Spotlight

The Best Way to Insulate a Floor

A homeowner in Arkansas weighs the benefits of mineral wool batts and sprayed polyurethane foam — but a third option may be the winner

This is Jim Wright's floor. Insulation, air-sealing and dealing with the possibility of a leak are all factors to be weighed for this western Arkansas home.
Image Credit: Jim Wright

Jim Wright’s house in western Arkansas has a pier foundation that elevates floor framing about 40 inches off the ground. Unlike a house with a basement, crawl space, or slab foundation, there is no enclosure at the bottom of the house, so the floor is more or less like another exterior wall.

How, Wright wonders, should this be insulated?

“I am considering two methods of insulation,” he writes in a Q&A post at GreenBuildingAdvisor. “The first method is 7.25-inch-thick Roxul batts (R-30) that are covered with 7/16-inch OSB sheets. The second method is 1.5-inch (R-10) of sprayed closed-cell foam.”

Not counting his labor to install the mineral wool batts, the cost of either of those options is about $2,000. Because of its thickness, the Roxul would have three times the insulating value of the foam. But it would be harder to install because the 2×8 batts are installed on 19.2-inch centers and all of the batts would have to be cut to fit.

“A big advantage of the closed-cell foam is that I would not be doing any of the labor,” Wright adds. “Also, it appears that the foam would seal better against air infiltration. The only apparent disadvantage of the foam is the lesser R-value. However, I’m not sure that I really need more than R-10 in the floor.”

Overall, the 68-year-old Wright is leaning toward the spray foam. “What say ye?” he asks.

The first job is to meet minimum code requirements

To meet the requirements of the 2009 International Residential Code, floor insulation in Wright’s climate zone should have a minimum R-value of 19, says GBA senior editor Martin Holladay. While the mineral wool batts easily get the job done, the proposed thin layer of spray polyurethane foam would not.

“Your spray foam contractor is suggesting an…

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20 Comments

  1. Expert Member
    Malcolm Taylor | | #1

    Insurance
    "More than one insurance company will give you a discount for either individual appliance shut-offs or leak detection systems."

    "More than one" includes the number two. I see a lot of advice on how insurance companies will offer discounts for certain building features but my insurance broker can never find them. It might be more useful to find out whether these policies are commonly available before suggesting them.

  2. Flitch Plate | | #2

    rodent den
    In western NY we call this kind of crawl space a rodent den: skunks, coons, possums, feral cats, foxes and ground hogs. One would have to put a perimeter and bury the bottom edge; lattice and hardware cloth if you want airflow. Treated chicken wire could be used but then carpenter bees would also do damage. Contrary to the words of the inexperienced, paint and other surface treatments will not deter carpenter bees. This 2014 spring is bumper year for carpenter bees in the north east.

    My raised insulated open crawl space buildings have 3/8" glued, screwed, caulked and seam taped OSB under-panels with dense paced cellulose blown in through holes in the subfloor above; which are then plugged with tapered wooden plugs made for this method. The perimeters have lattice/hardware cloth surround to allow air exchange yet keep out varmints.

    Once a ground hog establishes a den under a building, you have to become a warrior to handle the multigenerational assault from that one rodent's persistent off spring. County boys don't love big lawns; they hate ground hogs (big lawns take away the hog's cover so they avoid coming to the buildings to dig dens).

  3. Derek Roff | | #3

    bridges, breaks, and greenness choices
    Presumably, Peter Yost meant "thermal break", or something similar, when he wrote "Rigid foam has the advantage of providing a thermal bridge on your floor framing".

    In Martin's recommendation, "That should be followed by a continuous layer of rigid foam insulation at least 2 inches thick (foil-faced polyisocyanurate being the most environmentally friendly)", is a rigid mineral board product a reasonable option? If so, how would you rate the relative eco-friendliness?

  4. GBA Editor
    Martin Holladay | | #4

    Response to Derek Roff
    Derek,
    Thanks for catching the typo ("thermal bridge" for "thermal break"). I have corrected the text.

    Concerning your suggestion of installing mineral wool on the underside of floor joists to act as a thermal break, I have a couple of reactions.

    First, I've never tried the technique.

    Second, I'd be worried that any attempt to fasten the mineral wool to the underside of the joists would result in unacceptable compression at the joists unless you first installed OSB or plywood on the underside of the joists. Since you need a layer of OSB or plywood on the exterior side of the mineral wool as well, to keep rodents out of the insulation, your suggested technique might require two layers of OSB or plywood -- one above and one below the mineral wool. That's an expensive approach.

  5. Jesse Anderson | | #5

    caulk the ground facing panels?
    For air infiltration purposes, wouldn't the installer want to caulk the interface of the joists and ground facing panels? Also, would blocking at the unsupported ends make sense to tie the panels together?

    Interesting article. And regardless of the final choices, don't forget the panels cost money too.

  6. GBA Editor
    Martin Holladay | | #6

    Response to Jesse Anderson
    Jesse,
    Yes, you always need to pay attention to air sealing for all building assemblies. Caulk is one option, although an increasing number of builders are depending on tape. If you decide to use caulk, don't forget to address the perimeter of the floor. If you don't provide blocking every 4 or 8 feet, you can use tongue-and-groove plywood or tongue-and-groove OSB.

  7. Horst Fiedler | | #7

    Blown-in Cellulose
    One issue rarely mentioned with blown-in cellulose is that rodents love the stuff!. It's a wonderful material for nests, tunnels and living quarters. They will chew though sheathing to get to it. Best to seal up every opening with metal mesh, flashing, and urethane caulk especially along sills, corners, electrical opening, etc.

  8. John F Cross | | #8

    'My' Floor Insulating Solution
    I had a fiberglass bat, enclosed with chicken wire floor - absolutely filled with 'meece' nests (plural of mice).. I subsequently raised it, and installed a sip insulated 'down stairs' floor, removing the bats and chicken wire from the 'ceiling', and enclosing the crawl space - cross venting it with bin windows that open to the inside.
    If your situation were mine, I would carefully, foam your under floor, multi times to meet the R 20 requirement - in a manner that each layer, cured thoroughly. An uncured, thick layer would be a serious problem! In your location meeting code is probably more important than 'super insulating' the floor,
    I also believe that the Fab-form, footing form, would be the easiest/cheapest form - to then create a ?? 2 x 4 ?? enclosure wall (with bin window, venting).
    In my case, the building inspector, did pass and acknowledged my 'near code' crawl space as acceptable to him.
    To my knowledge, I do not have a moisture (in the structure) problem - no mold. I mostly close the bin windows in winter. Being directly on rock - I have kept beer, all winter, sitting on the crawl space floor. I do not expect a water problem, from above the crawl space.
    In my case, the wiring and hot/cold water are in the accessible, suspended ceiling (formerly under floor - before the raise). As I am on a drain back, lake pump water system - where power frequently goes off - I can shut off the pump and slope drain the water, when I leave for an extended period.
    It works! - especially with a mini-split, wood stove and inline water heater.

  9. Robert Hallenbeck | | #9

    thicker spray foam?
    While I'm not particularly a fan of spray foam insulation, it seems like the main argument against spray foam under a floor in Post and Beam construction is the insulation thickness. Is spraying a thicker foam to reach R19 (or higher) a reasonable solution in this application? Seems like this addresses air sealing and animal issues. Still a risk of damage from interior water leaks, however that seems no different than cellulose insulation.

  10. GBA Editor
    Martin Holladay | | #10

    Response to Robert Hallenbeck
    Robert,
    Q. "Is spraying a thicker foam to reach R-19 (or higher) a reasonable solution in this application?"

    A. That would certainly be a better solution than installing R-10 of spray foam, as was suggested by the contractor who advised the GBA reader profiled here. It would meet code requirements.

    However, unlike a solution that includes a continuous layer of rigid foam on the underside of the joists, your suggested solution still allows thermal bridging though the floor joists. (Admittedly, this isn't a major concern in a mild climate like that of Arkansas.)

  11. Conrad Blunck | | #11

    Rigid foam and felt paper?
    I'm facing a similar situation. Would one option be to simply fasten asphalt paper on the underside of joists and then install rigid foam on the underside of that? I understand that won't provide much insulation, but would it prevent moisture issues?

  12. GBA Editor
    Martin Holladay | | #12

    Response to Conrad Blunck
    Conrad,
    If you want to insulate a floor assembly by leaving the joist bays empty, and installing rigid foam on the underside of the joists, here are a few things to keep in mind:
    According to the 2012 IRC, the minimum R-value for floor assemblies is R-13 in zones 1 and 2, R-19 in zones 3 and 4, R-30 in zones 5 and 6, and R-38 in zones 7 and 9.
    Your suggested approach will only work if there is no air leakage from the exterior to the empty joist bays. To ensure that there is no air leakage, you need to perform meticulous air sealing work at the perimeter of the floor assembly (the rim joist area) and you need to carefully seal the seams between your rigid foam sheets with a high quality tape.
    The asphalt felt is unnecessary.
    After installing between 3 inches and 10 inches of rigid foam -- the amount needed to meet minimum R-values required by code -- you'll need to protect the rigid foam with a layer of OSB or plywood.

  13. maryjacksonuk | | #13

    I've used the GA4000 throughout my floors and walls. They have served me incredibly well so far and whilst the 75mm thickness may seem excessive, I'm pleased we made the investment. You can see the board a https://www.roofingmegastore.co.uk/roofing-categories/insulation/type/boards/celotex-insulation-ga4000-board.html.

    The only thing I would urge everybody to consider is sealing the gaps between their boards, particularly when using old wooden flooring. This can be done with cork strips and prevent any cold air from finding its way through the cracks.

    Thanks very much!

  14. Nick Defabrizio | | #14

    I stumbled upon this thread and found it interesting. People treat huoses on piers with open sides as if they are exotic animals. However, along the East Coast, in places like Long Beach Island, tens of thousands of houses have been built raised on wood pilings in a very humid environment (e.g., water table 12 niches below the ground level depending on tide). In the past, the pilings were not enclosed but, unfortunately, nowadays they are typically enclosed with "breakaway" walls so folks can store more stuff (most of the damage to raised houses from Hurricane Sandy was to these "storage spaces"). A few things to consider:
    (1) The higher up above ground your house is, the less ground moisture is a problem, though uplift and wind wash become bigger issues. In any area where there is wind, Simpson ties are important to join the beam to the post and the joists to the beam. This latter point may complicate air sealing at the bottom of the joists so you may be better off airsealing above the joists. You can also make and install wind baffles on the side of the prevailing wind.
    (2) Grade the land beneath the house to allow proper drainage away from the middle. Then cover the ground under the house with at least 6mils of plastic covered by a few inches of 1 1/2 inch clean gravel or a thin "rat" slab of concrete. This will dscourage some varmits but also reduce moisture coming up from the ground.
    (3) Consider exterior paperless sheet rock (Dense Armor plus) or Hardieboard to cover the bottom instead of OSB. Make sure it is painted and taped.
    (4) Insulate around the plumbing risers and maybe obx around them

    .

    1. Expert Member
      Malcolm Taylor | | #15

      Nicko,

      A small point, but in most climates if the plumbing is at risk of freezing, insulation won't do much unless the contents are moving. You either need a heat source or a large boxed in area connected to the house above.

  15. user-7155017 | | #16

    This is Jim Wright's floor. No it is the framing for the floor - left as is (dry exposed wood) the life expectancy is about 400 years. The soil or roof would have to fail first.
    trap this wood in anything - moisture will get inside the wrap - wood will rot. Doesn't even consider rodents etc.
    Insulating above the joists, continuous, is the approach to explore.

    1. Enteecee | | #17

      This is my thought as well. I'm building on steep grade, and digging is expensive there too, so piers are definitely going to be the way to go. The grade means that the floor structure will range from 3'-9' above grade, and it seems really ineffective to me to try working overhead in that condition. I can't find the Lstiburek assembly mentioned in this (pretty dated) JLC article anywhere else, but MISREADING it inspired what I'm thinking.

      Keep the floor framing light, with an Advantech or MgO deck. Then 3.5" - 4" polyiso with cross-laminated plywood floating over it. I know it'll be thicker, but for amount of labor and material, as well as ease of installation, I don't understand why this is hard to find examples of. What am I missing?

      https://www.jlconline.com/how-to/framing/raised-floors-for-the-low-country_o

      1. Expert Member
        Malcolm Taylor | | #18

        Enteecee,

        I'm a bit confused. Neither the description nor the illustration in the linked article show the assembly you describe. Joe L. is suggesting continuous rigid insulation attached to the underside of the floor system and protected by fibre-cement panels.

        1. Enteecee | | #20

          Ha! You're totally right, Malcolm, and I apologize. Until I read your response I didn't recognize how far I'd gone from Joe L's detail once I started drawing myself. I saw his and then tried to eliminate or at least minimize working from below which would be so awkward in this situation. I've edited my comment to reflect that and added a drawing of what I'm thinking of. Once framed, the whole thing is built up from above.

          The part I know I'm still unsure of (as opposed to what I might not know is wrong) is WRB location. I'm not sure whether to treat it more like a wall and tape off the "sheathing" subfloor at the risk of creating a tub, or to tape the top of the insulation boards so the whole thing can dry down at the risk of creating a condensation plane inside the assembly.

          Does this make sense?

  16. Deleted | | #19

    “[Deleted]”

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