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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 installation that doesn’t even meet minimum code requirements,” Holladay writes. “I have a problem with contractors like that — and I’ve been writing about the problem for years. It seems that the problem is particularly common among spray foam contractors. But that’s an issue for another blog.”

Holladay suggests filling the joist bays with “almost any kind of fluffy insulation” — for example, cellulose, fiberglass, or mineral wool. 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), and finally a layer of oriented strand board (OSB).

“That approach may be too expensive for you,” he adds, “but it’s the best way to go.”

Blown-in cellulose is a good choice

Even “crummy” kraft-faced R-19 batts would do a better job than the amount of spray foam recommended by Wright’s contractor, says Dana Dorsett. Instead of Roxul, Dorsett suggests cellulose that Wright, with the help of an assistant, could blow in himself with equipment rented from a big-box store.

The procedure he recommends goes like this:

  • Install OSB over the bottom of the joists.
  • Using a hole saw, cut holes measuring 2 1/2 to 3 inches in diameter in the OSB every 6 feet or so along the length of each bay.
  • Blow cellulose into the bays after blocking off all holes save the one currently in use with heavy rags.

With a single-stage blower and a 2 1/2-inch-diameter hose, Wright should be able to get the cellulose installed at a density of 2.5 lb./cu. ft., which would only settle 1/2 inch in 20 years if it settled at all.

The cellulose would be “quite a bit cheaper” than R-30 mineral wool batts in Dorsett’s area but more expensive than R-19 fiberglass. Wright should be able to buy the cellulose for about $350, and the store might include 24 hours of blower rental. Adding 1 1/2 inch of polyiso would be another $1,200, he says, with the total well under the $2,000 the foam contractor would charge.

“If you install the wood sheathing and pre-drill the blowing holes, with somebody to help with the blower you should be able to knock it out the blowing in less than a full day,” Dorsett writes. “The R-value of cellulose at 2.5 lb. density is near its optimal peak performance at about R-3.8 per inch, so for 7 1/4 inches you’d be looking at about R-28, comparable to the Roxul, but at a fraction of the price.”

Wright considered cellulose but initially rejected the idea

“Actually,” Wright replies, “my original plan was to use cellulose and blow it in as you suggested in both the walls and the floor; and, of course, the attic as well.”

But he decided against it when he discovered the cellulose should be blown in to a density of at least 3.5 lb./cu. ft. to avoid settling and voids. “And to do that you would need a high-pressure blower rather than the type available at the box stores,” Wright adds. “But there isn’t any place in my area that rents such blowers.”

Ultimately, Wright packed walls with cellulose by hand to an estimated density of 5 lb./cu. ft., and with Dorsett’s encouragement he’s back to favoring cellulose in the floor.

“As to the thermal bridging of the joists,” he says, “I was thinking of buying a few sheets of rigid foam and cutting them into 1 1/2-inch strips and stapling them to the bottom of the joists before installing the OSB sheets.”

When packed to a density of 5 lb./cu. ft., Dorsett tells Wright, cellulose has a slightly lower R-value than it would at 3.5 lb./cu. ft. And in reality, there’s no real need to hit the 3.5 lb./cu. ft. mark to prevent settling. A density of 2.8 to 3 lb./cu. ft. in an Arkansas location should be fine.

Dealing with potential leaks

Wright’s one remaining concern is the possibility of the cellulose getting wet if the floor were to be flooded by a leak.

If the cellulose does get wet, it should be removed, Dorsett says, adding, “It can dry through OSB, but it would take forever.”

If the prospect of removing a section of sheathing and digging out soggy cellulose isn’t appealing, Dorsett continues, “you could quasi-dense pack a fiberglass blowing wool in a similar manner, which doesn’t hang onto the water as readily as cellulose, and would make leaks easier to spot. Even minor plumbing leaks would drip through, unlike with cellulose, which would soak it up for weeks before it became evident.”

Wright would still have to replace soaked insulation in the event of a major leak, but it should be a smaller, more localized problem. “Simple spills wouldn’t be a major concern with either cellulose or fiberglass,” Dorsett says, “but major plumbing leaks involving tens or hundreds of gallons would be a problem with any fiber insulation.”

Our expert’s opinion

Here’s what GBA technical director Peter Yost had to say:

First and foremost, you need a continuous air barrier at the floor level in addition to a good insulation system. I would suggest a rigid sheet material on the underside of the whole floor assembly, either taped rigid foam, OSB, or plywood. Rigid foam has the advantage of providing a thermal break to interrupt conduction through your floor framing, while OSB or plywood is a better approach to protect against bugs and other pests getting into your floor cavities. Make sure that the tape you choose is compatible with the substrate you choose. (For guidance on pressure-sensitive adhesive tapes, blogs on GBA and BuildingGreen are good resources.)

In the photo you sent, it looks as though you have limited floor obstacles and penetrations to deal with, but getting a good seal around the carrying beam and all the plumbing and electrical penetrations will be key. I would take a look at this excellent LSU resource on insulating and air sealing pier and open crawlspace foundation systems.

Dealing with bulk water leaks in this sort of floor assembly is no small consideration. But since they are almost always coming from interior plumbing leaks, there are two good approaches:

  • Easy and accessible single-throw shut-offs for “hard-piped” appliances. We hard-pipe for 24/7 house pressure for washing machines, ice-makers, and dishwashers. And yet, it is not all that hard to plumb them with easy-access shut-offs so that you treat them like you do your switched lights: on when you need them, off when you don’t.
  • Leak detection systems. While they may not be inexpensive, consider whole-house leak detection systems, such as these GreenSpec listings: FloLogic or FloodStopper. They use flow analysis to monitor your water consumption around the clock and will shut down your whole house whenever a leak is detected. And if they seem expensive, consider the alternative.

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


  1. Malcolm Taylor | | #1

    "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. User avater GBA Editor
    Martin Holladay | | #4

    Response to Derek Roff
    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. User avater GBA Editor
    Martin Holladay | | #6

    Response to Jesse Anderson
    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. User avater GBA Editor
    Martin Holladay | | #10

    Response to Robert Hallenbeck
    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. User avater GBA Editor
    Martin Holladay | | #12

    Response to Conrad Blunck
    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

    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. Malcolm Taylor | | #15


      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.

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