The Best Way to Insulate a Floor

The Best Way to Insulate a Floor

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.

Jim Wright’s house in western Arkansas has a post-and-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 apparently disadvantage of the foam is the lesser R-value Measure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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 Material, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. 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

The Best Way to Insulate a Floor

«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 Heat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. 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 Building assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. 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 Movement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow. 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.

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