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Floor insulation, Roxul vs closed foam

The foundation of my house (1152 sq/ft) is post and pier and there is no perimeter enclosure.
The bottom of the 2x8 floor joists are about 40" from the ground.

I am considering two methods of insulation.

The first method is 7.25" thick Roxul batts (R30) that are covered with 7/16" OSB sheets.

The second method is 1.5" (R10) of sprayed closed cell foam.

The cost to me of both methods (if I don't count my labor with the Roxul) is nearly identical, about $2000.

The Roxul has the advantage of about 3x the R value of the foam.

However, the Roxul has a couple of big disadvantages as well.
I would be doing all the labor with the Roxul method and since the floor joists are on 19.2" centers, it would require quite a bit of cutting the batts, since Roxul doesn't come in that width.

A big advantage of the closed cell foam is that I would not be doing any of the labor. 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 R10 in the floor (The location is in western Arkansas).

I kind of leaning toward the foam (thinking about the labor with the Roxul, I'm 68).

What say ye ?


Asked by Jim Wright
Posted May 14, 2014 8:37 PM ET


17 Answers

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Jim, Have you considered sheathing the underside with Roxol R-12 Comfortboard? Of the three options it is the only one that eliminates the thermal bridging from the floor joists, providing a much better whole floor increase in R value. You could also do the same with rigid foam.

Answered by Malcolm Taylor
Posted May 14, 2014 9:15 PM ET


First of all, let's look at the building code. Most of Arkansas is in Climate Zone 3. According to the 2009 IRC, the minimum acceptable R-value for floors in Climate Zone 3 is R-19.

That means that your spray foam contractor is suggesting an installation that doesn't even meet minimum code requirements. 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.

I think that the best way to insulate exposed floors is to fill the joist bays with almost any kind of fluffy insulation -- cellulose, fiberglass, or mineral wool -- and to install a continuous layer of rigid foam on the underside of the floor joists. I'm partial to using foil-faced polyiso, because it is environmentally friendly and easy to tape. I think the foam should be at least 2 inches thick, and it should be further protected by a layer of OSB.

That approach may be too expensive for you, but it's the best way to go.

Answered by Martin Holladay, GBA Advisor
Posted May 15, 2014 4:40 AM ET


Unless you spray-foamed both sides of the joists as well as the subfloor (probably more than doubling the quantity of foam) the severe thermal bridging of that ~R1.5-R1.8 worth of joist that passes through the foam layer seriously undercuts the performance of that R10-ish foam. Even crummy kraft-faced R19s would outperform it if installed in an air-tight matter (sometimes easier said than done.)

Installing the OSB then using a hole-saw to cut 2.5-3" holes every 6' or so per bay you could DIY blow it full of mid-density cellulose using a box-store rental blower (you usually get free day of rental if you buy more than $100 worth of cellulose), which would be less work and a better fit than trying to trim & hang a bunch of batts. If you're going to take Martin's advice and apply a layer of polyiso you wouldn't need to fix the holes, but if not, save the rounds to use as plugs and use can-foam to seal/glue it in place. (You can use tape to hold it in place before foaming.)

To blow cellulose into floor bays this way you'll need some heavy rags to temporarily plug the holes other than the one you're currently filling from. A cut-off sweat shirt sleeve serves to limit blow-back where you're working. Insert the hose several inches into the hole,directing it an an angle to blow somewhat down the joist bay, not right at the sub floor. Start at one end, blowing toward the rim-joist until the blower stalls (you'll here it), then pull the hose back and direct it the other way, blowing until it stalls (which will take quite a bit longer, since it's filling more volume, but at lower average density. Then move down to the next hole and blow toward your starting point first (which won't take as much time since it's already filled at low density), then the other direct, etc. until you've done the whole thing.

With a single stage blower and 2.5" hose that will result in an average density of about 2.5lbs per cubic foot (so you can do the math on the total volume to know how much cellulose to buy.) At 2.5lbs density it will settle maybe 1/2" over 20 years if it settles at all. It's literally twice the density of typical attic open-blow, which typically settles about 15% over 20 years. The amount of settling varies with climate, since the primary driver of settling is seasonal moisture cycling, but there is a well defined density for any climate where it will not settle at all. That density may be as high as 3lbs in your neighborhood, but it's probably lower than that.

Drilling the hole can be a 1-person job, but blowing is best done with one person tending the blower keeping the hopper filled, with the other running the hose. The blowers typically come with a long wired remote switch for the operator can start and stop it without having to crawl out of their work space to control it. You'll need better-grade dust mask and some goggles to keep the cellulose & fire retardents out of your eyes- it's not toxic, but it is a serious amount of nuisance dust if you slip when re-orienting the hose while blowing.

In my neighborhood a DIY cellulose blow would be quite a bit cheaper than R30 Roxul, but more expensive than R19 batt. At recent box-store pricing it comes in at 45-65 cents/lb. and in a 14.5" wide 7.25" deep joist bay you'd be looking at about 2/3 lb per square foot of gross floor area (including the joist width), or ~40 cents/ft^2 give or take a nickel or so. (At distributor pricing it's a bit cheaper than that if you're buying it by the ton.) The big blue box store in Ft. Smith sells it at about 45 cents/lb: $8.25 for 19lb bags, and throws in 24 hrs of blower rental with a minimum purchase. So for 1152 square feet @ 0.66lbs per square foot you're looking at ~750lbs of goods at a material cost of about $350. Assume you'll use something like 40 bags, but buy at least 45 just in case it packs a bit tighter- they'll take the excess back. Then, 1.5"/R10 polyiso runs about a buck a square foot, which would then add another ~$1200, and you'd still be a few hundred USD below the $2000 budget you were looking at for the under-performing sub-code spray foam option. Figure on $200 for respirator mask & goggles, FSK tape & can-foam for air sealing, plus fasteners/furring for keeping the rigid foam in place, you're still under 2-grand, and you'd be at ~2x code-min performance rather than ~0.5x code-min.

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. Find out what blower model they rent- there's probably a lbs/hr rating published in the manufacturer's specs. De-rate that by half or more since you're an amateur. A lot of box stores rent out the Force-1, which is rated at 540lbs/hr, so if you can keep it humming along at at something like a 50% duty cycle you'll probably be able to pack in the 750bls of goods in about 3-4 hours.

The The R-value of cellulose at 2.5lbs density is near it's optimal peak performance at about R3.8/inch, so for 7.25" you'd be looking at ~R28, comparable to the Roxul, but at a fraction of the price. (At 1.2-1.4lbs open blown densities it typically runs between R3.0-R3.3/inch, at 3.5lbs dense-packed density it runs about R3.5-R3.6/inch, a hint lower at higher density.)

Answered by Dana Dorsett
Posted May 15, 2014 3:57 PM ET


Dana, your answer got me re-thinking about how to do the insulation.

Actually, 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.

Then I found out that for the walls, the cellulose needed to be blown in at a minimum or 3.5 lb/cu. ft. in order to prevent voids and to prevent settling at the top.
And to do that you would need a high pressure blower rather than the type available at the box stores. But there is not any place in my area that rents such blowers.

The same thing for the floor and also I was concerned about the cellulose getting wet if I ever had a inside water leak such as a pipe break, washing machine hose rupture, etc.
And if that happened, how would the cellulose dry out if it was covered over with OSB panels ?

Well, turns out that I did use cellulose in my walls (2x6). The method I used was rather unusual and took a long time but I ended up with a density of 5 lb/cu. ft.

What I did was to crumble the celluose through a 1/2" wire mesh to get it to about the fluffiness that a blower would do.

Then I would cover a stud bay with insulweb but only 18" high at a time. Then over the insulweb, I attached an 18" tall piece of OSB. Then I kept dumping the crumbled cellulose in the bay and pressing it down with the end of a short piece of 2x4. When that section was finished, I added another 18" and so on. After the bay was full, I removed the OSB pieces and the insulweb held the cellulose in place. After every 2 bays, I installed a sheet of drywall.

I kept track of the amount that I used in a bay and the approximate density was 5lb./cu.ft.
I don't know how much that may have increased the R factor vs 3.5lb./cu.ft., but I know there are no voids and I don't think there will be any settling.

Now, back to the floor. I think that I will use the cellulose after all. The big advantages to me are the reduced price ($330 for the cellulose vs $1650 for the Roxul, I alread have the OSB panels), and also the labor involved. With joist centers on 19.2", there would be a lot of cutting the Roxul.

As to the thermal bridging of the joists, I was thinking of buying a few sheets of rigid foam and cutting them into 1.5" strips and stapling them to the bottom of the joists before installing the OSB sheets.

That only leaves the possible problem of the cellulose getting wet if my floor got flooded from a water leak. Is that a valid concern or am I worrying about it for nothing ?

The floor bays are 17.75" wide x about 8' long and there are 90 bays.

How should I go about blowing the cellulose in each bay ?

I was thinking of drilling 3 holes adjacent to each other in the middle of the bay so as to make an oblong hole in order to insert the pipe to the ends of the bay, filling the bay from the ends back to the hole, then patching over the hole.

Would there be a better way ?

Finally, I'm still concerned about the possibility of the cellulose getting wet.

How would you handle that concern ?

Thanks for all the replies,


Answered by Jim Wright
Posted May 15, 2014 7:55 PM ET


There is no need to hit 3.5lb density to prevent settling in wall cavities in an AR location, but it probably needs to hit 2.8-3. It's possible to hit 3.5lbs density with a single stage blower, but it's slow going, takes a bit of experience, and more importantly, it takes dense-packing hoses that don't come with a typical box store rental unit.

I dense packed my own house to about 3.0-3.3 lbs density using a decrepit 40 year old single stage blower, and in my climate 3.5lbs would have been better. If I have to top it off in 25 years I'm OK with that.

At 5lbs density it's thermal conductivity is slightly higher (=R-value is slightly lower) than it would be at 3.5lbs density, but still pretty good: http://www.builditsolar.com/References/RvsDensityASHRAE.jpg

Adding strips of foam at the joists is a tried & true method of reducing thermal bridging. Since the OSB isn't structural you can go fairly thick on the foam if you wish, unlike wall-assemblies where the sheathing has a mechanical function of stiffening the wall against racking forces from wind.

If the joist bays are all blocked every ~8' a single blowing hole in the center makes some sense. Slip the hose in a good foot or more (all the way to the end if it'll make it without kinking) blow toward one end until it stalls, pull back a foot or so, blow until it stalls, etc. then switch pushing it as far as it'll go into the other end then pull it back to pack the last foot or so either side of the hole. This is a similar to how you'd do it with a narrower dense-packing tube, but it won't pack quite as tight, and it may be hard to get it fully into the end, which is fine. If you end up with a tiny gap between the insulation and sub-floor in 20 years it's not a thermal disaster.

Under the floor, if you have major plumbing leak you'll have to pop of the sheathing in that section and get rid of the soggy cellulose. It can dry through OSB, but it would take forever. If that's not acceptable 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 could soak it up for weeks before it became evident. (The stuff used for attics is fine- blowing it at a higher density improves the R/inch and increases air-retardency.) You'd still have to replace the insulation in the soaked sections in the event of a major leak but it would be a smaller, more localized area. Simple spills wouln't be a major concern with either cellulose or fiberglass, but major plumbing leaks involving tens or hundreds of gallons would be a problem with any fiber insulation.

Answered by Dana Dorsett
Posted May 16, 2014 11:06 AM ET


I don't think my last post made it because of the 'Access Denied' problem, so here it is again.

On using cellulose: What if I first stapled insulweb to the bottom of the joists, then blow the cellulose, then attach 1/4" hardware cloth to the joists.

Since the cellulose is the air barrier as well as the insulation, doesn't the OSB only serve to support the cellulose and prevent rodent intrusion ?


Answered by Jim Wright
Posted May 16, 2014 12:31 PM ET


Cellulose isn't really an air barrier. However, you subfloor is an air barrier.

Personally, I wouldn't depend on hardware cloth to keep out critters. I would feel better with the OSB.

Answered by Martin Holladay, GBA Advisor
Posted May 16, 2014 12:44 PM ET


OK, I think I've finally settled on how I will insulate my floor, but it brings up a final question.

I will cover the joists with the OSB sheets( I already have 30 of the 36 that I'll need) after I staple hard foam strips to the bottom of the joists.
Then in each bay (17.75" x 93"), I will blow in the cellulose as dense as I can with the box store blower.

Approx. cost: cellulose - $330, 6 OSB sheets - $53, 3 foam sheets - $33, total $416

As to the possibility of the cellulose ever getting wet from a plumbing leak, I'll just try to use preventative measures to keep that from happening and if it does, I'll just deal with it at that time.

So, here's the question:
I was planning to just make an oblong hole in the center of the bay and push the hose in to the ends of the bay (about 4') and work toward the hole but I wasn't thinking about air relief, so I assume that besides getting a lot of dust being blown back out of the hole, that the back pressure might inhibit the packing of the cellulose.

Is there a better way, such as making more than 1 hole for example ?
If more than one hole, where in the bay should they be and what would be the blowing strategy ?

Oh, one more question.
I read where Dana said that if I used blow in fiberglass instead of cellulose, that I would more readily be able to see a water leak as the water would drain through it more easily whereas the cellulose would absorb it.
So, for a 7.25" thick bay, what would the R value be as opposed to the cellulose and would the fiberglass block air flow as well as the cellulose ?
Any major disadvantages to using the fiberglass vs the cellulose or vice versa ?

Thanks again for all the replies,

Jim Wright (Arky)

Answered by Jim Wright
Posted May 17, 2014 10:48 AM ET
Edited May 17, 2014 11:14 AM ET.


Dana Dorsett said:
There is no need to hit 3.5lb density to prevent settling in wall cavities in an AR location, but it probably needs to hit 2.8-3. It's possible to hit 3.5lbs density with a single stage blower, but it's slow going, takes a bit of experience, and more importantly, it takes dense-packing hoses that don't come with a typical box store rental unit.

I dense packed my own house to about 3.0-3.3 lbs density using a decrepit 40 year old single stage blower, and in my climate 3.5lbs would have been better. If I have to top it off in 25 years I'm OK with that.

Dana, would you expand on the technique of using big-box-store cellulose blowers to pack to 3.5 lb density? Can you use a pvc adapter to attach a smaller diameter pvc pipe to the supplied blower hose? What diameter pvc pipe? Or do you need a more flexible pipe? Any other techniques?

Answered by Jack Woolfe
Posted May 19, 2014 8:45 AM ET
Edited May 19, 2014 8:49 AM ET.


With a single-stage blower you can use a 6-8' length flexible 1-1.25' flexible tubeing as a dense-packing hose. Don't use PVC plumbing reducers for the transition from the 2" hose to the dense-packer for two reasons: Cellulose flowing over PVC (and some other plastics, including your dense-packing hose) will generate a lot of static electricity, resulting in intermittent shocks to the operator and intermittent clogs at the reducer. It's better (but dramatically more expensive) to use copper fittings, and better still to fabricate an 8-12" long funnel-reducer out of sheet metal.

Air sealing the hose connections is critical to getting there too- hose-clamps alone won't often cut it- keep rolls of duct tape around. Also keep an electricians fish-tape or something similar for de-clogging the dense packing tube without having to disassemble the thing.

If dense-packing into mesh it's pretty easy to start at the very corners of the bays to ensure high density even at the corners, but that isn't absolutely essential in this application. When dense-packing into an OSB or gypsum sheathed bay you don't need or want to create an air relief (trust me, you'll get some blow-back even if you're using a rag or sweatshirt sleeve as a gasket to keep the blowback out of your face). You want the blowing pressure to drive as much fiber to the myriad air-leakage paths/cracks as possible, which will result in a much more air-tight (though not perfectly tight) assembly.

The R-value of fiberglass blowing wools varies, but will be comparable or slightly higher than that of cellulose when dense-packed or semi-dense-packed.

Drilling round holes with only ~1/4" annular clearance around the blowing hose or dense packing tube is far preferable to oblong holes, since that's something you can seal reasonably with a rag while blowing the insulation. That's enough clearance to be able insert a flexible dense-packing tube as well. With a dense-packing tube it's useful to cut the end at an angle of about 45 degrees or a bit more, which makes getting it into tight spaces or bypassing obstructions a bit easier. It's also sometimes easier to get the dense-packing tube in without kinking it or turning back if you run the blower in "air-only" mode. You want enough flexilibity to make the turn into the hole, but not so floppy that it wads itself into a pretzel when pushing it into an obstruction.

A guy named Rick Karg has a ton of tips on installing cellulose out there on the web. Google it. There are quite a few YouTube videos showing dense-packing (usually of wall assemblies) as well.

Answered by Dana Dorsett
Posted May 19, 2014 11:01 AM ET


Dana, Thanks for the tips and references. Very helpful -- as are all of your posts.

Answered by Jack Woolfe
Posted May 19, 2014 3:53 PM ET


Dana, now I'm really confused about the blowing.

If you only use one hole and pack rags around the tube as well, where does all the air go ?

Seem that there would have to be another outlet in the bay for the blowing air to exit.

Could you go into a little more detail.

Jim Wright (Arky)

Answered by Jim Wright
Posted May 19, 2014 7:28 PM ET



Do you have duct work in this space?

Answered by Lucy Foxworth
Posted May 19, 2014 10:04 PM ET


No, nothing in the joist bays at all.

Answered by Jim Wright
Posted May 19, 2014 10:22 PM ET


Oh, that is so nice. I am insulating around ducts, pipes, etc. It's a serious pain.

Answered by Lucy Foxworth
Posted May 20, 2014 1:05 PM ET


"If you only use one hole and pack rags around the tube as well, where does all the air go ?"

The air goes out every tiny crack, seam and hole that it can (much of it going into adjacent framing bays), until they plug up with the insulation fiber, which is part of the point of dense-packing. It's not a perfect air seal when your done, but it's a heluva lot tighter than when you started. The even with a single-stage blower the pressure is pretty substantial, but not enough to pop your nailed/screwed on OSB sheathing off. With a 2-stage it's pretty easy to blow out wallboard if you're not judicious about the settings. It's not as if a rag around the blowing hose at the hole will make a perfect air seal (or anything close,) but it can keep the flakes of cellulose and most of the fire retardent from blowing back in your face. The rag is more of a filter than a plug.

Insulating around ducts/pipes/wires isn't usually much of a problem with blown goods, but a total PITA when trying to fit batts snugly.

Answered by Dana Dorsett
Posted May 20, 2014 2:16 PM ET


Here's a vidi of a guy dense-packing a wall cavity from a single hole inserting the full-diameter 2" hose (no tapering to a narrower easier-to-insert tube the way most 2x4 cavities with potential wiring/plumbing/framing obstructions might need), pretty much the way I've described doing your floors:


This vidi shows dense packing with a narrower tighter-turning fill tube.


This vidi has 101 tips about what to look for to avoid problems when retrofitting cellulose in walls (not directly relevant to the empty-bay floors):


Most of the wall-filling in that last vidi is "2-hole method", with a hole at the top & bottom of each bay, with only the tip of the hose inserted. When using this method you start at the bottom angling the tip sligthly upward, and blow until you have cellulose coming out the top hole, then move to the top and blow down until it stalls. If you're luck you''ll hit the ~2.2 lb density range, which in a wall cavity will usually settle signficantly over time. though I've seen gutted walls insulated that way still completely filled 25-30 years after the fact, I've also dense-packed over walls previously insulated that way where it had settled more than a foot in 8-10 years. (Some folks are better at 2-holing it than others, and the humidity cycling in different walls will differ.)

Answered by Dana Dorsett
Posted May 20, 2014 2:50 PM ET

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