Anyone have any experience using Perlite underslab? A search didn't show anything. I am told that it can match performance of XPS with greatly reduced cost.

I don't know if you've seen this.  http://www.perlite.org/perlite_info.htm
It looks to me as if what you want is 22#/ft3 perlite concrete which  has an r value of between 1.85 & 2.12 per inch.  6 inches would give r11+or 8 inches would get about r15.  You would still need at least 2" of regular concrete on top of it.
Have you found a source?  What price have you gotten?

Actually I did some reasearch and it looks promising. There are many producers in the Country. This is for underslab, not actually used as the slab. The perlite comes in 4 cu ft bags, with a retail price of 10.75 per bag.
The bags are just laid on top of a gravel bed, packing them in neatly. The perlite in this form carries R3 per inch, or in place R18.  At retail cost that amounts to about $1.34 per square foot. Bags are 19x 48 inches, but have bulged sides, so CU ft Calculations are required.
One advantage is time placing and Skill level needed.   The material is also available in bulk quantities from some suppliers, as well as 64 Cu Ft Super sacks. The producer I talked to (Supreme Perlite in Portland) Said there is really no cost savings to the larger bags. ( some suppliers advertise there is.)
 The perlite institute show links to Producers.

It seems there are several ways to use perlite under a slab. 1. Make a lightweight concrete underslab. 2 place perlite bags, compact and flatten then pour slab (this method has been used on a few projects in Oregon). 3. Place loose perlite screed, cover ( with what I'm not sure), compact, pour final slab ( this method has been used on some German houses). Definitely the lowest cost is without cement and using big bags. I contacted a local producer and got prices of $90 to $125 for 60 cubic foot bags with coarser being more expensive. Buying it in 4 cu ft bags may be most economical as installation is simplified. Before I can use perlite under my slab I'll need to find some information about it's load carrying and deformation properties.
Well, I called another local supplier who gave a price of $4.60/ 4 cubic foot bag for what's normally used as concrete block fill and $5/bag of what's normally used as concrete aggregate.  It's definitely reasonable!

Wow! Thats quite a price difference, although the dealer that quoted $10.75 was a single bag price. You have to assume the savings for a truckload would be huge. Even at $10 it's cheaper than XPS. Because it is basically expanded glass, I would guess the load carrying capacity would be fine. Interesting....

As a reference point, I paid $0.35/sq ft for Type IX 2# density 1" thick borate treated EPS @ R4.2/in. This has a compressive strength of 25 psi which is comparable to XPS. At 4" thick, this would be $1.40/sq ft and ~R17.

If you install two layers at 2" thick and stagger the seams, this would be sufficient for your barrier as well. I assume you would have to purchase and install a separate vapor barrier over the perlite.

After seeing Liebler's price I did some more shopping and was quoted $6.18 per Cu Ft bag.  In place it is supposed to be 6 inches thick, or an R-18 as well. Price at this level is .77 per square ft.
Delivery can have a bearing as well.

A vapor barrier is required above the Perlite, 6 mil.  A big difference will be in labor. Just place, compact, cover, and pour.
I am trying to find documented verification of R value......

Where are you located? Is the ground temperature there cold enough that you really need R18 under a slab?

How do you keep it from pushing all over the place when the wet cement comes down on it?

Posted By MikeSolar on 26 Sep 2012 06:55 AM
How do you keep it from pushing all over the place when the wet cement comes down on it?

The bags of perlite are placed in the footing. neatly arranged. they are not emptied.

Posted By arkie6 on 26 Sep 2012 01:29 AM
Where are you located? Is the ground temperature there cold enough that you really need R18 under a slab?

For Net Zero energy application Slab on grade, with Passive solar. (Oregon)

Perlite is available in different densities. For the under slab insulation application you'll want the lowest density available. The r value is higher for lower density as well. From my shopping 3 #/ cu ft is about as low as is easily available. At 3# cu ft the r values I've found are all above r3/ inch.

Why wouldn"t perlite be an excellent choice for insulating the floor of a crawlspace as well?

we always use 2" Performguard EPS (2 lb/cf) under our slabs

Posted By Liebler on 27 Sep 2012 08:51 PM
Perlite is available in different densities. For the under slab insulation application you'll want the lowest density available. The r value is higher for lower density as well. From my shopping 3 #/ cu ft is about as low as is easily available. At 3# cu ft the r values I've found are all above r3/ inch.

Why wouldn"t perlite be an excellent choice for insulating the floor of a crawlspace as well?

Were you able to find documented testing at that R value? Other than manufacturers claims the only values I was able to find was r2.7 and that was
Buried in a report from ORNL.  Most manufacturers claim r3.  I did not ask which density they were recommending (it would make sense the lowest would have the best performance)

It would seem to me to be a good crawlspace insulation, especially after the fact.

I'm not understanding this discussion very well.

What is the overall benefit of perlite? I see a substantially lower R-value (say, 2.7 vs 4 for foam) and a cost that seems to be somewhat less or somewhat more than foam, depending. Because of the lower r-value, you need to allow for a greater space for the same insulating value and it's hard to believe that perlite would do well under a slab. Isn't perlite used as a water-retention agent in potting mixes?

And, isn't perlite finely divided, needing containment?

Perlite is much "greener" than any plastic foam. It is a naturally occurring mineral processed by modest heating. The "embodied energy" is far lower than any plastic. It is true that pelite is often used as a soil amendment, it improves both the drainage and water capacity of many soil types. Pertite does not absorb water, it acts in soils by creating many micro channels that can store water. Perlite is a "loose fill" material and needs to be treated as such.

Posted By ICFHybrid on 30 Sep 2012 10:26 AM
I'm not understanding this discussion very well.

What is the overall benefit of perlite? I see a substantially lower R-value (say, 2.7 vs 4 for foam) and a cost that seems to be somewhat less or somewhat more than foam, depending. Because of the lower r-value, you need to allow for a greater space for the same insulating value and it's hard to believe that perlite would do well under a slab. Isn't perlite used as a water-retention agent in potting mixes?

And, isn't perlite finely divided, needing containment?

The big savings is in the labor. No cutting and fitting sheets of foam, just place the bags neatly, compact and cover. Any gaps can be filled with an opened bag or two.
The containment is the bags, they are not emptied.  And a natural product without using nasty blowing agents.

From what I can calculate there will be pretty good cost savings as well.

Pertite does not absorb water,

A "Typical Product Data" panel for Perlite (from the Perlite Institute) says it absorbs 200% - 600% of its weight in water.

The big savings is in the labor. No cutting and fitting sheets of foam

There isn't much labor in fitting sheets of foam. At 4' X 8' it seems like you'd have to handle less of the foam sheets than bags of perlite.

The "embodied energy" is far lower than any plastic.

I'm sorta wondering about that as the Perlite has to be heated to a very high temperature to expand....