Using perlite/ lime cement as insulation

A mix of 1:7 cement: perlite has an R value of 11 at six inches, and is an insulating material that would not be damaged by water. It is also load bearing; not as good as regular concrete, but plenty strong enough for a residential floor.

I'm researching remodeling a basement with cinderblock walls and a concrete slab floor, and a garage with concrete walls and floor. Especially in the basement, moisture resistance is paramount. I can't dig out around the foundation to insulate on the outside. I could build a stud wall, with some sort of wall board and insulation behind it, but covering up my walls in this way makes me nervous; if a leak happened, I might not know until it had done a lot of damage to the insulating materials and stud work. Also, alternative wallboards and insulation bats are expensive; I wouldn't use ordinary drywall or fiberglass insulation due to chemical sensitivities and mold potentials. In any case, I would still have to do the floor.

I like the cement idea since it would not leave any cavities that I can't see, for water, mold, or critters to exploit. Lime cement should be fairly breathable, and will not become an impenetrable barrier to migrating moisture. It tends to suppress mold. It seems that there would be less chances for an air leak to form a condensation spring in the wall then with stud framing.

However, I'm not sure if it would work at all, and I'm not sure what the best way would be to attach it to a wall. I was thinking of pouring it in courses with a form, and using imbedded rebar, lathe, and the same sorts of ties used to support stone veneer to keep a 6-9 inch layer of plaster from coming off the wall. For the floor I was thinking of pouring it like a slab. I also have no idea if substituting a lime based mortar for a portland based mortar would work.

What do you all think?

Hey Gilbert,
Seeing that you are in Denver, I don't know where you would source a dump of perlite. Bags are costly. Maybe you can find super sacks in the industrial district. I'm in the San Luis valley, so I have access to Antonio Scoria and Espanola Pumice. I use it in many ways, though not thick cement/lime pours. I like loose cinder in earthbags with structural plaster skins for thicker insulative uses. I like it with masonry binders for thinner sculptable ferrocement type stuff. The Pumice pozzolana combines nicely with lime, but at those thicknesses, it wouldn't carbonate through with pure lime until your house most likely crumbled.
I've pure lime (with and without pozzolana) plastered, 50/50d, and added it to many cement stucco jobs. You're gonna wish you threw in a healthy ratio of cement if you do this (sorry permies). I don't like much lime, even pozzolonic, on horizontal surfaces. I do like to lay brick, pavers or flagstone floors on beds of Pumice/Scoria or even polyiso. If you are importing fancy hydraulic putty across the country or ocean, well I don't know anything about that... I
But seeing that you are in Denver, why don't you grab a cheap bunk of reclaimed or 2nds rigid insulation off Craigslist? Or, professionals on this board seem to like rockwool. though I haven't used it since I'm a salvaged 4" polyiso or Pumice guy myself when insulating. Even though I'll haul another trailer load of Pumice this spring, and I killed off my last barrel of putty today, in your house situation, I'd likely go 2-4" of polyiso for r12-24 and takes no time and is Cheap! Pouring under a ceiling sounds awkward unless you pull up the floor above. I've laid many pallets of Pumice Crete block, $$$2/each from Farmington, but so easy I shouldn't have even been paid.
Noticed that the Wally Bostitch hammer drill is like $45ish if you decide to tapcon and need one... If you polyiso, long internet tapcons with fender washers, and since you don't want to panel, eifs mesh and skim or lath/wire and sling it.
If you gutter your roof and drain away from your foundation, in Colorado that should keep your basement dry.
Conduit is Cheap. And I don't like cavities either.
--Earthbag Chris

Hello Christopher,

Thanks for the advice!

I'd rather avoid the polysio since some family members have strong chemical sensitivities. Also, I'd be worried it would create an impervious layer behind or in front of which water could do damage; would this be a concern? The more I read, and the more I tear down parts of our house, and talk to the owners of other moldy homes, the more I want everything to be breathable.

I've wondered about the expense of perlite. I've yet to research it. Asbestos free vermiculite might be another option; so might cinders from a local volcano.

What happens to lime on horizontal surfaces? Does it break up?

Does lime adhere to cinder block substrates?

That is an interesting idea about laying pavers on Pumice/ scoria. Do you stabilize these materials in any way? Are the pavers laid dry or mortared?

I've used some rock wood bats. It is supposed to resist mold. The reason I'm worried about any bat material, or any modular material for that matter, is that air can infiltrate around seams and contact a cold wall, forming a little condensation spring. Though I suppose I could plaster really well to keep cracks down.

Where do you get the salvaged rigid insulation? Is it from demolitions?

You'd think that here in dry Colorado, mold would be the least of my worries! But not only am I having to gut my house, but many of my friends are doing the same. These neighborhoods were post WWII building boom areas, and I think quality control suffered.

Happy Thanksgiving!

I've wondered about the expense of perlite. I've yet to research it. Asbestos free vermiculite might be another option; so might cinders from a local volcano.

If you go cinder, I'd look into those places that I mentioned, unless you find another volcano closer.


What happens to lime on horizontal surfaces? Does it break up?

It takes a long time to carbonate at decent thickness. It can break up depending on traffic. You said garage and basement, i like durability there. Its akin to a softwood pine floor, when there are more durable materials more suitable for a floor.

Does lime adhere to cinder block substrates?

Like a dream. The reason why modern Masons add lime to their Portland mortar (besides stamped prints) is because mainly of its plastic, sticky, workable properties. I'm still scraping it off my Glass

That is an interesting idea about laying pavers on Pumice/ scoria. Do you stabilize these materials in any way? Are the pavers laid dry or mortared?
I typically like screed cinder subfloors loose for maximum r value, ease of use, sustainability, etc. I've laid good brick tight and dry, irregulars mortared, big wood and concrete pavers with Adobe grout, and flagstone laid dry with mortar grout.

I've used some rock wood bats. It is supposed to resist mold. The reason I'm worried about any bat material, or any modular material for that matter, is that air can infiltrate around seams and contact a cold wall, forming a little condensation spring. Though I suppose I could plaster really well to keep cracks down.
Lay it tight and there are no gaps. A render is airtight.

Where do you get the salvaged rigid insulation? Is it from demolitions?
I suggested craigslist earlier since you live in the big city. You could also try commercial roofing companies when they do tear offs and apply new membrane. Or repurposed materials. They're up there too.

You'd think that here in dry Colorado, mold would be the least of my worries! But not only am I having to gut my house, but many of my friends are doing the same. These neighborhoods were post WWII building boom areas, and I think quality control suffered.
Detail that moisture well. In an uninsulated basement, gutter, drain away from foundation, vent appliances correctly. If you have larger basement moisture problems, I don't know your situation.

The new crop of blowing agents in spray foam is much more sustainable. I know you likely want to use more inert for your families sensitivities, but I was impressed with a basement Spray job that I saw. Is there à MgO airKrete company out there?

If this basement doesn't include bedrooms and family rooms, does it make more sense to just better insulate it's ceiling for the rest of the house? If so, then you could Lime wash or skim the block for piece of mind and aesthetics and devote your resources to other projects.

I use 6 to 1 ratio (by volume). 6 perlite 1 cement 1.5 water.

Why not use papercrete?

I am not sure you could use it for load bearing walls, but you could use it to insulate the inside of a cinderblock wall. The stuff is mold-proof, has an R-Factor of 2 per inch of thickness, and would not off-gas chemical fumes. You would have to experiment on how to put it up, like making it into 4x8 sheets lets say, or forming it up and pouring it into a wall, but it might be a low cost solution for you.

Okay...now that Travis has had more coffee...

I will admit that I have never heard of Papercrete before, until a few weeks ago when it was brought up by someone else on here. I looked into it a bit, and have to admit that I really liked the properties it had. Mold free, light weight, moisture resistant, and easy to make.

One application I can really see using it for is in the bottom of a concrete slab. On my farm, I LOVE radiant floor heat, and put Pex in all my concrete slabs just in case down the road I decide to heat the building above, but radiant floor heat in a concrete slab really needs a means in which to thermally keep the slab from trying to heat the ginormous heat sink that the earth beneath it is. That has always been 2 inch sheets of Styrofoam, but they are expensive to buy.

I have not tried this, but since papercrete has the same insulating properties of Styrofoam Insulation with an R Factor of 2 per inch of depth, I should be able to pour 2 inches of papercrete in the bottom of a slab, and then pour a normal concrete slab above it. This will enable me to save several hundred dollars on a radiant floor heated concrete slab. So I am really excited about this.

Curious where you came up with the R11/6" number? I think perlite+portland could work well, but I've been under the impression that it's closer to R1/inch, and you're putting it closer to R2/inch.

This perlite site has the unadulterated perlite at R2.7/inch. I think adding any amount of masonry binder will reduce it quite a lot due to the thermal bridging.

Hello, we are looking to use perlitecrete for stem walls under non load bearing straw bale walls (surrounding a timber frame). We have been talking to the Perlite Institute but they do not appeart to currently be staffed with anyone with any practical experience so we are having trouble getting a portland/perlite/water ratio for the concrete truck guy that makes any sense to him that will yield about r.8 to r1 per inch for the 18" wide stem wall. I was thinking you might have an idea or two. Thank you.
--Chris McClellan
Uncle Mud

hey mud, wish i could be of more help, but all i can say is that i think you could strike the right balance of compression strength to insulation value that you're looking for pretty easily in that application. i think the original poster's mix is a good place to start (1:7). i use as much perlite as i can for things like stove insulation (as much perlite as 1portland:12perlite) but this is super crumbly and wouldn't work for a stem wall. i've built an aircrete stem wall similar to what you're talking about, and based on my super unscientific stress tests it had 100psi and around R1/inch.

i've always found the data and methodologies in this field to be woefully small based on what i think i think is a very promising alternative to foam based foundation insulations.

sorry for the lack of concrete (pun incidental) help, keep us apprised of your findings!