Lava Rock as Natural Floor Insulation

This week we had 36 tons of lava rock delivered which will be used as natural insulation under our earthen floor. Preparation for the lava rock to arrive included having our underfloor plumbing and electric inspected, making boxes for plumbing valves, and mapping our plumbing and propane lines for future reference as it will be inaccessible under our earthen floor slab without digging up the floor. The lava rock will act as a combination drain bed and insulation layer for our radiant heated earthen floor. We don't have a crawl space in our house because crawl spaces can be a breading ground for mold.


Our local lava rock quarry Clear Lake Lava has a verity of lava products and we decided to have rock delivered directly from them because most landscaping yards don't carry pumice larger than 3/4 inches. Using lava rock that is over 1 inch; preferable 2 inches is desirable because smaller lava rock may settle and cause wear paths in an earthen floor. Sukita wrote an amazing book on earthen floor that presents a step by step guide to creating beautiful, natural, healthy alternatives to concrete slabs and other flooring methods called Earthen Floors: A Modern Approach to an Ancient Practice. If 3/4 inch lava rock is the only size that is available, Sukita's book recommends bagging the material to keep it from moving. Pablo Loayza who runs the Natural Living School mixes 3/4 inch lava rock with clay to lock the lava rock in place but this was going to be time and labor prohibitive for our large floor. We were able to find larger 1 1/2 inch lava rock so we didn't have to mix it with clay or bag it. Adding clay will reduce the insulation value of the pumice rock creating a lower R-value. When the lava rock was delivered at our building site, the driver told us that they call the 1 1/2 inch lava rock "gorilla rock" because the larger size rocks are hard to shovel and move when they have to clean our their trucks after haling a load. I took this as a good sign that the rock will not move under the earthen floor, it will however be hard to shovel and move into place. The other reason we went with Clear Lake Lava is because they test and certify their lava for comprehensive strength because their lava rock is used to make lightweight concrete.

According to Paula Baker-Laporte's book Prescriptions for a Healthy House lava rock should be tested with a Geiger counter for possible radiation levels. This is done by placing a sample of the rock with a Geiger counter in a large glass jar for 12-24 hrs and dividing the count by the length of time tested. She explains in detail how it works in her book, so check it out. When I took a workshop from her and her partner Robert Laporte, he also recommend testing any gravel that is being used for radiation. Find more information about them and building healthy homes at their webite Eco Nest.  For those who are are allergic to wood, Paula also has a section in her book that describes using pumice and concrete as a wall system for an alternative to wood framing.


Sukita writes in her book that lava rock has an estimated insulation value of R-1.5 per inch and expanded perlite is around R-3 per inch. I looked into using perlite but it has an increased embodied energy over lava rock because the volcanic rock is heated to high temperatures that causes the rock to expand like popcorn this also increases the product cost. Sukita suggests keeping perlite in bags to keep it from moving in a sub-floor application. When I went to my local garden store and checked out perlite I could easily crush it between my fingers and I felt that under the load of a floor it would crush at least some of the perlite into a powder.  In comparison, Chris Magwood writes in his blog post about using Poraver that he was estimating to get between R-1.5 to R-2 using poraver expanded glass with a lime binder. 

Source-  Building Energy Codes Resource Center

Source- Building Energy Codes Resource Center

We are using 7 inches of lava rock for our under-slab insulation which gives us around R-10.5 insulation rating. Our county (Nevada County) is a 5 out of 7 in the US Department of Energy climate zones. A cold climate is considered zones 5-7 and for cold climate zones the Building Science Corporation have target r-values for windows (R-5), foundation slabs (R-10), foundation walls (20), above-ground walls (R-40), and roofs (R-60). For more information on recommended R-values for your climate zone check out this great article titled How Much Insulation is Enough by


Poraver Expanded Glass Bead Insulation Sorce:

Poraver Expanded Glass Bead Insulation

I was excited about using a recycled expanded glass bead product called Poraver that Chirs Magwood from the Endeavour Center has used in his buildings and writes about on his blog and book Making Better Buildings. Poraver is from Canada and can be mixed with a lime or cement binder and poured in a sub-floor space or Chris has even used it as a structural insulated foundation wall. Poraver can also fill a floor space with no binder underneath a concrete slab. I would not recommend using poraver loose under an earthen floor unless there was a concrete slab over it to prevent wear paths in the floor.  The cost of poraver was prohibitive to purchase here in the States and so we chose to use lava rock also known as pumice.  I found that these recycled glass and expanded clay natural insulation products are developed and available in the UK and Canada because the UK doesn't have access to volcanic rock. But here in the States we do and volcanic rock is available from your local landscaping yard, gardening center, or gravel yard.


Many natural and sustainable building books feature ridged foam insulation in the sub-floor to prevent heat that is gained from passive solar or radiant heat from escaping. The reason foam insulation is used so widely in natural and conventional construction is that it produces a high r-value while being only one to two inches thick. Home Depot sells a product called R-Max Thermasheath Polyisocyanurate Rigid Foam Insulation Board that get and R-6 for 1 inch thick and R-13.1 for 2 inches thick. Foam insulation can also double as vapor barrier. Earthship Biotecture uses ridged foam as a "thermal rap" that helps to lock in the thermal mass and constant temperature of the earth that is gained by building their houses a few feet into the earth and berming earth behind massive rammed earth tire walls. Foam insulation is placed 4 feet behind the structures in the earth berms that as a "thermal battery". These "thermal batteries" give off heat when the air temperature in the home goes down at night. Earth Ships don't insulate under their floor slabs to access the constant temperture of the earth which is around 58˚ F.  Because our building site did not allow us to berm our project or bury it partly into the earth we opted to insulate our floor. The reason that we decided not to use foam insulation is that foams are highly flammable and because of this they are treated with a toxic fire retardants. Also foam is a material that is produced from oil and ridged foam insulation can be easily tuned through by insects and has been know to break down under the weight of a floor slab. If you are going to use foam insulation make sure that it is a Poly-iso foam because this is less toxic. I have read about soy based foams that have been promised to come on the market but have yet to see anyone manufacture it. There are also articles that say there may be laws passing that allows foams without fire retardants to be used in foundations and sub-floors where they will be isolated from the living space. I have yet to see any manufactures start make ridged foams with out fire retardants. Chirs Magwood's book Making Better Buildings goes into detail about why not to use foam products and some great alternatives. These alternatives will vary depending on geographic location and availability of materials.


Earthen floors are a viable, natural, healthy alternative to concrete slabs that can bring in a natural look and feel into a home or be as glossy and refined as a stained concrete slab you would find at Starbucks.  But earthen floors won't kill your feet or back and are be a joy to walk on and look at. As a proponent for earthen floors Laport also writes in her book how concrete can wear your body down by standing on it because it so hard as many people have experienced in schools, offices, and ware houses. Concrete has its' place in some applications and we have needed to use it in our footings and retaining walls to pass building codes.