The main reason why Gypsum is so light is due to the Gypsum product pounds per cubic feet. Gypsum is mainly packaged in 79.4 pound bags. Each one of these bags holds 1.23 cu. ft. of Gypsum. This means that per volume Gypsum in powder form no sand or water added eguals on average 61.5 pounds per cu. ft. and Portland Cement in powder form weights 94.5 pounds per cubic foot which makes it about 2/3’s the weight of Portland cement. Then we blend our sand which weights around 100 pounds per cu. ft. and water by volume which weights around 62 pounds per cu. ft. Please keep in mind that sand naturally has voids in it that water and cement will fill and sand also absorbs water which also makes it heavier then it’s base weight of 100 pounds. Ultimately the result of this mixing of Gypsum, Sand and Water results in a volume of 2.5 cu feet per mix which when still wet weights 118 pounds per cu. ft. However since the Gypsum does not remain in wet stage long, by the time your ready to install your finished flooring the much of the water used to mix your Gypsum has evaporated in the air leaving you with around 96 pounds per cu. ft. Now if you compare this weight to Portland based concrete with sand and stone which weights on average 140 pounds per cu. ft. depending on Hydration, you will see that Gypsum Underlayment weights 2/3’s of what Portland Mixed Concrete does.
1.5″ of Gypsum weights 14 pounds Wet and 12 Pounds Dry per sq. ft.

When we first pour the LEVELROCK RH Radiant Heat Floor Leveling Underlayment over your floor at 1.5″ average depth which will provide the minimum of 3/4″ coverage over the top of your tubes it will weight around 14 pounds per square foot. However after you ventilate and dry the floor sufficiently to receive flooring the weight to due to evaporation will be closer to 12 pounds per sq. ft.

Floor Deflection has a large impact on the depth that is required at a minimum. However the standard rule of thumb applies in 95% of case’s.

Radiant Tubing – Minimum amount above the tubes is 3/4″
Acoustical Board LEVELROCK SAM N12 1/8″ Mat – Require 3/4″ of topping
LEVELROCK SAM-N25 1/4″ & N40 3/8″ Sound Mat – Require 1″ of topping

Marino / Ware and Dietrich Corregated steel decking – Requires 1″ above the high point
3/4″ T & G Plywood Subfloor require 3/4″ minimum of topping
Concrete Slab or Precast Concrete – Requires to maintain a minimum of 3/8″ but may be featheredge at the high point or door ways.

I will give you the facts so you come to your own conclusion.

1. Thermal Mass – Both Gypsum Underlayment & a Mud Job have high thermal mass however because LEVELROCK is mixed in a wet form it doesn’t have the air voids that are found in Mud Jobs thus resulting in better transfer of heat.
2. Reinforcement – LEVELROCK RH Radiant does not require reinforcement unless a moisture barrier has been applied that impedes bond. Mud Jobs always require a moisture barrier and wire reinforcement because they are mashed into place and have not enough water to make a bond to it self let alone to the subfloor.
3. Installation – Because the materials are mixed and pumped in, LEVELROCK RH Radiant type products will use less man power and a fraction of the time to install. Mud Job Requires mixing Portland and sand together with little water then since it is not pumpable it must be man handled to the area that is to be installed thus shutting down your project for extended periods of time instead of the single day typical of LEVELROCK Installs.

Many Self Leveling Underlayments available today are preblended materials including the Sand. Unfortunately this additional drying and handling of sand takes the least expensive component commonly used a turns it into by volume the second most expensive. Then to add insult to injury you then have this bagged product that is 40% sand transported to your local seller. because there is so much handling involved you can expect to pay around $53 per bag installed plus any prep work which can easily top $.75 per foot. One bag will give you a typical yield of 6 ft per 1″ poured. LEVELROCK has only one similarity to the Self Leveling that is available in stores, that is LEVELROCK is also a self leveling and it has the same hard & smooth surface that you will get with the product that is available to the public. That is where the similarity ends. LEVELROCK is produced with every thing needed except the sand. The reason for this is that the sand that is mixed with LEVELROCK on site doesn’t need to be dried in advance, Dry sand costs 4 times as much as wet sand and because wet sand is found locally it is only transported once which is an instant savings. LEVELROCK will cost 1/3rd of the price installed and typically requires no floor prep. This is because LEVELROCK uses a larger sand which provides strength to the structure of the LEVELROCK. Since I know you will probably ask what the differance is between LEVELROCK and typical Gypsum Products I have listed those in the next Question!
The main similarity between the 35 year old Gypsum technology and LEVELROCK Underlayment is that they are both produced on site using local Sand and Water. All of the other components are packaged at the manufacturing plant. This is where the similarity ends. The 35 year old Gypsum Based products which are still used have high water demand and because of this they can experiance dusting and will take longer to reach there potential psi. The LEVELROCK products are next generation Gypsum Underlayments. They use less water and reach higher strengths rapidly and don’t have the dusting issue’s that plague other brands.