Does Gyp-Crete Dissolve in Water?

Annie Hall

The Gyp-Crete installer says Gyp-Crete is the best stuff to pour over radiant systems because it self-levels, is a great thermal mass, cracks less, and cures much faster.

The Lightweight Concrete installer says lightweight is the best thing to use because it provides a better thermal mass, is stronger, you can nail to it (avoiding tubes, of course), it holds up to wear, and it won't turn to mush if it is exposed to water. He says Gyp-Crete can't hold up in high traffic areas, and over the lifetime of your home, can turn to dust under your carpet or other flooring. He says give it 2 weeks to cure before you (slowly) turn the heat on and you'll be fine.

The Gyp-Crete installer says we use Commercial Grade Gyp-Crete.

The Lightweight Installer says Commercial grade Gyp-Crete is just gypcrete with some Portland cement added to it, and that if you want the right stuff you should use cement, pure and simple.

I would appreciate hearing from those of you with experience of both. Which would you recommend for a house, and why?

Mike T., Swampeast MO

I have not used the product.

From experience and study I know that gypsum is an extremely durable product with some very nice properties that would seem to make it ideal for radiant heating applications. Gypsum is also well known for dampening sound transmission and is exceptionally fire resistant. Gypcrete is NOT pure gypsum however; it is gypsum based.

High resistance to water is not one of the properties of gypsum as I have seen and understand. Water DOES NOT THOUGH immediately dissolve gypsum. To the contrary, gypsum is quite permeable to water. I'm not even certain if "dissolve" is the proper term for describing what happens when it is exposed to excessive water. Gypsum molds are used for pouring liquid slip in ceramic and porcelain production BECAUSE of their ability to pass water without being greatly harmed. As long as the mold is allowed to relieve the excess water from the previous use, it will retain a high degree of detail after many, many uses.

I can find NOTHING regarding water resistance at the Maxxon website. I VERY seriously doubt that spilling a glass of water on a floor with a Gypcrete substate will cause any discernable or permanent harm. A slow leak of a sink or similar is another matter, but such causes harm to nearly any type of flooring or substrate.

Maxxon states that Gypcrete is an ideal substrate for any finished flooring surface.

There does seem to be some debate of this, particularly regarding ceramic tile. Link below addresses every concern you mentioned--I cannot say however if such concerns have much in the way of legitimate merit. Sorry. I do though know that while the theory of a cleavage membrane sounds simple and great, achieving such in practice is difficult, expensive and troublesome.

http://www.tecspecialty.com/files/tec_bulletins/1Flex_over_Gypcrete_403.pdf

marc

light weight ?

first off gypcrete is a underlayment. must have something over it to protect surface. gyp can be poured with only 3/4 in. over top of tubing. 1/2 in. tubes = 1-1/2 in. gyp. i have yet to see concrete poured to an 1-1/2 that has not cracked all to hell. ask concrete guy to show some proof on thin pours, over sublfloor. what makes concrete lightweight. mix, air into mix, if air how well do you think this transfers heat. both products have there good points. gyp can be poured in one day. and it is flat. i think concrete institute does not have a approved way to pour concrete 1-1/2 thick and warranty it. oh yeah ask concrete guy about warrranty.

hr

Gyp

products do not like to see a lot of water! However, they do have sealers they advise you to apply. Savy tilesetters use only a waterproof membrane before setting on gyp.

I still have had problems with crumbling in wet entry applications. I suppose moisture enters from the edge or below would cause it to crumbly.

USG claims to have a mix of gyp that provides a hard wear surface equal to concrete, although I have not tried it. Try USG.com for more info. I still would be concerned about excessive moisture areas.

Gyp is a great radiant product when installed properly. It does a fine job of spreading radiant warmth, and I would classify it as a medium mass system, somewhere between 4" slabs and dry systems for performance.

Like any product, or application, understand the plus and minuses

hot rod

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Troy_3

Gypsum concrete

Gypsum concrete is Not self leveling! It is applied in a slurry that is much looser than portland concrete. The real benefit is it's flexural strength. That is, it will flex as the wood floor flexes. For slab on grade Portland is the answer. Over a wood subfloor Gypsum concrete is my only answer. Portland is harder which means it is more brittle. not what you want on a flexing wood floor. Portland also shrinks much more than gysum concrete. We have had gypsum floors under water and they don't dissolve. I've been told they actually get harder as long as they are allowed to dry out throughly. If it is in a constantly wet area the gypsum will break down eventually. Shower areas are not good areas for gypsum concrete.

Annie Hall

I'll check out USG

This will be installed over a wood subfloor. Sounds like I will be ok with gypcrete as long as it is sealed and the proper pads are installed before the finished flooring.

Thanks, Everyone.