Minimum thickness for concrete suspended slab

Eschen

Hi there,
We would really like to put radiant tubing in a thin slab on top of the subfloor on the main level of new build (log home) above our basement. It’s a relatively small footprint, 24’x28’ minus the stairway. The engineer has approved the weight load. The problem is we only have 1 1/2” to work with, due to the stair height. From what I have read the minimum thickness for thin concrete slab seems to be 2”, although I have also seen references to 1 1/2”. (Concrete would be the finished floor). We are not really interested in Gyp-Crete as there seems to be issues with it breaking down over time, and also seems to rely heavily on getting all the variables correct with the right installler.
Is our only other option at this point a panel system like Warmboard with tile over top?
Thanks for any suggestions!

STEVEusaPA

I think 1 1/2" might be the bare minimum, insulated underneath. I did my radiant with an 1 1/2" wet bed and tile, which (knock on wood) had been in about 10 years, no cracks (gonna knock on wood again).
Warmboard or a related product would be 3/4", then you would need a floor finish over it-which could be any proper hardwood 3/4" or laminate.
I'm also not a fan of gypcrete, especially moisture (water) hits it.

Jamie Hall

The real question with thin concrete on a subfloor is how stiff the subfloor is. This is a structural engineering question. Since you have already had an engineer look at it, he or she should be able to tell you what the deflection will be under rated load. If the deflection under rated load is less than 1/360 th of the span (for example, for a 20 foot span, less than half an inch), , any cracking should be minimal.

hot_rod

Is there going to be floor covering over it?
You want 3 times the aggregate size above the tube.
So with a 3/8 tubing and pea gravel mix you can get down to a
1-1/4" pour. Beef up the mix with some plastizers, fiber, etc.

Gypcrete may be a bit tighter number?

Joist sizing is critical as @Jamie Hall mentioned or you will have cracking.

Eschen

Thank you for the responses and questions. The idea was to have concrete as the finished floor. I can certainly ask the engineer about the deflection. I spoke with a well- regarded local flatwork guy who I had wanted to finish the concrete. He did not feel comfortable with 1 1/2”, said it would be different if we had 2”.

hot_rod

Yeah a thin slab as a finished floor might be tricky. Even if it developed thin hairline "road map" cracks it would not be pretty.

There were some high psi gypsum based products (Gypcrete) that could be used as a finished floor. That product doesn't seem to crack like concrete, and can pour down to a feather edge.

https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/usg-perfomance-flooring-brochure-en-IG2013.pdf

Zman

I think this could be done with 1 1/2". I have seen it done, but cannot get you there .
You need to find an engineer or concrete guru to help you spec the product. @Gordy might have some suggestions.

Eschen

Thank you hot rod for those links to products. I will look at those. And Zman, perhaps I will try to find a concrete pro in our area who can work with that thickness. I’m also now looking into various panel systems. But even withWarmboard at 13/16” we may not be able to get a layer of tile within 1 1/2”. It is a log house- so I’d prefer not to have wood floors too, for lack of contrast.

hot_rod

Can you get below the floors and install transfer plates?

Or look at the Viega Climate Panel.

Climate panel with cement backerboard and 1/4” tile may be the thinnest above floor option

Zman

In slab is really nice for log homes.You can go with a tight spacing on the exterior walls where all the heat loss is and widen them up in the middle. I would also recommend taking the supply to the outside wall first.
Contrary to what the log home salesman may have told you, the logs don't insulate very well and can be pretty drafty. Put as much heat as you can at the wall.

Eschen

Yes, hot rod, we can get under the floor. It is exposed joists at the moment, though their are a few wires for electric. I was concerned that staple up would not be as efficient, having to go through the OSB subfloor, vs. having tubing closer to concrete/tile. But I’ve also read that the whole floor system has to heat up anyway, so in the end it’s all the same? I suppose staple up really would be the simplest solution. I’ll look into the viega system, thank you.
Zman- good to know about the closer spacing near the walls. We will have a wood stove as well- but need to have electric form of heat for code and will be nice to have background heat when fire goes out.

Eschen

Oh- misread hot rod’s comment. Viega climate panel still on top. Okay will check that out too. Thanks for all your help- so many options out there- which is a good thing!

Zman

The wood stove in conjunction with high mass radiant and a log home can be tricky to control. When the stove goes out, it can take some time for the floor to catch up.
I would suggest prewiring several t-stat locations (you can install either t-stats or remote sensors), preferably on interior walls away from the stove. I would also plan on slab sensors. Wire is cheap at this point in the project.

hot_rod

This is a handy guide that explains options for radiant installation methods.
http://digitaladmin.bnpmedia.com/publication/?m=39071&i=467366&p=2

Alan (California Radiant) Forbes

If your joists are at least 2x8, Ultra-Fin is an option as well.

http://www.ultra-fin.com/

Heat transfer plates heat by conduction, Ultra-Fin heats by convection. Both work well.

We use more Ultra-Fin now because it's easier to install. One run per joist bay instead of two.

Eschen

Thank you all for your comments- and for the link hot rod. Hopefully will be honing in on a decision soon!

Eschen

It seems like a dry system like Viega or Raupanel would be more responsive and perhaps better for working in conjunction with wood stove. I believe the Raupanel is only 1/8” thicker than Veiga, and gets more recommendedations that I have seen. Some even say it works as well as Warmboard, even though the aluminum doesn’t cover the entire panel. I talked to our tile guy and he says he could do it with the 5/8” thickness of Raupanel.

Eschen

Hello again- I was hoping to get some advice from those who have installed Raupanel, especially with tile as finish floor. Troy_3 mentioned in an old post that his main challenge was an uneven subfloor, and I am curious how you dealt with that. We have somewhat of an issue with uneven floor (OSB left exposed and soaked up lots of water one winter during our long project). Planning on trying to level out with a drum sander. As for tile, an old thread discussed underlayment options over the Raupanel. I am wondering based on others experiences about Ditra vs Backerboard. The labor cost of each is a factor as I am not doing the work myself, and the crew has not done panel install that I know of. If Ditra works as well, it seems it would be nice to avoid having to screw down over panel, and avoid hitting tubes. Anyone have experience to share? Thanks. We will be contacting our local Raupanel distributor this week so they will no doubt have suggestions too.

hot_rod

I have seen flooring installers use a floor leveling mix to level floors before the put any rigid product down. Maybe a combination of sanding and using a leveling product if they are that “wavy”
A cleavage membrane or liquid type I think is always recommended by the Tile Council. Good info at the TCA and other tile association websites