# Warmboard on slab



## Eaglei (Aug 1, 2012)

I'm using warmboard radiant heat panels in a bathroom on top of a concrete slab . These panels are 3/4'' osb with an aluminum top layer that is pressed to fit 1/2'' pex .Instructions call for a layer of 6 mil plastic underneath the panels . Then each panel is to be fastened with nine 1/4'' tapcons or similar . My main concern is osb on slab and the nine penetrations through the panels and plastic . Techs said it will be fine as long as there's no moisture in the slab . We tested slab and there was none to speak of NOW . Customer is my plumber and friend and I told him I'm not comfortable with this set up . Has anyone ever used these panels in this same set up .


1st photo shows the panels installed on the ceiling . sorry about the upside down photos .


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## GO Remodeling (Apr 5, 2005)

I have never used warm board on a slab but have a few comments.

First, what is the finish floor material? Is the floor flat so there won't be voids under the panel?

Since you need to predrill the hole through the panel and into the concrete, you could apply a urethane tube glue around and into the hole to waterproof.

I think I'd use a thicker plastic, though. And turn it up the wall like a laminate installation.


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## Eaglei (Aug 1, 2012)

Floor finish will be tile on top of 1/4'' cb with thinset on top of warm board . The slab is dead level and flat . I like your idea of squirting in some urethane in the fastener hole , makes sense .


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## tenon0774 (Feb 7, 2013)

Just off the top of my head:

"Redguard" the bottom of the warmboard panels and thinset them to the concrete slab.
Weigh the panels down to the slab with buckets or actual weights until the thinset sets.

This may ease your worries of mechanically fastening the warmboard to the slab; and thus creating holes through your vapor barrier.

Problem with this is it is not a manufacture's approved technique.


I have used warmboard before, but never in this application. 

Just my two cents.

- Scott


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## tenon0774 (Feb 7, 2013)

What about electric warm floor,

Suntouch, Honeywell?

Float it in and tile on top...


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## Eaglei (Aug 1, 2012)

Scott I did redguard the bottom and edges and as olzo55 mentioned were going to squirt a bead of pl 400 or silicone into each fastener hole . Electric is cost prohibitive to go that route . Were zoning off a gas hot water boiler . Haven't seen you around hope all is well .


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## tenon0774 (Feb 7, 2013)

Right on.

It's been extremely busy for me for the last year, which is why I haven't been as active on this site as I would like.

All is well though, thanks.


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## Remodel LAC (Apr 22, 2015)

*Stay away from Warmboard*

The product is overly expensive and they won't take returns -- stay away! At the per-sheet price, do you want to end up sitting on them while hoping for another project?

If you decide to go ahead, Warmboard recommends 4 fasteners per sheet. I wouldn't use less than 6 for stability. Potentially more if cutting resulted in small sections at ends.


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