[Strawbale] PEX, Concrete Slab, Fear of Death (by contractor)

[Greg Haas]

Hello All,

Been away for a long time.  Used to be a frequent reader, then life 
took me in a different direction.  Tonight I spent two hours reading 
through the list archives back to July 2006.  My eyes are blurring!

Mostly I was reading about PEX tubing, radiant heating and floors 
because that's what I'm dealing with now.  I'm *not* talking, directly, 
about SB construction here, but I think many SB builders might consider 
this issue too.  I apologize for the off-topic part...sort of.

The discussions, I found, mostly concern efficiency, bubble-wrap/foil 
(don't do it!), nails, floating tubes, and lots more great stuff!  What 
I've not found though is something to help me with the fear of death my 
contractor is trying to put into me about impending cracks we'll get 
using XPS insulation (2"x4'x8') under the slab in our garage.

The "is it worth it" argument aside, his primary concern is cracking.

My bigger plan is to put the tubing in and as the year(s) go on, 
retrofit some of the rest of my (wooden) floors and install a boiler 
system and solar collector to replace my very old (1962?) gas burning 
forced air furnace.

He's alright with the idea of putting the tubing in.  But, as I 
mentioned, where we are at odds is the insulation.

I've read buckets of info on using, in my primarily heating climate, 
minimum of 2" XPS on top of plastic (polyethylene?) under the slab.  My 
plan detail is 2" down 8" vertically at the edge of the slab between 
the slab and the frost wall, then 2"x4'x8' sheets under the slab 
horizontally.  Then, in the spring (because it didn't happen before 
backfilling!!), excavating and insulating down 4' around the exterior 
perimeter (like I've already done around the rest of my house).

Ultimately, he'll do whatever I want.  He's said as much.  I win, it's 
my house, right?  But since he'll do whatever I ask him to, I need to 
know what to do.  Ain't that trouble?  But what I'm not finding much 
good reading about as a negative to in-slab radiant heating is this 
horrific cracking problem that he's describing (his brother too, they 
both shake their heads and laugh at me and wound my pride, heheh).  
These guys have a great reputation and I like all the work they've done 
so far.  They're fast, efficient and pretty thorough.

That reminds me of the other piece of this which is sort of political.  
That is to say, this family builds with concrete for a living and has a 
lot of respect in our community.  I respect them all.  I'm a little 
afraid to tell him how to do his job.  Do I tell him what water ratio 
to use?  Do I tell him how long to cure (keep covered with plastic, 
etc.)?  Will doing those things reduce the chances of cracking?  Enough 
that I might be able to show him the floor next year with little or no 
cracking in it?  He's describing a wreck, so anything less would be 
okay, right?  ;-)

So. What about curing?  Thickness of the slab?  Cutting?  Water ratio? 
*When pouring a slab on XPS what, if anything, needs to be done 
differently than a conventional on-gravel pour?*  Am I making a mistake 
using regular XPS as opposed to tongue and groove?  How do I tell a 
well-respected 'concrete guy' that I need the concrete to cure more 
than four days if that's all he says it needs?

Basically, I'm not going to convince him that what I'm asking is 
right.  Not till he sees the floor with no cracks, anyway.  But I need 
to be able to steel myself; to know that what I'm doing is right.

Then an aside: What's the easiest way to pressurize the tubing for leak 
detection?  Plug one end (I have a manifold with two "circuits") and 
put a garden hose on the other?  It's winter, there's a risk of 
freezing, I guess, although it is inside the new insulated garage 
inside a curing slab...

So here I reach the end of my long, semi-off-topic post after being 
'away' for nearly two years.

Thank you for your time, Happy Holidays!

all best,
Greg