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

[Jeff Ruppert]

Greg,

I think the concerns is not even so much that there is insulation, but 
that there is an impassable barrier that will retain the water in the 
concrete and slow the curing process down.  The concern is this.  If 
there is no way for water to drain out the bottom of the slab, the water 
will sit in the bottom of the slab until it is absorbed during curing, 
whereas at the top of the slab water is evaporating and the surface is 
drying out.  The difference in curing rates places stress on the slab 
and creates cracks.  This is a valid concern.  However, there are ways 
to mitigate this.  First, you can place a layer of sand over your 
insulation and pour the slab on the sand.  The water will drain into the 
sand and allow the concrete to cure more uniformly.  Also, this issue 
seems to me like it would more pronounced in drier climates where drying 
is taking place on the surface at a much more rapid rate than in a humid 
climate.  Or same goes for time of year.  Faster drying in the summer, 
slower in the winter due to temperature differences.  For example, a 
slab will dry out faster on a dry, hot summer day as opposed to a cool 
humid fall day.

You mentioned you put a sheet of poly down under the insulation.  You 
can get the same effect by taping the joints of your insulation with 
duct tape.  No need or both unless you have a real moisture issue...

You can test your pipe with air, not water.  If there is leak during the 
pour, you will know immediately. 

I have had the same concerns through the years regarding craking and 
when I show sand over insulation on our details, under a slab, I call it 
out as optional because so many contractors whine about it.  I tell them 
that it is there to mitigate the potential for cracking, and if they 
want to eliminate it, it is their risk.  Many times, a crack is not a 
big deal, as most slabs will crack no matter what you do.  The severity 
of the cracking is the real issue.  Cracks that are over 1/4" wide and 
slabs moving up and down are real problems.  These problems are usually 
due to poor subgrade conditions, and/or poor subgrade prep during 
construction.  Curing cracks are different and are usually not as severe.

Good luck,

Jeff Ruppert, P.E.



Greg Haas wrote:
> 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
>
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