I’m building a 32X32 workshop, single story. Location is upstate NY. Due to costs, it will be a floating monolithic slab (thickened edges, rebar, etc…) instead of footers/stem wall below frost depth.
It will not have a “real” heating system for 5-10 years; but I want to insulate and put pex in the slab so that when time and $ allows I can do radiant.
I’ve had the topsoil (+/- 12″) removed and bank-run gravel spread/tamped; so that will help minimize frost heave. However, with the slab moving up and down with the seasons, I wonder if the movement, and potential cracks in the slab at control joints, or elsewhere, can be expected to compromize the pex causing leaks when I go to circ water through it.
Am I dumb to spend my $ burying Pex in a floating slab?
Ithaca, NY “10 square miles, surrounded by reality”
Replies
Could you run the slab at a low temp in the meantime? that would probably prevent any real frost problems, I would think, even through insulation.
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Not an option unfortunately. I'd like nothin' better than to heat it right away, but it'll have to wait. I don't plan on even having the pex hooked up to anything, just stubbing out of the slab and taped off until I save more coin.Ithaca, NY "10 square miles, surrounded by reality"
Well, we're out of my area of expertise then, but I do know some slab guys up here who simply throw down rigid foam on top of slabs to keep them from freezing.. heat from the earth is apparently enough to keep it from freezing.So, with that in mind... how likely are frost heaves, really, if you insulate?I can't answer that. Just posing the question to others who may know better.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I assume you'll be using subslab insulationfor enrgy conservation. (at least 2" foam)
By using an extended 2" extruded polystyrene "skirt" out 2-4 feet beyond the footprint of the slab, you will prevent frost pentration under the slab edges and hence stop any frost heave.
Remember you still have to include slab edge insulation (at least 2" foam)
Experienced is on the money.
We do a lot of slabs and are north of Stray. 2 in rigid type 2 foam over a base of pit run or B gravel compacted to refusal with a diesel plate.( 800 lbs min)
We use A gravel on top for final grading and to get the top bump free, so the foam makes full contact with no voids. Compacted.
Surface drainage should be good around the structure.
Quality pex ie Rehau and Wirsbo are the best choices. Stay away from the cheap stuff.
Don't scrimp on the rebar and mesh either. Cheers.
It may be a bit off the subject, but each reply is suggesting 2" foam to be the choice for underslab insulation. I've been to several seminars where they talked about underslab insulation. Each one recommended against 2" foam. Reason being, if the surface of the foam, is scratched, denteded, cracked, etc... it then has the ability to retain water. After several years, the sheets of foam will be soaked through, loosing their R value.
Now, the same gentleman explaining this, was also marketing a new product (go figure). It was a bubble wrap type of product with foil inside of it. I believe it was called slab shield. It came in 3' by 100' rolls and didn't have the ability to retain water.
Anybody ever hear of this or use it???
upnorth8
Don't trust anyone marketing slab shield. The stuff is total crap.Rigid foam retains its R-value when wet.So does slab shield/insultarp and its bretheren. Of course, they are only retaining their 1.5R value, so they couldn't go down much more if they tried. Oh, wait, I forgot, they are "equivalent" to 2" of rigid foam, right?Total bull. Someone should start suing people making those claims.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
1. Dont know how foan will retain water since it is very small closed cells same as bubble wrap which has much larger void areas around its larger cells of air to hold water,
2. The pex can be installed for a floating slab application period this is a normal problem for the Professionals in the Radiant Industry, a. get a Professional who does this for a living and there are many, they will put pex in with sleeves and other methods for the joint seperation issue which is simply designed for movement, put complete system in slab structure that is floating and it will move right with the floor movement, good Luck Ken
"2. The pex can be installed for a floating slab application period this is a normal problem for the Professionals in the Radiant Industry, a. get a Professional who does this for a living and there are many, they will put pex in with sleeves and other methods for the joint seperation issue which is simply designed for movement..."
Hiring it done isn't an option due to funds... I had wondered about sleeving the pex. Would I put a sleeve to span all control joints of the slab?
I'm picturing like a 16" pipe (8" on either side of the control joint)
What material... EMT conduit? PVC? or do radiant mfr's make ready-made sleeves?
Can I cut the sleeve material legnthwise to get it over the pex, or do I need to thread it onto the pex prior to attaching pex to the reinforcing grid?
Thanks!Ithaca, NY "10 square miles, surrounded by reality"
"What material... EMT conduit? PVC? or do radiant mfr's make ready-made sleeves?"
I didn't have any joints to span, but when I poured my floor, I used foam pipe insulation where the pipe came out of the floor to the manifold, and where I brought it out the front of the slab for a (maybe) future snowmelt circuit in the apron/walkway to the house.
Figured the foam would give a fair amount of movement room, and it's cheap, easy to cut, and place.
Don
On dense compacted sand, the engineer I spoke with about my slab used a frost protection rule of thumb of 1" of EPS per foot of frost protection. ie: 2" of foam was equivalent to dropping your footing 2' below grade.
With a reasonable rebar spacing schedule, random shrinkage cracking shouldn't be much of an issue for PEX due to little differential height movement on either side of the crack and the elongation capability of the PEX without failure. (Apparently the stuff can freeze and stretch when filled with water and not burst.) The rebar or dowels spanning the joint take the shear loads of wheeled traffic or differential settlement rather than the buried PEX. Any shrinkage cracks that do develop randomly will not open up much at all if you have a good bar placement schedule. I was told that smaller dia. bars, 10m in Canada, (#3's I think you spec in the US) on a closer spacing are better than larger bars at a wider spacing, even though the total strength may be similar. I used a 9" spacing grid and had no complaints from the inspector on pre-pour inspection day.
I would be concerned about unsleeved working control joints, however, due to the long term abrasion of the PEX by the concrete over many shrink/expand cycles. All the activity would be concentrated on a relatively small piece of real estate it seems to me. I used the same reasoning where my PEX exited the slab to connect to my manifolds as well, using prebent 90 deg. sleeves on all tubing where it came up through the slab.
Split sleeves finally made sense to me; its d**n-near impossible to thread a close fitting sleeve over installed PEX with any bends in it (I found out the hard way), I also found it hard to calculate and position all my unsplit sleeves before installing and making any bends in the PEX during install and so went with split sleeves after getting a few unsplit ones wrongly placed.
Using zip ties to close up the sleeves after you install them allows very little cement paste works its way into the gap, I've found. Position the split down and the paste would have to work its way up to enter the gap, I figured.
I used cheap Polyethylene water piping for my sleeves figuring that normal polyethylene is about as close a material match to Cross-linked Polyethylene as I was ever going to get. (The Wirsbo lit. cautions against the use of adhesive tape to secure the PEX, citing the possibility of long-term degradation of the PEX due to contact with the stickum.) I didn't want any chance of dissimilar materials ruining my day sometime in the future. I bought the poly in a roll with an ID a bit bigger than the OD of the PEX to make it simple. I just cut it to length with a hacksaw and split it for straight sleeves or heated it and bent it as I slipped it over the stub outs where the PEX came through where the slab surface would be. A hot air gun worked fine for this. Although probably overkill, I deburred the sleeves to transition the tubing into the sleeves to avoid any chafing. (Don't you just hate it when you get chafed!)
I also suggest vibrating the concrete well on Crete Day especially where the tubing exits the slab, thats where most of my congestion happened.
For my control joints I saw cut them after the slab was strong enough to walk on without imprints (and the cut didn't "ravel" as I went.) (Within hours of finishing the surface, I seem to remember) When I do it again, I will form the joints into the slab with plastic inserts; sawcutting and sealing the resultant kerf space (to prevent foreign material entering and causing "dirt ratcheting") is just way too expensive to do by Diamond sawcutting. Scheduling the diamond saw, laying out the cut line, supplying water for the cut, picking up the rental machine, cleaning & returning the rental machine and making the cut are worth avoiding by planning the joint in advance and placing it before the pour.
I would definitely suggest installing your manifolds and making all the connections before pouring the liquid stone. Getting good tubing layout to the manifold would be a beech I think with only a short piece of tubing to play with. (The other end being held securely in hardened stone, so to speak.) Having the manifold in place allows pressurizing the tubing during the pour, also, reducing the chance of crushing damage during all the activity on Crete-Day. You could even put on a gauge to see if the install had any leaks and fix them before you even think about starting to pour, another plus. ( I watched my gauge like a hawk all through the pour to make sure no leaks showed up while I still had a chance to fix them. Had a couple of repair couplers on hand too, but never needed them.) I used compressed air at 60PSI to pressurize, thinking that any leak of air would not weaken my stone mix if a leak developed. (Like water filled piping might do.) The bubbling air geysering up out of the wet mix would tend to be dead give away if a leak showed up, I also think.
Wrap the manifolds in some plastic sheeting for the pour, they stay way cleaner that way.
Also suggest doing the tubing layout on paper before reeling out the tubing, it goes pretty simple then. Take many photos of the tubing once installed from above and archive them, once the stone is hard, the memory fades as to where all the tubing runs are.
This comes in very handy when partitions, or other penetrations in the slab become needed later. (I layed down a builders leveling rod to provide a scale for my photo shoot.)
If you've stayed with me all this time, good luck.
Sort of reminds me of the joke about the guy who talks so much his azz sucks wind.
if the surface of the foam, is scratched, denteded, cracked, etc... it then has the ability to retain water. After several years, the sheets of foam will be soaked through, loosing their R value.
This is absolute BS.
Packing bubbles are gonna insulate a slab? You're joking?
Joe H
Experienced and NRT are giving sound advice.
Extend the rigid foam 2 ft beyond the slab edge, avoid bubble wrap .
Usually it "pops" from the loaded wheelbarrows of concrete during placement.
If the aluminum foil comes in contact with wet concrete, it disolves.
In Canada all materials get a CCMC approval....to my knowledge no bubble wrap has passed any testing benchmarks. XPS (extruded) Foam is indestructible when buried. Looks the same when you dig it up 10 years later, more money, but worth every penny the way Oil is going.
Make sure to not pour your slab overly thick. You want the heat to conduct back to you, not be absorbed by an overly thick slab. We like 3 1/2" slabs, enough thickness to embed the pipe when cable tied to wire mesh. And not too thick, so the floor is reactive.
Short story, we were asked to price a placement, concrete only, for an owner builder.
He buried the SM insulation under 12 in of A gravel and intended to pour his 4 in radiant slab on top of the gravel creating a huge heat sink.
I tried to talk some sense to the man, but there was nobody home.
I passed on the work as I could not in any way afford to have my name attached to it.
Same thing with bubble wrap in my opinion.
Good point about sticking with quality pex.
It sounds like it's not so foolhardy to bury the pex as planned. I will contact Wirsbo (or whatever their new name is now...) to see what kind of disclaimers they have on their warrantee re slab constrcution. I'll post back if I find out anything from them.
Thanks for your help all.
Ithaca, NY "10 square miles, surrounded by reality"
PEX is used for deicing sidewalks and drive-ways without probs. your situation is similar, just make sure that you use the procedures that the manufactor recommends. Luck.
Good point. I hadn't thought about the ice melt scenario.
One concern I have is that the slab might move to different degrees from the slab edge (highly suceptable/exposed to freezing) and in the center (not as suceptable becaue it's in the middle of a building, which will temper the exposure to freezing under the slab, even when unheated).
In a driveway or sidewalk setup, the whole unit is exposed to the same temperature range and therefore is more likely to move at the same rate.
I don't want to overanalize this, but I also don't want to throw my $ away either... Ithaca, NY "10 square miles, surrounded by reality"
PEX is highly flexible, you would need an extreme situation where it would not conform to frost heave, just make sure that the tube is insulated where it comes through the concrete. I still think that your best answers will come from the manufators of the tube,[ I know we all hate telephones], but that might be the way to go. Luck.
google "frost protected shallow foundations". In a nutshell, you would bury insulation to gather heat around the slab so's it don't heave. Not that hard to do. That being said, if you have good drainage (think railroad beds-they ever heave with all that gravel?) you might be all right, depending on your soil conditions.
I've Googled for just that actually, but I had decided agaist the frost protected slab for 2 reasons...
1) I think it works great with a building that's heated all the time, like a house (been used in Canada and northern Europe successfully for years). My structure won't be heated at all for several years, and after that the heat will not be running constantly all winter either.
2) While the footer/stemwall doesn't need to be 4' below grade, you still need a footer/stemwall. The details I've seen for the frost protected slabs still show 18"-24" depth. This allows for at least 12" of fill over the insulation "skirt". Cheaper than going down 4', but not as cheap as the monolithic floating slab.Ithaca, NY "10 square miles, surrounded by reality"
What I've seen is short lengths of PVC conduit to protect PEX where it spans the control joints just is case.
PEX is tough stuff, but a little wiggle room here can't hurt.
Good info from all. Sounds like a little attention to detail is all I need to make this thing work.
I like the idea of foam insulation as sleeves....hadn't thought of that.
Much appreciated everyoneIthaca, NY "10 square miles, surrounded by reality"