Insulation Under Slab

[hausfxr]

Arguably the best thing you can do is put construction joints in every 12’ or so and lap one side’s ends of rebar over the joint. And that rebar that extends into the next slab needs to be greased and sleeved so the joint can move. And we simply drop the pex below the joint into the foam board at the joint, but I've never done a job with hydronics that did not have at least 3" (code minimum here) to 4" of XPS. Someone has already pointed out that the joints can be bad for small wheels (it’s one reason you see mostly thin control joint used over construction joints in big box stores, however, the base below those slabs is very well prepared and compacted with very large rolling vibrators, and the soil below that is tested and compacted as needed.), but a 20’ by 36’ slab on grade is huge and needs to be allowed to move. With a slab as large as yours you are guaranteed to get cracks and many of them, if not most, won’t follow control joints – stresses go in all directions. If you doubt any of this from your own experience, then you need to just revisit 50 or so slabs any concrete company has poured in the last 5 to 10 years after they are finished. You’ll see cracks in all of them, but especially those that were poured on grade.

Also, most people don’t realize how much organic matter there is in the top few feet of soil, and they think taking off that top darker layer is all they need, but once you cut off plant growth and all the organisms that give that soil structure, it will shrink for years, if not for decades later.

So, will your slab crack? Absolutely, and those cracks will be large and probably offset vertically over time. Fortunately, pex is pretty darn tuff, so the chances that you'll get a tear are slim - just not a chance I'd take myself.

 

[EricVonHa]

Arguably the best thing you can do is put construction joints in every 12’ or so and lap one side’s ends of rebar over the joint. And that rebar that extends into the next slab needs to be greased and sleeved so the joint can move. And we simply drop the pex below the joint into the foam board at the joint, but I've never done a job with hydronics that did not have at least 3" (code minimum here)

If we're talking about a concrete floor that is poured inside let's say a 40' x 50' building with a cinder block foundation, why would interior joints need to be sleeved together?

My thought is that zip strips or sawcut joints would be appropriate for an interior floor of a typical work shop or garage Mahal. Of course, pre-planning the water lines vs. joint locations is paramount. Though, most curious on interior slab sleeving for these smaller buildings? Part of me thinks that you may be referring to large-scale Warehouse spaces or exterior slabs?

 

[slowTA]

I'm glad I'm reading this now as I'm planning my build (32x50x12 pole barn) for this spring.

I'm still on the fence between radiant floor vs forced hot air as the price difference is huge and I'm closing in on the upper limit of my budget. My math shows $4,700 for in floor with Crete-heat insulation vs. $3,600 with foamular 250 and staples (+ $2,600 for the furnace at some other time) vs. $800 for a 125 kbtu heater that hangs from the ceiling.

I'm sure most here would do the radiant floor (as I've been planning for years). But now I'm wondering if its practical for someone who is only out in the garage for a few days each month during the winter. Since I'm doing a pole barn, even with insulated walls and ceiling, I know heat will just leach out everywhere so a huge heated slab of concrete probably is more economical... but is it really for my needs?

 

[finn]

I'm glad I'm reading this now as I'm planning my build (32x50x12 pole barn) for this spring.

I'm still on the fence between radiant floor vs forced hot air as the price difference is huge and I'm closing in on the upper limit of my budget. My math shows $4,700 for in floor with Crete-heat insulation vs. $3,600 with foamular 250 and staples (+ $2,600 for the furnace at some other time) vs. $800 for a 125 kbtu heater that hangs from the ceiling.

I'm sure most here would do the radiant floor (as I've been planning for years). But now I'm wondering if its practical for someone who is only out in the garage for a few days each month during the winter. Since I'm doing a pole barn, even with insulated walls and ceiling, I know heat will just leach out everywhere so a huge heated slab of concrete probably is more economical... but is it really for my needs?

I have posted on this subject before, and usually get pushback, since I, too, question the payback for a non continuous use shop, but I just ran the numbers on the operating cost for heating my 48x75x16’ stick built, insulated shop so far this heating season. All but 16x60’ is heated by a 96% propane boiler and pex in the floor, with the thermostat set to the mid forties, and a hanging 82%, 75k btu furnace set to about 60 degrees or a little higher when I am working.

Our heating season runs from early October through late March, and, although we are far north, we are by Lake Superior, and the lake moderates temperatures to some extent. January has been in the single digits to mid twenties.

So far I have had two propane deliveries, and the tank gauge read 30% as of yesterday, so we’re right at 450 gallons for about 110 days, or four gallons per day.

At $1.45/ gallon, that’s a little under $6.00/day.

Not bad, I suppose, for such a large space.

On the other hand, we have been gone for at least thirty days in that time frame, and I still have to maintain heat in that time frame, so figure it is a little more than $8.00/ day when I an at home.
If I average two hours a day for that 80 days, the cost to heat the workshop is in the neighborhood of $4.00/ hour.

 

[kj_mustang]

I never posted on my gas usage as it is no way for me to separate my shop/garage gas usage from the house. I filled the in ground tank in July. Used about 350 gallons so far. I am in a slightly warmer zone than finn and have a 32 x 56 shop/garage that the radiant floor heat stays on 58 all the time. The house has gas fireplace, range top, and a boiler for my hot water and in the last month radiant floor heat for the attached garage. I think I am doing pretty good on usage.

 

[yeldogt]

Put the tubing down today...

I'm not going to run mesh over it for fear of sharp edges puncturing the tubing.
I'm just going to have fiberglass added to the concrete mix and call it good.


Used LoopCad for the layout...

I just noticed -- they missed a loop .... or did you do for reason.

 

[yeldogt]

I'm glad I'm reading this now as I'm planning my build (32x50x12 pole barn) for this spring.

I'm still on the fence between radiant floor vs forced hot air as the price difference is huge and I'm closing in on the upper limit of my budget. My math shows $4,700 for in floor with Crete-heat insulation vs. $3,600 with foamular 250 and staples (+ $2,600 for the furnace at some other time) vs. $800 for a 125 kbtu heater that hangs from the ceiling.

I'm sure most here would do the radiant floor (as I've been planning for years). But now I'm wondering if its practical for someone who is only out in the garage for a few days each month during the winter. Since I'm doing a pole barn, even with insulated walls and ceiling, I know heat will just leach out everywhere so a huge heated slab of concrete probably is more economical... but is it really for my needs?

Radiant being slow to respond is not ideal when changing temps -- my place with propane we drop it down during the week when we are not around and now with internet control its easy to turn up. My other place is NG and we don't mess with it much.

 

[slowTA]

Radiant being slow to respond is not ideal when changing temps

Is being slow the only concern? Or would you risk cracking the pad by fluctuating the temp? I'm assuming you'll also be pouring a ton of heat into the pad to warm it up just a few degrees... to the point that it might be cheaper to keep it warm all the time (or at least not warming it up more than once a month).

 

[GYPSY400]

I think it's better to keep it moderately warm all the time.. I've heard it takes 8-12 hours to notice a difference in temperature..so keep the floor at about 50 and use a blast heater to warm it up for use.

Sent from my SM-G903W using Tapatalk

 

[Randy in Maine]

I keep mine at 50º F (air temp 4 feet up the wall at the thermostat) and that is about perfect in the winter. The floor and my feet are warmer.

I turn it on around Thanksgiving and off around April 1.

 

[stingry]

Radiant heat is best when left at a constant temperature. My shop is 3600 sq ft and is well insulated. It is heated with two zones, one seven loops and one five loops, and a 75k btu power vent water heater. I keep one zone at 63 degrees and one at 65 degrees, using a thermostat to measure air temp at eye level. When I fired up the system with both loops running, the air temp increased at one degree per hour until set temperature was achieved. I live in Western Nebraska and the average temperature for December and January is in the mid twenties. Cost to heat was approx. $100 from mid December to mid January, $80 for natural gas and $20 for electricity. I still have some inefficiencies to address which should lower the heat bill more, particularly having no perimeter insulation on the slab.

 

[finn]

Radiant heat is best when left at a constant temperature. My shop is 3600 sq ft and is well insulated. It is heated with two zones, one seven loops and one five loops, and a 75k btu power vent water heater. I keep one zone at 63 degrees and one at 65 degrees, using a thermostat to measure air temp at eye level. When I fired up the system with both loops running, the air temp increased at one degree per hour until set temperature was achieved. I live in Western Nebraska and the average temperature for December and January is in the mid twenties. Cost to heat was approx. $100 from mid December to mid January, $80 for natural gas and $20 for electricity. I still have some inefficiencies to address which should lower the heat bill more, particularly having no perimeter insulation on the slab.

The January temp in Nebraska is about ten degrees warmer, on average than we get (33/17 vs 23/11 day/night) and NG is considerably cheaper than propane (.32/therm vs $1.58/therm for propane @ $1.45/gal) if my math is right.

Is your shop metered separately?

 

[stingry]

Finn
No. Based my estimation on last years usage but would have been more if metered separately due to separate delivery costs.

 

[pv74]

Pictures of shop:
I built the little red 10X20 shed myself. It sits on concrete and has a 60 amp electrical service in underground conduit. The new shop has 125 Amps run in conduit. I also have conduit between my shed and the new shop.

My freezer, garden and poultry stuff go in the shed. Shop stuff will go in the shop.

Driveway between shop and shed is over 3,000 sq ft. I can drive around the shop easily with a trailer or just park under the overhang if I wish. Surface was scraped and graded, filled in in with approx 6" of roadmix… I'll have crushed drain rock brought in a bit later.

All wiring is in 3/4" EMT conduit...all 12 gauge.
All wall outlets are 20 amp on GFCI breakers.
It doesn't cost all that much more than Romex...just a little more work bending the tubing and calculating conduit fill.

Tubing was under pressure when poured and still is...
It hasn't dropped in pressure at all several weeks afterward...

 

[pv74]

Arguably the best thing you can do is put construction joints in every 12’ or so and lap one side’s ends of rebar over the joint. And that rebar that extends into the next slab needs to be greased and sleeved so the joint can move. And we simply drop the pex below the joint into the foam board at the joint, but I've never done a job with hydronics that did not have at least 3" (code minimum here) to 4" of XPS. Someone has already pointed out that the joints can be bad for small wheels (it’s one reason you see mostly thin control joint used over construction joints in big box stores, however, the base below those slabs is very well prepared and compacted with very large rolling vibrators, and the soil below that is tested and compacted as needed.), but a 20’ by 36’ slab on grade is huge and needs to be allowed to move. With a slab as large as yours you are guaranteed to get cracks and many of them, if not most, won’t follow control joints – stresses go in all directions. If you doubt any of this from your own experience, then you need to just revisit 50 or so slabs any concrete company has poured in the last 5 to 10 years after they are finished. You’ll see cracks in all of them, but especially those that were poured on grade.

Also, most people donÂ’t realize how much organic matter there is in the top few feet of soil, and they think taking off that top darker layer is all they need, but once you cut off plant growth and all the organisms that give that soil structure, it will shrink for years, if not for decades later.

So, will your slab crack? Absolutely, and those cracks will be large and probably offset vertically over time. Fortunately, pex is pretty darn tuff, so the chances that you'll get a tear are slim - just not a chance I'd take myself.

Code is 2" of foam in my area (I checked with the building inspector). I spent a week on the surface preparation alone.


Under the concrete is several inches of roadmix, which I personally leveled twice by hand with a screed board as for a brick patio. It was compacted three times with a rented plate compactor during the process. I also compacted and filed in around the sides of the building. Under the concrete, foam, roadmix and vapor barrier is caliche, which is the arch enemy of a backhoe. The building also has good drainage and overhangs on all sides.

The concrete crew knew beforehand that they were pouring over foam and had prior experience with it... They did a great job and seemed to know exactly what they were doing. A tool joint was placed at the garage door threshold where the foam ended and saw cuts were placed appropriately (not through my cutouts for the lift posts). Fiber was in the concrete mix (I verified that). Concrete over the foam is 4" (six inches under the cutouts and under the overhang).


No issues so far, but time will tell, but I am fairly confident that it will be just fine. As several people have told me "There are two guarantees with concrete: it's going to crack and no one is going to steal it".

 

[tweaek]

Your build looks great. I’m in SLC, UT south of you. Any ideas if your builder has does anything in the Utah area?

 

[pv74]

Your build looks great. I’m in SLC, UT south of you. Any ideas if your builder has does anything in the Utah area?

I used a local contractor who specializes in building pole barns, he seems to do business primarily in the Treasure Valley (found him through a Craigslist add). I did do a reference check and ended up doing a site visit beforehand to see some of his work. Overall, I'm very happy with how it turned out.

 

[tweaek]

I used a local contractor who specializes in building pole barns, he seems to do business primarily in the Treasure Valley (found him through a Craigslist add). I did do a reference check and ended up doing a site visit beforehand to see some of his work. Overall, I'm very happy with how it turned out.

Thanks!

 

[Denwood]

I'm glad I'm reading this now as I'm planning my build (32x50x12 pole barn) for this spring.

I'm still on the fence between radiant floor vs forced hot air as the price difference is huge and I'm closing in on the upper limit of my budget. My math shows $4,700 for in floor with Crete-heat insulation vs. $3,600 with foamular 250 and staples (+ $2,600 for the furnace at some other time) vs. $800 for a 125 kbtu heater that hangs from the ceiling.

I'm sure most here would do the radiant floor (as I've been planning for years). But now I'm wondering if its practical for someone who is only out in the garage for a few days each month during the winter. Since I'm doing a pole barn, even with insulated walls and ceiling, I know heat will just leach out everywhere so a huge heated slab of concrete probably is more economical... but is it really for my needs?

I have a rather unique situation in that we designed a 4800 square foot radiant over-pour, but we also have two air handlers to cover fresh air, cooling and heating. Both are fed with 96% efficient Triangle Tube boilers (redundant x2) so we can heat the space with radiant, or forced air..or both. The space is also serviced with six Ecobee3 stats (which store historical data) so we have gathered data on quite a few experiments. Four radiant zones each have their own Ecobee3 stat, as do the two air handler zones.

Temps here hit -35C in winter.

A few things I can say with confidence:

1. Insulating a slab is 100% worth it.

2. The most energy (both gas and electricity) is saved with the highest possible setbacks in winter. In my home shop, heat is off unless I need it. A Reznor hanging unit gets the small shop comfortable for working in 5-10 minutes.

3. We choose to keep the four (4800 sq/ft total) slab zones at much different temps. The loading bay (about 16x32 with 20ft ceiling) is kept at 8C..but it is separated by insulated walls from the ready of the space. The main working area radiant is set back to 10C off hours but never cools off to that. Each morning the air handlers bump temps to 21C, and do this in about 30 minutes.

In other words, unless you are in the shop a lot, radiant will use more energy than forced air..and recovery will be slow. YouÂ’ll also see a temp bump of a few degrees after a longer radiant run..again wasting BTUs. Insulating the slab in cold temps is more important than how you heat.

In order of efficiency for a shop that is not used daily:

1. Insulated slab with forced air on demand.

2. Radiant with low set temps, forced air to bump on demand.

3. Radiant.

If you are ever planning solar, then in slab radiant is the only way to go.

Here is a thread demonstrating startling savings on the radiant loading bay just by replacing door seals: https://www.garagejournal.com/forum/showthread.php?t=414538 There a few building pics there as well.

 

[Sparkynutz]

My 25x25 slab varies in thickness from 4.5" thick to 6" thick. Every time I thought I had grade close to level I moved laser to opposite side and different spots were off. Later I found out the auto leveling didnt work for ****. I ended up grading best I could by eye and an 8ft 2x4 compacting repeatedly after raking and more raking.
The other reason it was hard to get consistantly thickness was the fact I had a drain in front center 2/3 of garage with 2" slope to 12' out. I was trying for 5.5" thick but after installing a layer of 2" foam and rebar 12inch on center it ended up higher and lower here and there depending on my dips and how flat it layed. I cut the 25x25 slab into 6 squares so drain had cuts to it at the center of the front 4 squares.
The slab hasn't cracked yet and doubt it will. It hasn't even cracked where I cut it.
The foam definitely helped keep the garage above freezing during some -20* weather with minimal electric heating a few hours a day when working out there. Sometimes I wish I had installed piping for in floor heat but didnt have the money for it and knew even at a later date the money would be better spent elsewhere which is still true.
Pouring over the foam was not an issue one bit. The rebar spacing was tho. I had it left over and just wanted to use it up. 16 inch center or bigger would have been much easier walking and not catching feet on or breaking chairs or pushing chairs into foam.

Sent from my SM-G960U using Tapatalk

 

[slowTA]

Does anyone have a PEX stapler for sale. My building is going up in the next week or 2 (sooner than I thought)!

I figure a Garage Journal tool rental would be appropriate for this since I doubt many of us would use the stapler more than once in our lives.

 

[Randy in Maine]

I used something like this to attach my 1/2" pex to the 6"x6" wire...zip ties would also work I guess.

https://www.supplyhouse.com/Viega-15301-Wire-Mesh-Clips-1-2-and-5-8-PEX-Package-of-100

 

[kj_mustang]

Here is a company that rents one. I used it and it worked fine.

http://www.blueridgecompany.com/radiant/hydronic/501/rht-foam-stapler-and-staples

 

[slowTA]

Thanks, I found them too and I'll be placing an order soon.

 

[pv74]

Does anyone have a PEX stapler for sale. My building is going up in the next week or 2 (sooner than I thought)!

I figure a Garage Journal tool rental would be appropriate for this since I doubt many of us would use the stapler more than once in our lives.

Darn...I would have sold you the one I bought!
It is for sale, with the remainder of the staples if anyone wants it
I can tell you that the Malco stapler works and is worth every cent... Saved a lot of time and frustration. Also, it is built like a tank...

 

[slowTA]

pv74, check your PMs.

 

[thammel]

I have insulation board down, pex stapled to it and 5" of concrete on top. I have a two post lift and the floor is epoxied. The floor was put down 10 years ago. It has not cracked and is in beautiful shape. So this is a case against what you've heard.

Tom

 

[meathooker]

Just saw this thread. I live in Meridian and was doing the same thing at the same time this winter!

Do you race?

 

[wanderer]

My uneducated guess is that the concrete is more likely to crack with foam because it can’t pull moisture from the soil. You could mitigate this by leaving a soaker hose running to keep it from drying out so badly as it cures.

 

[thammel]

Good point about running a soaker hose. I kept my slab wet for many days while curing.
Tom

 

[slowTA]

Wouldn't the vapor barrier also prevent any moisture transfer?

 

[dweihs]

I am about ready for staple gun for pex. Any of you guys got one you are done with that you want to sell?

 

[Lonnies Performance]

2" foam, 2 years later, 6700 Sq.Ft. & no cracks so far... knock on wood. Lol.

4.5"-5.5" thick, with PEX tied to 6-8ga mesh over foam, over vapor barrier.

Do not use wire ties as the loose cuts or a broken one can float up to surface of concrete & end up in your finished floor. I used steel rebar ties to fasten my PEX.