New Radiant Install Dilemma

CameronG

Guys, I've been reading hard throughout the forum and see a lot of great advice being given. I was hoping I could get some feedback...
I'm building a house in North Jersey that sits on a slab on grade with no insulation underneath it. (kept original foundation)
I want to install radiant over the top of existing slab and then pour concrete over for the finished floor (modern home). I only have about 3" of space from existing slab to the door sils to do this.
On my 2nd floor I was going to staple down on top of wood subfloor and go with a narrow plank hardwood or an engineered wood.
My questions:
1-With no under-slab insulation I'm trying to figure out how to install insulation (1/2-1") on top of slab and still be able to install pex and pour enough concrete over it to stay at 3" so I don't hit the door sils. I realize that probably isn't enough concrete to prevent cracking and not enough insulation to maximally insulate. What's the minimum thickness of insulation to make it worthwhile?
2-Is it truly futile to lay the pex and pour 3" of concrete over with no insulation at all?

As a last resort, I'd consider laying 1-2" of insulation and going with an engineered wood floor/narrow hardwood in lieu of the concrete if this is a much wiser/efficient approach?
I also installed 2 HVAC units as a secondary means of heat, but was planning to solely use the radiant with a Navien combi unit.
Thanks

Zman

1" insulation and 2" concrete should be doable. Don't go without insulation. Consult with your concrete company to get the mix correct. A high strength fiber mix is probably a good idea.

TAG

Is the house built ?

So you only have 3" -- In the lower level the concrete will be the finished floor?

I have used Warmboard over a slab .... Insulation/ warmboard/ wood finished floor.

Make sure you do a proper heat load and get the output correct for whatever PEX system on the second floor

psb75

Warmboard is a great solution. It includes the insulation as well as the aluminum conduction and ready channels for easy install of pex piping. You could then do engineered wood or other flooring. Unlike concrete, it will then be a low-mass radiant system. It will have a much quicker response time than a slab. It comes at a premium cost though, for all of those benefits.

hot_rod

I've done 1-1/2" concrete pours. Use a 3/8" pea gravel stone and double the amount of fiber, maybe 3 lbs per yard. Pea gravel can be pumped in with a tow behind grout pump, no need for a large pump truck.

The rule of thumb is 3 times the aggregate size over the tube. If you went with 3/8" pex and 3/8 pea gravel you could pour down to a 1-1/8" slab, according to concrete experts.
So you could layer 1" and 3/4" foam if you want to maximize the insulation.

If this is going to be a finished floor, you may see some hairline cracking with thin slabs. Prevent that by not watering down the mix, pour it as it comes from the truck. Every time you add water you reduce the psi of the mix. Add plasticizers if you want concrete to flow like water

If you are pouring within a concrete foundation be sure to have edge insulation both for heat loss and so the slab can expand when heated without cracking. They are called "external restraint" cracks.

EzzyT

@CameronG How about radiant ceiling then all you’d have to do is insulate the underneath the new slab. 

Where in NJ are you building this home?

CameronG

C-The house is already built....it was on top of an existing slab from 1965

TAG said:

Is the house built ?

So you only have 3" -- In the lower level the concrete will be the finished floor?

C-Yes I would like concrete as the finished floor w a high polish and sealer.

I have used Warmboard over a slab .... Insulation/ warmboard/ wood finished floor.

Make sure you do a proper heat load and get the output correct for whatever PEX system on the second floor

TAG said:

Is the house built ?

So you only have 3" -- In the lower level the concrete will be the finished floor?

I have used Warmboard over a slab .... Insulation/ warmboard/ wood finished floor.

Make sure you do a proper heat load and get the output correct for whatever PEX system on the second floor

CameronG

@hot_rod Thanks for all that info....I will discuss with the concrete guy once he shows up

hot_rod said:

I've done 1-1/2" concrete pours. Use a 3/8" pea gravel stone and double the amount of fiber, maybe 3 lbs per yard. Pea gravel can be pumped in with a tow behind grout pump, no need for a large pump truck.

The rule of thumb is 3 times the aggregate size over the tube. If you went with 3/8" pex and 3/8 pea gravel you could pour down to a 1-1/8" slab, according to concrete experts.
So you could layer 1" and 3/4" foam if you want to maximize the insulation.

If this is going to be a finished floor, you may see some hairline cracking with thin slabs. Prevent that by not watering down the mix, pour it as it comes from the truck. Every time you add water you reduce the psi of the mix. Add plasticizers if you want concrete to flow like water

If you are pouring within a concrete foundation be sure to have edge insulation both for heat loss and so the slab can expand when heated without cracking. They are called "external restraint" cracks.

CameronG

@EzzyT The slab is existing can not insulate under it.
80% of the first floor has 15" ceiling and a shed roof-large open floor plan.
Montclair

EzzyT said:

@CameronG How about radiant ceiling then all you’d have to do is insulate the underneath the new slab. 

Where in NJ are you building this home?

EzzyT

@CameronG have you had a Heatloss analysis and a system design done? Are you sure that radiant floor alone will be able to handle the heating? You might need supplemental heat like panel rads, baseboard or hydro-coil.

Paul Pollets

Code calls for 2" (R10) insulation in most, if not all states, plus edge insulation, as HR mentioned. A 1" -11/2" concrete pour will surely crack. 2" lightweight concrete can be poured, if you have a local vendor. With engineered wood flooring, you're up 41/2"

CameronG

@EzzyT, I have not. I do have 2 hvac units w heat if needed to supplement.

EzzyT said:

@CameronG have you had a Heatloss analysis and a system design done? Are you sure that radiant floor alone will be able to handle the heating? You might need supplemental heat like panel rads, baseboard or hydro-coil.

CameronG

@Paul Pollets the slab is existing from 1965.
Im not doing concrete AND wood...
Finished concrete alone is my first choice, but if that won't work based on my limited height...I would ultimately settle for a wood floor on top of insulation/sleepers etc.
Thanks

Paul Pollets said:

Code calls for 2" (R10) insulation in most, if not all states, plus edge insulation, as HR mentioned. A 1" -11/2" concrete pour will surely crack. 2" lightweight concrete can be poured, if you have a local vendor. With engineered wood flooring, you're up 41/2"

EzzyT

@CameronG a Heatloss analysis with insulation laid on current slab along with whatever finish flooring used as your floor assembly is where you need to start. Then a design can be done to determine if radiant will work.
Montclair is only about 10 minutes away from where I live, so maybe we can set up an appointment for me to stop by and see what you want to be done .