Radiant Heat Uninsulated Slab on Grade

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Old 03-11-15, 06:18 AM
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Radiant Heat Uninsulated Slab on Grade

I have a 2200sq ft two story home in South Eastern Virginia built in 1986. I'm assuming, from the quality of more visible work, that there is no insulation under the slab and possibly no vapor barrier. A neighbor has reported that during remodeling, he found that there are actually two 6" slabs stacked with nothing else. I don't know how accurate that is for my home, I trenched around it to 18" deep and was still finding block. I have poor drainage (working on it) due to being a mile from the ocean the ground water is 12-24" below the surface.

The bottom floor is virtually unusable during the winter. Even my pets refuse to be on the cold floors (mix of laminate and tile). I have read plenty of back and forth on pros and cons of using slab as thermal mass, I haven't developed an opinion. Right now, I'm leaning toward insulating on top of the slab and installing PEX in a thin slab above that. The air temperatures in my area only get below freezing a couple of weeks a year, so I don't feel it is necessary to heat the whole slab. I'm not opposed to breaking up some of the slab, if I can figure out it's depth. Otherwise, what's the best way to insulate it thinly?

Are the radiant ceramic paints useful? With or without foam board? I won't know the depth until I start pulling stuff out. But, I'm thinking: 1/2" foam board painted with the ceramic paint, then PEX, then 1-2" concrete, no additional floor cover (just acid staining and sealing).

I know this isn't ideal, but I need to work with what I have and the budget isn't unlimited (no warmboard). Any opinions or suggestions?

TIA
 
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Old 03-11-15, 02:44 PM
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Alisa,
Are the radiant ceramic paints useful? With or without foam board? I won't know the depth until I start pulling stuff out. But, I'm thinking: 1/2" foam board painted with the ceramic paint, then PEX, then 1-2" concrete, no additional floor cover (just acid staining and sealing).
You can not use radiant barriers, Heat transfer is always the sum of conduction+ convection + Radiation. When objects touch only conduction dominates -- radiation and convection go to zero --

You would need the help of a architect/ mechanical engineer to design a thermal break, seeing as the slab is conductively coupled to the wet earth.

The material of choice is XPS (extruded Polystyrene ) for its hydrophobic properties.

See:
BSD-103: Understanding Basements — Building Science Information
BSD-108: Investigating and Diagnosing Moisture Problems — Building Science Information
 
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Old 03-11-15, 03:50 PM
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1/2" foam board painted with the ceramic paint, then PEX, then 1-2" concrete,
I would think that the concrete would be susceptible to cracking being that it's thin and not laid on a SOLID surface.

As HW has also said, I don't think the ceramic paint will add any value. Radiant barriers require an air gap in order to operate. Anything in contact with a radiant barrier is a 'heat sink' and sucks the heat right out of it.
 
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Old 03-11-15, 05:58 PM
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Any foam under the ground won't work. Termites are horrible here, as well as other insects. The same neighbor that I mentioned had tunnels all through his foam board. I don't remember if it was xps or eps. I haven't had any termite activity in two years, but I don't want to face that problem again. One side of the house, I added gypsum and sand to the soil. Added a French drain topped with stones. That side no longer has a problem with mildew or mold on the foundation. I'm making my way around the house, but obviously can't do much about the center of the slab.

I know that 4" is preferable for a thin slab, but I've seen where others have had luck with using the more flexible driveway resurfacer at 2". :-/ I may have to reconsider the warmboard. It's an open floor plan, so I don't even have the option of baseboard heat.

**Don't worry, I got an awesome price on the place. ;-)
 
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Old 03-11-15, 06:23 PM
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So the problem is that the temperature of the floor is uncomfortable?

If I were to add hydronic, I would use warmboard. You are going to need to pour a thick slab if you were to use concrete. If this is just to warm the floor up, I would consider electric radiant as well. That would probably require less thickness than warmboard.
 
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Old 03-11-15, 08:13 PM
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Yes, it's just the floor. I have a brand new hvac system and brand new insulated duct work. The air temperature is fine. The floor temperature stays around 60 degrees, no matter how high the thermostat is set. I can't just add carpet, I've had two scout troops, my own four kids, four teens and six pets in the space just today. Concrete floors are essential. The operating costs of electric radiant heat scares me. :-/

I will keep adding drainage and maybe look into a thermal inspection afterward.
 
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Old 03-12-15, 04:24 PM
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If you have ground water under the slab at 55f and it is moving, it will probably remove the heat from the concrete floor as fast as you add it. I would suggest that you insulate the floor and let your hot air system provide the heat .
 
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Old 03-12-15, 04:35 PM
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The operating costs of electric radiant heat scares me. :-/
Yes, it would scare me too if I was heating the home with it... but you will only be warming the floor a bit. Might not be as bad as you think!
 
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Old 03-13-15, 08:48 PM
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If you insulate the slab, you should include some electric radiant. It won't take much to keep the floor at just a few degrees above air temp.
 
 

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