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Radiant floor over existing slab

Greg1inCA's picture

What are your thoughts on the viability of a radiant floor system over an existing, uninsulated slab.  Is it a good idea?  Will it be hopelessly inefficient?

Would the following make sense:

Existing slab, 1" EPS foam, 3/4" plywood screwed down, 1/2" aluminum-backed radiant panel, hardwood flooring.

When I say hardwood, I'm only speaking generally... it is very likely that I will go with an engineered product.

Under floor heating (post #207298, reply #1 of 11)

To make this economic you need four inch thick polystyrene over the concrete, lay the pex in channels in the top, then finish with t&g glued OSB.

This will give you a fully floating floor, with no heat bridges. It will be quick to heat  and quick to cool.

If you prefer, cover the polystyrene with 3 or 4 inches of re inforced concrete, with the pex tied to the reinforcing mesh, this will give you a floor that is slow to warm and slow to cool, but if you never turn the heating off or down, that won't matter.

duplicate (post #207298, reply #3 of 11)

posted twice...delete

This is a remodel and I can't (post #207298, reply #2 of 11)

This is a remodel and I can't give up that much ceiling height.  The house was built in the 50's with zero insulation and is extremely leaky.  In addition to cosmetic and floorplan upgrades, a major goal of the project is to make the home more comfortable/efficient.  I run the forced air heater but the house never really feels comfortable.

I'm going to insulate, air seal and replace windows/doors.  This alone should be a huge help.  As I work out the details, I want to think about what I can do to make the floors more comfortable.  Space-wise, I figure that 1" of polystyrene isn't an issue.  3-4" is just too much.  Should I forget about a radiant floor?  

What is the current heat? (post #207298, reply #8 of 11)

What type of heat do you currently have?  Why do you want to change?

As for radiant, the slab and teh soil under the slab is a heat sink, no insulation is "necessary" .  Once the slab is heated, for teh most part, the heat it is not lost.  Now this is not true at the perimeter of the slab, you will lose heat to the outside at the perimeter, thus people sometimes even insulate the slab more or only at the perimeter with radiant.  I am no expert on the subject.

Which brings us back to why change your current heating strategy?

Forced Air (post #207298, reply #9 of 11)

I have a gas furnace.  The system is poorly designed but I think it makes more sense to improve it rather than go with radiant.  Now my focus is mostly on heat loss.  When I trench around the house to help mitigate some of the moisture issues, maybe I should insulate the perimeter.

the question is a tricky one (post #207298, reply #10 of 11)

while the apparent steady-state heat loss of a basement slab looks low in most load calculations, bear in mind most load calculations don't ask you much about the ground you're on.  If you're on ledge or wet ground, your downward losses can be huge.  even absent such a situation, the amount of energy it takes to heat up the ground is large, and is basically lost when you're done heating.  You might get some of it back, but you get it back when you're done heating and you still lose a lot of energy in the heat up/cool down cycle.

all heated slabs should be insulated underneath.  2" foam is code standard in most areas.  Here in maine 3" is code required for commercial slabs.

Perimeters of all slabs should be insulated with 2" minimum thickness.  though if you are installing on top of an existing slab, insulating over the slab is enough.

if you do go radiant you can insulate and then do a radiant layer and a finish floor.  I would shoot for 1" foam minimum and 2" preferred.  If you use a panel with aluminum on the correct side (top) you help out a bit because wood has an R value as well.  the absolute lowest profile for an overpour is usually 2.5' plus finish floor (1.5" gyp, 1" foam) and the lowest radiant profile of all in this circumstance is Roth panel, which is a foam panel with aluminum continuous top layer.  so you can do a 1" thick panel and float a finish floor over it.


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We trenched our block (post #207298, reply #11 of 11)

We trenched our block foundation past summer to resolve moisture and heat-loss issues. Stack-effect was a major source of heat-loss. Considered a number of possible solutions, but opted for surface bonding cement outside and insulation inside. SBC stops water, air, and much vapor. Stack-effect dramatically reduced as is energy use.

Two concerns: ground moisture (post #207298, reply #4 of 11)

Two concerns: ground moisture dries into the interior and warm air-borne moisture can condense on the cool surface of the concrete floor. Anything you put on the floor will inhibit evaporation of that moisture. I think the benefit of insulating a basement floor has less to do with heat than it has to do with moisture-management. We had a damp, dirt floor crawlspace we covered with stone, a layer of insulation, a vapor barrier, and a concrete slab. Now, ground moisture cannot migrate into the crawl and air-borne moisture is less likely to condense on the surface of the insulated slab. Radiant heating complicates--though it could improve--your situation.

Insulating concrete floor. (post #207298, reply #5 of 11)

With a concrete floor, covered with one inch thick polystyrene, topped by 1 inch OSB, underlay and carpet, you can expect your carpet surface to register the same temperature as the air in your room.

I have checked on the above with outside temperatures of minus 18C/-6F. 

With an outside air temperature of -18C, a mixture of double and treble glazing, 8 inches of polyurethane foam in the ceiling, a radiator surface temp of 60-65C an air temp in centre of room 22C the carpet surface temp to one foot from wall was 17C and the rest was 22C.

What about hardwood (post #207298, reply #6 of 11)

How does it look with outside air temperatures at 5C and 3/4" white oak flooring over the OSB?

Insulated floor. (post #207298, reply #7 of 11)

 If you wish for a white oak  t&g floor, I suggust that you lay this directly on top of floor grade Styrofoam/blue board, forget the OSB, this is merely the normal type of common floor.

The oak floor will have a surface temperature the same as the room air temperature, but will feel cooler and will have a wider cold edge. At -5C the edge will be about 19C.