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radiant floor heat in crete slab with hardwood

so all the research i have done leaves me more confused. i am thinking of adding radiant floor heat to my floor but am unsure of the actual facts......
i will be pouring a 4" thick concrete floor very quickly, i will then add poly to the top as a vapor barrier, then i will laminate the entire floor with 3/4" plywood then i will install 3/4" hardwood floors. does this install leave room for radiant heat to be effective? i have been told that the slab will be warm but i will not get a lot of radiant heat due to the 1 1/2" of wood...... thoughts?
i am also looking at DIY and the kit with the lines and manifold-fittings is $900, then later to actually put it to a heat source. i dont want to add anything onto my boiler for fear of taxing it so i would use an electric tankless heater.....

A. Spruce
Re: radiant floor heat in crete slab with hardwood

It is unlikely that you'll get any beneficial heat from what you are proposing.

Re: radiant floor heat in crete slab with hardwood

Concrete is a big conductor or heat. If you want to put in a radiant heat system, then you will need to insulate the concrete slab from the ground under it and the outside air around its perimeter, otherwise it will turn into a giant heat sink and suck your wallet dry. On top of that, if the radiant tubing is imbedded in the concrete, the 1.5' of wood on top will effectively insulate the slab from the house. All the heat will go down, not up.

To make it worse, a tankless electric water heater? These things should be outlawed in the name of energy efficiency. If the utility company charged you for the difference in the price for the larger transformer needed to supply you house and did primary metering where you have to pay for the excess losses that this type of heater causes at the transformer, you would find that they are no bargain. On top of that, if you don't spend a lot of money upgrading the wiring to your house, you will have a brown out every time the thing kicks on.

If you want radiant floor heat, put down at least an inch of rigid foam insulation over the slab, then the 3/4" plywood with the grooves for the tubing cut into it. Then the hardwood floor, be careful not to puncture the tubing, over that. You will need to allow for the extra inch of the insulation. Use your boiler to provide the heat, the longer it runs on each cycle, the more efficient it is. If you have a separate hot water heater, you could use it as the source, but beware of the cost if it is electric.

Re: radiant floor heat in crete slab with hardwood

well the slab is prepped with the foam isulation around the perimeter but not all through the slab......
i thought of pouring the slab then laying the tubes on top of it and laying the 3/4" plywood around it and hen hardwood over this that way the 1/2" tube is directly (1/4")under the floor but this puts direct heat to the wood wich is bad.....

i will not do radiant floor heat. now to figure out the hvac for the space....

Re: radiant floor heat in crete slab with hardwood

I don't think that plan is all that bad, except that you really need to insulate the tubing from the slab if the bottom of the slab is not insulated from the ground. Many people are under the false notion that heat only radiates or flows upward. Warm air rises, but heat flows uniformly in all directions.

If the slab is in contact with the ground, and the tubing is in contact with the slab, then the path of least resistance for heat will be downward. You will have a much higher R value above the heat source than below.

If you break that path with some insulation, then a majority of the heat will go in the direction you want. As for heating the wood, you are heating a very large area so the surface temperature does not have to get all that high. Baseboards, radiators and air vents must be kept at a much higher temperature because of the small surface area. The wood floors should not need to get any higher than about 80°F/32°C to keep the addition quite cosy. That should be about its normal summer temperature.

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