I am a builder working with a client. We are trying to find out if wood flooring (floating engineered) over a radiant slab reduces the overall efficiency of the system significantly.

My guess is that if the insulation below the slab was increased by the amount of R-value the floor provides then it would not lower the overall efficiency of the system. The heat transfer would be slower but not less. Is this correct?

They are inclined to go with a polished concrete floor. I say that's too hard on your feet and you should go with wood. This leads to a secondary question - is an engineered wood floor over gypcrete easier on your feet than walking on a polished concrete slab. The client is an old friend of mine so we are comfortable with these sorts of discussions.

Issues: Cost, Efficiency, Comfort

IMO, yes (with most heat sources) and no.

increasing upward resistance lowers efficiency by requiring a higher water temperature (temperature difference from water to the room) to drive the same number of BTUs into the room. If you increase both upward and downward R value, this increases the temperature the slab needs to be to drive the same BTUs into the room whether or not you end up with the same downward loss.

increasing the temp of the slab means you need higher water temperatures. this can effect efficiency. either a very little in this case, in the case of high temp boilers like wood or oil. or a little more, in the case of low temp boilers like condensing gas. or potentially quite a bit, in the case of heat pumps.

polished concrete is the best for efficiency and cost.

as for walking, it's hard to say. how much time do they spend on their feet? can area mat/rugs be used in front of standing areas like sinks? spot coverage wouldn't kill your transfer.

wood is usually not a dealbreaker though unless they are putting down multiple layers.

It is the floor surface temp that determines the heat gain that will be emitted into the building. Placing "insulation" (e.g., wood flooring, carpet, etc) on top of the slab will result in a lower floor surface temp (and a corresponding lower heat gain) unless you also increase the hydronic fluid supply temp. Increasing the supply temp will require more energy to provide the same heat gain which results in an efficeincy reduction. When designing a hydronic radaint floor heating system, one should not include the floor area that is "blocked" by insulating objects when determining the required upward heat flux and floor surface temp. While I wouldn't consider a thin wood floor to be blocking, you will certainly be giving up some efficiency.

Interesting. To get to specifics then:
1 1/2 xps insulation underneath. Then that 1/16" foam floor underlayment on top and 1/2" engineered wood on top of that.
Philadelphia PA
either a condensing gas hotwater heater or and on demand hot water heater.

We are going over an old existing slab that is not insulated and not at all flat, a carriage house retrofit.

Bad idea?

Personally I can not spend much time at all in socks on a concrete floor. I'm not overweight either.

do not use an on demand. mod/con boiler, or tank water heater if your heat load is very very small.

not a deal breaker either way, most likely. might need to tighten tubing on centers in high heat load areas. all depends on the room by room heat load calculations that are OF COURSE being done, right?

What NRT.Rob says... You really need to do a proper room-by-room heat loss analysis and then do a proper hydronic radiant floor heat gain analysis. Please feel free to use our DIY calculators for this or hire someone competent to perform these calculations for you.

Heat Loss Analysis Calculator

Hydronic Radiant Floor Heating Design Calculator

The design water temperature for my own radiant floor heating system is 120°F. I would have been lower but for the 3/4" pre-finished oak flooring on the main level.

A proper radiant floor heating design starts with a room-by-room heat load analysis, which will help an experienced designer specify the type of radiant floor and the design water temperature it may require under design conditions i.e. the coldest day of the year.

Yes concrete is best but we have cork, engineered bamboo, tile, and yes even 3/4" oak in this one house. Our clients decide the floor covering they prefer and we design the radiant panel to their requirements. If the floor will not satisfy the load we supplement with panel radiators or look to the nearly limitless output of a radiant ceiling.

www.BadgerRadiantDesign.com

Of course we will be doing room by room heat load analysis, now that you mention it. We were going to buy our system through radianttec. I'll see how extensive their design services are and if it seems scant maybe I could hire one of you.

This site is always quite helpful because of the people that post, though the SIP forums can get a bit argumentative.

www.BadgerRadiantDesign.com doesn't work, btw. www.badgerboilerservice.com does. I've looked at everyone's sites.

Good luck with that.

You will be well advised to stay away from radiant floor designs that include "bare tube" or 7/8" tube or non-barrier PEX tube or the folks that advocate their use. I would buy from Rob and I am a design/build contractor.