In-slab Radiant heat??? what can I cover it with?

Can I put an engineered wood floor over in-slab concret radiant heat and not affect the efficiency of the system?

Why do you want to do that?

There are lots of guys on here who know the math inside and out for this kind of thing....I'm not one of them. But I would guess it wouldn't help because the wood is not a good way to transfer heat. The concrete is because of it's high thermal mass. Not only that you need to keep the wood and concrete seperated. So you can't even rely on conduction to transfer heat for you.

With a layer of engineered wood you'd be putting about R0.75-R1 between the radiator (the slab) and the room. That will increase your water temp requirements, but in most cases it wouldn't be a large efficiency hit. Whether it is or isn't depends on the heat loss characteristics of the room and the type of boiler used. The effect on efficiency basement slab in an insulated basement with almost no glazing would be very small since it's heat load is also quite small, as compared to an above grade room with lots of windows and code-minimum insulation, where the effect would be more dramatic.

If you're running it on a gas or oil-fired 78-85% efficiency boiler the effect on efficiency is pretty much zero in all cases, but you may run into issues being able to get design-day heat into a very lossy room through R1 of flooring. The boiler runs pretty much the same efficiency at 140F as at 180F, and would be mixed down to the temp required by the slab in all cases.

But if you're running a gas or propane condensing boiler the effect could be fairly substantial if the temp increase causes the water returning to the boiler to cross the dew-point threshold for the exhaust gases (~125F). Basement slabs generally only need 80-90F water on the coldest hours of the coldest day of the year. If the wood flooring adds 10F to the water temp requirements you're losing maybe a percent in a worst-case. But if in a fairly lossy room it raises the water temp requirements from 120F to 130F or more you've given up at least 3%.

http://www.modsync.com/images/graph2.png

The other point to remember is you can't heat the wood to too high of a temperature or you will void your warranty. The typical limit is about 84F (at the wood). This probably won't be much of an issue with a hydronic system as you have a lot of thermal mass as well as losses to the earth (I don't know what you have for slab insulation). This is just something to consider and more likely is an issue with electric heated mats (since the heat source is right at the surface).

A slow/controlled heat-up is also a good idea with wood floors. Your system will naturally have a fairly slow response, but maybe ask your flooring manufacturer on what they suggest for how rapidly you can bring up the temperature once the heating season starts.

Void the warranty? Heck, walking on an 84F floor isn't exactly comfortable either! If you need the wood to be 84F to get enough heat into the room you have a VERY lossy room indeed! But you needn't worry about heating the wood up too quickly, given that it's a slab- the thermal mass of the slab inserts a huge ramped delay on the surface temperature (which is why PID-algorithm thermostats with both floor & air temp sensing are often needed with slabs to keep room temps from over/undershooting the setpoint.)

In the lossiest radiant zone in my house the surface of the wood floor never gets warmer than ~ 76-77F (when it's ~0F outside), but since it's a staple-up with 1.5" of plywood subfloor I still need ~130F water to get it there. (Had it been installed with the tubing in the upper layer of subfloor it could probably run on ~100F water on design-day.)

If you really want to keep the water temps down & responsiveness up even with, a dual-stage thermostat & a second hydronic loop for that zone, making the slab the primary radiation, and radiators (or baseboard- preferably cast hydronic rather than fin-tube convectors) the secondary will give you quick warmup response and lower operating temps. Panel radiators & cast-baseboard radiators can be on the pricey side, but they're wicked-comfortable compared to fin-tube. Fin-tube kinda craps out below ~110F (even dust-kittens can dramatically cut into it's very-low temp performance), but cast baseboard still does A0K with radiant-slab water temps. But as long as the slab can deliver the heat to the room through the somewhat insulative wood flooring there's not much incentive to go there.

Like I said, it probably isn't an issue since it is a hydronic system, but just something to keep in mind. It is more an issue with electric mats. Often they will have settings or sometimes a different t-stat for use with wood floors to limit the floor temperature as well as control the heatup. Since the elements are near the surface and can have little thermal mass (SLC or thin layer of thinset over the elements and this SLC/thinset may be thermally decoupled from the subfloor with some type of insulation). The combination can create higher surface temperatures (wood/heating element interface) as well as a higher rate of temperature change. That combination is not good for the wood if you don't try to control things a bit.