I am considering adding radiant heat to my kitchen floor, so I'm just gathering info right now. I know there is a product called RetroHeat, but I'm sure there are other "retro" floor heating products out there as well. (I'm looking to heat about 300 square feet.) Anyone have any specific suggestions on what products to look at and who to work with?
Thanks.

Yes, there's also low voltage Zmesh.

Thanks! It's good to check out all the options.

Doing a hydronic staple-up tubing solution between the floor joists using an existing gas-fired hot water heater would be another option- more expensive up front, but cheaper to operate in most US markets.

Really? I didn't realize you could use hydronic heat for a "retro" install. I guess it's a matter of seeing whether the operating cost savings would offset the higher installation costs.

I presume you're re-doing the flooring, in which case you can probably use a more-responsive above-the-subfloor system (eg. WarmBoard) which can be more expensive , but as long as there's reasonable access to the joist cavities from below there is usually a way.

If your main heating system is already hydronic it makes sense to use the boiler. If not, gas-fired hot water heaters have burner to spare for projects like these. Getting the necessary BTUs into the floor at domestic hot water temps is usually easy enough, but it still takes a bit of design work to know just how cheap you can go. eg. If you do-need or don't-need $500 of extruded heat transfer plates it's important to know that ahead of time, eh? Every good heating design starts with a heat-loss calculation- you need to know how many BTUs you need to get into and out of the floor first, then figure out how to get there. (And that's as true for electric mat heating as anything else.) The BTUs per square foot of free floor area you need, and the water temps available for delivering it determine what your options are.

I agree with Dana. If you already have the hot water source you can pull from without over taxing it, use it. On the other hand, if you don't and you'll have to bring in new, then do your homework on both designs.

Excellent feedback. Thank you!
Time to do my homework.