Drill and dowl the slab, after confirming where the pipes/electrical are. Then nail into the dowl.

Glad we now agree that 85 is an established threshold that radiant floor designers use as a recommended maximum for floor surface temperature. Foot contact with the floor makes the second way high temperatures lead to discomfort. 85 is evidently too hot for those expecting warm feet and will definitely lead to uncomfortably hot room temperatures, in efficient homes. There still seems to be confusion regarding water temperature and floor surface temperature. The temperature of the floor surface will be lower than the water temperature in the tubing. The water temperature entering the tubing loop from heating appliance, will be warmer than the temperature exiting loop, on its way back to the appliance to be reheated. The only mention I made of water temperature was that 125 seemed too high for an efficient home. Perhaps not for a long tubing run with short spacing but I still think it seems high and engineers generally don't like long tubing runs. I've personally installed tubing on a spec of mine and for other builder friends. We installed short runs with wide spacing for better zoning and they were for efficient envelopes. Few ended up being hooked up to an actual appliance. Why spend that money when the required AC can handle the heating too. The "at" which I corrected to "near" room temperature comments were in regards to floor surface temperature, which is a widely accepted fact for those involved in efficient construction. From All About Radiant Floors: "Why isn’t my floor warm? Homeowners who look forward to walking barefoot over warm floors are often disappointed by homes with in-floor hydronic heating. That’s because these floors are rarely as warm as most homeowners expect." From that article's case study (keep in mind difference of water temperature and floor surface): “On the design heating day [the coldest day of the year], the water flowing through the tubes in the floor will only be at 85 degrees,” said Robb Aldrich, an engineer at Steven Winter Associates" From energy expert Alex Wilson's article describing a level of home performance between passive house and international code minimum: "in a highly insulated house (and I’m talking about a really tight house with at least R-40 walls, an R-50 ceiling, and triple-glazed low-e windows), such a tiny amount of supplemental heat is needed that a radiant floor needs to be kept no more than a few degrees above the air temperature—or else overheating will occur. If a concrete-slab or tile floor surface is maintained at 72 or 75 degrees, it will likely feel cool underfoot—since it’s at a lower temperature than your feet. So you may not get that delightful benefit of a warm floor surface.

Polished concrete must be specified BEFORE they pour the concrete slab...not afterwards. It is the easiest time for the concrete finishers to get the right aggregate on the top before the whole slab 'sets up'. And then you have to decide on polished vs. 'finished' concrete. Polished is super tough. It is gorgeous in it's own right...but it can still stain and etch (depends on the aggregate and the additives used by the finisher and then the polishing schedule, etc). A 'finished' concrete = microtopping + a 'finish' applied. A finish could mean the low-end 'temporary' stuff like a wax or a polish (which you would have to maintain forever and ever and ever) or it could me a high-end epoxy (clear or otherwise) that NEVER needs maintenance other than mopping. A properly polished finish can be $7 (no sealants used) - $18/sf (multiple sealants used with multiple passes). A 'finished' concrete = $5/sf (microtopping + urethane finish) - $25/sf (3-5 stains used to create visual effects + coating of clear epoxy). If you want to use engineered hardwood upstairs then make sure the hardwood MANUFACTURER clearly allows for HYDRONIC in-floor heating. No one allows wood over electric wires for a 'whole home' heating system. It is up to the wood manufacturer to give the "OK"....not the heating company.

The issue normally isn't the 'engineered' part that is the problem. It is the wood part. It sounds like you are adding ELECTRICAL in-floor radiant heat. If you are, then wood is normally a no-no. The vast majority of wood manufactures do NOT allow their product over ELECTRIC in-floor heat. They are allowed over the more expensive HYDRONIC heating (tubing filled with water/oil/gel). Why not electrical? Because wood can burst into flames if heated 'too much'. The 'fire hazard' thing is real when it comes to electric wires (ahem...they are exposed wires....that's how they generate heat). Yes I understand you will be SINKING the wires into cement. Yep. Got it. To get away from the FIRE HAZARD you would need about 1" worth of cement sitting on top of the wires. If your home's flooring set up can handle that type of increase then go for it (check the clearance on the doors, base trim, exit and entrances, top of stairs...etc). That will reduce the chances of a fire hazard (lots of thermal mass for the heat to warm up = lower chance of wood spontaneously combusting). But the warranty on the wood is *probably void. Not always....but the majority of the time it is. For me, wood over electric heating = waste of money. It sounds like the heating in the home is provided by some other source and the in-floor heating is just to warm the toes and the tile. Skip the $10/sf upgrade (that would include the 1" of concrete on top...just sayin') and go ahead and upgrade your HVAC system to handle the home's square footage. It will be a little cheaper and will cost you less in the long run (because an upgrade often means HIGH EFFICIENCY model).