I am absolutely NOT an expert in this, but I can tell you our situation. First, we have concrete, too, but not on grade (3rd floor apartment). We also used heating mats, choosing Warmly Yours. (This decision involved the claims of electromagnetic fields reduction by someone else - I can't remember which one now - and coil distance and wattage per square foot.) I read a lot about trying to create a "thermal break" between the heating mat and our 12 inches of concrete. In the end, after reading a lot of posts all over the internet (reader beware, there is some pretty sketchy non-science out there), especially johnbridge and I think jcl, and after extensive discussions with my nephew who is a materials engineer, we did two things: 1/4" Wedi board and SuperTherm. Here's how we got to that decision (YMMV): So very many people told us that we didn't need any insulation, that insulation was stupid and unnecessary, that the heating mat would be absolutely fine without it. The two things we heard the most were: 1.) "I've installed 12 trillion-gazillion mats (or whatever extraordinary number they thought I would buy into), and I've never used it because you don't need it." 2.) "Heat rises." Regarding point #1, the thermal properties of wood underlayment versus concrete cannot be understated. What works over wood will be different over heat-sucking concrete. Even if you ask if all their other jobs were over wood, they very very VERY frequently lie. (Oh yeah, and I don't care what cocked-up thing you've inflicted upon other poor unsuspecting, non-researching saps....) Wood acts more as an insulator, concrete as a conductor of heat. Regarding point #2, lord this is stupid. Hot AIR rises, because it has more energy which makes the air molecules vibrate more, pushing the molecules apart, which takes up more room (for the same number of particles), which makes them less dense...so they rise above the cooler air. This is for AIR. When dealing with solids (and some liquids under some circumstances), heat acts more like electricity (both are forms of energy), and takes the path of least resistance. Meaning, that the heat from your mat will travel EITHER UP OR DOWN according to whether the concrete or your tile is more resistant to heat energy. The science behind which material involves density, heat capacity, etc. (It isn't exactly like electricity though, in that it isn't an all-or-nothing path decision. Apparently, with heat and different materials, it is a matter of what percentage - I'm over simplifying here - goes in which direction. Think about it: electrical conducting shields work absolutely, no matter how long the wire is live, but heat shielding can only slow down the transfer.) For anyone who owns a frypan with a handle that is made of a different metal than the pan body itself, this concept is in that pan. The body of the frypan is designed to transfer heat fast, the handle is made from a different metal that will absorb heat more slowly. The point: the handle WILL get hot eventually, just a matter of how much time. My nephew (a materials engineer, who doesn't work in this field but had more game than me for running the equations) figured out that my tile is waaaayyy more resistant than the concrete, that 70% of my heat would go into the concrete and only (about) 30% into the tile. All the people that said bare concrete would be "fine" were kind of technically sort of right: my tile would eventually absorb some amount of heat. I had two issues with this: how much more energy would I be paying for to heat the 12 inches of concrete AND how sloooowww would my tile heat up without insulation? So no contest, we needed a thermal break. (I wanted to be able to get my tiles warm in - at most - a couple hours. Without insulation, the mats would have to turn on about 12 hours ahead of time, just to hit 80 degrees by morning. No way.) I found a thread on johnbridge where somebody (I can't remember his screen name but I remember his real name is Eric, if that helps) actually mocked up several underlayment materials and measured how long it took for the top of the tile to move something like 20 degrees. (This was a while ago so some of these numbers might be a bit off, but the gist is the same.) So a good insulator would allow the tile to heat up faster because more heat is going up rather than down. He tested a couple products, and the clear, hands-down winner, coming in 40% less time than any of the others, was Wedi. (Interestingly, there was very little difference between the 1/4" and the 1/2". The guy that ran the test didn't understand why there wasn't a significant difference either, but we're talking about tenths-of-a-point level of small difference.) We went with 1/4" Wedi. The other thing we did - which was probably overkill - was because there are three different kinds of heat, and my nephew wanted to experiment with a ceramic (anti-reflective, anti-radiant) paint substance he read about. I understand the theory (waaayyy too involved to get into here), but the proof of this substance (SuperTherm) is not anywhere near substantiated yet. And dealing with the company sucked pretty badly (very unprofessional and difficult). But it is on half of my bath floor (we did a bunch of test to make sure it would adhere sufficiently to not screw up the tile) so I guess we'll see. The bath hasn't gone live yet and probably won't for another month. Don't let anyone tell you that you don't need it. Yes, the mat will work without insulation, but how well? How fast? (Yes, we are on a programmable timer. This was a big arguable point for the you-don't-need-no-stinkin'-insulation crowd, that the timed thermostat would cure all. That's fine as long as there are no exceptions to the original timer programming. But let me tell ya', my husband is ALL about exceptions. I swear he stays up late at night, trying to think up new ways that our routine won't suffice. He's adorable, but I need flexibility - in both lunch menus and floor temp.) I know this post is long, but your question involves some actual science (chem and physics), and knowledge is power, no matter what path you choose. And the amount of mis-information out there is staggering. And the surety with which people will tell you bad information is daunting. (And arrogant. And ignorant.)

I live in St Petersburg FL and my attic used to be like an oven. I used the cheap barrier long ago and it helped a little. I replaced it with a bubble wrap radiant barrier stapled to my roof rafters. That gave it a R value of about 12 and I installed two solar attic vents on the house to help remove excessive heat within this barrier. I attached 2x4 on the peak area of the roof in the attic and made it so all the hot air rises into this space. Then added an electric fan blowing outside air into the attic and the peak area. I added another fan on the other side of the house to exhaust the air out. My attic is cooler than ever. This makes my a/c unit that is in the attic to run less and keeps the house cooler. One more thing add another layer of insulation on top of anything that is there alreay. have it run the oposite way of what is up there now. Do you have an attic set of stairs or a hole with a lid. I installed a hatch cover with hinges and added a layer of insulation on top of it. This way when I leave the attic the hatch is closed and keeps out any hot air into the house and cold air in the winter.

If the manufacturer says it's OK over radiant, I'd trust that. Duro Designs told me in writing [via email] that their glue down was fine as well, though I don't know if they've changed the website to reflect that. When you think about it, there's no less flexible material than grouted tile, but that's commonly used over radiant. I think the reason there's so much trouble finding radiant approved flooring is that once the floor and heat system are installed, the homeowner is in the drivers seat: there's nothing to stop you from trying for a little greater efficiency by turning the water in the pipes up to 120 degrees if you like, and that can cause a lot of materials to fail. So the flooring industry tries to limit it's liability by saying 'fine over radiant, so long as the water in the pipes is kept at 85 degrees'. When you think about it, that's just silly: every floor gets hotter than that when the sun shines through the window or the fireplace is burning or near a heater. So why 85? Because it 'feels' warm to your skin. There's no materials testing, no scientific standard in play other than that. FWIW, I have radiant accross my main and upper floors. The tile is noticeably warm under barefeet, the laminate less so, the floating cork less, the carpet [despite special pad] not at all. I think a glue down cork will transmit heat at least as well as the floating, and would have used it if my heat pipes weren't the sort that rest on top the subfloor, rather than below. It isn't that the cork feels cool at all, it's just that the warmth doesn't make itself known in the same way. BUT be sure to insulate well below the heat: we all know that heat rises, but it also follows the path of least resistance. Cork is a resistor, a 'dam' for the heat, so you'll want an even bigger heat dam below it.

I'm assuming the cork will be UNDER the heat mats...and then the heat mats will have the hardwood OVER TOP? Cork is rated for in-floor radiant heat....but I always worry about putting electric heat mats DIRECTLY over a product that will CATCH FIRE! Yes. Electric heat mats have a long and glorious history of "shorting out" (although I must admit they are getting much much better) which can cause a spark. Cork doesn't like to burn but it CAN burn and it WILL burn if something is hot enough and allowed to sit long enough (like a sparking electrical wire sitting underneath hardwood). For that reason, I have always said that cork should be encased in concrete if heat mats are being used on top. I always err on the side of caution with fire ratings. The next thing you MUST do is to find out if the FELT material is rated for in-floor electric heat (many are not because they are made up of mixed fibres like PLASTIC = they can melt). And then, finally please check to make sure your engineered hardwoods are rated for ELECTRIC in-floor radiant heat. Many are NOT. Many ONLY ALLOWED the hydronic heating system (water/oil/gel filled tubing). I'm sorry to say but the BIGGEST problem isn't the cork or the hardwood or the felt. It is the ELECTRIC heat-mat system. More than 30% of flooring products on the market do NOT allow use with ELECTRIC heat-mat systems. The heat produced is often TOO HOT and the wires themselves are too delicate for use under many floors (delicate wires break = sparking = fire hazard). Ignore the heat-mat company's advertise. Investigate the OTHER materials used below/above the mats. I think you will be surprised what you find (or don't find...many do not allow this type of install). Personally if it were me, I would work with 1/4" cork and upgrade the wall heaters with lovely little baseboards that are efficient. The cork + new heating baseboards = wonderfully efficient. Save the $10K from the heat-mats and invest a little bit into upgrading the baseboard heaters. Cork will do the rest - both thermal and acoustic insulation all in one product. Skip the electric heat mats.