atelier, I can't thank you enough for the thorough, easy to understand response!! That's exactly the information I was looking for.

We'll have radiant heat all through the house but the floors where I'm planning on using the cork will be over a slab

I'm not sure why the Duro rep said not to use the floating floor (I wasn't in a position to ask) but I'm ok with that because I wanted a glue down floor anyway. My only remaining question for you is this... we were hoping to do most of our interior finish work ourselves, meaning this floor project (among many others) will likely be done by us. When you say to be sure to hire someone with radiant floor experience, is this something we will be unable to do ourselves? We're pretty competent at things like this (hubby especially) so I was hoping with the correct installation information we'd be able to install this ourselves. Thoughts??

Thanks again for your time and information.

cork2win,
Actually, in my experience, homeowners installing their own floor are the best ones at following installation instructions and specifications. Because it's their own money at stake. Most people are careful not to waste their own money and time by ruining their own floor...

Cork is tricky and does demand a very smooth substrate.
It might take a bit of practice before you can match the work of an installer who puts in cork professionally- but it's not impossible.

If you educate yourselves on the proper installation techniques, follow manufacturer's instructions, and take your time installing, you should do well. If you have any doubts or questions about what you are doing- try asking to speak to a technical or architectural representative with the cork company. If you've done enough background research and are fairly handy, you should be able to get good information on any of your outstanding questions out of these people. The manufacturer in the end likes to see a successful install.

You may even want the availability of technical help over the phone to be one of the factors in choosing from whom you purchase your cork.

I was suggesting you use an installer who's done this before only because that's what I would do if I were hiring someone to do the job for me.

Good luck with the project. How big of a cork floor is this going to be anyway? (gives me an idea of how feasible this is as a DIY project)

atelier, thanks again. We're looking at about 750 sqft, although this includes cabinet space and I'm not sure if you put flooring under the cabinets. Kitchen, dining, living area. I'd love to do more but can't really afford it. It will be a stretch getting DH to agree to do these areas. I'm guessing he'll object to having cork in the kitchen due to possible flooding issues (dishwasher overflow, etc) and would prefer tile, but the thought of standing for hours on tile just makes me cringe. My big attraction to cork is the softness underfoot, and the sound deadening properties, not to mention it's "green" nature.

You don't need to put in cork under the cabinets.

750 sqft is a fair-sized project for the DIYer, you'll get some good practice.
You might want to get some pointers about layout and chalking lines to make sure your lines run straight.

Go for cork in the kitchen- you won't regret it. A flood is just as likely to damage any flooring and your subfloor.

The reading I have done and from what I have been told, all say that the radient heat temp. must not exceed 82 degrees.

Hi, I am thinking about laying a cork floor in a kitchen/dining room and would like under floor heating - are you happy with how it all worked out or would you advise against laying cork over underfloor heating?

Cork+infloor radiant heating = inefficient heating system = increased price to run the heating system = waste of time/money/effort.

As a former cork flooring professional I have always stated that cork over in-floor radiant heat is a waste of time/money in the long run. Most people who want in-floor radiant heat want to remove the "cold" feeling on their feet and legs. Cork does this NATURALLY. Cork by itself, will do what most people want = take the hard/cold off the feet/legs.

If you want in-floor radiant heat then please choose stone/tile/concrete/porcelain. They do the BEST at heat transfer. Closely followed by carpet and thin engineered hardwood or thin laminate (less than 8mm).

If the "radiant heat" is just to keep the floors warm (floor warmer and not a whole-home system), you are better off leaving it off the list entirely and going with cork - all by itself. It will do everything you want it to do....and save you $10K in the process.

OK, BUT if installing cork over a slab which is directly on soil which is 50 degrees is the cork still "warm"?? Wouldn't you want the concrete under the cork to be warm? I now it is different than tile as tile is always cold. I don't want to waste money, but also don't want to regret not having radiant heat.

Cork does not give off heat, it STORES heat. That is what an 'insulator' does. That is to say your WARM FOOT steps on cork and stays there for, let's say, 5 seconds. During that 5 seconds the floor feels 'warm'. In other words it does not feel 'cold'. The cork is storing and then REFLECTING the body heat BACK INTO the foot. Thereby feeling warmer.

Compare that to concrete which is a 'heat sink'. That is to say heat goes into concrete and never comes out. Concrete removes heat from where ever it comes from. The heat 'sinks into' the concrete never to be heard from again. That is the definition of a 'heat sink'.

If you step on concrete with a bare foot, it feels chilly. If you STAY on concrete (ie. stand on it for 5 seconds) it begins to feel COLD. Not just chilly, but ICY COLD. That is a heat sink.

You are welcome to heat the concrete below the cork. Feel free. It won't hurt the cork. It will hurt your heating bill but not the cork. The cork will gather up the heat and slowly release it into the air. Slowly. Very, very, very slowly. Meanwhile the concrete underneath the heating tubes is going to gobble up that heat faster than the boiler can produce it.

In other words, the cork on TOP will stop the heat from ENTERING the house. A little will but not much - maybe 40% of what is produced. At the same time the concrete is going to eat and eat and eat all that energy you just paid for. And that heat will go into the ground BELOW the concrete. You will be heating mother nature.

For that very reason, cork is used UNDERNEATH heating tubes (in a concrete slab) as an INSULATOR so that the concrete does NOT absorb the heat. This helps radiate the heat upwards....into your nice hard tiles. Or concrete. Or engineered wood. Whatever the finished floor is.

Go ahead and use cork over top of hydronic heat. Be careful that the cork doesn't OVER HEAT and thereby MELT the glues holding it together. I've seen it happen. It isn't nice. The homeowners weren't getting the warm air in the room they wanted (it could only get to 19 C and that wasn't good enough). So they kept turning up the boiler. The room never warmed up (the floor was almost too warm). The glue then warmed past 28 C and the cork floor started to liquefy. It became cork oatmeal inside of a year. They were not happy.

OK thank you for that explanation and information!!!

Great information thanks

Not really great information. There's nothing wrong with putting cork over a radiant floor heating system. But it's all part of a system- if 4mm of cork is going to virtually stop your heating system from delivering heat- then you've got bigger problems- and it has very little to do with the cork floor. And good cork tiles won't "melt" - they barely even burn when thrown into a fire. By the way I'm also the owner of a hydronically heated house in Quebec with cork floors- 23 years and counting..

The binders that hold cork together have a maximum heat tolerance of 28C. The adhesive to keep the glue down tiles (4mm) of cork in place have a maximum heat tolerance of 28C. There are some adhesives that allow for higher temperatures but they are harder to come by because of air quality requirements in North America in the past 25 years.

I have now laid a cork floor over electric underfloor heating in my office and the floor is not cold but the room is. In other rooms where I have other floorings, the underfloor heating heats the room, it does not under cork. It looks lovely but Jesus I am happy I didnt lay it in my dining/ living area. I would NEVER recommend putting under floor heating under cork, it is next to useless

Most 'wood based' floors should NEVER be installed over top of electric in-floor heat. Whomsoever allowed the cork over the electric should be asked why they did that. A responsible cork flooring sales person (there are only a dozen or so in the USA) would NEVER allow their cork to be installed over ELECTRIC.

I'm sorry to say but your cork flooring warranty is now void. If you turn off the electric in your office and plug in a small ceramic heater you will have the warmest room in the house. The cork will keep the heat IN the room. Which is why the heat won't push THROUGH the cork from the outside.

I never switch the electric on as there is no point

Stephanie, I honestly don't know where you are getting these numbers from. Or it could be the products you're familiar with aren't very good. The shear and tensile strength testing data sheet I have for the adhesive I use had testing temperatures that went all the way up to 107°C, and the glue was still holding. It has a V.O.C. of 37g/L. It can be purchased at any decent wholesaler across North America. The typical cork binder in an agglomerated high-density cork tile is an inert polyurethane polymer and constitutes about 5-7% of the tile. Most polyurethanes don't break down until about 90°C. I have to chuck a piece of cork tile into boiling water before the binder softens, but the tile still mostly stays together.

From the Portuguese manufacturer and the manufacturer of Wakol W3540. As you know, the engineering community likes to be super safe. I'm sure the ACTUAL numbers are slightly higher.

I'm not sure which adhesive you are talking about, but I'm talking about Loba-Wakol.

No engineer specifies a safety factor of 3.0 - i.e. 300%

Take another look at your numbers so you can avoid spreading misinformation.

Wakol D 3450 is a fantastic product and is approved for radiant floor heating. It says so right on their Product Data Sheet.

Amorim, Supra, Granorte all approve their cork tile glue-down installations for use over radiant floor heating. I don't know about the other Portuguese producers, but I assume by now they are all using about the same binders.

Wakol D 3540 is rated for hydronic in-floor radiant heat so long as the cork-substrate interface does not exceed 28 C.

And If that is false then their chemist lied to me...He now works in Switzerland so it will be hard to contact him. He was ADAMANT about the 28C as the maximum temperature for D 3540.

https://www.corkart.pt/canada/wp-content/uploads/sites/52/2015/05/Corkart2-CorkTiles-Installation1.pdf

CorkArt does not recommend their glue down cork tiles over in-floor radiant heat. In this situation the manufacturer of the cork restricts the use of the glue down tiles over in-floor radiant heat...not the adhesive manufacturer.

Please let me know which adhesive you are speaking about that you used 20+ years ago.

Sure the recommended maximum operating temperature of a radiant floor heating system under a cork floor is 28°C or 82°F across the board.

What I had issue with is that you were suggesting that this was also the magical point at which everything starts to melt and furthermore that putting cork over a radiant floor is a very bad idea and a waste of money. Those are incorrect assertions.

I know how to successfully install cork over radiant floors. I have several installations over 2 decades to prove it.
When the heating system is well designed and the floor is installed according to specifications, there is absolutely nothing wrong with installing cork tile over a heated floor. If the system creeps up over 28°C once in a while - it's not the end of the world and the heating controls should be adjusted.
So if CorkArt doesn't want its cork on radiant then that's up to them- it just doesn't represent the position of most of the major cork producers. So I wouldn't use their product, but that's OK I have other manufacturers to choose from.

To answer your question: 23 years ago and today I use 3M Fastbond 30NF (and occasionally D3540)
My 23 year old cork floor over a hydronic system is still fine and still hasn't melted.