Thin underlayment for Coretec Plus?

6 years ago

Hey all. We will be installing 400 sq ft of Coretec Plus HD (8mm total, 1.5mm cork backing) on the third floor of our home. I'm considering laying an extra 1mm IXPE underlayment down. I know most say it's not needed, but the premium line really got me thinking a bit extra is nice. This floor will be playroom and guest bedroom, so softness and sound protection are definite pain points. 1mm seemed thin enough not to interfere too much, but just give us a bit of extra help.
The question is, is this a terrible idea? If not, the 1mm IXPE I found doesn't appear to have adhesives. Is there something I should be using for the seams? Do I need to secure the underlayment to the floor in any way?

Featured Answer

SJ McCarthy

6 years ago

CoreTec Plus does not ALLOW for it. As soon as you use it you void the warranty. The 1.5mm cork underlayment is as thick as vinyl can handle. Vinyl is brittle and thin. That means the click-edge is SUPER thin and VERY delicate. The moment you have a snick more movement (bounce from a nice squishy product) you run the risk of the edge breaking and your floor coming apart at the seams.

This is the single greatest drawback with vinyl. It just cannot handle underpad. It is not ALLOWED underpad if it already has it attached to the back. And the maximum thickness it allows (attached or added) is 1.5mm.

I'm sorry but your choice means you cannot use extra underlay. Besides, cork and IXPE make a HORRIBLE rubbing sound. It's like the sound of rubbing two balloons together. This is the most common complaint from people who add underlayment to underlayment. It makes some nasty noises. And sometimes (just sometimes) you can create an odd odour. Some chemicals react by creating an odour when the come in contact with each other. This is rare but not unheard of. And I've heard of this happening with cork and some artificial products.

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Comments (7)

Nicholas Timm
Original Author
6 years ago
Thanks for the comment. I eas wondering about the cork vs IXPE mix. The lroduct sheet does allow up to 3mm of additional underlayment.

"A second underlayment is allowed under any currently sold WPC product with attached underlayment in a residential

application. If installed over a second underlayment, this underlayment cannot be greater than 3 mm thick. IIC (ASTM E492-

09) and STC (ASTM E90-09) lab testing on certain WPC products tested with and without a second layer of underlayment, to

date, does not indicate that a second underlayment will provide additional acoustic benefit."

I really don't think I would want 3mm. For the reasons you stated, Inwas hoping just to get something really thin down. Not sure if they make anything else less than 3mm?
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SJ McCarthy
6 years ago
If you want acoustic then work with laminate and 6mm cork underlay. Or you work with WPC that does NOT have the underlayment and purchase the really expensive mass-loaded vinyl that is 1.2mm thick. The noise abatement is superb....for rigid surface flooring.
kaiegirl123
6 years ago
Hi @SJ McCarthy, the Coretec install instructions (https://pdmsview.shawinc.com/viewer/doc/5089) mention underpayments (the last 2 bullets in I. General Information on first page) in your experience are these typically added when moisture barrier is needed?
SJ McCarthy
6 years ago
A moisture barrier is not underlayment. Underlayment comes in two categories. For instance plywood or OSB are underlayments that are permanent. This is different from underpad type products such as 3mm cork underlayment (aka underpad). Cork is not a vapour barrier. And a vapour barrier is not an underpad. The two can be two different categories of product.

When it comes to attached underpad, I've seen (and heard) the 'rubbing' that can occur between the attached underpad and the extra layer of underpad. It sounds like two balloons rubbing together. It is highly annoying and it can be there for the life of the floor. Every time you step on the floor it gives a bubbly squeak. This is prominent in the morning hours or when the house is cool.

If you need a vapour barrier, the 6mil poly sheeting and moisture resistant seam tape is all that is needed. This is industry standard and it is cheap like borscht. If you think you need more underpad, then go for it. Try to stick with something as rigid as possible. And just remember what has been explainted: the two pads can and will rub against each other thereby producing a very upsetting noise.

I'm someone who doesn't believe in adding costs where it isn't needed. If a vapour barrier is all that is needed AND your vinyl plank already has an underpad, then I would leave it at that.

If you must have greater acoustic numbers then you want to work with something other than vinyl.
Denna Neff
5 years ago
@SJ McCarthy My CoreTec Luxury Vinyl Plank was installed throughout my "new" 1989 house Q1 2018. About 4 months ago, something that looked like "glue" (to me) was coming from the seams of the planks. THe original Contractor stopped by today to give it a look and he advised that CoreTec does not use glue, so I'll just call it "gook & moisture." It's awful and I have to SCRAPE the gook to try and get it clean but it's getting worse and I cannot keep up with it.

The Contractor advised that CoreTec does not use an underlayment and, from what I interpreted from your reply to another, that an underlayment is NOT the same as a vapor or moisture barrier.

My 1-1/2 store house is on a slab, stucco, built in 1989.

This issue began about 4 months ago and is increasingly getting worse. The worst areas are far from the walls that are against the exterior of the house (hope I described that properly.) For example, the worst seepage of moisture and gook is in the hallway which has a bedroom between the exterior wall and the hallway. Meanwhile, the seepage of moisture and gook is growing worse as time goes on and spreading slowly throughout the house.

While I have several dogs, they are not in the area of the foyer and hallway because I have baby gates so they are restricted to the living room that goes into the sunroom that has a doggie door to the fenced back yard. They have lots of room and no need for the rest of the house and that helps me keep it clean.

Would appreciate your thoughts. I know the company will try to find an issue that would not be their liability. I really just want what is fair. I'm going to have to have the entire main level re-floored. It's a 1-1/2 story on a slab, as I described above.

Thank you!

Denna Neff

Independent Senior Lady, 76 years old. :)
ezcamins
4 years ago
Did you end up using the 6mil poly + an additional underlayment for Coretec Plus? We are getting ready to install this in our basement and torn on the additional underlayment.