Micore 160 or Micore 300 for hearth?

There seems to be some confusion about which one has a better R value. I found some comments on this board that say Micore 300 is better, however, if I read the Technical Data Sheets correctly, Micore 160 has better insulating properties:

Micore 160:

k-factor: .39
R-value (1/2" board): 1.27

Micore 300:

k-factor: .49
R-value (1/2" board): 1.03

To my knowledge, a low k-factor is good, because it means less conductivity. So it seems to me that Micore 160 would be the better choice. However, and I am also weighing those factors, Micore 300 is mechanically stronger, which might (I am not sure if it would) be a factor when using it under tiles (with CBU over it, over course).

Anybody have any insights on this? I'm right now facing the choice of ordering one or the other.

Thanks everybody.

BeGreen:

When I discussed Micore products with my supplier in Salt Lake City, he advised me to purchase Micore 300 because he felt it was a better quality product. Depending on specific R requirements Micore 300 can be obtained in various thicknesses, or for that matter doubled up.

Here is what US Gypsum says about its products:

Micore 300

Micore 300 board does it all. It’s a superior substrate for fabric and vinyl-covered wall panels, office dividers and tackboards, and an excellent core for chalkboards, stove boards and other similar applications. With outstanding resilience, superior machinability, and high density, Micore 300 board has the best surface hardness of any Micore board. The board’s smooth surface laminates easily with various finish materials such as vinyl, fabrics and steel. It is compatible with most water- or solvent-based adhesives.

Thickness 3/8 inch (.375)
7/16 inch (.438)
½ inch (.500)
5/8 inch (.625)
¾ inch (.750)
Tolerances Thickness .015 inch sanded and coated
Thermal Conductivity k Factor = .49
R Value (1/2 inch board) = 1.03

Micore 160

For superior performance plus savings, Micore 160 board adapts to a variety of design applications. Ideal for open plan partitions, office screens, dividers, tackboards and various other applications. The board combines lightweight properties with the strength and stability of heavier boards. And that means easier handling, simplified assembly and lower freight rates. Micore 160 board also has good sound control, reducing noise in open plan interiors.

Micore
Mineral Fiber Board 160

Thickness 3/8 inch (.375)
½ inch (.500)
5/8 inch (.625)
¾ inch (.750)
Tolerances Thickness 0.015 inch sanded and coated
Thermal Conductivity k Factor = .39
R Value (1/2 inch board) = 1.27

I agree with both of you. I ordered Micore 300 today, because it is stiffer, which is critical for tiles. It is important to understand, though, that it has a lower R value than Micore 160, contrary to what has occasionally been purported on this board.

I wonder why, by the way, in their product description, they actually advertise the "high k-factor" of the Micore 300. Anybody buying this for insulation would see this as a minus. But maybe there are other applications where a higher k-factor is desirable.

Micore 300 is slightly more dense than Micore 160. As with all insulation, higher density means less air infused into that material and therefore not quite as good as an insulator.

On the plus side, it is a bit more rigid (i.e. durable) and less likely to compress under weight than Micore 160.