Sorry about this folks, but there was a heated debate a while back regarding pumice bricks vs. heavy silicate bricks to line up the fire chamber. Some folks contented that pumice (light weight) bricks were far superior in efficiency and would provide the best performances. As a manufacturer who use various material including the above I maintained it did not make a big difference and hence the can of worm and hot debate. I maintained that if it made such a big difference why would not all MFG use the light weight? Unfortunately I did not have the numbers to back me up then. Firstly, those advocating light weight bricks were right and ... wrong. Pumice bricks do have better insulating performance than heavy duty brick due to the amount of air in them. However, Our own internal testings between various material shows it hardly makes a difference between these 2 bricks on wall & floor of the fire chamber. Where it really makes a difference is in the baffle area. So there... While I'd love to share the hard data, it is proprietary so you'll just have to take my word for it.