Hey DupNorth,
First, my apologies for not jumping in sooner!! I have to be honest, this thread generated a lot of really solid responses from subscribers and visitors, and I found myself hanging back since the community was doing such a good job covering the ground. But I didn’t want you to have to make a purchasing decision without at least some additional input, so here goes.
On the foam itself, it’s worth knowing exactly what you’re working with… the HD36-HQ is a conventional open-cell polyfoam with a density of 2.8 lb/cubic ft and a 35 ILD rating, putting it firmly in medium-firm (leaning firm) territory. The upside of that density is durability. FBM’s own data sheet puts the typical lifespan at 12–18 years, so it’s not going to break down on you. It also has a support factor of 1.90 and 45% resiliency, which is decent bounce-back for a poly foam (though still well below latex). It’s not latex and does not incorporate the additives to classify it as memory foam, just a well-built, durable conventional poly foam. Now here is the caveat.
On the convolution question, it is important to understand what actually happens in that process. A full slab of foam is run through a convolution machine which creates two identical pieces with peaks and valleys, each piece the same height but with only half the foam material in each piece. This is a really important point that often gets overlooked. Because those peaks and valleys have removed half the foam from each piece, you now have significantly less foam bearing the load of your body weight. The peaks especially, being thin points of foam, will break down considerably faster under repeated compression than a solid slab of the same height would.
So while the convoluted HD36-HQ may feel softer initially at the surface, it will not have the same durability and longevity as the original solid slab, and it will not perform with the same support characteristics over time either. The “20-30% ILD reduction” you may see cited around the forums is really an oversimplification of what is actually a more complex change to the foam’s performance. If you are comparing the convoluted HD36-HQ to a genuine 28-30 ILD 2lb foam from another vendor, neither option will match the durability and long term performance of the original solid HD36-HQ slab.
A few other things worth thinking through as you decide:
Position in your build matters. Sandwiched between HR foam and soft Dunlop latex, this layer is really acting as a transition/support layer. The latex above it will heavily influence how it actually feels in use, so the convolution likely makes good sense there for both softness and airflow. But reduces longevity and durability.
FBM’s recommendation carries weight they know their own product, and if they specifically suggested the convoluted version for your layering, that’s meaningful even if they couldn’t quantify the ILD reduction exactly.
Convoluted seems like the lower-risk choice given it came as a direct recommendation for your specific setup. Worst case, a layer swap is always an option later. You just need to understand if you are using a convoluted layer, it does not change the density 2.8lb or the ILD 35, but the foam is not measured the same way, so the feel will be different. When the ILD is measured, a solid block of foam is used, not the convoluted piece. So the two pieces are still 2.8lb and considered an ILD of 35 (or original) but does not feel the same under load.
Hope that helps, and again sorry for the delayed response, you asked a great question and deserved a quicker answer!
Maverick