Conundrum: UFH Tog values and Underlay for Carpet

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We have installed between joist UFH, with fibrous boards above into two bedrooms. The boards (0.4 tog) + carpet (1.6 tog) have a total tog of 2.0. We've been recommended not to go much above 2.0 and that it stops working at a tog of 2.5.

So I'm thinking of either:
1) no underlay.
2) minimal 2mm underlay that adds 0.2 tog.

I'm looking at this. https://www.carpetyourlife.com/en/about-carpet/how-to-lay-carpet/do-i-need-carpet-underlay It says if text backing, then underlay, if ufh no underlay.... So a contradiction.

If we really need some underlay, I think max we can do is 0.2tog (2mm). The 2mm underlays say they are for laminate or hard flooring not carpet, but probably better than no underlay at all? This underlay has a tog of just 0.1. So we could add this, better than nothing, and we are well within reasonable over all tog values:
https://www.tradepriced.co.uk/duralay_low_grab_lvt_underlay.html

Our carpet is textile backing (lano smart strand forever), so trying to figure out in a heated bedroom floor, if we'll really notice the "uneeven" surfaces? Uneeven of what, the floor substrate or the carpet itself?
 
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Had some more feedback else where. The combined carpet + underlay 2.0 to 2.5 guidance is based 22mm chipboard over the heat source. By using 18mm Gifafloor (fiber board) we reduced total tog by around 0.6 (0.45 tog compared to 1.05 tog). Carpet manufacture says that actually the variant of carpet we want is 1.47 TOG, not 1.6. So we might be good for standard 0.75tog underlay - is this the best https://www.interfloor.com/underlay/duralay-heatflow-carpet/ ?

Do my above figures look reasonable?
 
That carpet and Duralay Heatflow for carpet will be about right. Don't expect much feel from the underlay, it as a very firm and dense product but is necersary when laying that carpet and will allow heat through while supporting the carpet correctly.
 
That carpet and Duralay Heatflow for carpet will be about right. Don't expect much feel from the underlay, it as a very firm and dense product but is necersary when laying that carpet and will allow heat through while supporting the carpet correctly.
I'm currently looking at 3 different underlays. King, Heatflow and Olympic 5.

King is 0.8 tog, 8.3mm and 42db and Heatflow 0.75 tog, 6.1mm and 31db. So for the sake of 0.05 tog, between those two, I think the King wins hands down.

So that narrows it down to King or Olympic 5 which is 0.5tog, 5mm and 23db. While 0.05tog between King and Heatlow would be insignificance, the King to Olympic is 0.3tog. I'm trying to understand how much better performance that will give for the UFH, compared to noticeable comfortable of the King. I have no experience of carpet underlays to make this decision, and erring on the side of UFH performance. I have samples on the way, to see if I can notice much comfort difference. if I can't notice comfort, I'll stick with UFH performance.
 
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The firmer and denser the underlay, the better the wear and pressure performance of the underlay.

The deeper and softer the underlay, the better the softness but at the expense of long term wear and appearance retention of the carpet.
 

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