I have a Speer Snugfit that I'm using with my double layer WBBB and I was looking for an option to extend the range of the SnugFit if needed. Most of my camping will be at or above 30 F, the rating of the SnugFit, but if I'm out when the forecasted low temperature is right at that mark, I want to plan for slightly colder so I'm not surprised. I've caught a few posts regarding using an Insultex (IX) insert, but it wasn't clear to me exactly what I should expect out of it or the best way to use it.
My plan was to either put the IX insert between the layers of the WBBB or between the Snugfit and the WBBB. Would one of these be these best way to utilize the insert or would there be a better approach? Would this compress it to much to provide much insulation? Would it be better to just use a plastic bag as a vapor barrier for what I'm trying to do and skip the insert?
Realizing warmth can be somewhat subjective, what can I realistically expect out of a single IX insert? I'd like to be covered down to 20 F.