Good questions from @oreoceruos, and good answers from @retiredjerry.
My situation is this, being from SoCal, and mostly camping here, my kit is tailored to warmer and dryer climates. No need for a synthetic overquit here, but I want to venture further away from home, and trying to learn as much as I can from others in advance.
I hope to use the gear I have as much as possible, and Ryan’s suggestion to another camper with a down quilt was to supplement it with a synthetic overquilt. This was for the reasons Jerry explained, that at certain temps the condensation would occur in the synthetic and not compromise the down insulation after days of cool/cold and damp weather.
Like Jerry, I’m speculating on the temperatures that I would be camping in outside my area of expertise. I would like to do a fall trip in the Sierra in the next 2 months, which will have cool, but probably dry nights. My goal for some trips to the NW might not be in winter initially, so maybe not needed there. We do get the chance to snow camp at altitude if it’s not a drought year, so maybe that’s the scenario for me to use my 30*F or 20*F quilt and take it down to between 0* and 15*F with precipitation. The only chance to snow camp in SoCal is one of the local mtns (Baldy, San Gorgonio, or San Jacinto) – when it’s snowing. When the snow stops is soon melts.
Since I’m basing this on Ryan’s suggestion, I have to report that in a recent email making recommendations from the REI sale, Ryan said: “When temperatures get into the teens (and especially, into the single digits), I start moving away from quilts and towards traditional mummy bags.” I have an old Western Mountaineering Super Apache DL mummy bag – 15*F with (I think) a Pertex shell. It’s heavy at 46.3 oz, and I’ve really moved on to quilts, but I could never give this one up. Maybe this get’s the nod if I’m heading into a snow storm rather than a lighter down quilt and synthetic over quilt.
Since this is BPL, the question is of course, which is lighter? In my case an HG Burrow 20F @21.1 oz + an Ikea synthetic duvet @23 oz is pretty close to my 15F sleeping bag @46.3 oz. I’m still interested to know if there’s a commercially available overquilt that is lighter, that could allow me to take my 20F HG or 30F EE quilt down to the 0-15 range.
Thanks as always for the advice.