Layering light down jacket over a down vest

I have done that at camp using a WM Flash jacket and a very light vest under it. ( only to about -7c / 20f)

I do synthetic vest, down vest over that, GTX over that, quite warm

Like you said, if the inner down is compressed it won’t be as warm, although some compression doesn’t totally ruin it.  For the same reason that over stuffed down will add some warmth although not as much as if it’s totally lofted.

I do the above inside my down sleeping bag and that’s very warm.  Plenty of room inside down sleeping bag so the down vest isn’t compressed too bad

I routinely wear a down jacket over a down vest and then an oversized Houdini wind shirt thrown over the whole thing.  Very cozy and down can be compressed a fair bit without losing much insulative power.  You can overstuff up to 2.5x and maintain the majority of warmth.  There have been a number of threads discussing this very thing.  The one with Richard Nisley chiming in is the most illuminating.

We’re having a conversation about this very thing here—

https://whiteblaze.net/forum/showthread.php/135646-Feeding-An-Underfed-Vest

I think the phenom is due to the fact that the vest hugs the core and provides torso warmth in the primary spots needed—and then the down jacket seals it all in along with the arms.

For this reason a good down vest could supply warmth while moving thereby keeping the arms free with more ventilation (thereby obviating the need for a bulky fleece outer layer)—countering the usual advice to never try backpacking in a down jacket due to excessive warmth and sweat.

Vest + puffy Jacket can be a great combination. I have used this combo with both garnents being down, and a mix of down and synthetic. Even though the interface between the vest and insulated jacket are compressed, I didn’t notice a drop in warmth.  There have been studies which found overfill and moderately compressed space did not significantly drop performance, but I don’t remembers where I saw this study / data.

If I may inject a slight, related detour:

Has anyone used the Uniqlo down vests?

They are very reasonably priced and I see them referenced on bike packing sites because they can compress down fairly easy. I use their t-shirts…no complaints but have never bought their outerwear. Utilization, would be as described above, or thrown on during a rest break while hiking or riding.

Not the vest but I do own one of the UL parkas

Quality varies from year to year and sometimes the UL isn’t all that ultra light. +1 for the shell fabrics compared to mine for instance

I am happy with mine but when I went to purchase a second and a vest the new ones were heavier with lower loft. Also they are cut very small and in Asian sizes so my XXXL just fits over my Patagonia Cap 4 winter base layer [ XL]

I wonder if in addition to the increase in thickness of insulation there is another benefit to the vest? Unless your down jacket is a very snug fit there is likely to be some air movement into and out of the jacket via the hem line and neck opening. A puffy jacket would help fill that space, limit air movement and reduce heat loss.

I have a Uniqlo UL down vest, it’s a steal at the regular price and I got on sale to boot. The fill is a little uneven between baffles but not bad, and the zipper works the opposite side (opposite for men’s styles that is). Otherwise well made, my size M weighs 5.5 oz and has about an inch of loft (the style with zipper, zip pockets and a collar, they also have a style with snap front and no collar). At that weight and loft I’m guessing the fill power is about 700. I layer it under my lightweight down puffy around town, makes for a more versatile insulation system than one bigger puffy as I can wear just vest, just jacket, or both. Haven’t had the need for it on the trail yet, but I’ve added the vest to my gear spreadsheet so it’s ready to be added to my baseweight calculations, expecting I’ll find it useful to take it on the trail.

And to another point made, my down puffy does have some room under it to allow for layering, and the down vest fills it nicely with no noticeable tightness or compression.

My understanding is that you really have to compress down quite a bit to lose warmth. That is why some places “overstuff” sleeping bags (and presumably jackets). It is still warmer, even though obviously the down isn’t as loose. Maybe somebody has a chart, but my guess is that you really have to squish it (from sitting on it, or cinching) to really cut down the warmth of down. So, for example, a jacket with three inch loft is probably 95% as warm if you squeeze it to 2 inches and 85% as warm at 1 inch. You probably aren’t coming close to that level of compression, which is why you are likely to be toasty warm. Not as warm as if everything was loose, but still very warm.

From memory Richard Nisley posted about compressing up to 50% with little or no difference in heat retention.

After putting in down to fully loft, additional down can be added but it will be half as effective

Up to 2.5 X total, then adding down doesn’t add warmth

An approximation of Richard’s data, actually it’s a curve but that’s close

For example, if you have 3 inches of loft that is fully lofted and you squish it to 2 inches, the first 2 inches of down (of the 3 inches) is fully effective, but the additional 1 inch is only 50% effective.  So, the total warmth would be 2.5 / 3 as warm as if it was 3 inches of loft.  2.5 / 3 = 83% the warmth as if it had 3 inches of loft.

If you squished 3 inches down to 1 inch, the first 1 inch of down would be fully effective, the next 1.5 inches would be 50% effective, the last 0.5 inch would add no warmth.  1 + 1.5 / 2 = 1.75. divide by 3 = this would be 58% as warm as if it was 3 inches

That’s only an approximate rule and the original 3 inches of loft was probably a little overstuffed to begin with, but that gives an idea