I have a cheaper down bag, a Kelty Cosmic. It weighs in at 2lbs 8oz. I'm ok with the weight, but the sack size is about 8"x 16" or so. That is the stuff sack that came with the bag. The bag is a 550 fill. The question; will it hurt the bag to get one of those compression bags with the straps to "suck" that dude down to a smaller size when on trips?

Compression won't hurt it. Overcompression, until the bag is like a rock, probably would.

As long as you don't store your down bag in between camping trips compressed you'll be ok. Keep it in a big roomy breathable bag, or hang it for storage. Your options for packing it for a trip would be to purchase or make a smaller stuff sack, or get the compression ties which will be adding a tad bit of weight.

Why do you want to do that?

Bill, 'cause that thing don't just stay shoved in there. It is a 550 fill bag, so it has more stuffing. I'm not asking to make it a baseball, it just needs to stay a little smaller to make getting to stuff and putting stuff back in such as lunch without so much hassle.

Tye
An often used technique is to use a larger stuff sack than the one provided, then to put the SB at the bottom of the pack and let the weight of the other items compress the SB down taking whatever space is available.
If you use a compression sack you end up with the shape of the sack and that may not correspond to your available space , therefore wasting some.
We call it the bowling ball effect.
These are a couple of pics I took to illustrate the point.


(the bricks simulate heavy items like food or water, no I don't take bricks with me hiking...)
Note that the items inside the clear bag are the same except on the second shot I have the bricks and some spare space left..

Franco

I stuff extra clothes or my down jackets in the stuff sack with the bag and compress all of it together. I tend to NOT put stuff that can potentially leak in with it though.

Cool demo with the clear bag and the bricks. Makes perfect sense.

Thank you Franco, that's exactly why I asked, and your demo perfectly demonstrates why a tightly compressed package doesn't always give you more room in your pack. Plus, it's easier on the down in your SB.

Tye, I just learned this myself this year from Franco and others here, and I was pretty amazed at how much more stuff I could fit in my pack. Give it a shot and let us know how it works for you.

I had a nice expensive bag get less lofty after just one use of a compression sack. I would just get a smaller stuff sack, but not a compression sack where you are really crimping the feathers....

OK, dang pier pressure. I'll hunt up a little smaller stuff sack.

BTW, the idea of letting soft gear get compressed by heavier gear and therefore filling the avaliable space, isn't mine nor new...
Just something I do but was done well before compression sacks came about..
Franco

I have 3 down bags of different temp. ratings. Two of them are decades old and have been stuffed in all sorts of ways and sizes. I do keep them un-stuffed in large bags when in storage. They still perform very well. Which brings me to another subject. When you decide to purchase another sleeping bag remember that it will last you a very long time so make a good choice concerning weight and warmth. You will not regret spending a bit more for good quality when you sleeping out in the wilderness.

Nice post Franco. I will heed your advice as I just got my first piece of Downy goodness. I will be taking the bowling ball affect in to mind. Thanks

Don't compress your SB. You'll probably damage 550 down and create a hard spot in your pack. Just drop your SB at the bottom and the other gear will compress it. Your pack will also be more cushy.

Just be sure you have a waterproof (test it at home) pack liner before doing that. A soggy sleeping bag (regardless of insulation) is not a good idea!

With down have given up stuff sack all together in favor of just dumping it loosely inside backpack encased in light bivy sack for protection.

I used compressor sack extensively with winter synthetic. This practice degrades synthetic insulation at an ALARMING rate, but is basically necessary with monster minus-twenty synthetic insulation.

Down is more durable and MUCH more compressible. Why bother unless you're talking a real artic-style down bag?

And if you've got such a bag, why take a chance on wrecking a $700 piece of equipment? Treated with care, down lasts a very long time.