Ideal Fabric for Summer/Liner Bag?

I took delivery of my first down bag Tuesday, and I was thinking of turning it into a flexible all weather system by adding a liner (for extreme cold) that could also be used by itself in warm weather. Something like this would be ideal, but I was hoping I could save a little $$ if I made it myself.

The question is, what light warm fabric would you recommend for this? I know fleece is common for this, but I'd like to know if there's something warmer for the same weight or just as warm for less weight.

You might want to consider an over-quilt rather than a liner. A liner will occupy internal space in the bag and in my experience is just something in which to get tangled if you are any sort of an active sleeper. You could make a down over-quilt from the kit that Thru-Hiker (www.thru-hiker.com) sells. There are plans and instructions available on the web. And, in warm weather, the over-quilt may be enough and could save some weight. I made one with a head-hole in the middle. This way I use it as an insulated poncho also and save the weight of a fleece or insulated jacket. The one I made is baffled and weighs about 17 oz with about an inch of loft.

That sounds really nice, but it's way over budget. Plus, a liner has the benefit of helping to keep the bag clean.

ETA: In my case, it's also a benefit to take up some of the internal space. I'm really thin, so I've got extra air space to heat.

I have to second Pika's experience with a liner. I tried one once (trying to extend a 32*F bag and keep it clean), I got so tangled up in the thing that it took me several minutes to get out of bed. After a couple nights of that nonsense, I got rid of it.

I usually don't have to wear my baselayers when hiking, so they stay relatively clean, and that's what I wear to bed.

Most people try to extend their sleeping bag's warmth by wearing all their insulating clothing inside (which will also take up more of the space inside. I have to do this when the temp gets down to the low 20s F. Some deliberately try to extend a lighter sleeping bag by investing in puffy (down or synthetic) pants as well as a puffy jacket with hood to use for sleeping. I personally don't approve--I want an extra allowance for really cold nights (like the time it got down to 15* in the Cascades in September). Remember that a warm cap, warm socks and a well-insulated sleeping pad are needed to keep you warm. The EU test for sleeping bag warmth for 20*F has the "dummy" wearing base layers, a knit cap and on a pad whose R value is 5.

A fleece liner will probably add at least 12 oz. to your bag. I have no idea of how much it helps the temperature rating.--you'll have to try it. Forget silk; there's no real warmth to it.

Are you sure you don't want to return the bag and exchange for a warmer one?

Yes. Strictly speaking, there's no need for a warmer bag in this part of the country except on very rare occasions (on the order of once or twice per multiples of years), and I'd only go out on those days for the novelty of it, and only if I knew my equipment could handle it. So, the liner part of the equation is really a just in case. If it couldn't also be used as a summer bag, I wouldn't even entertain the idea.

make a quilt! 2.5 oz Climashield Apex sandwiched between two pieces of breathable nylon.

I just looked on Ripstop by the roll.
$10 per yard of 2.5 oz Apex
$4.25 per yard for 1.1 oz ripstop.

So, 2 yards of Apex and 4 yards of ripstop = $37

Don't want to sew it yourself, look at a military poncho liner, otherwise known as a woobie.
poncho liner

The Thermolite is overrated in my opinion. I've used mine in winter and summer and it kind of failed at both. As a liner, it added maybe 5 -8 degrees to my bag. I generally sleep warm and have lived in cold climates my whole life, including working outside in sub-zero temps on a regular basis. The liner really added very little to warmth, far less than wearing my down puffy or a fleece jacket to sleep in. As for the tangling issues, we won't even go there. Suffice it to say, after one midnight bladder relief session, it ended up outside the tent and stayed there. As a summer bag it was OK, but on a 60 degree night I threw my rain poncho over me to keep off the breeze enough to sleep. That happened twice, so I'm a little leery of using it again when temps could be a little cool. I've been eyeing that woobie and they look like a much better solution for a winter summer dual purpose item.

I have an MEC Emperor Penguin overbag. Relatively light and it adds maybe 8F degrees to my +23F rated down bag. Ray Jardine sells quilt kits. Never seen one in person, but worth looking at. He uses a synthetic insulator. I'd try a quilt before a liner. I've used my down parka like a quilt, but it weighs as much as many bags and cost more than most. It's the one I'm wearing in my photo.
Before making something, I'd look for a used quilt on a site like BPL, here, hammockforums, one of the other UL sites or eBay.

You guys have convinced me that a quilt is the way to go. Since the weather has warmed up so much though (hopefully no more cold snaps), I'm probably just going to hold off until fall.

I've got another overnight, my second, coming up on Thursday, and I'm thinking of doing another on Saturday night (yes, two separate trips back to back). I'm definitely only going to use my new bag with full length sleeping clothes and a cap to keep it cleaner. Here's hoping it's not completely overkill, as the nights are already much warmer here. The predicted lows for each night out (in each place) currently stand at 67 and 52.

If you're only going overnight, assuming you bathe the day before you leave, you won't get the sleeping bag that dirty even if you have to strip (I suspect the cap will be too much). It's those multiple-day trips in which we get really greasy and ripe!

You can always zip open the sleeping bag and use it as a quilt, or even just sleep on top of it if it's that warm.

Some of us get more ripe than others...