A number of years ago, I bought a goose down liner that was made to be an attachment to a Moonstone bag.  The down liner has a 8oz fill weight (which I believe is 800 fill). 

So my thought was, why not use this as part of a convertible bag/quilt.  Make or buy a summer bag/quilt that I could attach to it that would bring the bag down to a 20-30deg bag.  In addition, on the trail I could remove or add the liner as the temperature changes. 

If I made a quilt or bag, it may be practical to just use synthetic insulation as I probably would only save an ounce or two by using down.  (as it would only need to be summer weight, since I would already have 8oz of down in the liner.

I am planning on hiking the Colorado Trail this summer and had this in mind.  From what I read, it is good to have a 20 degree bag. 

So, how much insulation would I need for the outer bag in addition to 8oz goose liner to get me down to 20-30 degrees, suitable for the Colorado Trail?  Any ideas or thoughts?  Is this a dumb idea?

I know those liners well- they were from Moonstone’s 800-zone series and were popular maybe 9 years ago when they were sold cheaply after the line was closed out.  Many people tried to convert them to quilts, but they were a little too narrow for that to work well for most folks.

What you’re planning to do is a better idea.  IIRC there are little grosgrain snaps the went around loops inside the main bag.  This would be pretty easy to either put into a preexisting bag or design into something you make yourself.

I think you’re on the mark in thinking you need a summer weight bag in addition to that down liner.  I suspect something around 40 degrees would do it.  This would probably be somewhere between a 2.5 and 5 oz layer of XP depending on how cold you sleep, or a commercial summer bag either in down or synthetic.

Thanks for your help AYCE.  I have been doing some research, looking around. 

Looking at my options for use with my Moonstone liner, I could still make a Climashield Quilt.

Here are some pretty reasonably price summer bags on closeout/good prices.  I suppose the syntheic filling wouldn’t have to be too amazing as the goose down would do a lot of the work. 

Lafuma 800 40+ 29oz. $54.93 (Thermolite® Extreme filling)
http://www.rei.com/outlet/product/767275

Sierra Designs Cochise 45+  $54.93 35oz (Climashield® XP)
http://www.rei.com/outlet/product/784403

Mountain Hardware 45+ Ultralamina 24oz, sold on ebay for $90 today (Thermic Micro filling)
http://www.rei.com/product/747887

Or I could just forget about the the 2/1 bag and get this 20deg. lightweight 750 goose bag for only $115.

REI Sub Kilo +20 29oz $114.03 (750 goose fill)
http://www.rei.com/outlet/product/780903

Any one have any thoughts?