Or, as you note, the Galaxy's expansion feature (a zippered gusset along the side allows the user to add up to eight inches of girth) gives you room for an extra insulation layer. Mountain Hardwear's Down Upgradeessentially a light down bag that can be inserted or zipped inside another bagsells for $180. By itself it's rated to 40 degrees. What it would add to the Galaxy's temperature rating is a little hard to estimate, but I think it's safe to say the Galaxy and the Down Upgrade would work well down to minus five.
Good enough for a Denali summit attempt? Probably not. For one thing, the combined bags will weigh nearly six pounds, and you'll be carrying plenty already. For another, a bag rated to minus 20 must be considered the minimum up there. I think you're going to have to review your trip budget and consider purchasing a bag just for this trip. But you needn't spend a tonL.L. Bean makes a good quality, minus 20 down-filled bag that sells for about $350. Or, if you want to splurge, get a Western Mountaineering Big Horn Super MF (minus 20, $530). Both weigh less than four pounds, a load difference for which you'll be grateful. And, that vapor barrier liner will work well with both, adding to the temperature range if it gets REALLY cold.
Support Outside Online
Our mission to inspire readers to get outside has never been more critical. In recent years, Outside Online has reported on groundbreaking research linking time in nature to improved mental and physical health, and we’ve kept you informed about the unprecedented threats to America’s public lands. Our rigorous coverage helps spark important debates about wellness and travel and adventure, and it provides readers an accessible gateway to new outdoor passions. Time outside is essential—and we can help you make the most of it. Making a financial contribution to Outside Online only takes a few minutes and will ensure we can continue supplying the trailblazing, informative journalism that readers like you depend on. We hope you’ll support us. Thank you.