Sierra Designs Mobile Mummy 800 4-Season Review

There are a few things that are hard to improve on: mom’s carrot cake, the original Star Wars trilogy and a powder day come to mind.  When I first reviewed Sierra Designs’ Mobile Mummy, I might have thought that Sierra Designs had peaked out with such a good product.  I was wrong, and the 4-season Mobile Mummy is here to prove it.

Sierra Designs Mobile Mummy 4-Season Features:

• Ultralight jacket hood
• Zipperless arm ports
• Garment-style shoulder design
• Centrally-placed, ventable zipper
• Draft collar, tube and curtains
• Stowable footbox
• EN Comfort Limit: 17^oF / -8^oC
• EN Lower Limit: 5^oF / -15^oC
• 3lbs. 2oz.
• MSRP: $479.00

The Iceman Cometh — and what is he wearing?!

I’d be curious to know if Sierra Designs has laid all their cards on the table by now: after the huge waves they sent through Outdoor Retailer and industry blogs upon the release of their new sleeping gear in 2013, how much innovation to their poor, overworked designers have left in them?  Hard to say – but I’ll definitely be keeping my eyes on them.  So, in fairness, the 4-season Mobile Mummy isn’t anything astonishingly new since it’s basically a puffier version of the 3-season model.  What’s really important in this bag, and the factor that will determine its success or failure, is essentially a proof of concept: can the quirks of the Mobile Mummy design hold up to harsh winter conditions that demand excellent thermal efficiency?  Let’s find out!

To begin with, the basic layout of the bag.  I’m testing the regular model which is good for chaps up to six feet tall; it has a long, 58″ center 2-way zipper with a baffle to provide ventilation options on warmer nights and to slip your legs out and ‘go mobile.’  The tail of the bag features two toggles that can be used to hold the fabric up and away while you’re walking around camp.  Sierra Designs’ cleverly designed hood and arm ports finish things off up top.

The primary concern that I had going into this review was to examine how thermally efficient this sleeping bag is.  One of the great thing about the way Feed the Habit tests gear is that we take new products blindly onto our adventures and we never quite know how things will hold up; this resulted in many cold nights, wet feet and a considerable amount of grumbling when a product didn’t pull through.  I felt a little bit of trepidation heading into the wild with an unproven bag in the wintertime, but Sierra Designs has rarely let me down before.

First of all, we can definitely check moisture management on the list of the Mobile Mummy’s great qualities – as it has been doing for two years now, DriDown continues to come in clutch.  I’ve mentioned this trip in several reviews now, but I headed over to Baker in October and we just got drenched in moisture from low-hanging clouds and an exhausting approach.  When I pulled the Mobile Mummy out of the bottom of my backpack (which I failed to cover) the fabric was damp in several spots.  I glowered at my climbing partners as we laid out our bags in the SD Convert 3 – they had both packed synthetic bags – but I needn’t have worried; when I crawled into my bag after a delicious meal of Mountain House lasagna, the damp down was still nicely lofted.  When I woke up in the morning, it had largely dried out.

So no surprises there – it handles moisture as well as all of the other excellent DriDown products I’ve covered.  The big concern with a design like the Mobile Mummy is whether or not all of those unique features are going to leak heat.  We’ve already examined this angle in the 3-season version, but the stakes are much higher in winter.

The arm ports are perhaps the most genius feature of the entire bag; they have a self-sealing baffle that is easy to push and pull your arm through.  When you pull your arms back in, all you need to do is wriggle your shoulders around and the fabric panel seals up the hold.  Both shoulder areas are actually quite remarkable to feel around within the bag – there is soooooo much puffy down action going on up there.  It feels, in many respects, like one of the most ‘cloudlike’ down products I’ve ever reviewed.  And, frankly, it has to – the excess down and fabric works hard to seal those ports and it helps maintain the balance between having ample shoulder room but not so much that you’re losing heat by trying to keep the space warm.  Sierra Designs wanted the shoulder area to be roomy, but to stay warm in the winter that means occupying a lot of that space with highly compressible down.

The next biggest possible source of heat loss is definitely that long front zipper; in many ways, it’s in a bad position because all of the heat from your chest and belly have the potential to rise up and simply escape through the zipper track.  I mentioned in my review of the 3-season model that the full-length baffle did a good job but felt inadequate at times when my body’s movement pulled the baffle out of place, exposing the cold zipper.  It seems to me that Sierra Designs has remedied this by increasing the size of the draft tube – the zipper track is now at the other end of around three inches of baffle which provides a good amount of protection from being exposed to the cold.  I’m happy with the new design and it actually never really became something that I thought about while testing the bag – that’s always a sure sign of a good design.

Finally, Sierra Designs made the hood without any elastic cords or adjustments and rather aimed for a hood that ‘just fits.’  I definitely appreciated this in the 3-season model, but I think it falls a trifle short in winter conditions.  When things get below zero I like to be able to pull the hood tight around my nose and mouth to minimize the amount of skin exposed to the air.  While the Mobile Mummy hood certainly fits well, it doesn’t provide the full-on coverage that I need in temperatures below zero.  That said, the bag is only rated 17′ F as its comfort limit, so I might be expecting too much from it in very cold weather.

Lastly, it’s appropriate to conclude with a few comments about the fit of the bag.  Frankly, this is the single biggest reason that I love the Mobile Mummy and continue to choose it as my go-to bag.  Sierra Designs built this bag to move with you while you sleep and it’s ideal for side and back sleepers.  You can curl up, stretch out (to a degree) or lie flat as a board and the bag will accommodate you.  The wider shoulders and torso are, to me, well worth the slight sacrifice in thermal efficiency for the better overall quality of sleep that I get with a more comfortable experience.

The Good

• Hey guess what – it passes my thermal efficiency test!
• The fit of the bag is very comfortable for various sleeping styles
• Self-sealing arm ports work like champs
• Full-length zipper provides plenty of venting options
• 800-fill DriDown continues to lead the industry
• Beefed-up draft tube does the trick

The Bad

• Hood design lacks coverage on very cold nights
• A trifle heavy as a result of its feature set

The Bottom Line: Mobile Mummy 4-Season

I mentioned at the start of the review that it’s always a little scary to head out in winter time with unproven products.  I should know by now that I can trust Sierra Designs – predictably, the 4-season Mobile Mummy kept me safe and warm through the full spectrum of winter conditions.  There are warmer, lighter bags on the market, but none of them can give you the same night’s sleeping experience as the Mobile Mummy.

Buy now: Available from Sierra Designs