Subscribe Contribute Advertise Facebook Twitter Instagram Forums Newsletter

Functional Designs Sweetie Pie Summer Bag Doubler

Functional Designs Sweetie Pie Summer Bag Doubler


by the Product Review Staff | 2002-07-04 03:00:00-06

Manufacturer’s Website:


Doubler Size

  • Temperature Rating: Summer Doubler +40 deg F
  • Shell Material: Black 50 denier Ripstop Nylon
  • Fill Material: Polarguard 3D
  • Total Weight: Summer Doubler 12 oz. (18.4 oz - measured)
  • Length: 58" (56" - measured)
  • Shoulder Girth: Additional 60"
  • Hip Girth: Additional 60"
  • MSRP: Summer Doubler $79

General Description

The Sweetie Pie Bag Doubler is light synthetic filled quilt that zips to a standard single- person sleeping bag to make a two-person sleeping system. Functional Designs intends the Doubler for use above the sleepers, with a standard sleeping bag underneath.

Other reviews of the Sweetie Pie bag doubler unblushingly beckon couples to paradise inside a single sleeping enclosure. So we forwarded our product sample to a particularly intimate review team and gave them free reign to design and implement a testing program that will see if the bag doubler lives up to its name or is just another piece of dry cake.

Our review team intended to test the bag doubler with their mummy bag of choice, the de facto standard Porsche of the lightweight backpacker: Western Mountaineering's 28-oz UltraLite. If the bag doubler and the UltraLite are able to serve the needs of two close sleepers, that would result in a sleeping bag weighing 46.4 oz for two people that is comfortable in most three-season conditions. Our review team had mixed results.


The Doubler is a handsome, well-made product. It is simply a flat quilt with Polarguard 3D insulation, offset stitched in 15" baffles and shelled in black ripstop nylon taffeta (1.9 oz/sq yd). The lining fabric has a warm and soft feel ("silky," says she). It is shaped like the business end of a ladybug - almost circular but then tailored straight across at the top and pulled down into a shallow tail at the foot end (see the photo, where the black Doubler is zipped to a blue sleeping bag).

There is no draw cord along the top. A set of YKK #8 zippers (the industry standard for sleeping bags) ring the outer curved edges of the piece, shielded by 1.5" wide draft tubes. The weight of the product as measured was 18.4 oz, more than 1.5 times the advertised weight of 12 oz (but still less than most sleeping bags it might replace).

The usable space in this bag (distance from zipper to zipper) measured 56" at the widest point (not the 58" claimed by the manufacturer), near where the sleeping couple's hips might end up.

Our product was the summer-weight Doubler, rated at 40 deg F. Functional Designs also makes a three-season version rated at 10 deg F and will soon release an expedition-weight Doubler.


Attempting to mate the Doubler with the Western Mountaineering UltraLite, our reviewers were dismayed by the fact that the UltraLite uses a non-standard YKK size in order to save weight, and so the two could not zip together. We learned that this is the case with most other bags in WM's ultralight series. The moral is this: YKK #8's may be the industry standard, but exceptions may exist on certain bag models, especially with lighter sleeping bags, so be sure to check!

So, our reviewers paid a visit to a local outfitter and tried the Doubler with several popular-brand mummy bags. Most mummy bags of a standard design (for example a North Face Kilo and a Mountain Hardwear Down Upgrade bag) accommodated the Doubler beautifully and made quite a lovely nest, even for a rangy sales clerk who was nearly six feet tall. Members of the gear shop staff oohed and ahhed at this unfamiliar but clearly useful piece of gear.

The bags that are most suited to a Doubler transformation are ones that zip very nearly to the bottom, because otherwise there's a single-bag pocket at the foot end -- in the very center of the setup -- that accommodates the taller person's feet and leaves the other person in cramped quarters indeed. Ones that zip around the bottom (like many semi-rectangular ones) are not appropriate: too much of the Doubler's length is spent traveling across the bag's bottom.

We reflected on the fact that, with the Doubler in place in the expected configuration, the couple would be sleeping under this $79 piece, using possibly a $370 down bag as nothing more than a mattress pad, probably in the process compressing areas of down that never otherwise get slept on. Perhaps it would be wiser to flip the whole thing over and sleep on top of the Doubler and under the bag. This "off-label" use might be entirely appropriate, enabling two people on a cold night to pool body heat under a single very warm bag with a minimal insulation layer beneath the bag to control drafts and improve cushioning somewhat. It would achieve great economy of weight, though the couple would have to conquer the breeze gap between their necks. In fact a couple might snuggle in comfort under bags of various weights in various climates with the summer-weight Doubler serving for all; on a given trip they would also have the option of placing the lighter or heavier bag above them according to the weather.

This flipped-over configuration (Doubler on the bottom) might seem problematic with a hooded bag. But the hood tends to cup nicely over the shoulders of one of the sleepers and it would give a bit of extra length to a setup that's a little stingy in that dimension. In addition, when the sleeping bag (whether hooded or not) is the top cover, its draw cord can be used to snug the top layer at the neck. With the sleeping bag underneath, its cord is not as easy to adjust and cinching it would not conserve heat as effectively: recall that the Doubler, which would then be on top, does not have a draw cord.

Overall Impression

The Sweetie Pie Doubler made a good first impression on us. It presented some unique possibilities in a sleep-system configuration for two people and was compatible with many popular bags. Still, many lightweight backpackers will do some arithmetic and wonder just what the Doubler will accomplish for them. If they are already using a pair of ultralight bags or blankets in the 40-degree range (e.g. high-fill down bags from Western Mountaineering, Nunatak, or Rab Carrington), each piece might already weigh less than the 18-oz Doubler, meant for such temperatures, or scarcely more (the typical range in this group is 16-19 oz). Such customers will not be motivated to attach the Doubler to some other bag that will inevitably be heavier. It certainly is true that the heavier a pair of bags is, the more weight a couple saves by substituting the Doubler for one of them -- and probably many backpackers don't have summer bags, so many will in fact stand to save weight. That's IF the Doubler can be married to the bag, which in the case of many of the lightest bags, with their slender zippers, it can't. Backpackers interested in the Doubler should do the calculation for their particular bags and make their decision accordingly.

The Doubler realizes an ingenious idea and executes it nicely. Perhaps the Functional Design people will see their way to making a variant that adapts to some of the bags the lightweight community favors, especially since their idea is conceived as a weight saver. An optional narrower-gauge zipper will make it that much lighter, and perhaps for the lightweight crowd they'll even use more delicate shell material (e.g. 0.85 oz/sq yd nylon). For now they're off to a good start: their Polarguard pancake will work well for some, and the design, developed further, would probably work well for most.


"Functional Designs Sweetie Pie Summer Bag Doubler," by the Product Review Staff. (ISSN 1537-0364)., 2002-07-04 03:00:00-06.