​Improved Modular Sleeping System (IMSS) Review

Improved Modular Sleeping System (IMSS) Review

In 1998, the US Military introduced their Modular Sleeping System (MSS). The system consisted of three key parts that could be used separately or together depending on the temperature conditions. Even though it was bulky, the MSS was (and still is) really popular with civilians. The MSS remained unchanged until 2007, when the Military updated it to the Improved Modular Sleeping Bag System (IMSS).

Components of the IMSS

Just like the MSS, the IMSS consists of three main pieces that you can use individually or combine. There are also two stuff sacks with the IMSS, as opposed to just one stuff sack with the previous system. Here are the key components of the IMSS.

1. Patrol Sleeping Bag

This is the lightest sleeping bag of the three-part system. It is rated for temperatures of down to 30F. However, remember that the Military lists survival temperature ratings. The comfort rating of this bag is closer to 40F, or 50F if you are a cold sleeper.

The sleeping bag is fairly basic in that it doesn’t have any baffles. It does have a drawstring hood (which was not on the previous MSS) and snaps for connecting it to the Intermediate Bag.

However, it has one very cool feature: there is a separate zipper on the bottom so you can open up the foot area. This allows you to adjust the ventilation for better temperature control in warm climates. In theory, you could also sleep with your boots on and leave them sticking out of the bottom.

• Temperature: 30F to 50F
• Outer Material: Water resistant rip-stop nylon
• Inner Material: Climashield continuous filament polyester insulation
• Size: 80" L x 34.28" W
• Weight: 2lbs 7oz

2. Intermediate Cold Weather (ICW) Sleeping Bag

ICW sleeping bag can be used by itself or over the Patrol bag. By itself, the bag is comfortable for temperatures around 30F. When used with the Patrol bag, the system is suitable for temps down to -10F.

Because the ICW bag is meant to be used over the Patrol bag, it is a bit wider than your typical sleeping bag. Large men will appreciate this because they won’t feel cramped, or compress the insulation. However, if you are smaller in stature, you won’t want to use the ICW by itself: there will be too much extra room in the bag, meaning it won’t trap heat as well.

As far as features go, this bag does have baffles to prevent insulation from moving around. There is a draft collar and hood for heat retention. The footbox has twice the amount of insulation in order to help feet stay warmer.

• Temperature: -10F to 30F
• Outer Material: Water resistant rip-stop nylon
• Inner Material: Climashield continuous filament polyester insulation
• Size: 80" L x 34.28" W
• Weight: 3lbs 10oz

3. Bivy Bag

The final component of the IMSS is the bivy bag with ACU camo design (the older MSS used Woodland camo). The bivy is made of waterproof Gore-Tex material and meant to be used over one or both of the sleeping bags. You can also use it with any other sleeping bag.

As far as bivy bags go, it’s pretty simple in design but really nicely built. The seams are completely sealed, there are snaps to fasten it to the other sleeping system components, and you can easily pull it closed from the inside.

Note: the Marine Corps version of the IMSS has a bug screen over the face area but the Army version doesn’t have this feature.

• Material: Gore-Tex
• Size: 80" L x 34.28" W
• Weight: 1lb 14oz

How Warm Is the IMSS?

It’s hard to get an exact temperature rating for the Improved Modular Sleep System. Even specs on the Military and vendor sites don’t always match up. However, most sources rate the system of down to -30F.

No, you won’t be warm at -30F.

Military temperature ratings are listed for survival of up to 4 hours. A more realistic comfort rating is around -10F.

Waterproofness

The IMSS bivy is made from waterproof Gore-Tex. Yes, Gore-Tex is actually waterproof. However, before you sleep outside in the rain with the IMSS, you need to know about waterproof ratings.

Gore-Tex has a waterproof rating of 28,000mm, which is considered very high. In technical terms, this means the fabric can hold up against a 28,000-millimeter (or 30 yards) column of water pressing down on it without leaking. This is tested by placing a sample of the material under a clear tube filled with water and measuring the height of the water column the fabric can support before water seeps through.

Obviously, rain falls from much higher than 30 yards. So, while you can expect a high degree of waterproofness from the IMSS bivy, it will let some water through in heavy rains. Likewise, if you press it against water, leakage will occur. So, you don’t want to sleep directly on very wet ground or in the rain without a shelter.

Size

The biggest downside of the IMSS is that it is very large and bulky. Even the individual sleeping bags are fairly heavy and bulky compared to their non-military sleeping bags with similar temperature ratings.

With the stuff sacks, the entire IMSS setup weighs over 9lbs. Even though the entire IMSS system packs into the bivy nicely, it packs down to a huge 19.5x23.5 x 10 inches. It will barely fit into a rucksack. You’ll either need to hang it off of your ruck, which isn’t exactly balanced, or pack the components separately.

Cost

While a lot heavier and bulkier than commercial counterparts, the IMSS is much more affordable—especially when you consider that it comes with two sleeping bags plus a bivy shelter. You can also buy the components separately, though it rarely ends up being as good of a deal. Just note that the Improved MSS system is foliage green (Patrol bag) and gray (Intermediate bag), and the older system is green and black.

Verdict

The IMSS is definitely not a sleep system for long-distance hiking, and it doesn’t have all the bells and whistles that you’ll find on many commercial sleeping bag systems. However, the simplicity is one of the main things to like about the IMSS, since it’s easy to use and holds up well. Just be realistic that it won’t keep you comfortable at -30F or dry in heavy rains.

You can get the Improved Modular Sleeping System here.