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Gear Review: Mammut Sphere Down 3-Season


The Sphere Down 3-Season is a down sleeping bag from the Swiss company Mammut and one of seven different down sleeping bags that Mammut currently offers.


The Sphere Down 3-Season is available in two different versions. One variant for a height of up to 180cm and the second variant for people with a height of up to 195cm. The Shpere Down 3-Season is currently not available as a women’s version.

The comfort temperature of the sleeping bag is -1°C and according to the manufacturer, the sleeping bag can be used till temperatures low as -24°C.

    185 Height up to 185cm
    195 Height up to 195cm
Zipper Left
Weight without bag 826g
Weight with bag 906g
Packing Size
Price ca. 530 EUR

Mammut uses 90% goose down and 10% goose feathers as filling for its Sphere Down 3-Season, both of which are certified with the RDS (Responsible Down Standard). According to the manufacturer, the total filling weight is 450g and the total weight of the sleeping bag without pack sack is 826g. At 826g, the Sphere Down 3-Season is definitely one of the lightweights among the three-season sleeping bags.

Special Details

The main feature of the Shpere Down 3-Season is the feather-light outer fabric made of prolightTX ™, which, according to, weighs only 330g. In order to keep the total weight of the sleeping bag as low as possible, Mammut does without a thermal collar. Heat collars are used in many sleeping bags to prevent cold air from entering the body part of the sleeping bag. In the foot area, Mammut uses additional insulation to provide more sleeping comfort and prevent cold feet. Mammut relies on wave-shaped chambers so that the down in the sleeping bag does not slip and cold bridges are avoided.


In order to test the thermal output of the Mammut Sphere Down 3-Season, we warmed up the sleeping bag and used a thermal imaging camera to see how much heat the sleeping bag emits and where cold bridges occur.

First, we spread out the sleeping bag so that the down can be puffed up properly and maximum isolation can be achieved. To warm up the sleeping bag, we placed three hot water bottles, which we filled with hot water, in the body area of the sleeping bag, as well as a hot water bottle in the head area. We put two glasses between the hot water bottles so that the warm air can be distributed throughout the sleeping bag.

After 15 minutes we took three pictures with the thermal imaging camera in order to see in which areas the Mammut Sphere Down 3-Season emits the most heat. The blue areas on the pictures mean that they have the lowest surface temperature and the further the color changes towards red, the higher the surface temperature at these points is. However, the images cannot be compared one-to-one, as the temperature range is redefined for each image. You can see this from the three temperature indications on the picture (red = highest temperature, green = lowest temperature, white = average temperature).

The thermal image with the overall view of the sleeping bag shows that the Sphere Down 3-Season does not show any clear weak points in terms of thermal output. A close-up view of the foot area shows that the seams give off more heat than the other areas. But this is not a surprise, as the down filling is steaming at the seams. Both in the general view and in the close-up of the head area, slight weak points can be seen in the thermal insulation of the zipper.


The Mammut Sphere Down 3-Season is a very light three-season sleeping bag that can keep up with pure winter sleeping bags in terms of thermal insulation. However, the low weight ensures that the thermal insulation has slight weaknesses in certain places. Despite this, the low weight and the small pack size make the Sphere Down 3-Season a perfect companion for hiking and trekking tours in cold temperatures.


You can find more sleeping bag reviews here.


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