Reverse Differential Hood
Our hoods are cut so that the inside of the hood is made with a larger piece of fabric than the outside. This creates a hood with loose down filled fabric which surrounds your head. That way you can can enjoy warm insulation around your head and face without having to cinch the hood tight. Also, the extra fabric can be adjusted to create a down filled ruff or bill across your forehead.
Mummy bags are made with an inner shell fabric cut narrower than the outer shell. This is called differential. Its purpose is to give the inner shell a smaller circumference than the outer shell. This eliminates extra fabric inside the bag that could lead to cold spots. All of our mummy bags are differentially cut to promote proper lofting and to protect against down compression when knees or elbows push against the inner fabric. Semi-rectangular bags are not cut this way so that they can be unzipped to lay flat.
A 3-season bag needs to serve in a wide range of temperatures. You will expect it to provide comfort during the heat of summer through the cooler temperatures of fall or early spring. To ensure a broad temperature range we build our 2 and 3-season bags with continuous baffles which encircle the bag from zipper to zipper. This allows you to shift the down insulation from one side of the bag to the other. During hot summer nights, you could shift the down from the top of the bag to the bottom so there is less insulation on top of you. When the weather is cold, you can move the down to the top of the bag. Just open the bag flat and push the down with your hands from one side to the other. This adjustment allows you to maintain a comfortable temperature inside your bag. We sew continuous baffle bags with 5 1/4" baffle spacing so that unintended down migration during the night is kept to a minimum.
Passive Top Collar
Heat Loss from the neck area is common while moving inside your bag at night. To help minimize this heat loss, our Megalites are outfitted with a Passive Top Collar. This amply filled 3-dimensional collar extends from the zipper across the neck to seal over your shoulders and keep warm air from escaping.
Full Down Collar
When a full seal from the elements is called for, we pull no punches. Our Full Down Collar is a 3-Dimensionally sewn down filled tube that wraps snugly about your neck. It is sewn with two widely spaced seams so that it is a solid baffle of down, and not some sort of flap. It uses a bungie draw cord to cinch it tight and in our Gore Windstopper bags the entire collar is made of Gore Windstopper fabric. You'll find the Full Down Collar on all of our Gore Windstopper bags and most of our MicroFiber bags.
Side Block Baffles
In cold weather you want insulation to stay on top of you without any chance of shifting. This task is accomplished with a side block baffle made with netting, that runs the length of a bag on the side opposite the zipper. Our winter bags are built with a V-Block Side Baffle. This V-Block design does two things, it provides a wide pie shaped chamber of down along the right and left sides of the bag, and by increasing the number of down filled chambers in a bag, the size of the chambers decrease. Smaller chambers provide better down control. Further, the shorter baffles that cross your chest lie flat so that down can't migrate to either side.
Fill Power Testing
To perform a certified test, a sample of down is sent to an independent laboratory. There the sample is placed in a large screened box and allowed to condition for 5 days before being tested. During this time the sample is stirred, mixed, and blown with a warm hair dryer. This conditioning is intended to stabilize the sample so that consistent results may be obtained.
Then a 1 ounce sample is drawn and placed into a measuring cylinder. A piston weighted to 68.4 grams is placed on top of the down and when it comes to rest the volume of the down is measured in cubic inches. In spite of 5 days of conditioning, the laboratory will publish their results with a plus or minus 5% error. This is a full 10% range and for an 850 fill test, results in an 80 point variance! Furthermore, conditioning actually improves the sample by drying and blowing out dust and other small particles while the down at the factory remains unconditioned! This is what we mean by an Optimistic fill power rather than a Practical fill power rating.
If two bags have the same internal volume, the bag with more loft will be warmer.
If two bags have the same loft, the bag with the smaller internal volume will be more efficient and warmer. If two bags have the same loft and internal volume, but one has a lot more down fill by weight, then check the density of down inside the chambers and the fill power claimed by the manufacturer.
Well filled down chambers provide better down control.