Auto windshield heat reflector pad
There are several types of auto windshield reflector pads. Some are folded cardboard with foil on one or both sides, some are a double layer of bubble wrap with a layer of aluminum foil laminated to each side and my favorite has 1/8” polyethylene foam with a piece of space blanket laminated to one side only.They come in all different sizes depending on the size of vehicle windshield. They will roll up to almost 2” in diameter and are around 4.5 oz. One nifty trick is to put this pad inside your sleeping bag so that it will stay aligned to your sleeping bag. If it does need a little adjustment, it is inside the bag and you can adjust it without unzipping your bag, losing body heat or waking up too much.
Tom’s experiment with Reflectix, windshield reflector pad, and fleece
I spent a couple of nights in the bush in September 2001 to experiment with reflector pads. Temperatures were in the mid 40s F (7ºC). None of this was very scientific, but the results were impressive. I used a 40" wide x 60" long piece of REFLECTIX tapered at the foot to 18" wide, a double layer of bubble wrap with aluminum foil laminated to both sides available (in different widths) from Home Depot, and an AUTO WINDSHIELD REFLECTOR PAD – 1/8' of polyethylene foam laminated to space blanket material. Comes in different sizes depending on the size of your windshield/body. I’ve heard that Wal-Mart has the biggest ones made for big trucks.
I sewed a piece of heavy fleece to the reflective side of the windshield pad but wearing a fleece suit would be as good or better. The fleece is important because it is fairly uncompressible and contains tiny air spaces necessary for radiant heat to pass through before it is reflected by the pad back to your body. Heat intensifies and, to me, felt like I was laying on some kind of solar cooker turned down to "warm". If your body is directly on the pad without the air spaces, you get the insulation benefits, but not the radiant benefits.
It was so comfortable that I did not need to be inside my bag so I just lay it over me like a quilt. Quilts seem to work well because they usually don't fall off because of the shape of the hammock. This was the most comfortable cool night I can remember.
Conclusion: Reflectix is an insulation manufactured mostly for the construction industry. It is used as a radiant insulating barrier in walls and wrapped around hot water heaters for additional insulation and also recommended as insulation for camping tent floors and insulation for hot and cold food containers. It is constructed of a double layer of bubble wrap with a layer of aluminum foil laminated to each side. It is around 5/16” to 3/8” thick and weighs about the same as foam pads of the same thickness.
The reflectix has several drawbacks. The raw aluminum will tarnish after a while, reducing it's efficiency. It is bulky and not as efficient as as the same thickness of foam and radiant reflector combinations. It's biggest problem is the heavy aluminum foil which is laminated to both sides. While it does reflect radiant body heat quite well, it also acts as a heat sink and the aluminum foil will suck heat out from under you wherever the aluminum foil is exposed to the cold air.
The windshield reflector pad is a much better solution to the problem. The closed cell foam is a better insulator than the bubble wrap and less bulky. The thin micro coating of aluminum reflective material is protected under a plastic coating, eliminating any tarnishing or any tendency to suck away body heat like the Reflectix did.
The dead air space between you and the reflector is an essential factor in the formula for radiant effficiency. If you sleep inside of your sleeping bag, the compressed sleeping bag insulation under you should have enough dead air space to allow the radiant pad to operate effectivelly. If you sleep directly on top of the radiant pad with your sleeping bag fully opened and laid over you like a big quilt, fleece material is probably the best way to get that dead air space between you and the reflector pad. For winter use, the fleece pants and fleece top make more sense since they are usable both day and night. Of course, fleece booties and fleece cap would really top it off. When wearing fleece clothing, you slip around quite nicely on the pad.
A windshield reflector pad facing up on top of a 3/8 inch thick urethane foam pad should take you down to around freezing. Ultralite backpackers will cut the full size 3/8 foam pad down to 36 inches long and keep the full length reflector because there is less weight and pressure on the pads under their legs. Total weight of this set-up is about 11 oz.
So far, this is the extent of our search for the perfect lightweight radiant reflector pad. The automobile windshield reflector pad offers attached insulation to prevent condensation build-up inside the hammock. It is flexible enough and does not interfere with the comfort of the hammock, nor does it pop out from under you in the middle of the night. Used in combination with additional foam pads of less than 3/8" provides comfort, efficiency, light weight and low cost.