REFLECTIVE SURFACES TO REDUCE HEAT LOSS HAVE BEEN AROUND IN CLIMBING GEAR FOR A LONG TIME, BUT HAVE USUALLY BEEN NO MORE THAN GIMMICKS OR CONCEPTS THAT DON’T REALLY WORK. EXCEPTIONS HAVE BEEN THE THERMAREST X THERM SLEEPING MATS THAT WORK VERY WELL, AND THE OCCASIONAL WATER BOTTLE COVER. OTHERWISE THE PRINCIPLE OF HEAT LOSS ARE USUALLY MISINTERPRETED, FAILING AT A HIGHSCHOOL PHYSICS 1.01 LEVEL.
.
TWO YEARS AGO RAB BOUGHT OUT THE TITANIUM LINED MYTHIC ULTRA SERIES THAT HAS EXPANDED TO BE A FEW SLEEPING BAGS AND SOME JACKETS. LIKE ALL OTHER ATTEMPTS THE INTENTIONS WERE GOOD, AND THE NEW TECH INVOLVED A STEP UP, BUT LIKE ALMOST ALL OTHERS THE IDEA FAILS IN THE EXECUTION.
TILT INVOLVED TITANIUM COATED FIBERS WOVEN TO FORM THE INSIDE FACING OF A SLEEPING BAG, MAKING A BEAUTIFUL LINING THAT IS VERY SHINY, AND CARRIES THE EXPENSE YOU WOULD IMAGINE. IT WORKS BETTER THAN SINGLE SIDED REFLECTIVE SURFACES BECAUSE IT STABILIZES A WARM MASS BETTER, BUT FAILS JUST AS FUNDAMENTALLY AS ANY OTHER BY THE WAY IT IS USED.
.
IN IT’S MOST BASIC PRESENTATION; FOR REFLECTIVE SURFACES TO RETAIN HEAT THEY MUST HAVE A DEAD AIR SPACE ON THE SIDE OF THE HEAT SOURCE AND IDEALLY ON THE OTHER AS WELL TO TRAP THE HIGH PERCENTAGE OF THERMAL RADIATION (HEAT) THEY TOUT. IF THE HEAT SOURCE CONTACTS THE REFLECTIVE SURFACE NO HEAT IS RETAINED, MAYBE EVEN LOST AT A HIGHER RATE BECAUSE THE METALIZED SURFACE WILL ACT AS A CONDUCTOR.
THE DEAD AIR SPACES ALSO STOP CONVECTION, THE MOVEMENT OF HEAT AWAY BY AIR CURRENTS INCLUDING BEING BEING PUSHED OUT PERPENDICULAR TO THE HEAT SOURCE BY EDGES EXPOSED TO THE COLD (WHICH IS THE WEAKNESS OF THE X THERMS MATS).
ALL THIS IS DEMONSTRATED IN THE WAY ROOFING REFLECTIVE INSULATION WORKS, NEEDING THE DEAD AIR SPACE OF A CEILING SPACE ON THE INSIDE AND A SPACE EITHER EMPTY OR FILLED WITH SOMETHING LIKE FOAM OR FIBRES BEYOND IT TO KEEP THE REFLECTIVE SURFACE STABLE.
.
TRANSLATED TO RABS SLEEPING BAGS, THEY HAVE INSULATED THE OUTSIDE OF THE REFLECTIVE SURFACE BEAUTIFULLY WITH 900f DOWN, AND HOPEFULLY YOU HAVE A TENT OR BIVY BAG TO FURTHER STABILIZE THE AIR FROM CONVECTION, BUT THEY HAVE MADE THE SAME OLD MISTAKE OF NOT BUFFERING THE INSIDE. YOUR BODY TOUCHING THE TITANIUM SURFACE LEAKS HEAT AWAY, CONDUCTED DIRECTLY THROUGH THE TITANIUM, INTO THE DOWN WHERE IT COOLS JUST LIKE ANY OTHER SLEEPING BAG, MAYBE AN INCIDENTAL AMOUNT SAVED BY BEING REFLECTED AROUND THE INSIDE OF THE DOWN. NOTHING IS ACHEIVED THAT ANY NORMAL AND MUCH CHEAPER BAG WOULDN’T DO.
.
WHAT IS NEEDED IS ANOTHER LAYER OF DOWN OR SYNTHETIC INSIDE THE BAG, BETWEEN YOU THE HEAT SOURCE AND THE TITANIUM LINER. WE HAVE HAD TWO WINTERS NOW TO EXPERIMENT WITH THIS, WITH MUCH GETTING IT WRONG, AND HAVE FOUND THE BEST SOLUTION TO BE AN ULTRALIGHT DOWN BAG THAT WEIGHS ABOUT 300g.
OF COURSE THIS WILL ADD WARMTH SIMPLY BY BEING MORE INSULATION, BUT THE KICK IS MUCH MORE THAN THE ADDED WEIGHT SUGGESTS. ANOTHER 120g OF DOWN SHOULD BRING ANOTHER 2-3 DEGREES OF WARMTH, BUT IN THIS CASE IT ADDS AROUND 10, SO SOMETHING BIGGER IS GOING ON NAMELY THE REDUCTION IN CONDUCTION AND CONVECTION.
.
BY REDUCING THE PLACES WHERE YOUR BODY CONTACTS THE TITANIUM LINING, THE REFLECTIVE PROPERTY CAN DO ITS JOB AND SEND RADIATION BACK TOWARDS THE SOURCE. IT’S NOT TOTAL BECAUSE THERE WILL ALWAYS BE PLACES WHERE CONTACT IS MADE, BUT OVERALL IT IS ENOUGH TO SEE A REAL DIFFERENCE.
HALF OF THE INNER BAGS ADVANTAGE IS ALSO THE REDUCTION IN HEAT LOSS THROUGH CONVECTION, SOMETHING THAT WOULD HAPPEN EVEN IF A REFLECTIVE LINING WASN’T THERE. THIS IS WHY INSULATED CLOTHING WORN IN THE BAG WON’T WORK AS WELL, BY NOT TRAPPING A SINGLE AIR SPACE, THAT THEN CANNOT CONTAIN THE DEAD AIR FOR THE REFLECTED RADIATION TO FILL.
.
THE USUAL CAVEATS THEN APPLY THAT THIS ONLY WORKS ON THE UPPER PART OF THE BAG, WHERE IT IS NOT COMPRESSED BY YOUR WEIGHT TO BE ALMOST NOTHING. UNDERNEATH THINGS CAN BE DONE, SUCH AS USING AN X THERM, OR EVEN BETTER A REGULAR MAT SEALED WITHIN AN ENVELOPE OF REFLECTIVE FOAM THAT SHUTS OFF PERPENDICULAR CONVECTION.
GET IT WORKING AND THE COMBINATION WILL GET YOU TO AROUND -20c, FAR BEYOND THE -8C RATING GIVEN BY RAB, AND WELL BEYOND THE OLD ADAGE OF 1500g OF BAG GETTING YOU TO -20c, IN THIS CASE GETTING THERE AT ABOUT 30% LESS OR AROUND 1kg. THAT’S REAL NUMBERS, AND IF YOU REPLACE THE 500g WITH FOOD, THE PARADIGMS FOR GOING HIGHER AND COLDER START TO COME TOGETHER.
