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Are Two Thermo-Lite Bivvies Better Than One?

Question:
I own two Thermo-Lite Bivvies. If I put one inside the other, what level of performance increase (if any) will I get? I’m thinking about freezing wet temperatures in the Cascade Mountains.

Thank you,
Dean
Answer:

Hi Dean-

This is a good question.  We have done tons of real world product testing with one Thermolite, but not with two, so my response is based more on my own understanding of the product rather than anything I’ve experienced in the field.  Because the Thermolite material reflects 80% of your body heat, adding a second one probably won’t drastically increase the reflectivity.  You’ll get a little boost, but it’s already so high that you’re unlikely to notice a huge difference.

However, a second bivvy will act as a barrier against the heat emissivity of the first bivvy, preventing excess heat from leaving the surface of the material.  Additionally, by experimenting with the positioning of the two bivvies, you can cut way down on the ventilation (for example, if the second bivvy is upside down inside the first, the Velcro side-ventilation will be closed off), drastically reducing on the amount of heat that escapes.  Keep in mind that Thermolite is not a breathable material, though, so cutting down the ventilation will increase the condensation inside the bivvy.

As a result, I wouldn’t recommend this configuration as a primary sleep system… in freezing weather, if you are in an emergency situation, two Thermolites will probably keep you warmer than just one, but I highly encourage you to bring a cold-weather sleeping bag if you are venturing out in the Cascades during this season.

Please don’t hesitate to contact me if you have any further questions.  Thanks for your interest!

Best,

Jordan Hurder
Product Specialist
Adventure Medical Kits

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