I have a little secret to share: I know a thing or two about sleeping bags and keeping warm. Might come as a shock I know, and it’s not to toot my own horn, but simply a statement of fact. Here is the secret to finding the perfect sleeping bag that will keep you as warm as the promised comfort temp rating: it’s the sleeping mat. (This article is mainly for 3 season hiking, same rules apply for winter camping, but there it’s also a question of certain techniques)
In my own experience and in my years of being in the outdoor industry with my own gear shop, 9 times out of 10 (I would say 10 of 10, but I always leave a little margin of error) when you are cold despite your bag being warmer (comfort temp) than the outside temp, it’s your sleeping pad. Yes I know you don’t feel cold from underneath, no doubt your sleeping mat has a high R-value, yes I understand you have all kinds of clothing and extra layers on. It’s still more than likely your sleeping mat, or it could be you bought a bag advertising it’s Limit temp (the temp you will freeze your ass off at) as it’s bag name. Example is the Haglöfs down LIM +1. A +10 bag marketed at a +1 temp. Just an example, though most companies market their bags this way. Limit temp, is simply put, where you will be so cold your teeth will clatter. Buy your bag at the comfort temp, not the limit.
With that said, if you buy your bag at the stated comfort temp, and you freeze despite the outside weather being warmer than the comfort rating of the bag, it’s your sleeping mat. There are many reasons for this, but the simple truth is that rating a sleeping mat is considerably more difficult than the more standardized rating of sleeping bags. Most comfort temps on most sleeping bags are fairly well measured, while sleeping mats can and do vary wildly. In my experience most thicker sleeping mats (air filled not cell plast) all get cold around +3 celsius. Once the temperature starts to drop, these pads start to get cold: regardless of r-rating. I think this might have something to do with how different companies measure their r-value. I’m not sure how it’s done, but it rarely matches up to reality. The exception being the Thermarest mats that all hold up quite well in colder temperatures. (Thermarest xTherm and xLite hold up to stated r-values).
I’m sure someone out there is going to tell me I have no idea what I’m talking about, and that sleeping mats are 100% accurate in their ratings. But alas, I’m not talking to you. I’m talking to everyone else that is freezing in their sleeping bags despite buying the thickest, most expensive bag on the planet. I know, because I’ve been there. Daily I have customers who call or write describing the exact same issue. Most of them have barely a thought on what they have for sleeping mats. My first response and question is always “what sleeping mat do you have?”. Don’t get me wrong, I’m more than happy to sell great, ultralight sleeping bags, but I don’t care for returns because of freezing customers.
So what is my standard advice? Try putting a cheap cell foam pad on top of your current sleeping mat first – on top not underneath your sleeping pad. If you are still freezing, try a different sleeping mat, perhaps an xtherm. If you have tried different sleeping mats, then start looking at the bag. Check the comfort temp of your bag, actual temperature where you are at (temps on apps are often taken in cities or towns where temps are higher). There can of course be other issues with your bag such as down clumping – make sure your sleeping bag is properly “fluffed” and that the down hasn’t shifted into clumps. Another issue is a sleeping bag that is too small, which means you squeeze too hard against the sides, not leaving any room for the insulation to leave insulate.
But at the end of the day, 9 out of 10 instances of people freezing outdoors in their “warm” sleeping bag, is due to a cold sleeping mat.