There has been some speculation around how much insulation is necessary for an over-system to perform it duty in managing internal condensation. Some have speculated that an Alpha over-system is not enough to perform, but this is absolutely un-true. The science of what is actually happening when using an over-system is rather complicated, with many variables, that I honestly don’t feel totally confident in speaking about. However, here are some of the variables that are part of the equation.
The ratio of R-value between the inner down system and the outer synthetic system. The outer system needs to have enough of an insulation boost to move the dew point.
The amount of moisture being produced. Everybody will produce different amounts of moisture during the night and I think different amounts of moisture might require different insulation ratios.
The amount of heat being produced. Everybody produces different amounts of heat. Some people really push out a lot of heat, and might not need as much over-system insulation for it to be effective. Others might run cold, such that the dew point for them is further in and they need more insulation to fully push it out.
The amount of ambient humidity. Honestly, I’m not sure how much this effects the microclimate inside a sleep system. It might effect it some.
When testing began with using Alpha in over-systems, I had a similar speculation that AD90 would not be enough in the ratio to move the dew point in many cases. I assumed that it would have a place in warmer systems, like maybe combining with a 40f down system for a 30f use. Or a 30f down system in 20f use. However, the AD90 over-systems have consistently proved to be plenty in much lower temps, indicating that the ratio is still appropriate. Personally, I have taken a 10f false bottom Wren down into negative double digits with an AD90 overquilt, multiple times, without any moisture in the down to speak of. As mentioned above, this is somewhat dependent on the microclimate that my body produces, but I would consider my microclimate to be on the side of “needing more” in terms of an over-system. I run fairly cold when static. I think I might produce a fair amount of moisture, but hard to say. I know that if I do not use an overquilt in winter conditions, it sure looks like I sweat a lot. I’ll get significant moisture in even one night. My results with the AD90 overquilts seems to track with the general feedback on them. They seem to do the job, universally.
So, the AD90 overquilt has really taken the spotlight, for me. Based on how my use has gone, I’d need to be in some really, really cold temps before I would feel like the APEX Merganser is the right choice. To be honest, I haven’t taken the 10f Wren / AD90 overquilt down beyond its limit yet. So, I don’t even know where that point is. I assume that -10 – 15f would be around the limit, but I also assumed that 20f would be the limit, so who knows…..
Really interesting post. I have problems with condensation on the exterior of my quilts (EE Enigmas 10F and 30F, both Long/Wide) as well and am struggling a bit with how to deal with the issue. I sleep fairly cold and am not sure how much moisture I put out, but it is enough to have issues in sleeping temps of low 20s F up into the mid 40s F. Condensation seems to be the worst on the side of the foot box facing the end of my tent, on top of the footbox, and on top of my chest area up to my head, with not as much over my legs. EE would typically recommend something like a 50F Apex quilt for use as an overbag, but in my case with a long/wide quilt I am concerned about compressing the down in my bags and rendering them less warm. A solution in Alpha sounds really attractive. Staying tuned.
Yeah, in my opinion, overquilts and bags should be sized specifically for that purpose. A large size regular system over a smaller regular system is often too snug in the footbox….and sometimes in other areas too. At those temps, an AD90 over system would likely be successful. Usually the dew point just needs a little nudge around freezing. It’s down in the low temps that you often need to be more concerned about the ratio, but keep in mind, everyone has a different set of conditions in their sleep system. It’s also worth noting that other components can play a factor. For instance, cowboy camping, out in open sky, is the worst scenario in terms of internal condensation. Tends to push the dew point inside, whereas using a breathable bivy, or bug net tent under a tarp, helps create a micro climate around you that helps move the dew point. Even setting up under and around vegetation can have an effect.
Dan – thanks for your comments, I am relatively new to backpacking, so still doing a lot of experimentation to get things optimized. Condensation on the outside surface of my bag is a tricky one. I first believed it was coming from my breath, and some of it may be. But even in a well ventilated tent(Dipole Li with vents totally open and at least 2 of 4 doors open, I still get condensation on my footbox. I have a Borah Ultralight bivy coming soon and will see how that works out and an Alpha overbag sounds good also, as sleeping a little warmer is a good thing for me. I will try the vegetation angle as well. Thanks for some new options!