Marty_JG wrote:The Mongar (like the Hubba-Hubba it is based off) is an excellent, large, 3-season tent but you need to be very, very careful about using it it in winter especially in exposed/windy conditions or if there is any risk of significant snow. The lightness and large surface area will not be working in your favour in inclement conditions (which can come in fast and without warning) so you absolutely need an escape plan and escape route based on your shelter failing at any hour of the overnight (think torches, mylar bivi, not being 30k away from safety, etc.). This is not to say you cannot use it over the next couple of months but you need to treat the situation like your life depends on it.
Completely agree......OK for sheltered spots in the glen but not really adequate if exposed to winter winds.
Conversely, limiting excursions to benign conditions however can often mean that the temperatures drop a lot colder than -5C e.g. still & clear nights where the temps in the glens can often fall below that on the summits.
My 'to go' bag for winter is -6C/ -13C rated which is OK for me down to -10C without resorting to extra clothing (fleece, down jacket, etc) when coupled with a TAR X-therm mat. IMO a SB that will keep
YOU warm at -10C should suffice for most Scottish winter nights, how this translates to a SB rating is really down to the individual.
You should also note that sleeping bag rating are established with a mat having a R value of 4.8, I doubt whether your "old therma rest" is close to this so any new bag will probably not perform to stated rating......putting a CCF mat under it will help a lot.
An alternative to a new bag is to just carry 2 SB's & use the second unzipped as an over quilt (having one bag inside the other is generally not a good idea as the insulation often gets squashed so it's not as efficient), while not exactly weight or pack volume efficient it works e.g. two +4C rated bags would provide a combined rating of around -10C. Definitely a low cost alternative to help decide whether winter outings are your thing.