You are correct about r5.0 being the standard baseline, the R scale is based on 0-10(there are ratings above 10 however, but that is mainly housing insulation which is different).R value is the measurement of resistance to conductive heat loss based on a standard time period of 8 hours. A typical/average person will not need a pad rated more than r5.0 for an 8 hour period with a good cold weather bag unless they are sleeping above the ground(i.e. the pad is not directly on the ground/tentfloor.) If you are sleeping on a shelter floor, or in a hammock, or not in a tent, or an open tent or shelter then you may find the 5.0 pad not adequate due to the rapid heat loss of moving air. The standard 8 hour time frame is an important factor to remember because you might be comfortable when you go to sleep at 9pm until you all of a sudden awaken at 2am to find yourself chilled. I can not find where I saw it a few months ago, but I recall seeing somewhere that there is about a .2r(roughly 2 degrees) "loss" per hour. So if it is 32F when you go to sleep on a prolite plus/3.8 pad and are using a 32F bag then you may get chilled inside a tent after about 5 hours, or even sooner because at that point the 10F differance of an average tent has become overcome by the conductive heat loss from your pad on the ground. Basically if you take an r value and divide it by .2 and then you multiply that by 2 you will get a temperature number in F. Then you subtract the hours so 16 for 8hours(8x2=16F heatloss from conduction)Then you compare that to your bag rating. If it is close to that number you will probally be fine wearing just a light baselayer, but if your bag is more than a few degrees below(higher temperature) that number then you will probally get cold during the night( this is assuming the appropiate r value pad is being used for the air temps.)
This is also why cold weather bags typically start at 10 or 15F and everything else above that is considered 3 season. Because after 5+ hours unless you have a 15F or below rated bag you will start to loose significant body heat, there is no way around it, and if the air temp is below 32F you will easily get to or exceed your bags rating at some point in the night. Just remember every 2 hours your losing 2 degrees, no matter the ratings of anything, where this heat loss "stops" is when it reaches your sleeping pads corresponding r value. So if it's 32F and you sleep on a 5.0r pad for 8 hours with a 32F bag you have only lost about 4 degrees. now same example but with a 3.8 r pad and now you have lost 16 degrees over 8 hours.
All of this is largely based off the EN 13537 system.
Generally speaking you must use a properly rated pad for the temps as well as a properly rated bag, and wear a light base layer and a hat to get the max out of your properly rated gear.
First you must understand that there are several limits involved with the EN system. There is upper limit, comfort, lower limit, and extreme.
Comfort is the thermally neutral point for women, though most people use their bags to the lower limit which is the thermally neutral point for men.
There are lots and lots of variables than can go into these equations.(I am sure you have heard of people that use a 45F summer bad in winter also by just wearing a bunch of layers) But bottom line is, if you use a properly rated pad and bag you can go down to the true rating limit, otherwise unless you add in variables you will more than likely get chilled while sleeping.
To answer the question, where I got my information. I have read all of my information from the standardized testing procedures of the EN 13537 rating system. Which can be found in lots of places online.
The way they test is by using a heated mannican which has bunch of sensors on it in a climate controlled room. And they use the associated R value pad to go with the bags proposed limit and test for an 8 hour period for heat loss/transfer.
Some people will find based on if you are a warm or cold sleeper that the bag ratings are off either for the better or worse. It is only meant to be a standard test to meet somewhere in the middle, ymmv. I know several people that can easily add 5F or more to a bags limit, where as some people need to use a 0F bag for 15F temps. I sleep a little warmer than average and I find myself still comfortable nearing my bags extreme limit of 11.3F(15F bag)
Keep in mind the ways heat is lost, anyway you can find to combat this will add to your warmth.
There are four methods heat is lost while sleeping:
Conduction — This is heat loss through two adjacent substances due to a temperature gradient such as direct contact of your warm body with the cold ground, and the primary type of heat loss that a mattress reduces. Without a mattress or pad, you will continue to lose heat until the ground beneath you (e.g. the planet) warms—a battle you cannot win. The higher the R-value of your mattress, the slower this process becomes and the warmer you will sleep.
Convection — This is active heat loss when warmed molecules move from one place to another and take the heat with them. In a sleep situation, this is generally encountered when a breeze blows across you or into your sleeping bag, moving away the warmed air around you. Your sleeping bag and tent reduce this method of heat loss.
Radiation — The transfer of heat energy between two objects via electromagnetic waves. This heat transfer does not require a medium - think about how the sun warms your face from way out in space on a freezing day. Redirecting those waves minimizes this type of heat loss.
Evaporative — Evaporation occurs when a liquid changes to a vapor. This “phase change" requires energy (heat) and your body is the source of it. There are two ways this cooling takes place.
Insensible or active perspiration is the direct loss of vapor through the skin, such as while breathing. This effect is enhanced greatly when in an extremely dry environment due to the driving effect of a humidity gradient. For instance, the extremely dry air of deserts and high altitudes can vaporize sweat before it has a chance to accumulate on your skin. When inhaled, it pulls more moisture from your body as well, raising the need to be hyper-vigilant when it comes to staying hydrated.
Sensible or active perspiration has a powerful and straightforward cooling effect, via the formation of sweat on your skin. On a hot summer day this is great, but sweating profusely because you are overdressed and overheated in winter can wet-out your insulative layers, requiring vast quantities of heat from your body to dry them with disastrous results.
I am by no means an expert, just have done a fair amount of reading. As with any gear your mileage may vary. But based off what I have read from research, and feedback from people on multiple backpacking forums I find this to be pretty accurate.
Remember to be thermally neutral/comfortable(not chilled even the slightest) you need the appropiate R value pad for the temperature, and a properly rated bag to match.
Variables to consider: Tent, baselayers, hat, additional layers or thicker baselayers, eat something high in calories right before bed, stay well hydrated. If you wake up in the night, eat something like a handful of trail mix or some chocolate. Your body uses alot of energy to keep you warm, and alot of people get chilled at night because they hiked 15miles and got to camp at say 5pm, ate dinner at 530 and then didn't go to bed until 9. So around 11 or 12 your body is now lacking enough calories to properly maintain your warmth. Eat a cliff bar, trail mix or something right before bed and you will see a significant difference. Things that also help are putting hot water in a nalgene inside the bag, or adding some of those chemical handwarmers into your sleeping bag.
Hopefully yall can make sense of all that, pulling from what I remember.