The inflatable sleeping pad I bought last year is leaking air and I'm debating the wisdom of taking a replacement to Alaska in September. When working, it's more comfortable than a foam pad. But I worry it will provide no insulation when deflated. I have also never used a camping air mattress below 50 Fahrenheit.
Is there a risk of hypothermia from a leaked inflatable pad?
When working properly, do inflatable mattresses provide sufficient insulation for cool / cold temperatures?
It would not be wise to take a leaking inflatable sleeping pad on any trip where you actually need a sleeping pad, and cannot hike out and replace it easily.
The pad is already leaking. You have to weigh the probability and consequences of the leak getting worse, maybe much worse, on your Alaska trip vs the cost of replacing the pad now.
Hypothermia at 50 degrees or so is unlikely if you have warm clothes, a change of warm clothes, rain gear, a sleeping bag rated for significantly colder than 50 degrees F and a tent. I doubt you will get hypothermia even with a completely deflated pad. And you can always put on more clothes, or spread some clothes under you in your bag. The only caveat about hypothermia at night at 50 degrees F or so is that if it is likely to rain continuously for several days, but that is a different question. There is another caveat, by "below 50 degrees" you didn't mean below 40 degrees, did you?
As for comfort, you will be much more uncomfortable if your inflatable pad seriously deflates than if you used a foam pad. Also, if you are really uncomfortable with the foam pad, get tireder than you normally do; that often guarantees a sound sleep on anything!
So, unless you are on an extremely tight budget, I recommend that you replace the leaking pad.
The answer to your last sentence is yes, but again, bear in mind that 50 degrees F or so is not really cold. I've used an air mattress when the water bottle outside the tent froze solid, and I was perfectly warm.
(Not completely frivolous aside, see the movie Alpha for inspiration. 20,000 years ago people didn't have inflatable or foam pads, and managed to sleep well. We should be able to do anything they could do!)
Most backpacking sleeping pads are insulated, some are even down-filled. When they're deflated they lose a lot of their insulating properties. This can be detrimental in cold weather, because when you're laying down most of your heat loss is by conduction through the ground.
If you take your mattress to go camping in the cold, it will give you cold spots if it deflates.
When camping in extreme cold it's recommended to use two pads: an insulated air mattress over a closed cell foam pad—this system is what the expeditions on Mt. Everest sleep on—the combination of the two provides you with the best insulation against the ground.
You don't mention what temperatures you expect to encounter. From experience temps of -5 to -10 C [ 23 to 14 degrees F] are 'safe' but unpleasant without a sleeping pad. You end up putting most of your clothing underneath you. You end up turning over frequently, as the ground side gets cold.
I have done this at colder temps making a several inch bed of spruce boughs. This works down to temps of -30 [ minus 22 degrees F) or so, assuming you have a reasonable sleeping bag for those temps.
