Foam sleeping pad redundant if packing a backpacking Air Mattress?

I see some people recommending brining both, but wont the air mattress insulate you enough from the ground to just use the sleeping bag on top of it? Or is dual layering with a foam pad really necessary?

it depends on how cold.  also the type of terrain and how far from the car you will be.  air mattresses can pop or leak and be difficult to repair in the field.  I have seen them leak or pop.  A foam pad won't break and can be used alone and keep you alive.  If you just had an air mattress and it broke, you were 10 miles from the trail head, and it were below freezing you're in for some uncomfortable times.  You could sleep on the rope or your pack or whatever, but packing at least a 3/4 length foam pad is prudent in cold conditions.  YMMV.

Having both will feel like a luxury and if/when the air mattress pops (has happened to me 2x in the alpine) you'll be glad to have the foam. 

Not only does it provide insurance against leaks, but I've found that a combination of a 3/4 length 5mm Evazote closed-cell foam pad and a three-season air mattress (1" Therm-a-Rest) is warmer and just the same weight as a four-season air mattress (1.5" Therm-a-Rest) used on its own. The only problem is that Evazote pads are hard to find in the US. You can get them at MEC in Canada, though.

Some rando climber's ​thoughts​​​.

When I was in Alaska (in summer, at lower altitudes, i.e. not very cold), I would snatch my therm-a-rest out from under my stuff and throw it in the cook tent during the day so we'd have somewhere to sit.

...yeah, it makes a difference. My bag was thick enough that I don't think I would've been bothered without it, but it definitely added considerable warmth over just my inflatable. Eventually I just cut the bottom quarter of it off to use as a seat (sorry, other dudes, just room for me now. I know you have mats, too, dammit!) without having to come back to notably chillier sleeping bag.

Depends on the r value of the air mat. If you are on snow then an r value 4 or 5 is preferable otherwise the mat will be cold.

Also as mentioned above the redundancy thing is nice if on snow. Personally on snow or in winter I bring an air mat with an r value of 5.5 and make sure there is at least one foam pad in the group I can borrow if needed if I get a flat. 

I've decided I will be bringing both from now on. For me, it's not a matter of insulation; the comfort of an air pad is pretty clutch at night but I also enjoy having a foam pad for stretching/yoga after long days. Or just to sit on around camp. I guess this is one of those personal decisions and weight considerations factor in heavily...but popping an air-mattress in the field really sucks. And those patch kits aren't a guarantee. 

Dr. Doom has some good thoughts on the matter. Personally, I find air mattresses more comfortable, but almost all I have had have malfunctioned, Exped, Klymit (sp?), etc. My NeoAir has escaped any leaks so far, but I imagine its only a matter of time. If I only have an air mattress I either ensure the temps won't be too bad or I have a backup pad/rope, etc. I've slept on the rope a couple times and its not that much worse than thin closed cell foam. YMMV.

Do you not bring any repair supplies, or have you just not had any luck with them?

I know there are specialty repair kits out there, though I'd wager the old standby of Tenacious Tape and Seamgrip would do the job.

David M wrote: Do you not bring any repair supplies, or have you just not had any luck with them?

The problem is that you have to locate the leak in order to fix it. It only takes a tiny pinprick to deflate an air mattress, and locating a tiny pinprick isn't always easy. You may have to submerge the entire mattress in a tub of water, which can be a challenge if you don't have a mattress-sized tub handy.

I once tried Tenacious Tape. Cleaned the area with isopropyl alcohol and applied tape in the field..didn't work. So when I got home I went all out with proper repair supplies. Still had a slow leak. Since then, I keep a repair kit, but given how vulnerable air pads are to puncture, and the possibility of a repair failure, I'll stick with the one air one foam pad (when conditions allow it). Sometimes every ounce counts and one pad will have to do; but to answer OP's question, sometimes redundancy is worth the extra weight. My .02

David M wrote: Do you not bring any repair supplies, or have you just not had any luck with them?

I know there are specialty repair kits out there, though I'd wager the old standby of Tenacious Tape and Seamgrip would do the job.

Well, ideally the leak site is obvious (ie you just watched your buddy put his pack into the tent, it tips over and something sharp attached to the outside punctures your pad). Ideally you are not totally werked, dehydrated, hypothermic, your fingers are not frozen, and you're thinking clearly. Ideally, your first patch holds air. Sometimes not everything is ideal. Just finding the leak in the field can be a pain. Sometimes it's the valve and that's not easily repaired. Handling tape with gloves on doesn't work, frozen fingers aren't much better. The repairs don't always hold, especially if they're between baffles. I've had repairs not go perfectly in the comfort of my home, I think planning on pulling one off in the mountains is a gamble. 

So if it's an overnight trip in mild weather, every ounce counts and you could probably survive sitting on your pack and ropes if need be...your call on bringing just the air mattress. In cold weather or in the expedition setting I think it gets to be a less enticing proposition.

Obviously if you can't sleep well without an inflatable (that's many of us) then you need to bring one, just consider some kind of backup. A good "middle of the road" option is a full length foam pad and a torso length Prolite (old school self inflatable). Compared to the newer inflatables it is not as light, compact or cushy BUT the Prolite is more durable, easier to repair (flat surface), less effort (2 breaths to top it off), and the open cell foam inside still provides some insulation if it does take a shit. 

Martin le Roux wrote: Not only does it provide insurance against leaks, but I've found that a combination of a 3/4 length 5mm Evazote closed-cell foam pad and a three-season air mattress (1" Therm-a-Rest) is warmer and just the same weight as a four-season air mattress (1.5" Therm-a-Rest) used on its own. The only problem is that Evazote pads are hard to find in the US. You can get them at MEC in Canada, though.

Evazote is on Amazon Prime for cheap :-) This is basically the classic Karrimat. 

If you want thinner foam, ​Exped​​​ has this one you can cut to size, or double up.

​​Gossamer Gear​​​ sells it too in 1/8 and 1/4in, but they're short at 60in.

I am a backpacker who started doing a little climbing. I have slept on a lot of different things in different conditions, over the years. I am an old guy too. If you spend some time online you will find that there are entire backpacking pages devoted to your question. First, what you may carry in warmer weather is different in what you would need in colder weather. Secondly, everything else is personal preference: how much you want carry, how much you want to spend, and how much cushion you want when you sleep. I would suggest start cheap with something like Thermarest Z lite pad and REI’s Stratus inflatable. I own several others of different types, but I use these the most, separately and in combination. See what you like.

If you are lucky you can find the leak, but more times than not it isn't obvious. I don't use a foam mat often but my CCW pack uses one as a frame so it is there for an emergency.

Definitely personal preference (and potentially temp related). There are those who adhere to the statement that "not all ounces on your back are equal" - and that sleeping pad ounces can be far more valuable than most if it means a good nights sleep.  
For cold weather or puncture protection I'll often put a closed cell under an inflatable.  But honestly for comfort (or noise reduction of a neo-air) I'll put a closed cell on TOP of the air mattress and find that to be a SUPER comfortable combo for me.