I've always footprinted outside of my tent but found water inside my tent after really heavy rainfall. This has led me to wonder whether an additional liner inside of the tent would serve useful as a second footprint.
Have you done this? What are your thoughts on this double footprint approach (if pack space and additional weight aren't a concern)?
I've used a double liner (one inside the tent, one outside) before, and it was very effective. When I was in high school, I went on a multi-day backpacking trip where it rained every day. This was in the forecast, and bringing the double liner was the only thing that kept the water out.
You could also double check that you're setting up your tent in a site that will be less likely to have water running along the ground in a heavy rainstorm.
If you only have one liner, I'd recommend you put it outside then tent, because that will help protect the tent floor from wearing out when it grinds against the ground as you lie inside.
Footprints:
My point is this: if you have a waterproof floor on your tent, you don't need any footprint. The trouble is, you are subjecting your tent floor to the abuses of rocks, sticks, sea-shells, brambles, or whatnot - meaning it will quickly get micro-tears and perforations.
Enter the footprint: a cheaper, replaceable protector for your tent floor. Thus, you want to put it OUTSIDE to protect the floor of your tent, which is (or should be) waterproof already. The footprint itself is usually waterproof (and in fact. some tents can be set up with just to footprint and the rain-fly as an ultra-light body-less option.)
Now, if both your footprint AND your tent have lots of wear and holes, then neither will help much (even together) in a deluge. Sure, ANOTHER footprint inside would help (assuming it is structurally sound) but if you need it, you might re-consider how your floor got to this state of disrepair in the first place, and re-adjust accordingly.
If you put down a ground tarp that is significantly larger than the tent (and by significant I mean more than an inch or so), tuck any extra under the tent, otherwise rain will hit the tarp and then go straight to the tent-floor, opening the door for tent-floods.
I have never put a tarp inside a tent, but I have also never had an issue with tent-flooding once I learned to tarp outside and tuck under.
I do both. I use some Tyvek I got from a construction site (one of the workers rolling the outside of the house gave me enough for inside and outside for $5!). I use the outside liner to protect my tent from sticks and other sharp objects that can puncture the floor. On the inside I use it, well, because I can be a bit of a pig, I suppose. Dirt, crumbs, etc are easy to get rid of by folding it and dumping it outside the tent. No sweeping to clean out the tent, either. It works great and Tyvek is VERY lightweight material.
My tent is old, maybe 400 nights of use. If I put it down on damp ground water will seep up through the floor whose PU coating is well worn. Since the tent is still serviceable other than that I am thinking of making a bath-tub floor to be used only when the ground is wet or when it might rain.
I think I will put it inside the tent so I can make it from one piece of material without any holes, folded corners sewn only on the top, less than 3 yrds of material for my one man tent. I am still thinking how to support the sides of the bath-tub within the tent. I could add a few additional loops to the above nylon mesh interface of the existing tent and then just tie the sides of the bath-tub up to these loops.
I think just some inexpensive $4/yd, 70 denier nylon, with a decent PU coating should be sufficient.
While on a month long canoe trip we were using well-used tents, a tarp under the tent was usually all that was needed. But we did have a few nights where adding an extra tarp to the inside to keep sleeping bags dry was a great quick fix.
Most people on the trip had their own tarps, though we were sharing the camp's tents.
