I have never used a footprint under a tent floor. I have made some for floorless tents, though. There are some interesting questions about tent floors and footprints. If you have a floor, why do you need a footprint if the floor isn't defective? The thing that got me to thinking was a statement, I don't remember where I read it, that the tent floor fabric may lose its waterproofness when pressure is put on it. That is similar to the old idea that tent walls will leak if you touch them while they are wet. I wonder if this doesn't go back to the days before coated fabrics. I have never tried to put pressure on a modern coated fabric to see if water will come through, so I don't know that this is true. In any event, the question comes down to whether you want to protect the tent floor from abrasion and punctures or do you want to protect yourself from water leaks. A waterproof liner inside the tent on top of the floor will protect you from water leaks, a footprint won't. I'm not a big fan of polycro, but it seems that using that as a tent floor liner might be a good application. If you want to use Tyvek, which I'm not certain is really waterproof, you will likely have to seam it, since Kite Tyvek is only available in 60" width, as far as I am aware. There is Tyvek repair tape available, but I don't know how this would work for seaming. Housewrap is normally Typar and is heavier and crinklier. There is a Housewrap called BlockIt that is much softer and is available in 10' width. I had some and wanted to make a footprint for a floorless tent, but used all that I had in construction. I want to get some more, but at $100 a roll, I am in no hurry. The best thing is not to pitch the tent in a hole, but site choices don't always allow this. Window film might be a good approach for a safety backup barrier.