How to keep the inside of the camping tent dry on rainy days?

You did not quite specify whether the moisture was internal from condensation of body vapors or whether it was penetration from seam and joint leaks from the outside. If condensation, you must try to increase ventilation which will at least get humidity inside the tent close to the level of humidity outside the tent. If penetration, then you should purchase a good seam sealer from an outdoor gear shop which is like silicone rubber. Then apply it to all seams and joints, dry for a day, hit the tent with water or rain at home before you go afield to test whether you have solved the problem. The alternative is to purcahse a more expensive tent with a full-cover rain fly and factory sealed seams. I only purchase backpacking tents in the class of $300 or more. No leaks. If smaller in size, these can give you some condensation moisture on the walls.

Most of the moisture gets into the tent because we heat up the air inside the tent with our body heat and our exhaust from out breath is loaded with humidity and it condenses on the walls of the tent. They make super soaking camping towels, use them to get all the moisture out. Sop as much as you can and squeeze it outside and repeat. If it is clear the next day let the sun dry the inside out (and your bags and stuff). Ground cover the size of your tent helps and a good rain cover is priceless.

Most of the moisture gets into the tent because we heat up the air inside the tent with our body heat and our exhaust from out breath is loaded with humidity and it condenses on the walls of the tent. They make super soaking camping towels, use them to get all the moisture out. Sop as much as you can and squeeze it outside and repeat. If it is clear the next day let the sun dry the inside out (and your bags and stuff). Ground cover the size of your tent helps and a good rain cover is priceless.

My suggestion is you need a better tent. Keeping you dry is really one of the most important reasons for using a tent (the others being protection from wind, protection from insects, and privacy). As others explained, the moisture could be from the tent leaking. Sealing the seams might help. Use a sealant type recommended by your tent manufacturer. The other potential source of leaks is the floor. You should use a waterproof footprint or ground sheet under the tent. It is important that this ground sheet be slightly *smaller* than the floor, about 2" short on each side. This is to keep the rainwater dripping off your tent from collecting between the ground sheet and the floor. The last source is moisture in the air itself, either from your breath or the humidity. Without adequate ventilation this moisture will condense on all cool surfaces and drip down onto the floor and your gear. This is why a separate rain fly covering the tent is a better design than a single-wall tent. The rain fly should provide coverage for your vents, windows and doors so you can leave them open for ventilation, but still prevent rain from getting in. The moisture will collect on the rain fly and drip to the ground outside the tent. If your tent does not have a rain fly you might consider setting-up a tarp over your tent so you can keep your windows open for ventilation without letting in the rain. Don't just throw the tarp over your tent, suspend it at least a few inches above and at an angle so any rainwater or condensation can drip to the ground, not into your tent.

My suggestion is you need a better tent. Keeping you dry is really one of the most important reasons for using a tent (the others being protection from wind, protection from insects, and privacy). As others explained, the moisture could be from the tent leaking. Sealing the seams might help. Use a sealant type recommended by your tent manufacturer. The other potential source of leaks is the floor. You should use a waterproof footprint or ground sheet under the tent. It is important that this ground sheet be slightly *smaller* than the floor, about 2" short on each side. This is to keep the rainwater dripping off your tent from collecting between the ground sheet and the floor. The last source is moisture in the air itself, either from your breath or the humidity. Without adequate ventilation this moisture will condense on all cool surfaces and drip down onto the floor and your gear. This is why a separate rain fly covering the tent is a better design than a single-wall tent. The rain fly should provide coverage for your vents, windows and doors so you can leave them open for ventilation, but still prevent rain from getting in. The moisture will collect on the rain fly and drip to the ground outside the tent. If your tent does not have a rain fly you might consider setting-up a tarp over your tent so you can keep your windows open for ventilation without letting in the rain. Don't just throw the tarp over your tent, suspend it at least a few inches above and at an angle so any rainwater or condensation can drip to the ground, not into your tent.

I car camp, so I always have a duffel bag of old bath towels. I roll them up along the inside edges of the tent in case the seams leak during a rain storm. They make good throw rugs inside the tent too. I stretch a large blue tarp over the tent and stake down the 4 corners. It's about 4' bigger than the tent all around and the rain runs off away from the tent,no mud by the entry and you can open the windows to let the tent breathe during the rain.

I second all the excellent information the previous answers have given. I would add that the dryest tents for heavy rain use are the ones where most of the ceiling and walls of the tent itself are made of mosquito netting, not nylon. The rainflies of tents like this are always designed to cover those mesh areas completely so rain will not get in -- the huge advantage over nylon walled tents is ventilation: condensation moisture can't collect on the mesh and you will stay much cooler and dryer inside. We camped out in the Florida Everglades last month in my Marmot backpacking tent with a full mesh roof. Even though it was very humid and we had nights of almost continual heavy thunderstorms, the inside of the tent stayed cozy and comfortable with no condensation or rain drip. We noticed when the weather cleared up in the mornings that the other people camped around us had to lay damp sleeping bags and gear out on picnic tables to try to dry it out while our gear was completely dry,

Yes by all means, spend 300.00 or more on a tent or it will leak. Or 6 Coleman's just to make sure the bases are covered

I second all the excellent information the previous answers have given. I would add that the dryest tents for heavy rain use are the ones where most of the ceiling and walls of the tent itself are made of mosquito netting, not nylon. The rainflies of tents like this are always designed to cover those mesh areas completely so rain will not get in -- the huge advantage over nylon walled tents is ventilation: condensation moisture can't collect on the mesh and you will stay much cooler and dryer inside. We camped out in the Florida Everglades last month in my Marmot backpacking tent with a full mesh roof. Even though it was very humid and we had nights of almost continual heavy thunderstorms, the inside of the tent stayed cozy and comfortable with no condensation or rain drip. We noticed when the weather cleared up in the mornings that the other people camped around us had to lay damp sleeping bags and gear out on picnic tables to try to dry it out while our gear was completely dry,