Two Walls. Twice as Good as One?
Actually it all depends
Many people view single wall tents and double wall tents as merely a different style or feature. Sort of like the difference between a tent with a vestibule and one without. The reality is that these two categories of tents are actually quite different and best suited for very different conditions.
Let's start with tent construction in general. A tent must provide shelter from the elements, and this can be broken down into three categories. Keeping wind and water out (such as a rain storm), letting moisture out (such as the moisture produced by respiration), and letting air in (for the purposes of ventilating the tent).
Single wall and double wall tents approach each of these importantfunctions differently. Asinglewall tent is constructed of just one layer of fabric. When you are inside the tent there is only one single layer between yourself and the elements. This means the one layer must simultaneously prevent any water or wind from entering from the outside, while at thesame time allow moisture to escape from the inside. Modern fabrics are capable of doing this but often when you ask a piece of equipment to perform two different functions you will not get the same results as two single purpose pieces of equipment. This explains one of the major complaints of single wall tents...they just don't breathe well allowing moisture to accumulate on the inside.
Double wall tents are actually two tents in one. The main tent is made usually made of a
water proof and breathable bottom and a mesh top. In warm fair weather this allows you to be sheltered from the ground and enjoy a nice breeze (and view). The second part of a double wall tent is the rain fly. This is a water proof cover that will go over the top of the tent in inclement weather. Double wall tents allow up to three configurations: tent without fly; tent with fly; or fly alone (used as a tarp or sun shade). This style tent breathes very well and does a nice job of keeping out the elements.
So it might sound like the double wall tent is the winner. Not so fast. Although single wall tents tend to perform poorly in hot or humid circumstances, they do have their advantages. Single wall tents are quieter to sleep in (no flapping rain fly), they are smaller in your pack (half the material), they set up and tear down faster, and they are lighter (less material again). This makes the single wall tent a very popular choice with ultralight backpackers, and hikers.
Ultimately the choice between a single and double wall tent comes down to personal preference, and personal preference is informed by direct experience. If possible try to borrow or rent one of each before making your purchasing decision. Some large outdoors stores will also have floor models set up so that you can crawl in and try them out. When in doubt I would recommend calling a local state or national park and asking the Rangers in your area what type of tent they themselves prefer. Often times these outdoor professionals will have the best recommendation for your particular location.