Ref. No. 1270 Alif
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
It is compulsory to cover the private parts of the body during namaz. For males, from below the naval to below the knees is compulsory to be covered well. The right hip & the left hip, the right thigh & the left thigh, and posterior etc all are counted one part each.
Uncovering 4th of one part (or more) is deemed to equal the "exposure of the private parts" and, therefore, prohibitive to prayer. If the uncovered part is less than 4th of that part then the prayer is valid.
Likewise, covering the private part (Aurah) with thick and opaque clothes that do not display the skin color underneath them without close observation is compulsory. If a person performs prayer in wet clothes that reveal the color of the skin clearly then it is prohibited and the person must repeat the prayer performed while wearing them. And Allah knows best
Darul Uloom Waqf Deoband