I recently downloaded the texture pack of Mtgr23 and all of his replacement textures (architecture, landscape, trees, clutter, plants, etc.) are compressed with dxt5. After reading an article about texture compression, I learned that dxt5 is not really needed for all textures (except for e.g. 3d models).

Since compressing in dxt1 makes it half the size with no quality loss, I'm interested what textures exactly need dxt5. Is there a guide somewhere that explains it in more detail? Do I lose quality if I recompress some of the textures with a program like thecompressonator?

Here is something about dtx comppresion http://cs.gamesas.com/constwiki/index.php/Choose_the_right_DXTC_compression_algorithm

if the texture has already been saved out and compressed, then recompressed again further down the line, it will introduce quite a but of extra artifacting.

The difference between DXT1 and DXT5 is that DXT5 has an alpha channel while DXT1 doesn't have one. Alpha channels are needed for various things. On diffuse textures (the 'image' you see in game) it is used either for transparency effects (like on cobwebs) or parallax effects (can be seen in some caves, on the walls of the prison cell you start in etc). Most diffuse textures don't have an alpha channel and thus can be saved as DXT1 since transparency is only needed when it's needed and parallax isn't used very often because it doesn't look good in all cases and it gives a performance hit. You also need to change a setting in Nifskope to make the effect work (which means unless the replacer comes with nif replacements as well there can't be any additional parallax effects).

On normal maps (a kind of bump map) it is used for specularity. For example shiny swords have a bright alpha channel, but also the glossy Ayleid ruin walls. Many normal maps make use of the alpha channel and whether it is needed or not lies in the eye of the beholder (unless the alpha channel is completely black - then you can just save as DXT1).

If really all the textures are saved as DXT5 you can say for sure that many of them don't need that kind of compression, especially when the replacer doesn't come with nif replacements.

thank you all for the kind advice!

I generally use:

DXT1 - Diffuse maps (no transparency)
DXT3 - Diffuse maps and Icons (with transparency)
DXT5 - Normal maps

I like to use 8:8:8 and 8:8:8:8. Not for everything, but some things do benefit from it.