Porosity is a key component to hair moisture. Porosity is the measure of absorption in the cuticle layer of your hair. Hair can be described as high, average and low porosity. The cuticle layer of your hair is arranged like shingles on a roof. A tight, compact shingle pattern makes penetration difficult yielding low porosity. A loose, raised shingle pattern makes penetration easy yielding high porosity. In between is medium porosity.
Low porosity hair is resistant to water, heat and processing. Products tend to lay on top of the hair shaft rather than be absorbed. Good deep treatments for low porosity hair include alkaline solutions like baking soda in order to open up the hair shaft. Those with low porous hair should avoid thick creams or butters as they will not be absorbed and leave hair feeling greasy.
High porosity hair absorbs and releases product readily. For this reason it is more prone to dryness and damage from heat and processing because the cuticle remains open. Highly porous hair requires the use of oils, butters or silicones to seal the moisture into the hair to avoid evaporation. Those with highly porous hair should use acidic solutions like vinegar to close the hair shaft.
Average porosity hair will absorb and retain water and products. While you have a given hair porosity, you can change the condition of your cuticle through mistreatment. Excessive heat,over-processing or harsh detergents can cause damage to the cuticle layer resulting in high porosity hair that is dry. It is also said that frequent prolonged henna treatments can seal the cuticle layer resulting in low porosity hair that is also dry.
There are simple tests to help determine your hair porosity. Grab a single strand of hair with one hand then using the other hand slide two fingers up the hair strand from the end to the root with the strand between them. If the strand feels smooth or polished like rubbing wood with the grain, you have low porosity. If the strand feels bumpy or ruffled, like when you rub velvet against the nap, you have high porosity. Another way to test porosity is to place a single strand of clean, dry hair in a bowl of water. If the hair sinks to the bottom you have porous hair. If the hair floats on top of the water you have low porosity hair. If the strand floats in the middle you have medium porosity.