# Why are tertiary alcohols very inactive in acid catalyzed hydro-alkoxy additions?

...is too bulky and the reaction would take a very long time to proceed. The alcohol would have to move around slowly until it finds the right orientation with the cationic intermediate for the backside attack. By the time it finds the right orientation so that orbital blockage is not an interference, the good leaving group $O {H}_{2}^{+}$ would have fallen off (no joke)---the water substituent on the protonated alcohol is not nearly as good of a base as a tertiary alkyl group would be (pKa of alkyl >> pKa of water).