Bed bugs tend to be centered on the bed. There are exceptions of course such as mattresses that have very little ribbing and mattresses on the floor. Both of these circumstances tend to push the bugs to the baseboards and surrounding furniture. But in the case of an ordinary mattress with ordinary ribbing with a box spring and a frame with legs the bugs are concentrated on the bed. As the infestation ages the bugs tend to push out from the infested bed in concentric circles.
The difficulty with mattresses is that it is illegal to apply residual chemicals to the mattress. The box spring also has many crevices, solid pieces of wood, and corrugated cardboard that is difficult to treat. Even a steamer is a long shot in getting all the bugs out of a box spring. As a result encasements have become popular. The theory is that if you starve the bugs in a sealed bag they do not need to be treated. The other bugs located off the bed are then addressed with residual chemicals.
These encasements must then be left in place for up to 2 years. And when I inspect these items in the course of business I often note they are full of holes. The biggest culprit is the fabric is crushed between the frame and the hard wood edges of the box spring causing holes. As far as I know there are no studies that ascertain the usefulness of these encasements over a 2 year period. Holes in the encasements are not conducive to bed bug treatments as the bugs will simply leak out over time. As an experiment the next time you stay in a hotel lift the bed up and inspect the box spring encasements – they probably have holes where they touch the frame.
Even if the encasements are successful in their initial task a secondary difficulty arrises with the next infestation. A bed encased preemptively will no longer harbour bugs on the bed as all the good harbourages are gone. As a result the bugs move off the bed to the next closest items such as night stands, dressers, baseboards and nearby clothing. The result is that there is an increased likelihood that, even in relatively mild cases, the bugs will transfer to other suites directly via the baseboards and the infested clothing will be worn in public areas causing those areas to become infested. Therefore I question the use of encasements as a matter of public policy. Perhaps they cause more harm than good.
I suggest that if you insist on using encasements preemptively that thin strips of corrugated cardboard with the holes showing on the edges be scattered under the bed, between the box spring and mattress, between the box spring and frame, and by the baseboards. If you do become infested the bugs will start by harbouring in the cardboard. In turn the threat to other suites and nearby clothing is reduced. Prevention is always better than treatment.