I get messages from people who find that they cannot walk through a doorway or up stairs, etc. And it is hard to explain why this is, and why in most cases by far the easiest thing is to re-rez the house.
So I am going to try to do that here, in a very simplified way, and only in regard to doorways (floors are another matter, though the same principles apply).
First let me explain what the three Physics Types (PT) are:
1. Prim — this is the legacy physics type, which any cube or other shape you rez from scratch will have. All SL content originally had this physics type.
If you make a hole in a wall made of a prim with Prim physics shape, you can walk through the hole. This is how most non-mesh doorways in SL were always made. You made a hole to walk through.
2. None — None is the same as Phantom, except Phantom makes the whole object phantom and with None you can make only one prim phantom. With a doorway wall set to none, you can walk though not only the hole but the whole wall.
3. Convex Hull — this is the default for mesh. Think of it as shrink-wrap. You cannot walk through the doorway because the hole is shrink-wrapped. (The shrinkwrap can have random smaller holes in it but not usually ones you can walk through.)
Now when the new “mesh accounting” system came into being I found I could cut the prims on my houses by 40% just by making MOST of the prims Convex Hull (CH). (I did this in the Features tab of edit.) The reason the prims dropped is because when you link two regular cubes (walls), together they are not two but ONE Land Impact. So, yay!
But shrink wrapping the whole wall the doorway was in blocked the hole, so you could not walk through any doorways.
So I had two choices: change the walls and doorframes to None — so anyone could walk through them as if they were phantom — OR to make them Prim, so people could could walk through them like regular prims.
Often, making them prim was the way to go BUT I found that sometimes, for reasons I don’t think anyone fully understands, a wall that has been hollowed and cut to make a doorway will shoot way up in prims. Recently I found every prim like this in a house was 17 Land Impact!
So what I sometimes do is make the wall None but slip regular prims inside the walls to block people from walking through.
(I will also sometimes do this with mesh walls if I find that adding regular prims inside walls has lower land impact than uploading the wall with a physics shape — this is too complicated to explain here, but has to do with the fact that adding a regular cube prim to a house can often be lower LI than uploading a large mesh with a physics shape.)
Here is what causes the problem. When you unlink a prim with None Physics Type, it stops being None and becomes Convex Hull — so any hole you walk through is closed.
This is what causes the blockage. And if you know how, and it’s just one prim, you can just link that prim back — without changing the root prim — and change the Physics Type back to None (or Prim).
Bottom line: If you intend to unlink and relink something, check the Physics type and make sure you restore it after relinking.
But if you have unlinked a lot — and sometimes people unlink (and then relink) the entire house — and there is no easy way to fix the problem except to re-rez the house.
And re-rezzing is very simple unless someone has modified his home and failed to take copies, so if you mod anything, be sure to make regular backup copies.
And that was the SIMPLIFIED explanation! There is a lot more to it. This is just a way to explain that when I say re-rezzing is the easiest solution by far, it’s because it is.