Joining rooms made with archimesh creates one sided model I'm a real blender noob (maybe 4 hours of experience). I created model of a house with archimesh addon. Once I was happy with the layout, I joined all the rooms into one model using CTRL + J. I tried to texture the outside of the house but it does not have vertices, edges nor faces. 

Here is image of the faces my model contains when rooms are joined:



I took a look at backup of the model where rooms are not yet joined and I noticed that rooms didn't have "backside" then either:



I want to see the edges of "outside" of the house (and each room) so I can mark seams to start texturing. Please help a noob.
Archimesh uses a so called solidify-modifier to make your walls thick (instead of "real" geometry that you can manipulate in edit mode)

you have multiple options to change the materials:


you can apply the modifier: go to the panel on the right and click on the blue wrench icon. there you can click apply (green box in the screenshot)
you can enter a material index offset of "1" in the modifier panel and simply add a second material to you walls