First, Keola, welcome to the site. Second, congratulations on your house.
GCF has given you one way to identify interior faces, but I think there are others. One of these is to have the object in wireframe view, as your first image, and rotate your mesh so that you are looking at the object through the floor. In face select mode, the faces comprising the interior walls can be more easily seen through the floor, and deleted. Note though, that the interior faces may not save you all that much, considering the method you used in building the roof. If you're attempting to model a typical "liveable" house, (by which I mean one that is well maintained) you may be able to save considerable geometry by making the roof with a large, flat, sloped plane, rather than constructing it as you have done with rafters and what seems to be individual boards in the surface. Constructing the roof as you have done in the screen shot, you have considerably more geometry in the roof sections than you have in the walls, where you have not shown the interior studs of the walls. [Note that if you are modeling a derelict house, where part of the roofing materials are missing, you may want to have the rafters and individual boards of the roof.]
As to the "invisible" faces, this sounds to me to be most likely caused by "flipped normals". There are to ways I correct these, both ways being done in edit mode, and textured shading. Textured shading is selected in the drop down menu of the shading icon just to the right of the interaction button (where you choose whether to use "edit view" or "object view". "Textured shading" is selected by selecting "texture", the option indicated by the red-and-white circle icon. If there are only a couple of instances of flipped normals, I simply select the faces, and flip the normals. If there are a larger number of faces, I use the A key once or twice to select all of the faces, and choose the option to recalculate normals. For more information on Normals, see the Wikipedia pages http://en.wikipedia.org/wiki/Normal_%28geometry%29 and http://en.wikipedia.org/wiki/Vertex_normal.
The exact location in the User interface where one would find the options to flip normals changed in more recent versions. It has always been in the tool shelf in edit mode, and in more recent versions is under the "Shading/UV" tab.
Your idea of implementing reusable components like walls and rooms is a good one, but I suspect you will find that it is difficult to have these components UV-mapped in advance. One of the reasons for this is that many game engines prefer the number of textures to be minimized, so you will likely find it expedient to make your mesh, then prepare a single texture in an image processing package [I use GIMP; cf. http://www.GIMP.org] as a collage containing all of the bits of the building, and map the various surfaces of the mesh to the appropriate locations on the texture.
For myself, I find that I can usually make a wall (or even a simple building) quickly enough from scratch that I'd have it constructed before I remembered which ~.blend file contained the various components of the vuilding, and finding the right component in the ~.blend file, though I do make use of the technique of re-using blender components for more complex meshes.