When we were constructing one of the additions, I wanted this one to feel like it WAS an addition and I decided to use shiplap on the walls to create that effect of having it look like it was added on at some point. There are many ways to do shiplap. I prefer a more rustic look in both material and installation. Here is how we brought it to life.

Prepare the wall
Take perfectly good walls and paint them black! Yes, black. You can purchase the cheapest paint you can find for this. Why black? Well, when you hang your boards, everything looks uniform because there is a black base underneath. By uniform, I do not mean straight or equal gaps. I mean it looks like it is supposed to be that way. I wanted it to look a bit rough and not perfect.
My husband would tell you we had perfectly good walls and then I paid someone to hang grade 2 wood on them. That’s exactly what I did but with four kids, a dog and a cat, nothing wears better than shiplap on the walls. You cannot hurt it! You can ding it up and I will call it character.
There are a couple of things that you need to do to make it hang correctly besides the black paint. You need proper baseboards. You need squared off base boards that stick out further than the shiplap will. You cannot install shiplap on a typical suburban baseboard or it won’t sit or look right. Same thing with any crown moulding you may have. It has to sit right beneath it or if you do not have any, you can add a piece on top of the shiplap to finish it off.
Once the boards are all up, you use a brush and paint them. You cannot use a roller because the paint will not go on correctly.
And there you have it! Ship-lap walls. We used #2 spruce on our walls. It is full on knots and character!