We have a 400 foot building that we refer to as the back house. It has
electrical, phone, heat and a fireplace. No plumbing. The idiots who owned
this house before built it and much like everything else they did, they
screwed this up as well. I think.

We just had a new roof put on that building and the house and are getting
ready to have them painted. But the painter noticed several pieces of
siding that were rotting on the back house. We knew of two but there were
three more pieces.

Husband didn't like the price that the painter (he also does siding,
windows, roofs, etc.) had quoted so decided to do this himself.

Well... Long story short, whoever built this building just slapped the
siding up over the 2 x 4's which is likely why it is rotting. Daughter said
she thought there should be Tyvek up underneath. My mom thought tar paper.
I looked online and it said specifically for a shed which is basically what
this is, to use house wrap. Not sure what house wrap is.

So my question is... How bad is it not to have anything underneath? And
what (if anything) should be underneath? The building is mainly used for
storage of seasonal items and we don't really have a lot of those. Also
some lawn equipment but we have a gardener now so we don't use those things.
We do have a garden shed on the side of the house so any of the smaller
things are in there. Lawn mower won't fit though.

I would hate to have to pull all of the siding off and do this over. We
don't even need a building this big. Husband had intended to put in a floor
(floor currently is cement) and walls but gave up after putting a wee bit of
insulation and wallboard around the fireplace. His friend had suggested
putting in beadboard over some insulation so that it wouldn't have to be
painted. But I can't see that happening now as we no longer have a way to
haul stuff like that.

If we leave it like it is... Would this be a really bad thing? We've been
here for almost 10 years now. I kept sweeping out dead worms by the
fireplace and there were tons of spiders in there. I sprayed for those and
no more. But I figured out how they were getting in. There was a hole
rotted all the way through by the fireplace.

Oh and we are in WA state. I saw someone else here mention that this siding
is sometimes used in the Northwest. My parent's house has it and it was
built in 1962. House is still standing and their siding never rotted. I
think husband helped in rotting two pieces of ours when he pulled off some
gutter. Why? No clue. New gutter has since been installed but the damage
already occurred. Painter said we might need to replace a couple of other
sections of gutter but wasn't sure that was the cause of the damage.