Apologies, because some of this is redundant to the earlier discussion of PVC.  However, we have again faced the assumption that plastics are a poor substitute for wood, or "real materials." 

That said, we've used PVC and HDPE in a number of venues and roles, and for all of the trim, mopboards, and thresholds in the Little Easy, and not everyone understands why.  we feel that in some contexts, and when managed well, plastics belong at the table with hard woods, and in most respects are a significant improvement over pine and spruce for interior and exterior applications.

Our setup for finishing Seaboard to a satiny smooth surface

Our setup for finishing Seaboard to a satiny smooth surface

Why Plastics?:

Here's Why:

We use polyvinyl chloride (PVC, available as and from KOMA) and high-density polyethylene (HDPE, available from Vycom as Seaboard).  There may be others just as worthy but we haven't met them yet.  (Nb.  We have worked a fair bit with another PVC, Azek, but there is little to commend it, and nothing that merits interior use).

Plastics do not absorb fluids or gases.  The most relevant threat is water, the bane of wood in just about any application.  Plastics do not absorb water, which means no warping, twisting,t splitting, or cupping.  It also means there will be no expansion and contraction cycle following exposure to even trace amounts of moisture.  This means no swelling, shrinking, popped nails, or cracks.  

No absorption means no rot and no support for mildew or mold.  Once KOMA is painted it will never, ever mold or foster moss or algae.

PVC and HDPE are UV-stable so they won't age when exposed to sunlight, whereas even indirect sunlight gradually breaks down all organic materials, notably wood.

These plastics are completely inert, so not only will a quality paint not check or alligator, but it will also not separate from the substrate.  The trim and paint *should* only show age from external forces like abrasion or impact.

Regarding the Little Easy, one element of the design brief was that the house should be ultra-low maintenance so the elderly homeowner, who is stubbornly self-reliant (like certain carpenters we might know) needn't be beholden to anyone for upkeep and can save her spare change for Fedco.  Assuming the little old lady doesn't beat mercilessly on the trim it will never need maintenance, replacement, or repainting.  (Dents incurred can be easily filled with DAP's Crackshot or something similar and painted over, but they aren't likely).

We recently faced the opinion that plastic cheapens a house, and I'm sure it would in the wrong hands.  However we aren't using plastics out of any expectation of ease or short-term economy.  In order to achieve the other design brief - simple but highest quality - we are milling every edge and sanding every surface.  The joints are beveled and then bullnosed over or rounded in.  The net effect will be a finer finish than any building we've ever worked on.

The other riposte about the cheapness of plastic is that it is more expensive than pine.  With the effort applied to create the perfect finish, it's a lot more expensive than most trim applications of pine.  

I think it's worth it.  

However, note that there is not a single stick of plastic in my house.  It's not appropriate in the context of historically informed restoration and maintenance.  (We do have vinyl siding but there is so little to commend modern wooden clapboards that I don't expect that to change.  As we replace the vinyl we will be reintroducing corner boards and the details - character - that vinyl had steamrollered out of most houses over the last fifty years).

However, we do hope that we get to work on plastics again on some future project.  We don't think it deserves it's cheap-o reputation.

The trim for Hannah's Loft is one single piece of PVC with 8 sides on 2 planes and rounded edges

The trim for Hannah's Loft is one single piece of PVC with 8 sides on 2 planes and rounded edges