I finally built a raised bed for veggies. 4x10ft x 18 inches tall. Just planted some winter veggies, so I'm designing a greenhouse type cover for it, which will likely be a hoop design with poly. Was wondering if there's any benefit (or detriment) to enclosing the 18-inch tall wooden sides with the poly to prevent heat loss through winter. The wooden sides are 1.5-inches thick.

The good news is you live in zone 8 so it won't get as cold as it does here. Hoop houses are season extenders, not greenhouses (think coldframe). If you are growing cabbage, kale, beets, carrots, onions..., they will do great. Everything else will die, just a little later than normal depending upon the thickness of the poly. The wood sides might have more insulating value then the poly but it may be easier to cover the sides with the poly just so you have a way of holding the poly down.

What are you using for your hoops?

Correct, it's not a greenhouse as it won't be heated other than by the sun.
I'll be using electrical conduit or chain-link fence tubing for hoops, depending on whether I decide to make it strickly for the one bed or large enough for a second bed.

Isn't that expensive and hard to bend?
I used to repurpose old watering hoses that the lawnmower had run over. Cut it to the lengths your hoops need to be. Run a heavy duty wire (14, 15 gauge, about the thickness of a coat-hanger) wire through each tube, with maybe 10" end sticking out each end. Bend it any shape you like, stick the free ends of wire into the ground. You can double the wall with a second set of hoops that fit snugly over the first sheet of plastic. Easy to take down again when the weather gets hot.

Electrical conduit is cheaper than pvc and lasts longer.
https://www.youtube.com/watch?...

I have old chain-link fence. Might make it too heavy to open though. Still designing in my head.
https://www.youtube.com/watch?...

I've been watching a lot of tubing bending videos lately too - my 16x20 high tunnel is going to need to be recovered within a couple of years and I want to replace the PVC structure that we used for the top with either EMT or the chain-link fence posts - which seem to be kind of surprisingly easy to bend using a hoop bender.

CaptObvious, I think I would lean toward the electrical conduit for the size of bed that you are covering and with the issue of wanting to be able to open it. And be sure to use UV protected greenhouse film, unless you want to re-do the plastic every year.

I agree on the UV rated greenhouse film even though you would only be using in winter. It will last far longer.

And UV is another problem with using PVC. It becomes brittle when exposed to sun and in this climate does not last long with exposure.

What are you planting that will require frost protection? I am also in zone 8a and do not provide protection for winter crops. We had a lengthy stretch of below freezing temps last winter. I was not home at the time so couldn't even provide frostcloth protection. The only crop I lost was the broccoli. Spinach, mustard, lettuce, celery and the root crops all shook off the freeze and continued on. It opened my eyes to how much cold these winter vegetables can endure. My raised beds are metal and even that didn't seem to transfer too much cold to the root systems.

I had thought of similar but to use in the heat of summer. It could provide protection from some bugs, pets and even be used for shade cloth in the heat of summer.

Please do keep posting on what and how you continue.

Hi! Larry. Just for my two cents worth. I have a wooden frame cold frame, the bottom wooden frame is 2x10,the sides and top are polycarbonate panels. In the coldest part of winter Jan,- Feb. ( zone 6b) I wrap the bottom with black plastic. If we are expecting really cold weather, I wrap the lid and side walls with large bubble wrap, just stapled to the inside frame. Will try to send some pictures. Hope some of this will be helpful.