Replies

1. Expert Member
   BILL WICHERS | Feb 09, 2021 12:51am | #1

   If you can use entirely exterior insulation, this will help you. You want to keep any interior surfaces above the dew point, and you want to avoid any insulation that is vapor permeable. You'd never want to use batts in a greenhouse, for example, because they'd quickly get soggy, never dry out, and then you'd have problems. Rigid foam is much better suited to this applications (as is spray foam).

   You could try an interior side vapor barrier, and everything drying to the exterior too, but it wouldn't be as good as all-exterior rigid foam. Note that you could also put all-interior rigid foam and achieve the same thing. What you don't want is in-between temperatures such as you'd have with some rigid foam on the outside and some batts on the inside. Think about how we normally use enough exterior rigid foam to keep mositure levels in the wall in check, but then whip out your handy pschrometric chart and go all the way over to the near 100% humidity side. You'll see that you need MUCH MUCH warmer temperatures now to prevent condensation. That's the issue you have to design around.

   If you're just building a space inside your house, you can use poly to isolate the room, then keep it under a slight negative pressure so the mositure doesn't leak out into the rest of the house too much. I had such a setup a long, long time ago and it worked OK to grow stuff in the basement under lights (orchids).

   I suggest you look at greenhouses and see how those are usually built. Even if you won't have all the clear area for light to come in, you'll have the same moisture concerns to deal with. Cheap greenhouses are often built from treated or painted lumber, with rigid foam on the outside and polycarbonate glazing (for better insulation compared to glass).

   Avoid drywall in high humidity areas. You'd be better off with wood, or just leave the interior side of the studwalls open.

   If I were to build a greenhouse, and I'm assuming that's what you might be considering, I'd build it with regular (non-treated) framing, except for a sill, and I'd use triple wall polycarbonate glazing. I would use reclaimed XPS or type II EPS on the outside of the studwalls, probably with some plywood sheathing for racking resistance between the studs and the insulation. I'd use a sprayer to paint EVERYTHING on the inside with several coats of white paint, and I'd install the glazing after that.

   Run all your electrical in PVC conduit and use fixtures and devices rated for use outdoors, which means they'll be ok if you get some water on them. Be sure to allow for ventilation too.

   I'd try not to use any membranes. With high humidity all the time, any little leak will gradually let moisture through. You'll never have any drying potential in a building like this, so a lot of the usual building science stuff doesn't apply the same way. You have to plan for stuff to be wet, basically.

   Bill

2. 
   charlie_sullivan | Feb 09, 2021 08:04am | #2

   One nice option would be interior foam insulation, with fiberglass-mat drywall:

   https://buildgp.com/product/densarmor-plus-interior-panels/

   As for ventilation, it depends on whether you expect the plants to produce plenty of humidity, and you need to bleed some of it off in winter with an HRV, or whether you are expecting to be having trouble maintaining the humidity high enough in winter, in which case you want an ERV.