Insulation Questions

[Jayhawk_Aviator]

I have a 40x50 metal shop and am located in Kansas City area.

I had 2" of closed cell foam spray on the ceiling (they sealed everything up), and they sprayed the bottom 3' (up to the first girt). I did that to really seal the bottom edge with the ridges. I am doing the horizontal framing and installing R19 batts around the rest of the sides.

Had a couple of questions:
1) Do I need to install a vapor barrier on top of the batts on the side? I will be drywalling / metaling the sides but wasn't clear on whether that's needed or not. As noted, I think the building is sealed up top via spray foam (no open ridge vent, gable vent, etc.). Th trusses are obviously exposed.
2) Haven't decided if I'm doing a ceiling (to the bottom trusses) or not, but if I do, I assume I should NOT insulate on top of the ceiling? My understanding is if you insulate the ceiling (not the roof), you will want to have gable or ridge vents to facilitate airflow. Since my roof is spray foamed, the whole space is sealed and I wouldn't want to also insulate the ceiling as then the space between ceiling and roof would have no airflow (and not be totally conditioned), resulting in problems, correct? If I want to add a ceiling for anesthetic reasons, I assume I could do that provided I don't insulate it?

Thanks.

 

[DC73]

The building science gurus are getting away from calling them vapor barriers. They now use the term vapor retarder. There was an excellent article by Martin Holladay in the January issue of Fine Home Building magazine about vapor retarders. Here is his short version of the answer as to whether a vapor retarder is needed:

What’s the short version?
• Most buildings don’t need polyethylene anywhere, except
directly under a concrete slab or on a crawlspace floor.
• The main reason to install an interior vapor retarder is to keep
a building inspector happy.
• If a building inspector wants you to install polyethylene in a
wall or ceiling, try to convince him to accept a layer of vapor-
retarder paint or a smart retarder instead.
• Although most walls and ceilings don’t need a vapor barrier,
it’s always good to include an interior air barrier. Air leakage is
far more likely to lead to problems than vapor diffusion.

Here's a link to the entire article: http://www.finehomebuilding.com/membership/pdf/174534/021264084-87-EnergySmart.pdf

As far as ceiling insulation, I don't think it matters if you insulate the ceiling or not. It could help if you need more insulation than the amount of spray foam under the roof provides. 2" of closed cell spray foam is about R13 so you don't have a tremendous amount. You could also add extra insulation via batts installed directly against the spray foam. If you insulate both the ceiling and the roof, you have effectively created a "room" that might need to be conditioned. So, if you ever install heat or air conditioning, you might need a small duct into the "attic" to prevent condensation or musty smells.

DC

 

[Jayhawk_Aviator]

Thanks. I should have noted that I do heat the interior space so I worry about the "room".

Sounds like the answer is no vapor barrier on the walls. Thanks.