Vapour barrier - where should it be installed??

[colliderman]

Hey all,

I'm about to start my build but am stumped about the proper way to install my vapour barrier. I'm in Ottawa Canada, so a cold climate where the standard way (and required by code) is to install the vapour barrier on the warm (interior side) of the room.

My existing construction is a standalone 28' x 28' building. I've attached a floor plan of the studio that is hopefully clear enough to describe what I'm talking about. The existing structure had drywall installed on all walls and ceilings and a continuous 6 mil poly vapour behind the drywall. (Orange outline)

My original thought was that I'd remove the existing vapour barrier on the walls, put the new vapour barrier on the new interior walls where there is double stud to the exterior, and then snake it back in to connect to the existing vapour barrier where the double stud walls stop (south side). This is shown in the Green outline. The problem with this is that I see no way to have a continuous vapour barrier between the two rooms' ceilings. Keeping it continuous between the north walls of the new rooms seems like it'd be difficult as well.

So my next thought was that I'd install the vapour barrier around the north, east, and west walls, as shown with the pink outline. In this scenario each room would have its vapour barrier installed on its ceiling. This way seems a bit odd to me though and pretty complicated.

Lastly I suppose I could keep the existing vapour barrier where it is and hope for the best! This doesn't seem like a smart option, although it sure would be the easiest.

Any help on this would be greatly appreciated! Hopefully the image describes what I'm talking about clearly enough, but I can provide more detail if necessary. Thanks in advance!

[Soundman2020]

General rule of thumb is that the WATER barrier goes on the exterior wall, just under the outer cladding, and the VAPOR barrier goes on the warmest side of the wall, up against the sheathing. You can have only one water barrier, and only one vapor barrier, so you would need to remove the existing one. You can't have two vapor barriers inside the same wall. For Canada, I would imagine that the warmest location would be your inner leaf, so it would go on the studs just before the first layer of sheathing on that leaf, so you seem to have the right idea there.

That said, check with your local authorities to find out what the correct location is for YOUR area.

I saw an interesting video about this recently on YouTube: I'll see if I can find it again.

Yes, you do need to keep the vapor barrier continuous. That's rather important. I'll take a closer look at your PDF, to see if I can figure out how to do that, but that's very necessary.

It seems you are trying to have a continuous vapor barrier around the entire studio, but a better way of doing it might be to give each room its own vapor barrier. Thus, you only need to keep the barrier continuous for each individual room, not for the entire studio.

- Stuart -

[colliderman]

Hi Stuart, thanks a lot for your reply.

You are exactly right that in my area, code calls for the vapour barrier to be installed on the warm side of the wall, so under the interior wall sheathing is typically where it is installed.

I think there's actually a formula to calculate the percentage of insulation which is allowed to be inside the vapour barrier (for example the vapour barrier can be installed in any location as to there not being more than 30% insulation on the warm side of it, or something like that) which means that in some situations it's allowable to be installed inside a double stud wall, but I think I'll just stick with the conventional approach. I'm really not sure how I would attach it to the outer side of the interior walls anyway..

The issue with giving each room its own vapour barrier is that the southern most walls (in the pdf) aren't exposed to the exterior. This is what's got me so confused. In the areas where there isn't double stud construction, like the entry/washroom area, and the loading/hvac area the existing vapour barrier needs to stay where it is, on the warm side of the exterior walls.

Then it makes me wonder about the walls between the two rooms that run down the center of the building. Would they be considered exterior walls? I wouldn't think so. Especially once I take into account that I have radiant heat in the slab, so the space between the control and live rooms will actually be heated.

And then when I start to think about the ceilings it really throws me for a loop! I am quite confused as to the correct approach here.