How to slow moisture through a gap at the bottom of a wall where there is no vapor barrier?
ashtont
| Posted in General Questions on
I am completing a total renovation of my 81 year old house and have a question about a vapour barrier in walls without a traditional vapour barrier and where there is a gap in the drywall at the floor.
At issue is that the wall height is about 8’3″ meaning that two sheets of standard drywall did not fill in the gap at the bottom of the wall. The drywaller went that way to save cost and figured that the baseboard would cover the gap at the bottom of the wall, which is about 1″-2″ now that the finished ceiling and hardwood flooring are installed.
The wall details are:
-1/2″ drywall
-2×4 construction (original framing) with Roxul R14 in the cavities
-3/4″ ship-lap (original)
-Typar housewrap
-two layers of Roxul R8 (3.5″ thick, total, R16) not yet installed
-siding (probably Hardie panels)
We do not have a vapour barrier in the wall because of the exterior insulation and because I have had issues with mold in the past if they are not perfectly sealed, which they never are. However, the building inspector wanted me to use a vapour-retarder primer to slow the rate of moisture passing through the walls and wanted us to acousti-seal behind the sheets of drywall to act as a vapour barrier from the bottom of the wall. Due to the gap at the bottom of the wall, I am worried that any moisture may pass through the bottom of the wall too quickly and become an issue in the winter if R16 is not warm enough outside to keep the sheathing from freezing.
I live in Canada in a ski town at a fairly high altitude (3,675 feet) where we get about an average of 60 inches of snow per year in town and the average winter temperature is about 19F. We are in climate zone 6.
Do I have enough exterior insulation to negate any vapour issues?
If not, what should I do to this gap?
I thought about using Tuck tape to close it up but can’t get it under the floor to properly seal it off, and also about using acousti-seal on the back of the trim where it meets the wall and also where it meets the floor but am not sure that’s best either. Perhaps I would need to add more insulation on the exterior to prevent freezing of the sheathing?
Do you think I even need anything to slow the moisture through this gap?
We start installing trim and exterior insulation this coming Tuesday (the 11th) so I’m hoping to figure something out quickly. The picture included shows what I’m talking about.
Thanks for any help!
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Replies
A half inch of wood is more vapor retardent than vapor barrier latex on wallboard, so as long as it's reliably air tight it isn't really an issue. Polyurethane caulk or acoustic sealant sticks to anything and remains flexible for decades (centuries?).
With R12 rock wool on the exterior of R14 cavity fill, any tiny remaining air leaks aren't really very risky. That's plenty of dew point margin beyond the IRC chapter 7 prescriptive for zone 6
(which is R7.5 for 2x4 /R13 construction), and you have the additional benefit of enhanced drying toward the exterior.