How to effectively move moist air in basement when using foam board
I’m about to kick off a basement renovation. Previous owner finished the basement, but did it poorly. My goal is to reduce humidity, have more consistent temps, and reduce noise from above. I’ll do this by adding insulation (including 2” XPS foam board which will act as a vapor barrier).
I’m in Richmond, VA (zone 4A). Our house was built in 1972.
I’ve done a ton of research and feel confident about the overall strategy. For brevity, I’ll avoid going into all the details, but the approach is 2” XPS foam board on the walls to act as a vapor barrier and insulation, and frame on top of it and add more batt insulation. For the ceiling, just frame and add insulation.
I want to focus on one question in this post. There’s a lot of information about basements and I’ve searched a ton but I’m having trouble understanding how moisture moves through material & air which leads me to this question: will the 2” XPS foam board help with humidity if air can seep into the room from other places?
Ok, some detail: see the first picture (sorry for my bad drawing). That’s my layout; I’m insulating the office and playroom. The walls are all cement block, and the exterior of the house is brick. The bottom walls are below grade, but the top walls face outside because the ground slopes down (which is great because moisture isn’t as much of a problem). Moisture definitely seeps in from the bottom walls; after tearing down the current walls, I can see a lot of efflorescence.
I’m going to cover the walls with 2” XPS foam board, attaching it with small amounts PL300 adhesive which leave a gap between it and the cement block (see 2nd picture). This gap allows the cement to breathe, so moisture can escape.
I’m having trouble understanding where it will escape to though, and generally how moist air will move in this system. For example, if I don’t seal the top of the foam board, it’s useless right? The humid air will simply travel into the ceiling which doesn’t have any vapor barrier, and likely just go into the room.
Even if I seal the top of the foam board (probably with sealing tape), look at the interior wall between the office and the playroom. That’s a cement block wall, which means it’s hollow. These are old walls which have crumbled some – should I assume that air can travel from the bottom wall to the interior wall and out into the playroom? If so, I’m doomed. I can’t put up a vapor barrier from the playroom side because the stairs, and bathroom, and a bunch of stuff is in the way. If I can bet on air not traveling through it, will moisture still seep into that interior wall and come out?
Let’s say I put up 2” XPS foam board on all the walls except the interior one (on both sides, meaning there is also no vapor barrier to the left of the stairs). And I tape the top & bottom of the foam board, which will create an shared air gap with all the walls – will this force moisture to move from the ground to the air in the gap and eventually move to the top wall and to outside? Or is there another way to approach this?
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