Replies

1. GBA Editor
   Martin Holladay | Feb 28, 2017 04:32am | #1

   Barbara,
   Q. "Do I adhere the vapor barrier up onto the walls before installing the rigid foam insulation board or do I put a vapor barrier over the foam board after it is installed?"

   A. It doesn't really matter. Most installers would probably extend the vapor barrier up onto the walls before installing the rigid foam.

   Q. "Right now I have about 12 inch of extra plastic at the perimeter walls but not adhered. At one time I was planning on running the plastic up the walls to within 3 inch of the top but was afraid of condensation forming. I have a lot of vapor barrier left over."

   A. Don't worry. It really doesn't matter how far up the wall the vapor barrier extends.

   Q. "I'm confused as to how I should treat the brick foundation walls. I do not plan on introducing conditioned air from my HVAC into the space. I hope the dehumidifier is all I need."

   A. In most cases, a dehumidifier is unnecessary (or is only necessary as a temporary measure until conditions stabilize). I'm not in favor of running a dehumidifier in a crawl space for years at a time unless it's absolutely necessary, because a dehumidifier is an energy hog. For more information on this issue, see Building an Unvented Crawl Space.

   I advise you to adopt one of the two code-mandated methods for conditioning your crawl space. The two methods are described in the article I linked to.

   Q. "I live near Charlotte, NC and we have very hot humid summer. If I insulate with unfaced polystyrene foam board insulation (XPS) what thickness/R value would be adequate for this area?"

   A. You are located in Climate Zone 3. According to most building codes, the minimum R-value for your crawl space wall is R-5 -- but a higher R-value won't hurt.

   Q. "How far up the walls do I pull the plastic over the foam board insulation, or does it matter?"

   A. Over or under? It doesn't matter. How far up? It doesn't matter.

   -- Martin Holladay

2. 
   BarbinNC | Feb 28, 2017 08:54am | #2

   Martin,
   Fantastic, thanks for your answers to my questions. Also, I've read a great deal of information provided by this site and I am grateful. I continue to read. You'd think by now I would have a grasp on all of this.

   I'm just having a hard time wrapping my head around the plastic vapor barrier being impermeable and; therefore, trapping moisture in the brick foundation. Thinking then, over time, problems would surface.

   I hung out in my crawl space (yep, never again out of sight, out of mind mentality) during a hard rain and was able to detect an area under our exterior deck that was water saturated; therefore, that side of the foundation wall (east side) at the ground level was wet (not visible water but extremely wet extending out 3 to 5 inches from the wall). This is also the side that has the greatest below grade foundation wall. I also noted that side of the exterior foundation never receives sun and the soil under the deck takes an extremely long time to dry out. It is always wet or moist. It is also encumbered by restricted air flow underneath. The grading is adequate but we are on the receiving end of a high hill located approximately 100 feet from the house on that same side so I think a lot of run-off would occur during a rain. Although there is never any visible signs of water standing during or after a rain anywhere inside or out. There is a french drain on the interior of the crawl space but no sump pump. No visible signs of water have ever drained at the end of the drainage pipe.

   I placed plastic over the interior side of that particular side of the crawl space brick wall and monitored. Condensation quickly developed. This was end of 2016 summer. I panicked and that's why I am so hesitant to move forward.

   Does creating an air space between the brick foundation and the plastic help? Since the foam board is semi-impermeable would it then provide a little time for the brick to dry to the interior as opposed to zero drying capacity if using plastic. Seems like the plastic would immediately trap moisture and the moisture build-up would outpace the drying over time. This could be the cause of those funky smells people report after several years of encapsulation, unbeknownst at the time of sealing.

   Just being overly cautious.

   My crawl space is so white and bright, nice & very clean, I like to visit often. I'm thinking of hanging a chandelier to enhance the ambience of my frequent visits. I also have a pan and tilt surveillance camera that I can monitor the situation from upstairs. I like to know if there are any unwanted intrusions (liquid or not). I'm just trying to make that environment inviting to lesson the dread of my workplace as it takes a toll on my knees and manicure.

   Barbara

3. GBA Editor
   Martin Holladay | Feb 28, 2017 09:42am | #3

   Barbara,
   The crawl space under an exterior deck should not be connected in any way with the crawl space under conditioned space (living area). You need to have an airtight wall between these two types of crawl spaces.

   The crawl space under an exterior deck is outdoors. The crawl space under your conditioned space should be included in your conditioned space.

   Most outdoor decks receive either direct rain or wind-driven rain, so of course the crawl space under a deck is wet.

   -- Martin Holladay