b'PSYCHROMETRICSrelative to all the others. It could have beenasked the owner about it, and they said it was aSo, the solution to the stains on the roof blocked venting at the eaves; it could havecrawl-space access. So I popped the hatch andsheathing in Figure 5 was to improve drainage been a poorly installed vapour retarder atfound myself looking at a lake in their crawlfrom the crawl space, but I only found that the ceiling level; it could have been related tospace. For whatever reason, the drainage ofwith a psychrometric survey. Notice, though, occupant behaviour. groundwater in this area was poor. The waterthe difference between this example and the One of the diagnostic tools to help us deter- in this lake would be warmed by the spaceprevious one: defining the vapour-pressure mine the most likely cause of the problemabove and would steadily evaporate and diffusegradient in the rotunda pointed to blocked (and why I bring it up in this article) is aup through the occupied space into the attic,venting that needed to be addressed. In this psychrometric survey. In this case, I tookdriven by the vapour-pressure gradient ascase, it helped identify the moisture source in measurements (see sidebar) throughout themeasured by a trusty sling psychrometer. the unit. house again, looking at absolute humidity ratio instead of RH. This provided a localizedFIGURE 5: CHARACTERISTIC MOISTURE STAINING ON THE UNDERSIDE OF ROOF SHEATHINGprofile of humidity ratio, which correlates to vapour pressure, so I could identify vapour-pressure gradients within the unit. Doing this led me to the laundry room (not a big surprise), so I suspected occupant behaviour as the likely suspect. I checked to see whether the dryer was exhausted to the interior, as some occupants do this in an attempt to reclaim exhaust heatbut not in this case. I asked the owner about their laundry habits, but these were quite normal. I asked if they had teenagers or pets (higher laundry loads for both situations for similar reasons), but not in this case. I was having trouble figuring out where the moisture was coming from, and then I noticed a floor hatch in the laundry room. I A Leaky Roof LetsIn More Than Just Rain604-882-9734rcabc.orgSPRING/SUMMER 2020 17'