Posted By cschmelz on 24 May 2011 01:20 PM
Posted By Dana1 on 11 May 2011 01:53 PM
BTW: For climate data & ground temp info purposes, what's your ZIP? (A few decades ago I lived briefly in heart of the Columbia Basin- 98823 and summered a time or two 99140 back in the '90s.)
98942 for Zip
I'm thinking that for the solutions given (create hole is far duct in crawlspace, then exhaust crawlspace via HRV/ERV and intake to return air) that an ERV would be better than an HRV as we are VERY dry most of the summer (dew points <20dF) and in winter I am using a Honeywell TrueSteam humidifier to make the house more comfortable (or baseline rel humids inside are 25% or so).
For straight ventilating the crawlspace it would make more sense to use a HRV to remove humidity as effectively as possible, right?
With a ground vapor retarder and foam insulated crawlspace walls the absolute humidity (dew point) of the crawlspace air will be exactly that of the conditioned space air. But without insulation on the floor of the crawlspace it'll run cooler than the conditioned space, and closer to the dew point.
Deep well temps (except on volcanic anomalies) in your area run in the low 50s, and at ~1500' in Selah it'll be seasonally cooler than that. Putting going up to R15 on the walls (1" xps or closed cell foam on the walls, + mini-studwall with unfaced R13 batts), and 2-2.5" of EPS (R8-R10 bead-board) over your ground vapor retarder, protected by a 1.5-2" rough non-structural concrete "rat slab" over it would have long term benefit on energy use, while raising the temp of the crawl giving you plenty of margin on dew point issues.
If your baseline RH is 25% in winter, a round of whole-house air-sealing (half of which may well be the band joist & foundation sill & vents in the crawl, would likely raise that to over 35%, and you could turn the TrueSteam off. In a typical year the binned-hourly mean (not hi or low) temps for January winter temp in Selah is in the mid-20s but some years are can be significantly cooler than that. The higher you raise the RH, the more mold potential you will have in the studwalls. The dew point of 70F 35% RH air is ~40F, so any wood in the walls that averages below 40F for a month is susceptible. The dew point of 70F 50% RH air is 51F, which brings the susceptible wood an inch closer to the interior. It's very important to make the interior as air-tight as possible (more important than vapor barriers) if you're going to be running over 35%. Health professionals place the human-health optimal zone between 30-50%, so there's really little health benefit to going much higher than 30%, and the comfort level at 35% is MUCH better than at 25%, yet the difference in comfort between 35-50% is barely perceptible.
Your average wintertime outdoor dew points are about 20-22F, which is more humid than mine (which average ~12F), yet in a not-super-tight house it coasts along at ~35% without adding humidity, only dropping below that during extended cold spells (multiple consecutive days with high temps below 10F). This is with 3 people showering cooking, breathing etc. For a 1-person household the RH would surely be lower. If you can seal up your house well enough that its about or under 2.5 air exchanges at 50pascals (a standard blower door test) your natural ventilation will be sufficient to be healthy, but the indoor relative humidity will rise. Then you can control the HRV via dehumidistat to lower the RH to under 40% should it start to rise, and set the TrueSteam to blast away only when it drops below 30%. If you run the HRV with a timer, adjusting the duty cycle downward in winter is an effective way to maintain a minimum RH without having to add humidity.
ERVs make a lot more sense in areas with high latent air conditioning loads- your problem is the opposite. An ERV would allow you to run higher ventilation rates without overdrying, but air-sealing the house would be over all a better approach, for both the structure and the humans within.