Do you put plastic on your
Do you put plastic on your floor Joist?

My question since the crawlspace is not conditioned I can leave the insulation in the floor

Im an HVAC contractor so im
Im an HVAC contractor so im interested in the subject at hand. What if you had both a supply and a filtered return in the encapsulated crawl space, a dehumidifier, and an air scrubber inside your air handler. I assume that air quality is the problem with having a return in the crawl space blowing back into the living space poor air quality.

I live at Jersey Shore.
I live at Jersey Shore. Crawlspace has been encapsulated. Ground table is up.Puddling occurs underneath. Was advised to either use a dehumidifier or install 2 sump pumps . Want to try dehumidifier first before spending more thousands of dollars …?

So, my questions are, 1) how long would the payback be if I did choose to go the full encapsulation route ? and 2) is there anything else I could be doing to reduce the humidity coming into the living areas since it is a vented space. Would closed cell foam be a logical next step for under the floors or would that create more issues ?

Thanks…

Great article by the way. I agree with all of what you said, I just can’t say it near as eloquently.

When I said crawl and
When I said crawl and exterior dew points would swing roughly together, I didn’t mean the crawl is more connected to outside than units above. I was just saying that unless there’s a significant interior moisture source, crawl moisture must be driven by infiltration, regardless of whether it’s at the foundation or from units above. I was simply refuting the notion that crawl moisture was driven by an accumulation over the summer.

As you know, a significant internal moisture source can be diagnosed by monitoring outdoor and indoor dew points in the units, evidenced by dp spikes that don’t correspond to outdoor conditions. That would be the best scenario in terms of remediation. But again, the high RH you noted in the crawl was associated with a not-so-high dew point.

BTW. since crawl perimeter is tight, I don’t see how running exhaust fan in warmer months would be helpful. An exhaust fan can be useful if a crawl is more connected to the outside than the rooms above. Essentially, it tips the pressure balance to ensure crawl is dried by house air. In this case, it might make sense to run the exhaust during the cooler shoulder months when crawl temp drops too low. This would pull in warmer air from the units above, thus raising crawl RH.

Based on everything you’ve said (and excluding an internal moisture source, e.g., inadequate spot exhaust in the units), I think you need to focus on preventing crawl from getting too cool. One option would be to use a humidistat set to, say, 60% RH, to control the exhaust fan(s).

To those who would bypass diagnostics and go straight for a dehumidifier, I’d say that’s an expensive band-aid rather than a solution. It’s usually better to attack the problem rather than the symptom. In this case, the problem appears to be crawl temperature, not excess moisture.

Here’s a link to Lew’s excellent mold presentation at the 2015 Hot Dry Climate Forum: http://www.bit.ly/2frLBMo. Contrary to the title, the focus is not specific to dry climates!

Looking at the temp./RH in
Looking at the temp./RH in crawl spaces vs. the living space above, for the past couple of weeks, shows me that the RH in the crawl is following the RH above, with just a bit of a lag and with swings not as wide as above. I am confident that there is no moisture entering the crawls other than what is in the air from living spaces above. I also do not see any potential for hygric buffering.
That is a handy dew point calculator. I have been using a psychrometric chart to determine dew points and changes in RH with temp.
There has been some condensation in the crawls from potable water pipes being at the dew point. That can be stopped by insulating all of the cold water piping.
My main concern for the crawls is possible mold, especially with the potential at >70% RH. I am also looking for guidance on ventilating the crawls during warmer months, so as to not introduce lots of moisture by ventilation. I could probably answer the question if I continue to monitor the crawl spaces remotely for another year, and see how quickly the dew point in the crawls responds to changes in the outside air, without the fan running.
Is there a particular article or book that you would recommend from Lew Harriman on this particular subject?

Brad wrote: > it is my
Brad wrote: > it is my understanding that an RH of >~70% can allow mold to grow

I think that’s a good guideline, although Lew Harriman opened my eyes to the complexities involved in mold growth.

I don’t buy that the high crawl RH you observed is residual moisture that migrated down to the crawl over the summer. Partial vapor pressure differentials are highly cyclical. There can be significant lag if a building is reasonably sealed, as well as hygric buffering (storage), but I can’t see that playing out over a period of weeks, let alone months.

Unless there’s a significant interior moisture source (foundation moisture, inadequate spot ventilation, etc), I think you’d likely find that the crawlspace and exterior dew points swing roughly together. But again, 54F dp isn’t that high. The only reason it manifested as 80% RH is because the temperature was allowed to drop to 60F. (BTW, here’s a handy dew point calculator: http://www.dpcalc.org)

Allison wrote: “for those not
Allison wrote: “for those not in a cold climate or building on a slope, the expense of digging that hole doesn’t make sense.”

Well, I’m in a warm climate, and preparing to dig a big hole for a basement home on a flat lot. Check back with me later to see how the cost compares, but it definitely will cost less than building the same size home on grade (slab and roof would be twice as large). OTOH, when compared with two floors above grade, yeah, a basement probably does increase the cost a bit.