Comments on: Vented Crawl Spaces and the Psychrometric Chart Are Not Friends

By: Psychrometric software

Psychrometric software — Sat, 20 Jul 2013 21:27:34 +0000

Recently I wrote an
Recently I wrote an application which makes calculations with psychrometric chart very easy. I thought it might be useful for you guys as well. You can download it for free at psychrometricchart.net.

By: Taak

Taak — Tue, 14 May 2013 12:55:45 +0000

With psychrometric chart
With psychrometric chart software you can calculate the dehumidifier capacity for your own situation. Have a look for the software at psychrometricchart.net

By: Curt Kinder

Curt Kinder — Sun, 10 Mar 2013 20:24:31 +0000

By and large, I view a
By and large, I view a dehumidifier as a bandaid for building science compromises - expensive to run and must be maintained.

Better be sure any insulation applied in a crawl has provision for termite tunnel inspection; in other words, don't give termites a place to hide their work.

By: David Butler

David Butler — Thu, 07 Mar 2013 05:25:04 +0000

Jeremy wrote: <
Jeremy wrote:
"in my mixed climate, humid but shorter cooling season and longer drier heating season, what is the best solution?"

In your climate, a basement would be best (in my opinion). But for crawls, the joist cavities should be insulated, and the perimeter sealed based on accepted design practice and local codes.

AE's original research on closed crawls was conducted in eastern NC where the trade-off favors insulated walls. As you move further north, the trade-off begins to favor floor insulation. At what point cannot readily be modeled. It depends not only on climate but the relative cost of heating vs. cooling BTU's, and whether HVAC is in the crawl. Also, the difficulty/cost of properly insulating the floor cavities is a factor.

AE's follow-up study did little to clarify these trade-offs other than to raise the issue.

Although crawl spaces are easier for the trades, I'd rather have an edge-insulated on-grade slab in a warm climate. It's not only more efficient (love that mass) but costs less to build. Either way, vented crawls are an abomination in humid climates.

By: Skye Dunning

Skye Dunning — Wed, 06 Mar 2013 23:09:25 +0000

The other common option is to
The other common option is to supply air from the HVAC system. 1 cfm/30 sq.ft. Unless it's really well sealed we find that this option does not work so well.

From above:

"After the crawl is closed, supply the crawl with an RH sensor with an alarm set for 75%."

You should see RH levels in the 60's in the summer. The alarm will alert you before any problems from the dehumidifier failing (which it will at some point).

By: James Groman

James Groman — Wed, 06 Mar 2013 22:53:47 +0000

I've sealed my crawlspace
I've sealed my crawlspace (mostly), and I installed a dehumidifier just to be sure it stayed dry. I don't think it runs often, and it feels dry, and there is no mold growth, but I don't like to depend on a dehumidifier. Any suggestions? (From Greenville, SC)

By: Ted Kidd

Ted Kidd — Wed, 06 Mar 2013 22:01:17 +0000

OMG, all my firmly held
OMG, all my firmly held beliefs are being shaken! If I walk out the door, will the sky fall on me?

Next you'll tell me my mother was wrong about washing in hot water to sanitize the laundry!

YOU DOGMA CRUSHER!! (Keep up the good work!)

By: Jeremy Begley

Jeremy Begley — Wed, 06 Mar 2013 22:00:04 +0000

Good stuff Allison. The study
Good stuff Allison. The study on the Advance Energy page points out a interesting phenom for cold dry climates "While moisture reduction was consistent, energy
savings varied with climate, insulation and duct
placement. The research makes it evident that in cold climates, it’s better to insulate the floor
above the closed crawl space than to insulate the foundation walls."
Good stuff!!
SO in my mixed climate, humid but shorter cooling season and longer drier heating season, what is the best solution?

By: Adam Gloss

Adam Gloss — Wed, 06 Mar 2013 21:40:52 +0000

Thanks for pointing out that
Thanks for pointing out that this is for HUMID climates. At the ACI NW conference in Seattle this year, David Hales from Washington State University shared studies comparing vented, sealed and conditioned crawlspaces in the Pacific NW. Due to our climate conditions, the vented crawlspaces actually performed well (as long as they are properly ventilated). And on a cost/benefit basis can outperform sealed and even conditioned crawls.

While there may still be reasons to seal and/or condition crawl spaces in the NW from time-to-time, vented crawls are perfectly acceptable here. Unless of course there are water problems in them, but that is another topic altogether!

By: Skye Dunning

Skye Dunning — Wed, 06 Mar 2013 20:09:33 +0000

+1 on crawlspaces.org. There
+1 on crawlspaces.org. There are a few traps here, especially in new construction, so beware. There are plenty of important details but my main warnings would be:

1. For new construction, track wood moisture content & take measures when it starts getting above 15% or so.

2. After the crawl is closed, supply the crawl with an RH sensor with an alarm set for 75%.

3. What ever you do, don't forget to communicate with the pest control contractor (trust me on this one;)