https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/ Building science knowledge, HVAC design, & fun Wed, 21 Jul 2021 00:35:54 +0000 hourly 1 https://wordpress.org/?v=6.6.2 https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7335 Fri, 25 Jul 2014 22:22:15 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7335 Curt wrote: “Air at 50
Curt wrote: “Air at 50-60% RH that wanders into a cooler crawl space may well exhibit 75% RH.” 
 
Point taken, but if crawl is sealed and insulated, it won’t often be cooler than house during periods when house RH is that high. Although I can see this happening in swing seasons, when you can have extended periods of wet cool weather with relatively high dew points. Not coincidentally, this is when not-so-tight homes are most likely to need supplemental dehumidification, and is the strongest argument for making envelope tighter in moderate and hot humid climates.  
 

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7334 Fri, 25 Jul 2014 07:02:02 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7334 Air at 50-60% RH that wanders
Air at 50-60% RH that wanders into a cooler crawl space may well exhibit 75% RH. 
 
A reasonably careful blower door depressurization for leak location shouldn’t damage a 10 mil barrier. Nowhere is is said that the depressurizatio need to be to -50 PA (or any other specific pressure)

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7333 Fri, 25 Jul 2014 07:00:04 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7333 Mark wrote: “if the
Mark wrote: “if the crawl is tight how does it get several air changes a day without wind to drive the pressure differential to a significant number?”  
 
Air changes are driven not only by wind (which is always a factor over time) but mostly by stack effect. A well-sealed house might have an average natural air change rate of, say, 0.10 ACH-nat (verified by a blower door), which means an average of one change every 10 hours over the course of a year. Sometimes more, sometimes less. 
 
Addressing site/footings/roof drainage issues will minimize hydrostatic pressure that otherwise would overwhelm even the smallest imperfections in the moisture/air barrier (primarily from the ground and below-grade walls).  
 
“depressurizing the crawl would displace & distort the barrier causing damage to it.” 
 
In order to effectively use a smoke pencil to find leakage paths, it’s necessary to use blower door in depressurization mode. It’s true this can lift lighter weight floor barriers if there are leakage path from below. I’ve seen it happen. But this doesn’t necessarily cause damage. It’s up to the technician to adjust the pressure accordingly to prevent damage. A company I used to work for used 20 mil, which doesn’t lift very easily, and holds up much better to foot traffic (e.g., hvac or plumbing service techs). 
 
“Are you saying that the tightness concern is more between the crawl & the exterior, than the crawl & the underlying ground moisture?” 
 
No, both are equally important. I see no conflict between the two statements you quoted. The idea of a closed crawl is to first minimize bulk water loads (from foundation and ground) as well as minimize infiltration loads (through walls and rim band) so mechanical dehumidification isn’t necessary. As long as the crawl stays below about 65%, there’s no risk to the structure. The house needs to be lower than for comfort reasons.  
 
Putting a small amount of supply air in the crawl is a defensive measure, and may be sufficient to resolve (minor) elevated RH levels. 

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7332 Fri, 25 Jul 2014 04:05:27 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7332 “Even if crawl is
“Even if crawl is tight as house, it will still experience several air changes a day.” 
David, if the crawl is tight how does it get several air changes a day without wind to drive the pressure differential to a significant number? 
 
“if your crawl RH climbs to 75% without mechanical dehumidification, you still have a moisture source.” 
David, there is always a moisture source, the ground & masonry under & behind the barrier. If RH climbs there is a path or paths between the source & the crawl. 
 
Seems logical that depressurizing the crawl would displace & distort the barrier causing damage to it. Please explain how you would do that. 
 
On one hand you said previously that “In humid climates, it’s unrealistic to expect crawl dew point to be in line with house, at least not without mechanical dehumidification, which is would be expensive, and unnecessary.” 
Now you are say, “Stop the source and you won’t have to throw good money after bad by purchasing another DH, not to mention the ongoing electrical cost to operate it.” 
Are you saying that the tightness concern is more between the crawl & the exterior, than the crawl & the underlying ground moisture? 

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7331 Fri, 25 Jul 2014 02:32:24 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7331 @Paul S, if your crawl RH
@Paul S, if your crawl RH climbs to 75% without mechanical dehumidification, you still have a moisture source. Stop the source and you won’t have to throw good money after bad by purchasing another DH, not to mention the ongoing electrical cost to operate it. 
 
If there’s bulk moisture below the poly, you have a site/footing/roof drainage issue (or a high water table). Either way, your crawl (apparently) isn’t sealed very well. I suggest you hire a technician to depressurize the crawl with a blower door. Aided by a smoke pencil, s/he should be able to identify the primary leakage paths.

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7330 Thu, 24 Jul 2014 23:47:49 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7330 SZMIKWICS, 

SZMIKWICS, 
Is your standard foundation waterproofing strategy a corrugated pipe laying flat on top of the footing?

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7329 Thu, 24 Jul 2014 23:16:50 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7329 For a 1500 sq.ft., 48″
For a 1500 sq.ft., 48″ bottom to subfloor, encapsulated crawlspace in central NC (new construction in 2010) we used a 4″ supply duct off the plenum (with a cape backdraft damper) coupled with a small (30-pint) dehumidifier set at 65% RH. It would run intermittently mostly during the cooling season. The crawlspace was encapsulated with a 10mil fire-retardant poly barrier with a 0.03 perm (US) rating.  
 
Question: should we continue with this strategy? 
 
Cons:  
the small dehumidifier was recalled by the manufacturer about 3 years into its life. We are having second thoughts about putting another one in and dealing with potential recalls or failures, or worse yet, risks associated with an electrical appliance in the crawl. 
 
Pros: 
As nice and comfy this crawlspace might seem to be, I am not crazy about recirculating air between it and the house so the supply-only strategy works better for me – keeping the crawl under slightly positive pressure and counting on air exfiltration to the exterior. Btw, house is balanced with an ERV. 
 
Without the small dehumidifier the RH creeps up to 70-75% region and I am not comfortable leaving it like that in the long term. 
 
One practical solution: install a pex-based residential sprinkler right in the vicinity of the dehumidifier (a 3/4″ water supply line is nearby). Very simple to install. Am I a bit crazy? 

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7328 Mon, 21 Jul 2014 07:18:15 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7328 @Mark, nice comment, although
@Mark, nice comment, although I take minor exception with this:  
 
” If the crawl is encapsulated seasonal weather changes should not have significant effect on its ‘climate’ (temp/rh).” 
 
Even if crawl is tight as house, it will still experience several air changes a day. In humid climates, it’s unrealistic to expect crawl dew point to be in line with house, at least not without mechanical dehumidification, which is would be expensive, and unnecessary. 
 
In any case, if crawl is closed, a minimal insulation wrap (R-4) should prevent condensation as long as all seams are sealed.  
 
I totally agree with your comment that code should require ducts to be inside the envelope, and regarding mice.

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7327 Mon, 21 Jul 2014 05:36:29 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7327 David,  

David,  
I’m not suggesting not insulating ductwork, but just emphasizing that when a good thermal boundary is installed & a crawl is encapsulated, the need for highly insulated duct goes down.  
I’ve seen the damage heavily sweating ductwork can wrought.  
The question is what to do to prevent sweat. (the air meeting dew point) 
The answer is lower the humidity in the space, raise the r-value on the duct or some combo of the two.  
The first of those is the most important. If the crawl is encapsulated seasonal weather changes should not have significant effect on its ‘climate’ (temp/rh). 
 
When the ductwork is inside the thermal envelop the necessity of tight joint & insulation is less. 
Frankly it should be code that it has to be inside the thermal envelop. 
 
As an aside, I’ve never seen a crawl that was not vulnerable to mice. Using anything but metal ductwork in a crawl is insane. I’ve inspected too many to ever believe differently. 

]]> https://www.energyvanguard.com/blog/a-line-in-the-sand-the-dew-point-duct-duel-2/#comment-7326 Fri, 18 Jul 2014 20:58:23 +0000 http://energyvanguard.flywheelsites.com/?blog_post=a-line-in-the-sand-the-dew-point-duct-duel-2#comment-7326 Mark Bishton wrote: &quot
Mark Bishton wrote: “Speaking of throwing money around; how cost effective is using highly insulated duct in an encapsulated crawl, like the one in the last picture?” 
 
Not sure what you mean by “highly” insulated (I don’t think it’s possible to determine the R-value from the picture), but I disagree if you’re suggesting that *any* insulation would be a waste. 
 
Uninsulated supply ducts in an encapsulated crawl or attic will sweat in all but the driest climates. Even ducts in directly conditioned space will sweat if system airflow is on the low side or interior RH is not closely controlled. 
 
Furthermore, notwithstanding the unfortunate moniker ‘conditioned crawl space’, I can’t imagine that insulation wouldn’t be cost effective. It’s not as if you need the crawl to be at room temperature.

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