Crawl space fan placement?
#11
Our crawl space has too much moisture with a touch of water. I’m interested in hearing from the experienced folks here.

The questions are where would it be best to install fans to move the humidity out from under the house, how many fans, and how many CFMs are likely to be necessary?

This 20 year old house (roughly 1800 SqFt under H/A) is built on a stem wall on acreage where the grade runs downhill from east to west. Water primarily runs around the house although 2 places run straight into the eastern stem wall.

There is a slight downhill from south to north.
The stem wall in the southeast is 2 courses rising to 3 on the northeast and rising to 6 or 7 on the west wall.
There are 3 crawl space accesses:
1. Western end of the South wall
2. Western end of the North wall
3. Northern end of the West wall.
Accesses 1 and 2 are more or less in line at opposite ends of the house.
Looking down on the house from above, the stem wall would appear nearly as a rectangle with a plus sign connecting the perimeter.
Each section has a roughly 6’ opening which makes each section accessible to the other—the openings are bridged by precast concrete.
There is, however, one area in the southwest corner which is more like a vault—it is fully enclosed on three sides and has one small ventilation opening.

There were 2 single block ventilation holes on the east wall which were letting water enter the crawl space and which I have just sealed.
All the other walls contain one or more single block ventilation holes dispersed by the engineers.

- Above the stem wall are 12” I-joists, 1/2 subfloor, 3/4” T&G sturdi-floor, vapor barrier and finally 3/4” real wood flooring.
- Between the I-joists, we have 6” of blown in cellulose insulation sitting roughly 4” below the subfloor.
- The house is perfectly comfortable, inexpensive to air condition and doesn’t feel the least bit humid.

However, the insulation shows humidity ranging from 30-90% and the bottom of the I-joists also have high humidity.
Ever since Hurricane Irma it’s been wetter than we’ve seen in many years.
The humidity in the insulation is the greatest concern because it could create issues.
I estimate the maximum height of the crawl space to be 48” and the minimum about 24”.
I estimate there are 5 single-block ventilation openings at 24” or higher (no water intrusion issue) and there are 3 roughly 4’ SqFt accesses.
I believe I’ve sealed the only ventilation openings allowing direct water in.

Engineering ideas especially as it relates to installation and location of exhaust fans in the crawl space?
Gary

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Liberty, Freedom and Individual Responsibility
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#12
Not engineering that hasn't already been done...

This is code: Keeping in mind it is the minimum. More is better and vents open year round but you will still get some condensation just because the cround under your home is cooler than the outside of your home.
 Section R408.1, Crawlspace Ventilation

The minimum net area of ventilation openings shall be not less than 1 square foot for each 150 square feet of under-floor space area, unless the ground surface is covered by Class 1 vapor retarder material. Where a Class 1 vapor retarder material is used, the minimum net area of ventilation openings shall be not less than 1 square foot for each 1,500 square feet of under-floor space area. One such ventilating opening shall be within 3 feet of each corner of the building.

It helps a lot of you can move the air with a fan pulling air out of at least one vent. Not sure if you are getting ground water or condensation or both. 

Non vented and encapsulated spaces are acceptable if:
Section 308.3, Unventilated Crawlspaces
  • Mechanically circulating air is established between the upper conditioned area of the home and crawlspace. The air-circulating device must move at least 1 cubic foot of air per 50 square feet of crawlspace area.
  • The crawlspace floor area must be completely sealed with a vapor-retarding material. This means lapping the edges of the vapor retarder up against the inner foundation walls, overlapping separate sheets by at least six inches, and sealing up those seams.
  • All crawlspace walls must be insulated to appropriate R-values for the regional climate. 

Dehumidifiers will help too but you really should insulate the walls and lay a vapor barrier on the floor or you'll work the dehumidifier(s) to death.. Exposed foam generally does not meet fire code and is required to have a thermal barrier like drywall but I haven't done a lot of reading on crawlspaces. Usually they are referring to the interior of the home and I'm not sure if Crawlspaces are considered "interior" by definition. Might want to google it. I believe foil faced foam is acceptable but I have also seen a product from Owens Corning in new homes called Formular 150 and 250 which isn't faced. Again, just nailed to the block/concrete walls. There are insulation products that come in a roll that are fiberglass with a plastic facing. I see this in unfinished basements on new homes but I'd be concerned about it wicking ground moisture. Wet insulation has almost no insulation value. You nail it to the walls using concrete nails with plastic retainer rings so they don't rip through the insulation. The nice thing about foam is that it won't wick ground water.
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#13
The house was built to code when constructed and basically meets current code. Code isn’t really an issue unless there’s been new knowledge gleaned WRT construction techniques in the last 20 years.
Having said this, it appears the current amount of ventilation within the stem wall meets the circulation net of 1 SqFt per 150 SqFt.
Notwithstanding, LOML and I are looking to improve circulation and remove wet air from under the building. It is for this reason I described the stem wall layout in detail.
The questions remain:
Where would you place fans within the stem wall system?
Gary

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#14
Where are the vents now? If you install exhaust fans, will you be able to locate them so they are actually drawing air all the way through the crawl space and not just circulating the air in a section or corner?
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#15
Hi Phil,
The east wall is no longer vented.
The south, west and north walls each have an access opening roughly 5-1/3 Sq Ft and at least one other vent opening provided by a single cinder block.
I believe the answer is yes.
I can locate fans anywhere under the house—just not sure where to best locate them and how many are needed.
Gary

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#16
Would any of the openings be large enough to place a simple Home Depot or Menards-sourced box fax?

I'm thinking that running such a device for 1-2 days while monitoring humidity levels may provide some real-world data that would guide you.

Something as simple as a $25 remote humidify monitor like this may provide some decent insight:

https://www.amazon.com/ThermoPro-TP65-Th...=8-1-spons

Consider testing with the fan in both push and pull configurations.
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#17
Last weekend I ran a 3-speed air mover under the house for 3 days pulling air out. It’s probably a 220 CFM unit.
It rained on and off all weekend though. It is Florida and is always high humidity.
Next week, we’ll be able to let the fan run all week.
I might have another fan I can set up to pull air out another spot.
Gary

Please don’t quote the trolls.
Liberty, Freedom and Individual Responsibility
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#18
(09-05-2018, 06:48 PM)Gary G™ Wrote: Last weekend I ran a 3-speed air mover under the house for 3 days pulling air out. It’s probably a 220 CFM unit.
It rained on and off all weekend though. It is Florida and is always high humidity.
Next week, we’ll be able to let the fan run all week.
I might have another fan I can set up to pull air out another spot.

Do you have a humidistat?

I'd think the ultimate goal is to equalize humidity in the crawl space with the outside humidity?
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#19
The average daily humidity year round is about 76% here!
Gary

Please don’t quote the trolls.
Liberty, Freedom and Individual Responsibility
Say what you'll do and do what you say.
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#20
(09-05-2018, 08:52 PM)Gary G™ Wrote: The average daily humidity year round is about 76% here!

Do you think it is higher under the house?
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