How agressive you get in adressing the crawlspace is dependent on current humidity level averages.
Cheat code:
If the ground is dry and dusty, you can get by with just a vapor barrier of 6mil or equivalent. You can by 1000sqft rolls and even 2000sqft rolls from Lowes. Material cost on that sqft should be $200 or in that area. Factor in that between overlap and running a few inches up each exterior wall will eat up about 25% of the material overall. The pre-estimates mentioned already are spot-on.
Wet(tish) crawlspace with a light mildew smell to it would need extra modes of air movement added and a designated breaker wured in to accomodate.
Very dank, moldy and complete ground saturation is a disaster waiting to happen. Most won't notice until there's odors in the house and in extreme conditions the hardwood floors will buckle for seemingly no reason. Basically in a nutshell, you're experiencing dewpoint in a contained area for several hours within day. Usually water intrusion from outside source is the culprit here but so could be plumbing leaks combined with just the right temperature. This is where a true encapsulation comes in. I started doing these about 11 years ago with a company based out of Davidson NC that was backed by a material company out of Colorado (Basement systems) that helped pioneer encapsulation in the Carolinas. Personally ive trained two other companies how to install it, one of them later called me out of the blue and hired my co-worker and I sight unseen the second they found out we were available to the highest bidder. Doorways are doorways. Full encapsulation includes a 20 mil floor liner, 12mil wall and pilar held by mechanical fasteners, fully covered but left 4" down from sill plate for termite inspection, and sealed together with caulk and 4" white vapor bond tape. Vents will be completely sealed retaining an appropriate R value and humidity will be controlled by a dehumidifier set at 55% after the mold issue is resolved and floor insulation is removed and disposed of. Insulation is then made unnecessary because what has happened is the crawlspace has been made part of the conditioned living area and kept at low humidity by use of a dehumidifier and conditioned by residual heat/cooling from the duct system (in which will never be 100% sealed anyways. 85% best case scenario). An encapsulation IS the absolute best way to use every bit of this resource otherwise gone to waste.
Let's hope it's dry.
It's not necessary to use 20/12 mil for the vapor barrier, when converting to a closed crawlspace, for the system to be efficient and effective.
There is no ROA on anything thicker than 6 mil plastic unless the reason for sealing the seams is to cover up and seal out a high concentration of mold on the ground surface.
Even with a 100% + vapor barrier and forced air ventilation of a crawlspace, there can still be a wood moisture problem due to humid air and cool crawlspace temperatures.
The biggest payback of the vapor barrier on the walls is during the cold winter when the brick curtain wall gets cold and drops below the dew point.
It's also important to get the vapor barrier high enough on the walls to keep any water intrusion from severe weather events to enter the crawlspace under the vapor barrier.
Data supports that meeting the NC code requirement of 12" min overlap on all seams is sufficient for moisture control without sealing such seams, as long as there is adequate mechanical fasteners holding the seams to the soil. This prevents the vapor barrier from being disturbed during normal maintenance and inspection of the space.
The common guideline for crawlspace moisture is wood moisture content, not relative humidity.
Increased ventilation doesn't work in our environment, and can accelerate mold growth leading to wood decay, especially when localized crawlspace temperatures are below their dew point temps.
Sandy and or dry soil also has a very small impact on crawlspace moisture. Again the problem is moisture laden air entering the crawlspace with areas below their dew point condensing moisture.
Wood over 19% moisture content is a concern.
Over 21% MC can grow the type of mold that eats the wood structure. This is commonly referred to as "rot". There is no such thing as "dry rot". The mold that eats the structure of the wood is commonly called "brown mold" and is a blend of several fungi.
Under 17% moisture content is great.
The mold that grows on the wood with moisture contents from 17-19% lives on the surface, and mainly consumes the sap and not the wood structure.
Water intrusion is a major concern as it helps to supply humidity to the crawlspace, although in very very very little amounts. This is due to having very little energy to cause evaporation and allow the wood to soak any of that up. The major concern with bulk water intrusion is erosion and undermining the foundation.
The main is humid air from our climate.
Once the crawlspace is sealed and converted to a closed type, the floor insulation is pointless as the temps above and below the floor are nearly identical and the air mass itself acts as insulation.
Fiberglass insulation looses 36% of its R value when the moisture content is raised 1.5%.
Yes I know, some of that is contradictory to the IBC but is the problem we face with our humid climate in the SE.
A lot of variables go into it, but there are energy savings averaging 15% by converting to a closed crawlspace and running the correct size dehumidifier for your crawlspace.
Feel free to pm or call me about any specific questions. I'll be glad to help any way I can. I've been working in crawlspaces in eastern NC part time since 1999, and full time from 2009 to today. We convert roughly 150 houses a year to the closed type with a dehumidifier.
No pissung contest in any way shape or form. I respect your knowledge 1 billion percent and a huge thanks for sharing sir!
