If I were to take less risk I get a soil test to review it, then depending on moisture content index and other properties come up with a slab of a hygroscopic moisture management system like limecrete or hempcrete and/or modify the soil and keep the space ventilated. Test only cost around $100. I'd also design to keep the floor assembly breathable in any direction since there are conditions where high humidity is not always below the floor and vapor drive is not always vertical and is also dependent on pressure gradients. Some people live in climates with indoor humidly at 100% and heat at a 100F. Changes of fungi increase exponentially above 30% RH if drying does not occur in 48 hours or less. I have a thread "Indoor Air Quality and Healthy Buildings 2017" also "Breathable Walls" with supporting data. There is a good thread called "Raised Earth Foundations" by Jay C Whitecloud you may want to review too.
For the house in question, we both still need to know what is under the floor, and why the bottom-most layer is OSB. And what the owner/builder's goals are- to use all "natural", is he willing to use plastics and other petroleum-based materials (like urethane foams, polystyrene, etc.) and what about insulation?
I am interested in your soil floor options. They are new to me.
I am not certain I agree with all completely, but it's because we can have different options at each stage for each assembly.
The real issue on the barrier location is not where the humid/wet air is, it's the surface temp of the dividing material - in other words, is it going to be at dew point (condensation occurs). The WUFI is a great resource for design professionals; I've found I've never needed it in my work of investigation, assessment, and corrections. For large complex projects it's valuable at the start of design to avoid errors.
I am a fan of the treated space. However I am not opposed to well-ventilated crawls, but usually they are not. Nor are they insulated correctly. Nor sealed against moisture intrusion. So, it's easier to design as if it's treated space instead of properly vented. (Aside: why is venting and air movement so difficult? even for roofers, insulators, hvac, and many designers?)
If one looks only at the flow of water (in all forms, vapor, liquid, solid), and starts there, it is possible to prevent both water damage and mold growth.
The number of 30% RH is not a correct number for cause accelerated mold growth. I am not sure where it originated but I do know it's been out there a long time. Many indoor environments have much higher, up to 60% and never have mold growth problems. Again, one must ask where the water ends up? And what direction it's flowing (including mass movement in air).
I hope we can get other contributions to this discussion!
All of my natural home designs have no plastic vapor barriers or house wraps and perform extremely well in wet/cold mixed climates.
Kayia, I'm confused, when you say "trailer" are you talking about a mobile home or RV with steel joist? I have a 2007 48 foot toy hauler with plywood floor that does fine so far after traveling the nation and being in all sorts of wet climate zones including the PNW. I did get some mold growth there under my seats, that place is mold heaven.
John is correct OSB will normally not withstand heavy street moisture/water pressures even when coated with a sacrificial layer or polyurethane I would hope a low voc. Please post the mfg you are referring to. Plywood should do a better job since the phenol resins are layered usually 3-4 plys vs embedded in wood strainds. I'd agree the steel zinc coating or galvanized, clad, passivated coating on AL are as well sacrificial and will eventually corrode. It is probably 2014-T3 clad you are referring to. In aircraft design I also do it has all been replaced with carbon composites that last 3Xs longer without corrosion inspections(15 yr cycles vs 5 for AL). Ironically, the AL I have under my plastic holding tanks common to the street are holding up just fine. There purpose to protect the tanks and plywood not control moisture.
Kaiya Hiya wrote:I feel like I need to catch up. I'm not an "experienced" builder. How could I access this WUFI? I'd like to stay as organic as possible. I was not happy about using the marine grade paint but mores just looked at it as something I had to do. There will be roux insulation. It could very well end up in northern midwest climates. I am not familiar with the radon blanket material... I just did a quick search and it brought up horse blankets? Were you suggesting the radon blanket go above the floor cavity under the interior subfloor? I'm not sure that radon will be an issue as it's ventilated under the trailer and there is no basement, but if it is a decent vapor/air barrier... it may be an option. Is it a blanket in the sense that it insulates as well?
The only time you need to design to radon is if your soil type has been found to produce it otherwise it is a complete waste of money just like plastic barriers people assume are needed for all soil types. If they tear and they will at 6 mil as proven by field test, the pressure across the hole causes more flow then if it were not there at all. Now they are recommending 20 mil min, those that subscribe to this theory anyway. If I remember right the 20 mil is now code min in the PNW that came up with the idea based on many findings of torn 6 mil.
John, I'm not so sure a moisture barrier would be the best on the interior of the osb. It may have ben a good use though had I painted it and then wrapped it in house wrap so the house wrap is the bottom most layer. It's too late for me to do that but maybe it will help someone else. It's a good idea.
You were quite confusing in your OP, I had no idea you were referring to a mobile home/rv that should have been stated so we know the application which is far from putting a permanent vapor barrier on the ground. It also sounded like you were going to coat OSB with poly. Huber does not coat their products with poly, they use a resin and catalyst. You do not want to modify their product for warranty purposes and because you are not a Huber Engineer. In this mobile situation the products must be mobile since they will encounter a wide range of conditions.
So lets start from the begging. Are you referring to Advanteck flooring? If so it is not rated for continuous exterior weather conditions. It has a Class 1 500 day warranty and the application assumes it will dried in within that time frame and not see any continuous weather or, it will swell, rot, in time, probably right after or before 500 days. The 500 days is primarily for the T&G/edges. I did not know they made a "Marine Grade" again, please provide a link if you want some help so we are clear because using Class 1 panels that are not exterior rated will be the biggest mistake you made, for get all the other layers you are wanting to stack around this dilemma.
GP makes a Platinum series 3/4 plywood sheathing that has the ADA rated "Exterior", Zip tapped seams. The glue is fully water proof and the wood veneers are of a higher water resistant grade C. Typical of what would be found in the higher end lines of mobile homes and RVs like mine. Followed by ZIP tapped 10 mil plastic vapor barrier. Followed by floor joist/w Roxul, followed by T&G any sub floor not taped and breathable so drying can occur to the interior. If you keep the interior RH levels to 30%-40% as the PHDs recommend I noted you s/b fine.
This assemble is toxic enough with the glues and plastics that won't last no need to make it worse with unnecessary additional petro based tar paper or house wraps typ of mainstream construction.