moisture barrier and vapor barrier in one sandwich

Hello, My subfloor sandwich plan looks something like this from the bottom up (the bottom layer will take the most moisture), moisture resistant 1/2" osb coated in marine grade poly, floor joists and insulation (roxul) followed by vapor barrier and 3/4" subfloor. My thoughts are to throw in tar paper with overlapping layer simply played over the 1/2" osb on my bottom most layer for moisture control. I don't want it to be so completely sealed that moisture cannot find it's way out, nor do I want to create a scenario where condensation moisture is trapped in and cause rotting of my floor joists. Is it possible to use a moisture barrier and vapor barrier within the same cavity on each side of the insulation? I am anxiously awaiting your wisdom. Thanks so much!

Kaiya, what is under this floor assembly? dirt? gravel? concrete slab? And how much space between the bottom floor layer and the ground, is it an actual crawl space? The crawl space conditions are what is most important, more than the floor materials. The OSB as a bottom layer closest to the enclosed space (moisture) will not last very long without water damage and/or mold growth (also considered damage). You do not want moisture to 'find it's way out' thru your floor assembly into your living space unless you mean trapped water such as from a leak or flood from above (broken pipe, etc.). Your moisture and vapor barrier should be the bottom-most layer, no matter what. The floor assembly should not have a vapor barrier in the middle, since one day it will likely trap moisture/water/vapor on the wrong side. Water vapor should rise or evaporate from the bottom of the floor assembly up into the house. Vapor will not typically migrate down into the subfloor space (aka crawl space) because that space is usually wetter (higher humidity) than the living space (the direction of vapor pressure will go toward the living space). Wet goes to dry. So, ventilate the crawl or put a vapor and moisture barrier like plastic directly on the ground before you build your floor above it. Typical accepted concept is to treat the crawl like a living space, so ventilation is not usual anymore, but sealing that soil is required, as is insulating the crawl space walls, floor and foundation. then you need no vapor barrier in your floor assembly.

Welcome both of you to Permies the best site on the net for natural building. Good post John. Only parts I may differ on is the plastic barrier anywhere could be a microbial breeding ground depending on what chemicals are in the plastic and materials/soils in contact, and the boundary conditions. We do have software like WUFI that can accurately predict fungi and bacteria based on climate, layers, and material properties. I'm not certain this trend we are on in the US of air sealing crawl and other spaces with these types of materials is a good idea. Nor do I agree that a plastic vapor barrier is assumed necessary on all soil types, and the only means to handle vapor. I'd like to see more data that proves the design?

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.

Thank you I greatly appreciate all the input. Although, I haven't seen it been used I think the moisture resistant sob double coated in marine grade poly should prove to be pretty resistant. I should also expand that I'm working on a trailer as my foundation. Many people use sheets of aluminum or galvanized aluminum however I chose against that. Laying just taped 1/16 of an inch aluminum under my insulation did not sit well with me as durable or good protection. My bottom layer needs to have some r value which is why I chose the marine grade poly osb but also wick moisture. It will be ventilated below the trailer but not really in the floor cavity because it sits about 15-18" above the ground however when it's moving there is a lot of water, pressure, rocks etc hitting it (another reason why I chose against the aluminum, and that tape never really stays, galvanized would eventually also corrode the trailer). Since it will be a small space and susceptible to condensation due to temperature changes there are small protected outlets for moisture to be gravity fed down and out. The seams of the osb lay over crossmembers on the trailer to keep moisture from getting up into the trailer but allows for it to find it's way out. I want it to be as moisture resistant as I can. Unfortunately changing the sob is no longer an option, and I like it so far... but I know back in the day tar paper has been used for moisture control (just overlapping it a few inches on the bottom layer, not sealed) and thought that would be useful in this scenario but I'm afraid if I use the vapor barrier on the interior and the moisture barrier that it will backfire and trap moisture between the two. I've heard a moisture barrier and a vapor barrier are good, but both can cause joist rot which is what I'd like to avoid... Any ideas?

location. Due to location and other information it's very difficult to answer building envelope questions without know your location. It's critical. If it's a habitable dwelling, code would vary; i.e. the coast in the NW is a "hygro=thermal" zone and information about envelope does not apply compared to other areas. Need to know.

By the way, I really like the idea of laying some plastic over the soil/ground underneath. I think that will be helpful. Any ideas as to what type of plastic it should be?

Terry, thanks for the welcome! I just found this group hours ago and am already intrigued enough to be involved. I too do IAQ/IEQ for many years, along with other issues with enviro health and sustainability.
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!

Kaiya, one of the best available sheeting products these days is the professionally installed radon blanket material. I do not know where to get it, I have a contractor who installs it. It has hardware and adhesive tape for installation to seal it completely. It acts as a true air barrier - to keep radon gas out of course.

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?

O.k. sorry found the radon blankets, reading up now.

Also, remember that a moisture barrier can be one that breathes, like a house wrap (those pink and yellow ones, like tyvek); they keep out liquid water. The vapor barriers do not breath but their seams must be sealed properly.

All, it is impossible to verbalize fluid flow and dynamics (CFD) without the proper tools and the best tool out there for buildings is WUFI. If you have ever ran the simulations you know this. It is much better than all the heresay debates you'll find all over the internet of the subject matter. I have alot of experience with running the models in several different industries that pay me well, Flowmax, Solidworks, etc, and the simulations if done by an experienced pro provide better results than unfounded opinions. Many of the models I have run provide much more than determining design choices and are based on finding's in the field of many designs in many different climate zones. So again, you cannot design for moisture flows without having the proper tools. Enough of that since we are getting too far off what our OP is probably capable of understanding unless an Engineer with this type of education and experience. I'll point you all once again the PHD building scientist and microbiologist I noted if interested, since the undisputed data I noted @ 30 RH and 48 hr drying times are in those threads. Read them!

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.

Hi Terry, thank you! yes for visual purposes similar to an RV. O.k. that's good to know on the plywood. The manufacturer is Huber. It's a specifically made moisture resistant osb. I would have gone for dryly by GP but their products are basically extinct to the typical residential builder, building a small space. So, that was the best I could do. That's really interesting that your natural home designs are in great condition without any vapor barrier or house wrap. I'd love to build a hobbit type earth home. That will be the next project. If I could incorporate those principles it would be great. I'm not sure how applicable they would be though to what I'm able to do in this type scenario, because it will need to move? I'm all ears though. I guess I was opting for the vapor barrier because it will be a small space and I thought it would make it air tight as well as help control moisture? Although, with that said, there would really be no use for it in the floor??? I'm not so sure. So, I'm still wondering do I put my tar paper in the floor for moisture control to help prevent the floor joists from rotting from moisture build up? The joists are wood 2x6. I would really like for this structure to last as long as I possibly can so it's not another generations problem, so I'm trying to think through this well. I think what is sounds like the consensus is no vapor barrier and yes to the tar paper??? Would Input something else in place of the vapor barrier under the subfloor? And no, I am not any type of engineer so all those kinds of running tests are not something I would really do... Although the WUFI sounds like it would be useful and interesting. I don't know how to have access to it? Many thanks!
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.

Kaiya, yes I very familiar with Huber, a relative new product that integrates an air/moisture barrier. They are some cases you will find on the net of it not living up to it's claims but I consider it one of the better OSB products out there.

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.