what kind moisture barriers and and where do i need them?

[Naturalist3]

Well im building a 12'x20' cabin in the my 55 acres.  After finding this site and seeing some pictures I realized my plans were very simliar to the 12x18 Little House Plans Kit, http://www.countryplans.com/nyikos.html. 

The base of the cabin will look almost identical with the same elevation off the ground.  Do/should I use a 6mil vapor barrier over the joist before laying down my 3/4" plywood floor.  Is it okay to have it directly against subfloor?

I am still trying to figure out the way to go with siding.  I'm thinking just using T1-11 sheets, but not sure.  Is there any other type of material to use for siding that could be cheaper?  The T1-11 seem to run about 20 bucks a sheet.  When using a T1-11 board do I need to wrap the wall studs with house wrap?   

Then, there is the roof.  My father-in-law is building me trusses, the are roughly 4 to 4.5' high in the center and giving a foot and a half overhang.  He is making enough to put them 16" oc.  This makes each face of the roof about 8.6' x 22'.  I plan to use metal roof panels 3' x 9' on 2x4 purlins, probably 2' oc.  The cabin will not be insulated and roof/trusses exposed to floor of cabin.  I plan on using a wood stove with the pipe going through the roof to heat.  Possible using a window a/c unit when I get my own generator.  The question is, do I need to put down a barrier of some sort between the cabin/purlins/metal roof?  I can also put a small vent on each side of the gable in the peak.

If anyone could help me out on this I would greatly appreciate it.  I have performed and assist in basic construct, but have trouble understanding the barriers for moisture and where exactly I need them.  I plan to take many pictures as I build and post some.  Looking to having I at setup by spring. 

Thanks, 

Erik   

[MountainDon]

   Naturalist3

First a question, will there be any building inspections, or local codes that you need to meet? That could influence how you build.

You do not need a vapor barrier in the floor if you use T&G plywood or OSB sub flooring. The adhesives used pretty much do the job of a vapor barrier.


If your planning on using metal roofing over purlins and having that roofing exposed to the interior you will more than likely have an interior condensation problem. If you are not planning on insulating at all my personal recommendation would be to use 7/16 or heavier plywood or OSB sheathing to sheet the roof. Then apply roofing paper and then the metal roofing. Forget the purlins.


As for the walls... If you install wrap over the studs and then apply the T1-11, that leaves the wrap exposed on the inside. Building wrap deteriorates with prolonged exposure to UV. That might not be too much of a factor on the inside, but I am not certain. Building paper is too readily ignited for my likes to be exposed on an interior living space. If you are not inspected and not insulating you can likely forget the house wrap or building paper. My personal choice would be to sheath the exterior with OSB panels, or plywood. Then building wrap or building paper. Then the finish siding of your choice. That would enable you to better seal any door and window openings as well as make a more airtight and water resistant wall. In most places code would require that as well. Building wrap or paper on the exterior is meant to perform as a barrier to liquid water from the exterior as well as an exterior air infiltration barrier. It's called a weather resistant barrier.

Vapor barriers are applied on the warm side of walls or ceilings. In places with cold to moderate climates that normally means applied on the inside of the insulation. In a hot climate where the interiors are air conditioned the warm side is on the outside. So the rule changes.

[rocking23nf]

I always think of it in dollars, house wrap is around 80$ for a 100 feet roll(in canada). Assuming your house is 1 story, you have 64 running feet of wrap you need. Its a drop in the bucket when you compare to the price of the complete house.

[Naturalist3]

No I am not under any code restrictions.  But I definetly do not want any interior condensation problems, so I think I will deck the roof.  My father-in-law called me today and said he knows a buddy who does roofing and has many bundles of shingles leftover from many projects that may be enough for what I need, and I can have them for free.  If I do get them for free I will definetly go this way.

Are all osb sheets the same?  Can I use the same sheets for walls and roof decking, or is there a special type to use for one or the other.  My father-in-law says you dont want osb on a roof.  But I tried to tell him, from my reading, many builders seem to be using it.  I thought i read something about it being accepted and interchangeable with plywood.  He says its not near as good and will swell like crazy if it gets wet.  He doesnt think it is allowed by codes.  I think a lot of what i hear from him is his hard-headedness ways.  Although he have done several renovations and repairs to homes and has much experience.  He confuses me quite a bit when he tells me what to use for certain things.

I'd rather not have the house wrap showing if it cathes easily and I will be running a wood stove in there, thanks for the tip.  It would also add strength to use another layer.

Thanks MountainDon!

[MountainDon]

OSB is stamped just like plywood, and the same product is accepted for walls and roof sheathing. It will be stamped indicating what it will span, usually up to 24".  The big stacks of it at the big box stores are all approved for wall and roof, at least the ones I've seen. Floors require thicker, 3/4 minimum, and should be the tongue and groove type. (T&G)

Yes OSB will swell when it gets wet. Plywood can also delaminate, although perhaps not as readily. The idea is to prevent that with properly installed building paper ('tar' paper) or building wrap. So once the floor is down it's always best to be able to get the walls up and the roof on as quickly as possible.

Building paper comes in two types #15 (walls) and #30 (roofing). It's tried and true. The big advantage to the building wraps is that it goes on quicker; less labor is important for a contractor. For building on your own I like building paper. Thcan be disadvantages to the building wraps. If you are planning on not insulating the walls I would go with paper. My reason is that water vapor from the inside could make its way through to the backside of the wrap and condense there. Building wrap will not allow that moisture to pass through. Building wrap will only let vapor through. So you end up with water trapped between the wrap and the sheathing. Bad. If you use building paper any condensation in there will wick thru the paper and eventually evaporate.

[davidj]

Are all osb sheets the same?  Can I use the same sheets for walls and roof decking, or is there a special type to use for one or the other.  My father-in-law says you dont want osb on a roof.  But I tried to tell him, from my reading, many builders seem to be using it.  I thought i read something about it being accepted and interchangeable with plywood.  He says its not near as good and will swell like crazy if it gets wet.  He doesnt think it is allowed by codes.  I think a lot of what i hear from him is his hard-headedness ways.  Although he have done several renovations and repairs to homes and has much experience.  He confuses me quite a bit when he tells me what to use for certain things.

My vague memory from when I looked into this is that OSB is okay if you're enclosing the eaves, but if you're having open eaves you have to use plywood.  The OSB isn't rated for exterior use even in sheltered locations.

[John Raabe]

That's right, you should use ACX plywood or 1x4 shiplap for exposed or open eaves. It will hold up much better over the years (and looks nicer too).

We planed down 1x4 cedar for our eaves. If I were to do it again I would just bridge the transition between 1/2" sheathing and 3/4" T&G with a taper strip. With a heavy shingle you can't see the transition.