Insulating cabin floor

[Jared]

Right now I'm saving to build a small cabin on my dad's 50 acres. I've decided it'll have a shed roof,  with an 8' wall in back and 10' in front. I'm not sure about whether it'll be a 16x16, 20x20 or 14x14. What I'm definitely not sure about is how to insulate the floor so I don't lose heat when we stay in the winter. It'll only be for weekends, but maybe for an entire week, who knows? There's a link on countryplan's homepage to a 10x14 walk-through of building a small cabin. But he doesn't insulate the floor. I want to build like that one,  with the floor joists resting on rails that are resting on pavers and pre-cast footings. Would I lay the rails, then plywood, joists, insulate and plywood over that? What could I put on the ends for ventilation?
Jared

[martyv(Guest)]

I'm building something similar.  I just bought all the materials for the floor.  I'm using 2X10 floor joists from the advice I received on this forum.

My cabin is 16X16.  I'm laying out the floor in two sections, 8X16 each.  After I lay out the joists and tack the rim joist on one section I am nailing down 1/4" OSB on top, then I'm flipping it over so that is on the bottom.  I'm nailing that down to the 4X12 foundation beams and then filling it with insulation, On top goes 3/4" tongue and groove OSB sub-floor.  I'm not ventilating it at all.

[jraabe]

Jared:

If you are thinking of the Little House project that started the CountryPlans.com site (http://www.jshow.com/y2k/), those shed roofed buildings are most certainly insulated in the floors, walls and ceilings. The plans show this in much more detail if you don't already have them.

Martyv:

What you are planning to do can be done if you are lucky enough to have dry weather all the time during framing (and no leaks and such forever after). Most buildings (including the Little House plans) have a decent height space under the joists so that you can get under there later to put in wiring, plumbing and insulation after the roof is up and the place is fully dried out.

At some point you will need to get under there and work on things. Buildings (barns, sheds and other "temporary" buildings) often don't have this and slowly sink into an organic soup over the years. If the building is worth saving, then the owners go to the expense of jacking the whole thing up, cutting out the rotting stuff and building a proper foundation under it at that point. This is more expensive, of course, than doing it that way in the first place.

[Jared]

John, would you recommend a real foundation in the first place? Or, since this is my first building project, going ahead and doing it with the pavers and pre-cast footings?

[jraabe]

If you are doing a building for 100 years or more, I think I would do the standard reinforced concrete foundation.

That said, a properly built post and pier foundation in the right soil can last just as long with only slightly more attention.

Insulating the floor with a non-vapor barrier material and then screening the access under the house will keep critters from tearing things up. Concrete foundations do this initially of course.

Far more critial to the lifetime of either building is to maintain the roof, keep the gutters cleaned out, make a solid chimney and keep it clean and all those boring maintainance things.

[Jared]

This cabin won't be for 100 years or more. If it lasts that long, great. It's not my intentions. I'm doing this small cabin to learn to build walls and to have a place to stay when we want some cheap fun and a place to stay if I decide to undertake framing my own house. So I think I'm going to go with the post and pier foundation. Once our house is done, it'll probably be an oversized playhouse for our three and sometimes four kids. I want a woodstove and maybe a sleeping loft. Do the little house plans cover something like that with a shed roof?
Jared

[martyv(Guest)]

I'm sorry if I'm hijacking this thread.  

My floor is on two 4X14 beams which are on posts on concrete piers.  Similar to the little house design.

It is not dry by any means but the entire floor will be elevated off the ground with air passing easily under it.  I do expect it to shift some over the years and I will need to shim it occasionally.

Do you think the sandwiched floor will need to be ventilated?

[jraabe]

Jared:

The Little House plans have a shed roof option for each of the three sized cabins. They each have a post and pier foundation and should be what you need for that project (http://www.jshow.com/y2k/listings/29.html).

Martyv:

Remember that you don't want to build any insulated part of your buidilng with sealed vapor barriers on both sides (OSB and plywood are vapor barriers). This is especially true in the floor where spills and leaks will run into the insulation and moisture may be unable to get out. The bottom side of the insulated floor should have an air barrier but not a vapor barrier. As I describe in the Little House plans you can use Tyvek, pegboard, or standard kraft faced insulation (held up with wire or strips of lath) with the face slashed to allow vapor transmission. If you want to do your OSB sandwich then drill vent (drain?) holes in what will be the bottom side of the floor.