sub floor - vapour barrier question

[skiwest]

Building a cabin on piers and beams. Floor joists will be insulated. For subfloor going with 3/4" OSB for the 24" OC then putting on 5/8 fir plywood for extra stiffness for later when we tile part of it. I have hated putting plywood down later so going to do the whole thing at once.   Will put down some sort of wood floor later , probably Douglas Fir , but that might me in a couple of years.

Now where is best place to put VB. On top of joists or between the OSB and the plywood?

If it matters under the joists to hold the batt insulation in will be 7/16" OSB. Over kill I know but wanted something solid to keep critters out. OSB will be 4' off grade.

[glenn kangiser]

Insulation goes vapor barrier to heated side.  If condensation were to occur it would be at the vapor barrier so you would want it at the bottom od all the flooring - top of the joists.

Also John has mentioned that you need ventilation to the bottom to let moisture out - slits in things such as Typar etc.  Possibly some saw cuts in the osb under the floor joists to provide this.

Hopefully you will get other opinions on this also.

[skiwest]

For the ventilation under floor I guess could leave gaps between OSB and cover with hardware cloth.

I hadn't though of the condensation issue.  But isn't it the same really.  On top of joists means that condensation occurs on under side of OSB.  Between OSB and plywood , occurs on underside of plywood.

[Pala]

Wouldn't 6mill poly on the ground and crawlspace vents be the norm here?

Did i miss something peculiar?


chuck.

[John Raabe]

Go ahead and put down your OSB and plywood subfloor as normal - that is your floor vapor barrier. This is due to the glues in the material and this VB is in the right place on the warm side of the insulation.

When you insulate make sure you don't seal up tight the bottom or cold side of the insulation. You do want to keep air from blowing through the insulation, but it needs to have some air exchange and you don't want it to trap water from a leaking pipe or such.

Tyvek or other air barrier (but not VB) products work well if you have unfaced insulation. Or, like Glenn mentions, slit the foil or kraft faced vapor barrier on faced insulation.

As Chuck mentions, you do want to control moisture coming from the soil and raising humidity in the crawlspace. In moist areas of the US a 6 mil poly VB over the soil does this. However, this is a different issue than the location of vapor barriers in the structure.

Control excess wind carrying heat away by skirting strategies.

[skiwest]

Have been debating the VB on soil thing. Initially wasn't but now will.  Have sloping away ground on all four sides plus other than 6" of silty soil on top the rest is coarse sand gravel .  Actually under cabin less than that after scrapped away soil with sticks or any organic matter.   After a heavy rain a puddle will last all of 5 minutes.  The trouble with the per test is that it way too fast.

The only time I think VB on Soil would be a good idea is winter/ spring.  With ground frozen can have top layer of soil damp as water can't get through frozen ground.  

[skiwest]

But back to VB in floor .  Isn't its purpose , just like in walls , to stop any moisture from from getting into the structure/ insulation and condensing where it shouldn't.  I guess OSB would be pretty good if ensured lots of glue at edges.  but would VB do any harm?  Its already bought so no cost saving no putting it in.

Will be shirting crawl space and putting in vents.  Glad i went with a high peirs as last winter the snow hieght got over 3 feet.  If went low I think the vent would have been covered with snow.

[John Raabe]

No, a poly VB between the layers of the floor will do no harm but will not add much of anything either.

[Chuckca]

Could you use a mesh screen to hold the insulation and keep out the critters?

[youngins]

Thats what I did:



However, I loosely laid some tar paper down before the insulation to keep Red from chewing on the stringy pieces of insulation.

Chris

[John Raabe]

Chis:

Nice solution clearly shown. Thanks

Hope Red isn't too disappointed.

[youngins]

Red-



Looks pretty happy to me..

[PEG688]

Humm,  Red does seem to be a little peaved about the whole setup

[youngins]

Actually, this is the "Let me in - or your shoes become chew fodder" look: