vapor barrier confusion

[mdshunk]

I'm converting a garage in a rental of mine to living space. To make the floor the same height as the rest of the house, I've built the floor up with 2x10 joists sitting atop strips of pressure treated shims (since the floor was pitched). I intend to insulate this floor cavity, and I'm confused about the vapor barrier for this application. The original floor was concrete. The "48 hour, tape down plastic test" seems to indicate that the concrete has a vapor barrier underneath it, since no condensation was evident under the taped down section of plastic. Should the fiberglass insulation that I install in the joist cavity have a vapor barrier or not? If so, which way should it face? Should there be any air gap between the concrete and the fiberglass? If so, how much? The last thing I want to do is grow mold inside the floor cavity.

[joed]

The vapor barrier rule as I understand it is, vapor barrier on the warm side.

[mdshunk]

I understand that too, but I also understand that you aren't permitted a double vapor barrier. That would trap moisture in between the layers and start to grow nasties. I think that the vapor barrier under the slab, and the a vapor barrier on top of the insulation would constitute a double vapor barrier. I don't want to set up my own little underfloor terrarium.

[suemarkp]

Can you put 6" R-19 in the floor joists (vapor barrier towards living space) and vent the gap between the insulation and the concrete? Hopefully, the shims on each joist are substantial (at least 3/4") so you don't have to vent each joist cavity, just a few of them.

[joed]

I would personally not consider the vapor barrier UNDER the concrete as a second vapor barrier.

[Wgoodrich]

Vapor barrier under the concrete is a vapor barrier thus no vapor barrier should be installed above the concrete in the project you design. You may put a product such as tyvek down so it may breathe as a separation between fiberglass and concrete but not necessary. You may install fiberglass directly on the concrete but make it friction bat with no vapor barrier filling the joist space allowing that space to breath. Leave a 1" air gap between insulation and floor decking for an air gap.

If you have intermittent water or known moisture problem at top of concrete such as not high enough for seasonal high water table etc. Then you may add treated 1x slats on the concrete then the floor joists again treated then the insulation laying on the slats allowing for an air gap at bottom between concrete and insulation but still leave the air gap just below the floor decking also and still no vapor barrier.

HOpe this helps

Wg

[mdshunk]

Thanks for that info, WG.

I'm puzzled on two points. First, the reccommended air gap between the insulation and the subfloor. What is it's purpose? Second, would vinyl flooring be considered a vapor barrier, or is vinyl flooring known to respirate in some manner?

[Wgoodrich]

Vinyl flooring does not perk moisture. It would act as a vapor barrier. If this vinyl is installed as flooring it would be dry on top with a sealant of glue below thus not allowing moisture to enter the living envelope. Water would then wick out the sides of the concrete if any or would be trapped before it enters the concrete by the vapor barrier below the concrete.

Problem is when a floor joist system is installed above this area now you have a cavity you need to allow moisture to escape that gets in through the floor from above due to spillage or flooding etc. This is why the air gap between the insulation and the floor decking. Gives an air space to allow the fiberglass to dry back out and avoid mold and rot. The decking will obsorb a certain amount of light moisture adding to aid in dissipation of moisture in that floor space cavity.

Vapor barrier should be below concrete floor if not present then directly on top of the concrete trapping moisture before it enters the cavity or living envelope if possible.

Installing a second vapor barrier traps moisture without an outlet between the two vapor barriers.

Remember wood can get wet and dry out with little damage but it can not stay wet. Wood also wicks out the ends transmitting moisture when needed if only a periodic wet.

Best I got

Wg