My home is in Seattle. It has a typical vented crawl space. I am getting different answers from different people, including Owens Corning who doesnt want to say one way or the other. When installing new r30 fiberglass batt insulation pressed up against the sub-floor in a vented crawlspace, do you use faced or unfaced insulation, (with the faced part pressed up against the sub-floor)?????????? It appears if we were in florida, the answer would be unfaced, in the NE the answer would be faced. Would seattle area be faced or unfaced?...i have not been able to find the answer in seattle/king county building code search.......... Vapor barrier on the ground....6MIL or 8Mil - black bottom/white on top. Thicker seems better to me does it make any difference????????? THANK YOU

The "right" thing do do would be to put down a ground vapor barrier and insulate the foundation walls with rigid insulation, not fluffy stuff between the joists, and make it an unvented, conditioned crawlspace. This is particularly true if (as in most vented crawlspaces in Seattle) you have plumbing below the joists that might freeze in a cold snap (ask me how I know).

If you're insulating between the joists anyway, it's safe in your climate to put the facers on the bottom rather than at the sub floor. All batt insulation needs air barriers on both sides to hit their R-performance. If you're not installing OSB or plywood under the insulation you might as well use the facers for that function. Kraft facers are "smart" vapor retarders and will become sufficiently vapor open to let moisture out if it needs to, but is otherwise moderately vapor retardent when dry. It won't create a moisture problem no matter which way it's oriented, but you'd have to install something else under an unfaced batt as an air barrier. Don't use foil faced batts here, only kraft.

The vapor retardency of 6 mil poly is low enough to protect against ground moisture and gases, but it's not as rugged as 8 mil or 10 mil. But even 10 mil poly isn't as rugged as EPDM roofing, which is also used in this type of application.

Hi and thank you for the response/contribution and detailed info. I agree with the condition/closed space, however not an option at this juncture...i can only imagine what you have had to deal with..yikes!

Question to your outline: My initial plan has been to use faced insulation with the facing up against the subfloor. I would just leave the batt insulation 'open' or in other words not encapsulate it...re/nailing OSB or plywood to the bottom of the joists. To my understanding the craft facing always goes toward the warm side of the house, however it sounds as thou you point out it could go either way./? ....at least in this Pacific NW area?

However my other query to your last note regarding 'needing' "something else under an unfaced batt"....would that be the case if using unfaced (both sides) insulation against the subfloor, is there a need then to attach plastic or as referenced OSB etc to the bottom of the joists? in the case of using faced insulation.....be it faced up against the subfloor (or as you suggest turned down) i assume/? that there is not a need to put OSB etc attached to the joists to encapsulate it...correct?

I had one installer tell me that if I put craft faced insulation up against the subfloor that it would create a moisture problem...because we are here in the NW..and that it was "industry standard" to just use open faced insulation and 6mil vapor barrier.......i want to say 'i disagree'

you are describing a standard fiberglass insulation detail that is primarily used for rodent housing - intentional or not. You can install the batts as well as possible, but rodents will find their way in, and within a couple of years the insulation value will be -0-. You could try Roxul batts which may work better. Whichever you use, the insulation must fill the entire depth, and the bottom of the joist bay must be -well- sealed with plywood.

Posted By Pierce_Sea1 on 20 Apr 2016 06:09 PM
Hi and thank you for the response/contribution and detailed info. I agree with the condition/closed space, however not an option at this juncture...i can only imagine what you have had to deal with..yikes!

Question to your outline: My initial plan has been to use faced insulation with the facing up against the subfloor. I would just leave the batt insulation 'open' or in other words not encapsulate it...re/nailing OSB or plywood to the bottom of the joists. To my understanding the craft facing always goes toward the warm side of the house, however it sounds as thou you point out it could go either way./? ....at least in this Pacific NW area?

However my other query to your last note regarding 'needing' "something else under an unfaced batt"....would that be the case if using unfaced (both sides) insulation against the subfloor, is there a need then to attach plastic or as referenced OSB etc to the bottom of the joists? in the case of using faced insulation.....be it faced up against the subfloor (or as you suggest turned down) i assume/? that there is not a need to put OSB etc attached to the joists to encapsulate it...correct?

I had one installer tell me that if I put craft faced insulation up against the subfloor that it would create a moisture problem...because we are here in the NW..and that it was "industry standard" to just use open faced insulation and 6mil vapor barrier.......i want to say 'i disagree'

You definitely DON'T want to put 6-mil polyethlene sheeting under the joists or on the under-side of the batts, since the much cooler crawlspace will be below the dew point of the interior air at times, generating a condensation event inside the joist bays, and a higher risk for mold. You could safely put polyethylene directly in contact with the subfloor (assuming you don't have vapor impermeable flooring, such as vinyl everywhere) but it's unnecessary- it buys you nothing.

Putting OSB on the underside of the joist bays would be fine, since it's a "smart" vapor retarder, and becomes more vapor open whenever it takes on moisture. That characteristic allows the cavity to dry toward the vented crawl space if it ever takes on moisture, but is otherwise fairly low permeance when dry. Asphalted fiberboard would be even better, since it's even higher permeance, but isn't currently in common use in the PNW.

The only vapor-impermeable air barrier acceptable to put on the under side of the batts would be foil faced rigid foam board. An inch of foil-faced polyisocyanurate would provide sufficient dew point control from conditioned space moisture drives on R30 batts in your climate, but a minor plumbing leak would be as big a problem for rigid foam as it would be for 6-mil polyethylene. OSB or fiberboard would be more resilient.

The rodent issue Bob brings up is real enough, but that won't be fixed by using rock wool. Rodents (particularly squirrels) will chew through nearly anything, including half-inch OSB, but OSB would discourage most mice & bats.

If it were my house (and it isn't) and for whatever reason could not insulate at the foundation walls I'd be more inclined to sheath the underside of the joists with half-inch OSB and blow the cavities full of ~2lb cellulose. The cellulose would settle a bit over time, but the overall performance would still be there. Insulating and air sealing the band joists with R8-R10 rigid foam prior to installing the OSB would keep the performance high even after settling. The cellulose can be installed by drilling 2.5" holes every ~6-10' in the joist bays, and patching them with a 4" square of OSB glued over the holes with a sealing circle of construction adhesive around the hole.