I am builiding a new garage for my business in New Hampshire. The framing is done. The garage has 2 foot deep trusses running across it to support the ceiling and a floor above. I don't plan to finish the room above there too soon.

My tenative insulation plan:

• Have 2" of closed cell foam sprayed on the bottom of the floor deck
• Insulated the remaining depth with fiberglass or celulose.
  

The dilema:

Since the garage is going to be warmer than the attic, should I vapor barrier beneath the insulation?

Thanks in advance, ck

you obviously intend to heat above laetr, so do not install any vapor barrier in the garage/attic seperation

Please correct me if I am wrong, but I thought that the closed cell foam was, itself, a vapor barrier.

At 2" thickness closed cell foam is a class-II vapor retarder (but only class-II). But let me see if I understand the stackup:


/roof\truss/

----attic space-

----attic floor-

----2" foam-

----batt or cellulose-

----(maybe vapor retarder, maybe not, there lies the question)

----ceiling-


Yes?

In that case, assuming the foam is ~R12-14-ish you will run into condensation issues in the batt or cellulose if it's more ~R13- worth, maybe less. (You don't say WHERE in NH, the max fiber depth without condensation issues in Berlin will be lower than in Portsmouth.) It's less fussy if the 2" foam is applied to the ceiling layer, and the fiber insulation applied atop that.

Wall or ceiling, stackups work more reliably if you put the highest vapor retardency materials closes to the warm surface and never place a more-retardent material to the cooler side of a less retardent layer, but there is wiggle room.

If you're committed to applying the 2lb foam to the floor decking and putting big-R between it and the ceiling, place a class-I vapor retarder (6mil or thicker poly sheeting, or 1/2" foil-faced iso above the gypsum, with seams glued/caulked, mastic sealed, or FSK taped) on the warm side of the fiber, or no further than 1/3 of the way into the total R value. Then be EXTREMELY fussy about air-sealing that vapor retarder layer. (Forget about using recessed lighting, and foam seal all plumbing & electrical penetrations & boxes.) It needs to be more vapor retardent than the foam, and drum-tight to guarantee seasonal dryness of the fiber.

In a wetting event (minor roof leak) the assembly will still dry toward the attic through the semi-impermeable foam, but you have to make sure you don't accumulated moisture in the fiber layer from air & vapor infiltration from the warm side. In a major roof leak it may take a VERY long time to dry though.

Putting 2lb foam on the cold side of the assembly is easier to deal with when it's the bulk of the R-value, and the wintertime dew point of the conditioned space air occurs within the foam layer.

The presence of a vapor retarder in the ceiling/floor has no effect on whether you later convert the attic into conditioned space, since after you've made the conversion both sides will be the "warm" side. In ANY NH climate some amount of warm-side vapor retardency and air-sealing is warranted, but in coastal & southern regions vapor retardent paint is sufficient. Class-I vapor retarders on the warm side aren't usually necessary unless you have an 8000HDD+ climate, or have a class-II vapor retarder on the cold side (as in the above stackup.)

Under no circumstances should you put multiple class-I retarders in a stackup, which is a setup for "never-drying" rot & mold issues.

First, thanks to those who take the time to answer these questions.

Dana1, without my premise of spraying 2" of foam, what would you do in my position? I am required to drywall this ceiling since it is a wooden frame and I will have vehicles in it.

I am in Northwood NH, which is between Concord and Portsmouth. Our temps are modestly colder than the seacoast area.

Here is what I decided to do:

Using a foam gun, I am sealing all of the framing seams-1/4" to 1/2" bead. Then fiberglass insulation carefully placed. Finally a layer of 1" Polyisocyanurate which is foil covered to create a thermal break and taped to create a vapor barrier. I will apply foam on the outer walls within the truss area.

That should work well. At the end of the day you should have that area well insulated to eliminate the "cold floor" issue that is common in a bonus room over a garage.