Hello, I've been looking for some answers to what seem like very simple questions, but the more I read the more confusing it gets. I am finishing a pole building that will be a metal fabrication shop located in North Idaho. It has hydronic heat in the slab. The main question is whether or not to add a vapor barrier. The 4 different scenarios where it needs insulation are: #1. On top of a flat ceiling under a vented attic space. The roof is metal over OSB and the ceiling is also metal. I have available at a very good price some R-30 unfaced fiberglass batts that I would like to use. I find conflicting information as to whether I should add 6 mil poly under the batts or not. Some information seems to indictate the metal ceiling is more than enough vapor barrier and the poly could trap any moisture. #2. One area of the ceiling will have storage above it and so there the underside of the roof will need to be insulated. Again the roof is metal over OSB. If I fasten the unfaced batts up between the roof joists should I add the poly underneath it? I realize spray foam would be the way to go here but unfortunately it simply is not in the budget. #3. The exterior walls are metal siding with no sheathing underneath. Most of the interior walls will also be metal. Poly on the warm inside of the unfaced batts, or not? #4. And finally an office space where the interior walls will be sheetrock and otherwise the same as the other walls. Any different considerations here? Any guidance would be greatly appreciated.

Situation #1: If the ceiling is metal and air-tight, it's more vapor retardent than 6-mil poly, but you'd have to seal the seams with an appropriate caulk or duct-mastic (a good practice in any event.) Skip the poly there. Without an air barrier on the top side of low-density R30 batts it's cold weather performance will be well-under it's 75F ASTM C 518 rating due to air convecting a bit too freely through the fiber. At -10F outdoor temps it could be as low as ~ R15. In the vented attic space where the insulation is on the attic floor that loss of R at low outdoor temps can be remediated with as little as 3" (R10) of blown cellulose, which while air-permeable, is much more air-retardent than low density fiberglass. An overblow over batts restores the full-R of the batt, and fills in any gaps, improving the total performance far more than would be explained by going from R30 to R40. (Code-min is probably R38 anyway, eh?)

Situation #2: If you're going to insulate the roof deck with fiber insulation in the cathedralized ceiling portion, code requires a ventilation space between the OSB & insulation, and an interior vapor retarder. To reduce convective loss of R there you can use relatively vapor-open rigid-foam such as 1/2" unfaced XPS or unfaced 1" EPS sheathing (not thicker) above the batts, but installed such that it does not block the vent gap. Even corrugated cardboard vent chutes etc will make a difference here so long as the fiberglass isn't allowed to convect freely into the vent gap.

Situation #3: Metal interior skins detailed to be air-tight is key to preventing interior humidity from getting into the cavity and condensing. Adding poly buys you nothing here. Fitting the batts carefully so that there are no gaps and that it fills the cavity completely from the interior metal to the exterior air barrier is key to making it perform. Any voids in the cavites give rise to higher convection rates within the fiber layer (or in some cases, completely around it), than you get with a fully-filled cavity. Compressing thicker batts is far preferable to batts than hang to one side leaving a thin cavity on the other. With the metal siding mounted on lateral furring, use vapor permeable housewrap on the interior-side of the furring to make the exterior air-barrier for the fiberglass, leaving a furring-depth ventilation gap, trading away a small amount of R value for a vast improvement in drying capacity to protect the susceptible wood.

Situation #4: Some sort of interior vapor retarder is required, but be aware air tightness is far more important than vapor permeability. The moisture transport of square inch of air leak from the interior is worth 50+ running-feet of wall by of vapor diffusion through standard latex paint. Rips or even pinholes in poly vapor barrier that leak air can make the poly a bigger problem than a solution. Use foam/caulk to seal all electrical & plumbing penetrations (or route the power & plumbing only on interior walls.)

Thanks Dana1 for a very thorough answer. One thing I am not clear on is in my situation # 2 where I need a ventilation space between the OSB and insulation. Does this mean a vent space that is actually open to interior or exterior air? Or is it just an air gap? This building is built with trusses and has roof girts 16" O.C. running parallel to the ridgeline so there would be no easy way to create vents above the insulation that would flow to anywhere. FYI the county I am in does not require building permits or have codes. One other question regarding fiberglass batts in a wall. You mention it being far better to compress them somewhat than to have them hanging to one side and leaving a cavity on the other. In that case is there a way to calculate actual R value from a compressed bat, say a 9" R-30 compressed into a 7" cavity? Thanks again for the help.

The county may not have codes, but you're still required to follow the laws of physics, eh? ;-) (In particular, the physics of water as vapor, liquid, or solid.)

If you don't have a means of venting the roof deck any moisture that gets in the cavity ends up in the roof deck in winter, since the roof deck will below the dew point of the air trapped in the cavity. If there is an interior side vapor barrier there won't be much moisture getting in there from warm-humid interior air, but it means the assembly needs to dry toward the exterior- no mean feat through metal roofing. With even a modest amount of air gap an venting at the gable ends the risk of rot conditions in the roof deck go down. If the roofing is mounted on purlins and cross ventilated the roof deck can dry toward the exterior, as long as any underlayment or roofing felt, etc. is at least somewhat vapor-permeable, 0.5 perms or higher. Most roofing felt is over 1.5 perms when dry, higher when the humidity is higher. Ice & Water Shield etc, is extremely low-perm.

The R value per inch of R30 batts goes up as you compress them, but with fewer inches it's less than the labeled R-value. According to Owens Corning data an R30 batt compressed into a 7.25" 2x8 framing cavity ends up at about R25.

See: http://numsum.com/spreadsheet/show/21111

Notice that an R22 batt is only R19 when installed in the 2x6 framing it was designed for, and an R19 batt is only R18. Batts suck, and low density batts like R22s and R19s suck worse. An R19 batt has exactly the same amount of material as an R13 batt used in 2x4 framing, but it's density is much-reduced. While it may perform at R19 with an average temp of 75F (the testing criterion for labling), the induced convection within the low density batt makes it perform much worse when it's very cold or very hot out. The loss of performance is worse in roof assemblies than in walls, and at -10F outdoor temps you'd be lucky to get R10 performance out of it.

Eave to ridge furring strips over OSB would be a standard way to vent.

This is speculative:

While metal doesn't pass moisture, it's not so clear that the joints couldn't be made leaky enough to serve as vents. Perhaps with assistance from DrainWrap for horizontal movement of air. You only need some amount more airflow than can leak into the wall from the interior side. Ie, a well air and moisture sealed interior side (taped metal, poly, smart membrane, etc) means you need little venting on the exterior side.

When covered with snow & ice steel roofs are not so leaky-venty eh? (At least not in a GOOD way.) Getting some amount of air space between the metal & wooden roof deck is necessary, and less than 1/4" using a rainscreen-mesh type underlayment isn't going to cut it in an ID panhandle climate, even if it's adequate for draining off the drip-drip in a reasonable amount of time.

Snow - very permeable (which is why the ridge vent works). Solid sheet of ice on a metal roof that covers all the joints - I'll guess very rare (and also a problem with small ridge vents). But I agree, it you want lots of air flow, look at a big gap and vents that cannot get covered.