...Getting ready to have the attic insulated, the insulation contractor says vapor barrier not necessary over my living space. (?) I have never heard of this before. My house is 1900 sf, flat ceilings throughout. We are planning on approx R48 (6 inch between the 2x6 joists, then 10 inch laid crossways over that). I have already removed what was up there before (remodel project) so the attic floor is now bare.

What do you think? Barrier or no barrier?

thanks

Mark

IMO, depends on climate. But in either case, I'd put in an air barrier.

jonr,

I live in the pacific NW 97141, about 1 1/2 hrs west of Portland, on the coast. The climate here is a lot of wet stuff, but not a lot of extreme temps.

Thanks for your reply. I tend to agree, but I would like to see if there are any other remarks as well!

Mark

In Tillamook OR with a vented attic there's essentially zero wintertime condensation risk from interior moisture drives and there's no need for an interior side moisture barrier.

But NOW is the time to air seal every seam electrical, flue & plumbing penetration, before the insulation goes in. Replace any recessed light fixtures with GASKETED AIR TIGHT versions rated for insulation contact. Use 1-part can foam for most of it- you'll need metal at flue penetrations. You'll also need to use rock wool where the insulation contacts any flues.

Blown cellulose would be more effective than cross-laid batts since there are no gaps or seams that could open up with blown goods. Wrapping flues with unfaced R13 rock-wool batts first allows you to blow cellulose into the rest.

BTW: I haven't been through Tillamook since about 1980, but in the early 1970s I was on a crew repairing & reinforcing the trusses on those WW-II vintage blimp hangers ( they were being used as a plywood mill at the time- not sure if they still are.) They're tall enough inside that with their minimalist-venting mist/clouds would form at the peak of the arches inside, and some of those wooden trusses were SCARY punky after 25+ years of ultra-high humidity.

Thanks Dana, good points about air sealing, that is in the works! I have never heard of wrapping the flues with rock wool, though. Will the inspector sign off on that? normally the flue requires 2 inches of airspace for solid fuel appliances, one inch for gas and pellet. Seems like a much better method than a sheet metal firestop alone.

One of the two hangars has burned to the ground, abt 6-7 years ago. The other one houses a cool antique air museum, with lots of WWII aircraft, including some ultra rare birds. The remaining blimp hangar is still in the guiness book of world records as the largest free-span wooden structure on the planet!

Thanks for your great, valuable, helpful, and accurate answer(s). You, sir, are awesome!

Mark

The clearance requirements written into code are usually clearances to combustibles. The combustion point of rock wool (=spun slag from steel making) is higher than that of steel, and higher than the melting point of the material. (If it's ever that hot in your attic it means you were too close to a nuclear explosion.)

An R13 rock wool batt is nominally ~4" thick when fully uncompressed, designed for a snug fit in a 3.5" 2x4 cavity. A single wrap is good enough for maintaining 2" clearance to fiberglass or cellulose.

Bummer about the hanger burning down- it (and it's surviving brother) are impressive structures, with those tall parabolic trusses & all- not exactly your average Quonset hut! Working at the top of the trusses was a bit gut-wrenching, safety lines or not. IIRC it's over 150' from the slab to bottom chord at the center. Whenever a chunk got away, it was a long countdown before impact. (Fortunately that didn't happen very often.) Turning it into an aircraft museum is a fitting use though!