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Vapor Retarder for Blown Insulation


Kevingoff
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I recently inspected an attic with approximately 3" of blown insultation. I informed my client to add to R-49 (new energy code for Michigan - Zone 2 attics). There was not a vapor retarder under the existing insultation. Does blown insultation like cellulose require a vapor retarder? Do you veterans recommned removal of existing insultation, add vapor retarder, then blow in additional insultation?

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In most cases, the paint used on the walls and ceilings is as effective a vapor retarder as foil or kraft facings on batt insulation. I don't know about your area, but around here cells is a real good vapor retarder and Fred Lugano, a Vermont insulating contractor and frequent contributor to Fine Homebuilding swears that it's as effective at retarding air movement as any other method used.

I never see vapor barriers with cells around here. Then too, the heating climate here is radically different from yours. Is a poly barrier reguired behind drywall in new construction where you are?

OT - OF!!!

M.

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This is very interesting stuff.

I'm seeing a collision of what the building codes want to see regarding vapor barriers, and how things actually work. I've yet to see any evidence that kraft/foil facing on fiberglass, or poly sheet under the drywall does a damn thing, other than trap moisture in the wall or attic.

Wasn't there a guy @ JLC called "DensCell Dave", or something like that, hailing from Vermont that did dense cellulose packs in rafter bays? He and I had an interesting email conversation several years back where he convinced me that vapor barriers are not necessary, and if they're installed, they don't work.

As far as the recommendation to upgrade the insulation in this existing attic, personally, I wouldn't put in a vapor barrier. I don't know how that plays out in your market though.

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That's one of Fred's oft-repeated claims - that dens-packed cells in a roof/ceiling plane don't require vapor retarders because the cells themselves are so effective at stopping air movement. He's been packing rafter bays in Vermont for years and is still in business. As cold as it gets up there, there's got to be something to it or he'da been sued outta business years ago.

OT - OF!!!

M.

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Yeah, he's the guy. He convinced me.

I've got a buddy in Michigan that's done the same thing for many years. We even took apart one of his 5 year old houses this winter so I could look myself; not a speck of mold, moisture or anything other than bone dry cellulose.

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