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Question DetailsAsked on 11/8/2017

Can I use 30# felt as water barrier between sheetrock and insulation in LR ceiling?

New construction with traditional materials -- requires no poly sheeting, minimal synthetics. Want to guard against damage from any moisture condensation under attic floor resulting in water damage to LR ceiling. Attic over ceiling is unheated and vented. LR is heated. 1x12 pine attic floor, has gaps from shrinkage. Standing seam roof. Outside temps -35 to 95 degrees. Heavy snow. and high winds

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1 Answer


Nope - not unless you want wet insulation and possible ice buildup in the winter. Felt is not feasibly sealable at penetrations and lap joints (there is really no tape which sticks properly) and it is not water vapor impermeable - it is a semi-permeable product to both water vapor transmission and actual water seepage through it.

You need a true vapor barrier on the interior side of the ceiling envelope - right under the drywall, stapled to the underside of the attic floor joists or trusses would be the usual way. Properly done all seams would be lap jointed and double-taped (both sides) and all penetrations through the vapor barrier for wiring, pipes, lights, etc would be sealed - ditto to penetrations through the top of the walls.

Normal vapor barrier (not knowing any local code variations) would be 6 mil polyethuylene vapor barrier.

Oh - and the insulation in the attic should be unfaced, though generally paying a paper facing faced-side down on top of the vapor barrier (between the joists/truss bottom chords) would not hurt - but you do not want any vapor barrier poential (which all facings are) possibly blocking any water vapor flow (which would be normally from the warmer ceiling to the attic in winter when condensation is usually the biggest issue.

Answered 2 years ago by LCD

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