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Community and Q&A

Re-insulate from the outside with a vapor barrier or not?

MG34MxFyst | Posted in Energy Efficiency and Durability on

I live in northern Indiana with a wide array of temps and weather conditions. Our home was built in 1973 and this past winter I repaired some drywall and noticed 1- no vapor barrier and 2-huge voids in the cellulose insulation — some entire stud areas missed completely.

So my plan is to re-insulate from the outside, since all my rooms have been recently smoothed (texture) and finished. I plan to use unfaced R-15 fiberglass and R-3.3 (1/2-inch) foam board in place of the 1/2-inch black tar board on the outside as sheathing.

I wondered if I should try to add a vapor barrier from the outside, tucking the film into the wall cavity (against the drywall) and around the stud then lay in the batt and covering with foam board. But I am worried I will create a wet wall with no way for it to dry. Or should I leave out the inner vapor barrier — or the foam board — or is it all ok?

I have called a half dozen insulating companies and all say either vapor barrier is not needed or it is a horrible idea. I just need a clear direction to proceed.

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  1. Riversong | | #1

    You should not use an interior vapor barrier, and particularly not if you're installing foil-faced (vapor-impermeable) exterior foam. This would create a double vapor barrier and trap moisture in the wall envelope.

    In climate zone 5, your walls should be at least R-20 by energy code standards. If you can manage to fit 1" foam board you'll have noticeably better energy performance and you could use extruded polystyrene (styrofoam) which allows some vapor diffusion to the outside. I would also suggest leaving the existing cellulose in place and dense-packing all cavities with additional cellulose. Cellulose is far better in every way than fiberglass, except you need to either hire a contractor or rent a blower. You can cut holes in the exterior foam board and then patch them and seal with either canned foam or housewrap tape.

    Depending on door and window and corner trim and siding type, you may be able to use 1" exterior foam without altering trim details. If there is no structural sheathing on the house, you may have to let in steel T-bracing, such as Simpson TWB, for shear bracing.

  2. GBA Editor
    Martin Holladay | | #2

    Robert is right; no interior vapor barrier, and you need thicker foam. R-5 foam (for example, 1 in. of XPS) will keep your sheathing warm enough to prevent condensation problems.

    Even thicker foam would be even better.

  3. David Meiland | | #3

    If I were removing the existing blackboard I would probably spend some time air-sealing inside the walls... around the electrical boxes, the drilled holes for wires and pipes, etc., whatever you can do to prevent easy movement of interior air into the walls, and easy movement of air from the crawl or basement up into the attic.

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