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Retrofit insulation of an exterior wall

John Divincenzo | Posted in General Questions on

I want to insulate my exterior wall and have the room gutted. I am in the Poconos of Pennsylvania and in Zone #5 I believe. I have 2×6 walls with Tyvek wrap, OSB siding and vinyl siding. The walls had R-19 faced fiberglass batts that were installed poorly. Currently all of the wall cavities and studs are exposed and are in perfect condition. The studs, tyvek and insulation have absolutely no moisture issues at all. They look like the day they were installed 22 years ago. I want to insulate but have questions about vapor issues. I sealed the rim joist with closed cell spray foam. I’m going to seal the top and bottom plates as well. I also plan on spray foaming all wall studs inside from top to bottom to stop airflow. I also figured that foam would provide me with approximately a 1/2″ airspace between the tyvek/sheathing and a poly iso layer that I was thinking of installing. That airspace would also clear vinyl siding nails that are protruding and exposed in the wall cavity. The aforementioned poly iso foam board layer would be approximately 1 1/2-2″ in the wall cavity and spray foam the edges for an air seal. I think the poly iso would be act as an exterior vapor barrier. Would this allow the siding to still dry from the exterior? I was thinking that airspace between the sheathing and the poly would isolate the exterior vapor drive from the interior vapor drive. I also was going to address the thermal bridging that occurs through the wall studs by attaching 1 1/2″ poly iso strips on the wall stud faces. Then I wanted to install 3 1/2′” of R-15 Roxul thermal batt insulation to finish filling the wall cavity. Then I was going to apply an air barrier/vapor retarder layer of the Certainteed Smart Vapor Membrain product to allow drying to the interior. The interior face of the wall will be fiinished with 1/2″ sheetrock. Am I over engineering this job or should I just Roxul the whole cavity and Membrain for my vapor retarder and be done? There’s not much info on this type of renovation . The info available seems to be about exterior applied foam and drainage planes. Any feedback about experiences relative or similar to my project installation would be appreciated. Jay

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  1. GBA Editor
    Martin Holladay | | #1

    Your plan sounds complicated.

    It starts out like a flash-and-batt job -- without the batt.

    Then you leave an air space.

    Then you switch to the cut-and-cobble approach.

    Then you switch again -- this time to Roxul mineral wool.

    Then -- surprise! You switch again, finishing off with strips of rigid foam on the interior side of the studs.

    Whew! That's exhausting!

  2. GBA Editor
    Martin Holladay | | #2

    Here's a simpler approach: fill the stud bays with dense-packed cellulose. Install a continuous layer of foil-faced polyiso on the interior of the studs, followed by drywall (or furring strips and drywall).

  3. wjrobinson | | #3

    Martin is advocating the "aj plan" and I agree. Simple and works providing your home does not have high humidity after the reno. If it does while renovating you should install IMO Panasonic bath fans and wire them to the light switch so they are used. I like installing an electronic timer switch and set it for 20 minutes per use.


  4. Expert Member
    Dana Dorsett | | #4

    Although, for clarity, shouldn't that be, "Install a continuous layer of rigid FOIL-faced polyiso on the interior of the studs..." rather than "...foam-faced polyiso...".

    AJ: I like to use occupancy sensor switches with internally adjustable time-outs for bath fans, since it doesn't require re-setting the timer on the way out the door. They automatically detect when you've left and start counting down from the last occupancy sensed, not before. The cheap ~$25-30 Leviton IPP 15-1L series works just fine in this application- it's a manual-on version, with a few time-out presets that count down from the last time motion was detected, 30 minutes max. Auto-on versions can also work in some situations, but you don't need the fan to come on if all you're doing it brushing your teeth or re-stocking the linens, so the manual-on versions are usually better.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for catching my typo. I have corrected it.

  6. wjrobinson | | #6

    Dana, in the past I have used the bath fans for exhaust only ventilation. So I need them to come on and like them to come for teeth brushing. I like what you posted about switches though, thank you.

    In most situations, like showering, someone enters and starts a shower, so 20 minutes has worked so far, and the owners know to change that if they desire or just hit the switch for another 20 if needed which I do on occasion myself. I love the switches and the fans too.

    Dana, Martin, set up typo editing rights for Dana. Not only is he the numbers person, he is the spell check monitor extraordinaire

    Dana, greenbuildingtalk ... remember when you opened the door here at GBA? Do I get one brownie point for cracking that door?.

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