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Installing Insulation from Exterior

Cshaugh61 | Posted in General Questions on

Renovating an old farmhouse originally built in 1850 w no fewer than 6 additions over the years.  In several places I will be removing the outside siding, adding sheathing and then new siding.  I plan to include house wrap atop the sheathing before siding install.

I would like to use this opportunity to insulate outside wall without tearing down inside walls.   If I install fiberglass (or rock wool) insulation do I used faced or unfaced?  I know facing goes toward the inside so .. how do I do that and attach it?  Do I need a vapor  carrier if there is tyvek (or similar) House wrap?  If I used unfaced how is it attached?  One note.   The studs are typically 24 inches on center ( +/- 3 inches lol) and are real 2 inches by 4 inches.   Not dimensioned lumber.

I would much appreciate advice on this. I’m baffled. (Pun intended)

Lynchburg VA

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  1. canada_deck | | #1

    What do you know about the current wall assembly in all of the additions (e.g. was a plastic layer added behind the drywall)
    One main thing you need to watch out for is sandwiching your wooden wall between two vapor barriers.

    You might want to find a clever way to mark the studs before covering up your walls. It's very useful to know where they are when you try to install siding and if the spacing is uneven then that will be important to know. A single screw that is set at the center of each stud at the bottom of the wall may do the trick. If the wall allows, you might be able to put the screw in a location that will always be visible.

    What are you doing for siding and are you planning on installing a rain screen?

    1. Cshaugh61 | | #4

      I'm planning on vinyl siding (cuz Hardie plank is 3x the cost) and from what I understand a rain shield is unnecessary. I live in VA so not a terribly hot nor cold environment but I'd still like it to be efficient

      1. Cshaugh61 | | #7

        Also, the wall assembly (additions) has no plastic layer, no house wrap, mostly not dry wall but, rather, something resembling tongue and groove hortizontally on the walls - at least in most cases.

        the siding is a hodgepodge of masonite nailed directly to wall studs (which is damaged and water soaked in places) and rough cut wooden siding (resembling 1x6's overlapped from top down). Likely cut out of black walnut as they are prolific and wild growing here.

        I plan on OSB sheathing followed by house wrap and then vinyl siding.

        One question:> Also for Malcolm below - are both sheathing seam tape and house wrap needed or is it overkill? the OSB is just plan sheathing OSB not zip system or anything like that. AT least that's what i've used so far for a small part - much more to do so not against changing if warranted.

  2. mjhil | | #2

    A little over a year ago I insulated one wall from the outside on my 1970 house and I used rockwool to re-insulate and cut out the existing poly vapor barrier with a utility knife (I'm in zone 5). I used zip sheathing with their tape and liquid flashing for seams, window, and foundation connection (see Matt Risinger for details). I also used A Quickflash panel for my ERV outlet that was on that wall sealed to the wall with liquid flashing. On top of the zip I used planed down furring strips with cedar siding painted with linseed paint.

    If I had to do it all over again I would probably use exterior insulation even though adjoining brick and a front door complicated that route. Other than that, everything worked really well, was straightforward, and it is very airtight.

    Naturally an 1850 farmhouse will be far more weird, but a similar approach could work - though making batts work would be a challenge for sure.

  3. walta100 | | #3

    Consider after your new sheeting is install drilling 2 holes in each bay blowing in cellulose it will completely fill what are likely to be odd sized bays of your old house.

    1 Using Zip sheeting and making it air tight at every penetration. This with an air tight ceiling would transform the comfort levels.
    2 Adding two inches of exterior foam. This would make the sad R13 wall into a pretty good R23 wall.
    3 Add drainage gap / rain screen
    4 Now is the time for new windows and doors.

    My guess is none if this is in your budget and if you did the math, it would cost less money bulldoze the place and start over on the lot.


  4. Expert Member


    Your interior probably has enough layers of paint that it performs as at least a class 3 vapor-retarder, so I wouldn't worry about using faced insulation. The big weaknesses of old houses is their air-leakage, so once the sheathing is off before insulating try and seal up any holes you can at electrical boxes and plumbing, etc. I would detail your new sheathing as an air-barrier by taping the seams and perimeter, and sealing all holes. Cover it with a house-wrap, carefully flash your windows and you will see a great improvement.

    Good luck with your project!

    1. Cshaugh61 | | #8

      I think you are suggesting both seam tape and house wrap, correct? Or is it one or thuther? And, which kind of seam tape do you recommend? (That does the job and doesnt break the bank.)

      1. Expert Member
        MALCOLM TAYLOR | | #9


        You definitely get a better job if you tape the sheathing, but taping the perimeter (especially at the foundation ) of the house-wrap works too. Another option is a self-adhered house-wrap.

        I use Dupont Straight-flash, but there maybe better choices for adhesion or price. Perhaps others have a better suggestion.

  5. Cshaugh61 | | #6

    Thanks much Malcolm. It's a hodgepodge of stuff and these sound like great suggestions

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