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Looks like studs for my exterior walls are just 2x4s. Dense-pack won’t be enough?

Apollo S | Posted in General Questions on

I am waiting for summer, so I can address insulation of the house. I knew it was bad, since living area was built in ’38 and gut-rehabbed in ’55, but only last weekend I realized how bad my walls are.

So I ripped out some drywall to do electrical and discovered that all I have is 4″ deep cavity. 2X4s all around, then T&G boards, then felt, and finally siding. Zero insulation but foil stapled in between the studs. No wonder my gas bill was $550 last month (I am in Boston).

Looking at what I got, what is the max R value I can achieve with having contractor densepack with cellulose? I am wondering how long I can last with so-so insulation before I save up to rip off the siding and can do EPS and sheathing.

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  1. GBA Editor
    Martin Holladay | | #1

    It's possible to achieve R-3.7 per inch with cellulose. If your wall has old-fashioned 2x4s that measure a full 4 inches deep, you could achieve R-14 or R-15 in the middle of each stud bay.

    If your wall has planed (3.5 inch deep) 2x4s, you could achieve about R-13 in the middle of each stud bay.

    Your whole-wall R-value (taking into account thermal bridging through the studs) is likely to be no more than R-12.

    Don't be too upset, however. If you hire a contractor to dense-pack your walls with cellulose, the work will significantly reduce air leakage through your walls. Removing your siding to add exterior foam will never be cost-effective, unless the siding is so deteriorated that it has to be replaced.

  2. Apollo S | | #2

    I need to remeasure, but I think I only really have the 3.5" cavity. As you see in the picture, it isn't going to be an easy job.

    Would it make sense to put 1" polyiso with foil facing on the inside and put another 1/2" layer of drywall over it?

    Another curious thing is that it looks like crawlspace to 1st floor was balloon framed and later they put in bricks and cement into that gap all around. I understand I have no other choice but to spray foam on band joist (which is likely thermal transfer disaster with concrete in there). It still puzzles me why they jammed bricks and cement in there. "Pest block"?

  3. D Dorsett | | #3

    If you have foil on the exteior side of some or all of the stud bays, you can't put a layer of foil faced foam on the interior side of the assembly unless you can remove the pre-existing foil or you'll have a moisture trap.

    In climate zones 5 or higher (Boston is zone 5) foil on the exterior side of the stud bay could result in quite a bit of wintertime moisture accumulation in the insulation layer. In zones 1 & 2 it would be fine. In zones 3 & 4 it kinda depends... It may be possible (but a major PITA) to remove the foil without gutting the wall if you can gain access via either the basement or attic in an balloon framed house. Even if you had to drill to gain access it's possible. (

    In a house in Worcester we had to remove a laminated 1/2" blanket of kraft / horse hair / kraft that was woven between the studs, bisecting the bays diagonally in order to dense pack it. (The blower pressure wasn't sufficient to rip the double laminated kraft paper.) We fabricated long hooks out of mild steel rod to shred and pull the stuff out through a hole drilled through from the basement, later using those holes for dense packing, but it was time consuming. If it's not too difficult to yard out the bricks at the bottom of the bays that would provide ideal access for inspecting the bays and scraping out the foil. It doesn't have to be completely removed, but it has to be most of it. You could then staple blowing mesh and dense pack it from that location.

    The bricks probably WERE there to limit rodent access. In balloon framed houses without blocking it otherwise gives bats / voles / mice / squirrels / rats a 50 lane superhighway between the basement & attic.

  4. Apollo S | | #4

    That foil you see is stapled not glued to the studs. I don't even have a slightest idea why the heck they did that in the 50s or what was the logic. it is foil to the inside and craft paper to the outside. Then you have rock lath about inch thick with foil backing in some places and no foil in other places.

    If not foil faced polyiso (I can source from recycler, who can get me factory 2nds) would you cut out strip around entire wall, rip out the foil, and then do 1" XPS plus 1/2 drywall over it?

    I am looking for most cost effective way to do it. Cellulose I will need to wait for MassSave (state program) to blow it in during summer.

    Crawlspace was absolute disaster. I removed bags and bags of garbage. Insulation had mice nests everywhere. It was disgusting. I guess they kept that swill down there and found it easier to brick in the perimeter. It is too hard to remove those bricks.

  5. Lucy Foxworth | | #5

    You can do a Mooney wall and insulated more to the interior.

    That's what I'm going to do on my house. I'm going to add 2 inches of Roxul to the interior and then apply drywall over that. My walls are plaster and a pain to remove so I'm just going to leave them in place and build a frame that will support the Roxul comfortboard. You can even build a new wall and do the equivalent of a double stud wall and then dense pack with cellulose or use the batt insulation of your choice.

    You can also do a room at a time as time (and money) allows.

    If you put polyiso over the wall, and then drywall you limit what you can do later because you don't want to make the dreaded "foam sandwich" with the risk of trapping moisture in the wall should you decide to add foam to the exterior at some later date when re-siding.

  6. Apollo S | | #6

    But I did EPS, that should be fine as opposed to less permeable polyiso?
    Ripping out that rock lath would be really tough project. Stuff is so heavy! Disposal charges alone would be bad enough. I was thinking about using removed lock lath for fill in a raised patio I am planning, but I am not sure how smart or dumb is that.

  7. Lucy Foxworth | | #7

    Dana is the permeability expert. He can answer you better in the regard.

    Plaster and lath are a serious pain to remove so I'm just going to leave it in place at my house. You can still have your walls dense-packed with cellulose and do the Mooney wall interior to that.

    Later should you decide to re-side and insulate the exterior, choose something vapor permeable like Roxul and add a rainscreen.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    take out the foil
    vacuum up
    get a battery operated big caulk gun like the Ridgid or another, caulk air leaks, via adding a fan to help find leaks, cold weather helps in finding them with back of hand, face etc, match... incense sticks
    add the 1" of interior foam tape seams, caulk it in place by first caulking frame it is attaching to.
    Caulk all frame members where they intersect, sill, tops sill, jack to stud, headers etc. If it joins, caulk.
    blow in cells this summer with your rebate.
    oh yaa, add drywall... you know that part
    add pool table
    add card table for Wednesday card game
    add beererator
    go for a tandom hang glide at Torrey Pines


  9. Apollo S | | #9

    That sounds like I should rip out the rock lath.

  10. Lucy Foxworth | | #10

    To do the absolute best job, yes, you should rip out the rock lath. BUT, you do have options:

    1. If you don't want to gut every wall, you can dense pack the wall as is. You will still have a much more comfortable house just by doing that. Well done dense pack cellulose will provide some air sealing as well. And then put extra time and effort into working on the rim joist, other air leaks and the attic. Later you could do exterior insulation.

    2. You can do the Mooney wall like I talked about, dense pack first, Seal those holes, leave the wall intact. Pull off the trim. Install a Mooney wall to decrease thermal bridge from the studs, use a vapor permeable insulation like cellulose, denim, mineral wool, or even fiberglass batts to get extra insulation. Install new dry wall, replace trim.

    3. If you gut the walls, one way to minimize thermal bridging is to fir out the walls with EPS and a 1 x 3 as shown in this April/May 2015 Fine Homebuilding article called "Breaking the Thermal Bridge. (I think it is members only article). I'll post the link.

    4. Or the do the double stud option after gutting the walls.

    You have to decide based on time and money, how much disruption you can tolerate in your home.

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