GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Best way to add insulation to exterior of a 2×4 wall?

ezigmond | Posted in Energy Efficiency and Durability on

Hi all,
I have a single level ranch home built in 1978. Brick exterior on front, T1-11 siding on three sides. Stemwall was built wide enough to support brick. I want to take advantage of the width to add more insulation to the front wall. I know the house was built with plywood bracing in the corners, fiberglass insulation in the cavities, and asphalt compressed sheets over that, then the  brick/and or T1-11 siding. What’s the best way to handle this ? I live near Boise, Idaho .

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1


    With side walls, you can remove the siding and add a WRB flashed to your windows and exterior insulation over that. This site plenty of details for all aspects of this.

    For your front brick, I'm assuming you don't want to remove it. If this is the case, probably your best bet is EIFS over the brick. I doubt this would be worth the cost since your walls are already insulated. Much better use of your money is to air seal that wall and leave the brick alone.

  2. ezigmond | | #2

    Thanks for the reply. I forgot to say that I do want to remove the brick since it shows signs of water damage to the mortar, which will give me about another 3-4 inches on the front of the house to build a bigger insulation cavity for add rigid foam insulation to it.

  3. Expert Member
    Dana Dorsett | | #3

    On the T1-11 side how much depth can you add? Are you willing to re-mount/reflash the windows?

    If you have 6" of depth to work with you could take a Larson Truss approach leaving the T1-11 in place and filling in the truss cavities with open cell foam or blown fiber, more than doubling the wall performance. T1-11 painted with exterior paint is usually a class-II vapor retarder, and would protect any exterior sheathing on the trusses from interior side moisture drives.

    If you have 3.5" - 4" of depth to work with, installing 3" of reclaimed / surplus roofing polyiso (or 2" reclaimed polyiso + 1" foil faced polyiso on the exterior) strapped in place with 1x4 furring through-screwed to the studs with pancake head timber screws would have comparable performance.

    This is a bit of a drive from Boise, but the price is right-enough to be worth the drive if you have a truck big enough: Running this search every week or so may dig up closer sources:

    With either approach you have to pay attention to flashing details at both the windows and foundation.

  4. ezigmond | | #4

    on the front side with brick, 3-4 inches. The brick and old siding are being removed and replaced with vertical siding to match our addition. thanks for the craigslist lead. This was the direction I was looking at going. There aren't many windows to deal with either. The only sheathing is on the corners for shear strength. the rest is the old school fiber asphalt sheets under the t1-11 .Pictures attached.
    Thanks !

    1. Expert Member
      BILL WICHERS | | #6

      If you’re planning on replacing the T1-11 now or in the near future, I’d put polyiso under the siding everywhere, an entire reside. I’m working towards doing that on my own house since enough of the T1-11 is showing signs of deteriorating that I think it’s best to just replace it all.

      Note that T1-11 can act as the structural sheathing too, not just siding, so make sure you have enough of the plywood panels you mentioned are in the walls to do the job.


    2. Expert Member
      Dana Dorsett | | #7

      With the windows all tucked in high under a deep overhang like that you could probably even get away with NO window flashing, but you'd still need to flash the transition at the foundation.

      It looks like you originally had a vented crawlspace, now sealed & conditioned (?).

      The asphalted fiberboard is moisture resilient and has real R-value (about R2 for the 3/4" goods). It can be left in place even if you strip the T1-11. If stripping the T1-11, take a look at the condition of the back side of the siding near windows. If there are signs of wetting it's going to be worth adding flashing when doing the foam-over. If you are replacing the windows entirely it's worth properly flashing them despite the mitigating factor of the deep overhangs.

  5. GBA Editor
    Martin Holladay | | #5

    Here is a link to a relevant article: "How to Install Rigid Foam Sheathing."

  6. ezigmond | | #8

    The foundation is vented, I still have the styrofoam plus on. the inside stem walls of the crawlspace on the underside of the floor have insulation. I would consider leaving the fiberboard but I'm afraid of what underneath considering the failed mortar joints under the windows.

    1. Expert Member
      Akos | | #9


      The failed mortar joints bellow the window comes mostly from bad detailing of the original brickwork. Using bricks as a sill is always a bad idea, they just don't hold up, there should have been a piece of flashing above them to drain the water properly.

      I would poke behind your fiberboard in a couple of places to check to see if there is water damage. If it is in good shape there is no point in taking them off and loosing the free R value they provide.

  7. ezigmond | | #10

    One of the bedrooms we removed the paneled walls and found mold on the drywall, removed it and found the fiberboard had rotted away to where we could see the backside of the brick, its coming down , too much visible damage, plus we don't like he look.


  8. PAUL KUENN | | #11

    "plus we don't like he look."
    Great idea! Put all the money on insulation. That will pay you back. 60s-present buildings with expensive stone or brick on the front and all the rest vinyl, wood or aluminum siding looks so poorly chosen.

  9. ezigmond | | #12

    Our addition is the fourth photo in the above postings. That is what we are shooting for.

  10. ezigmond | | #13

    would the ZIP R+ insulation panels be good alternative to using rigid foam and OSB and house wrap?


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |