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Exterior double wall

89G4ebLjTX | Posted in Green Building Techniques on

Hi, we just bought a 1955 ranch house with stucco siding (which also covers the basement blocks to a depth of about 4″ under grade) and 2×4 exterior walls insulated with urea formaldehyde foam some time ago. A friend with an infrared camera determined that the old foam looks like swiss cheese inside the wall cavities, with large pockets where the foam didn’t reach. The previous owners had huge gas and electric bills even though the house is only 1,400 sq. ft, a perfect rectangle with straight exterior walls.

I’m thinking about adding exterior insulation, either 4″ of rigid foam sheets and siding over that, or a new 2×4 frame on the exterior filled with fiberglass or foam and siding over that. The house has 2 ft overhangs/sofits, and I’ll add overhangs on the gable sides of the house to top the new frame.
I’d appreciate if someone could comment on the idea. I have the following concerns.

1. Should I remove stucco prior to installing either the 4″ of rigid foam or a new frame? It would seem to me that stucco may act as a thermal bridge and bring cold temperatures behind my new exterior wall.

2. If I use 4″ rigid foam nailed to existing studs, how would I attach it to the studs, long nails? And how would I attach new siding? I’d have to use 6″ nails to go thru 4″ of rigid foam and 2″ into the studs???

3. If I build a new 2×4 frame, can I use some sort of L-bracket to support it since it won’t be resting on the foundation? I could attach the brackets to either the sill plate or perhaps screw/bolt it to the basement blocks.

Thanks for any comments/suggestions.
Allen

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Replies

  1. David Meiland | | #1

    It always helps to mention your climate zone.

    The bottom of the stucco is buried? If so, it's going to act as a wick and draw moisture up into whatever new wall assembly you build. I would remove the stucco, pour a new grade beam around the outside of the existing foundation, frame and sheathe a new wall on that, and fill the entire frame with cellulose. There may be some merit in foam over the sheathing, depending on other details, to prevent cold sheathing.

    As you mention, you would need to do some work on the roof overhangs to make everything look right, but you'd end up with a really nice wall. In my jurisdiction I'd need to look at where the shear was, and would probably need some engineering to make the inspector happy.

  2. GBA Editor
    Martin Holladay | | #2

    Allen,
    I vote for a different strategy.

    Cut away a 4-in.-wide strip of the stucco near the mudsill to prevent capillary wicking from the soil.

    Install rigid foam over the stucco. Secure the foam with cap nails temporarily.

    Install vertical 1x3 strapping over the studs, 16" or 24" on center depending on the stud spacing, screwed through the foam into the studs with 6-inch screws. Attach your siding to the strapping.

    Of course you need good flashing details and air-sealing details to make this work.

  3. user-659915 | | #3

    Martin's advice is sound. Watch out for the window sill flashings with your extra wall depth, although with your deep overhangs these will probably not be too critically stressed.

    I have to ask though, have you (or a previous owner) addressed the other aspects of the thermal enclosure you are likely to find substandard in a 1955 home? In many climates (what is yours?) walls are not the major cause of poor thermal performance, however poorly insulated, and you may have been unduly impressed by your friend's IR imaging. Is the roof insulation R38 or higher? Has the crawl space been sealed? These are far less expensive interventions than new exterior insulation and new siding and will have much bigger impact on your energy use. Next priority: are the windows and doors double-glazed or fitted with decent storms, and are they properly weatherstripped? Don't touch your walls until all these other items are done.

  4. 89G4ebLjTX | | #4

    Thanks to everyone for great suggestions. We live in Central Ohio. We're adding 16" of cellulose insulation to the attic, replacing the original single-pane aluminum windows with modern windows, and insulating rim-joist bays with fiberglass batts. Still, we'd like to do something about the exterior walls too. Yes, the stucco is burried a few inches below the ground, I never even thought of the wicking action. How do I remove a 4" strip of stucco? Is there wire underneath stucco, and if so would the circular saw cut thru stucco AND the wire? Thanks again for great advice.
    Allen

  5. user-659915 | | #5

    Good move on the attic. Be sure to carefully airseal all ceiling penetrations before adding the cellulose. Likewise be sure to airseal the new windows into their openings. Do you have a basement or a vented crawlspace? Is there subfloor insulation and if so what kind and in what condition?

  6. David Meiland | | #6

    You can cut the stucco with a dry diamond blade in a skilsaw. It will cut the wire and the stucco easily. Once you make the cut, break/pry off the part you're removing.

    Did your friend email you any of the IR images? I'd like to see a few...

  7. GBA Editor
    Martin Holladay | | #7

    Allen,
    Fiberglass batts are a poor choice for insulating rim joists. Since they don't stop air movement, they allow warm humid interior air to contact the rim joist. In winter, when the rim joist is cold, frost formation, condensation, or moisture accumulation are possible.

    It's best to use spray polyurethane foam or pieces of rigid foam insulation, sealed at the edges with caulk or canned foamr.

  8. J99aAMQzYo | | #8

    Cutting Stucco - A few notes:

    1) Cut a small patch first to determine the applied thickness back to the existing sheathing, tar paper, whatever. Then set the saw for about an 1/8" LESS than that. If it doesn't get all the way through the wire lath that's okay. Better to have to snip that manually that to cut through any of the existing backer.

    2) Since the stucco is cement based, make to sure invest in a higher quality cartridge-style respirator and not just disposable masks. The quantity of dust you'll create will be pretty sizable and excessive inhalation of the dust can lead to silicates in your lungs (which the body can't breakdown .... ever). SImilarly, make sure any kids, neighbors, etc. are warned and not around.

    3) Since you're in a (light) snow area, consider cutting the stucco 6" or more above the actual grade to lessen the times when it will be in direct contact with frozen ground, snow, etc. How high you can (or can't) cut will depend on the test patch you cut and where the existing framed wall bottom plate is. You don't want to cut any higher than that plate and expose it to the elements. You actually want to stay about 2" below if you can and still get away from the ground.

    4) Once you expose the underside of the stucco, you'll need to install a bug screen detail to prevent bottom-up intrusion. Then you may want to consider a flashing detail to prevent rain splash back into the wall cavity. However, any flashing detail would also have to incorporate weep holes at set intervals to allow any bulk moisture from above to escape.

  9. 89G4ebLjTX | | #9

    Once again, thanks for the great ideas and advice. I don't have the IR images to share, we looked at them using the large viewfinder on the back of his camera. It appeared that the foam didn't reach in some places, and there are lots of holes distributed throughout the foam, my friend said this isn't the worst urea formaldehyde foam job he has seen.

    I especially appreciate the ideas on removing the stucco, this will help a lot. I'm thinking that after removing the stucco, I may remove the sheathing behind it (someone said it's something called "weatherboard"). The framing has diagonal braces on all corners (seen on the IR camera), so I think it's structurally sound. After removing the sheathing, I'll also remove the old Urea Formaldehyde foam and re-fill the cavities with modern insulation, cover the cavities with 4" of rigid foam using Martin Holladay's technique above. Do you think that the 1x3 straps attached with 6" screws to the studs will be sturdy enough to support heavy siding such as fiber-cement Hardyplanks?

    Yes, there's a full basement (heated with living areas in it), and I have easy access to the rim joists, at this time there's no insulation there of any kind, must've been freezing there in the winter.

    Thanks,
    Allen

  10. Dan Beideck | | #10

    If you are interested in this project because you are interested in reducing the greenhouse gasses associated with wasted fuel use, I would encourage you to avoid XPS and closed cell spray polyurethane foam. You can read more about why on this post on greenbuildingadvisor, https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/avoiding-global-warming-impact-insulation It could take more than 40 years of reduced fuel consumption to make up for the greenhouse gasses associated with the manufacture of these products!!

  11. Hoping for Heaven | | #11

    I was so impressed with the quality of the posts I'm going to 'join up' try to learn and contribute!
    Great job with this 'project' . i've got the similar situation in Central California
    105F summer 32F Winter.

    Freddy

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