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Community and Q&A

Exterior (north) stone wall

Edward Kramer | Posted in GBA Pro Help on

Indianapolis Zone 5a
My own outside weather station reads mid 70% humidity July through October which is 10% higher than Indianapolis historic values.

House is ranch style one story slab on grade. South facing wall is stone on the outside. The wall construction is as follows from outside to inside:
stone (avg 3 inches) – no noticeable air gap or weep holes.
sheathing – not plywood or OSB– looks like particle board with black facing the inside.
fiberglass batt – removed
3 mil plastic – removed
drywall – removed.

It looks like the builder intended the house to dry to the outside. Yet, mold, condensation and musty/ wet smell is experienced. The wall is also very cold in the winter.

Questions 1 – what is the best construction of the wall considering I will not be removing the stone or sheathing? I could possibly drill weep holes.

Question 2 – is there an inexpensive alternative that works just as well?

Questions 3 – I have contacted some spray foam companies. They are reluctant to put closed cell foam on sheathing in wall cavity.

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  1. Expert Member
    Dana Dorsett | | #1

    The blackened particle board is asphalted fiberboard. It should be black on both sides though it's made both ways. When asphalted on only one side, the black should be facing the stone.

    The sheathing should also not be touching the stone, which becomes a wicking path for liquid moisture. Where the sheathing is plywood rather than asphalted fiberboard it's an even bigger problem, since the plywood is more susceptible to moisture. Asphalted fiberboard is fairly vapor permeable (about 3-10x more vapor permeable than 1/2" plywood, depending on humidity) and 3-mil polyethyene is highly impermeable. So yes, this assembly as-built can only dry toward the exterior, but it isn't set up to dry very well at all.

    In an air conditioned house the plastic sheeting will drop below the dew point of the exterior air during hot sticky days, which causes moisture condense inside the studwall. Even on less sticky days, direct sun will drive the moisture out of dew or rain wetted masonry claddings into the wall cavity to condense on the plastic. The fiberboard isn't much affected by the additional moisture, but the rest of the wood become susceptible to mold/fungus.

    Weep holes at the bottom with corresponding vents at the top of the stone veneer will definitely help, but it's probably not going to be enough on it's own, especially if the house is air conditioned.

    Installing an inch of cut'n'cobbled foam board (any type) or a flash-inch of closed cell foam up against the fiberboard will keep the moisture at the sheathing and stone veneer, limiting the amount that makes it to the interior. In your climate, with an inch of foam there does not need to be an interior side vapor retarder to manage wintertime moisture drives, since average temperature at the foam/fiber interface will be above the indoor air's dew point, and it will not accumulate moisture over the winter. Standard interior latex paint on wallboard would be sufficiently vapor retardent to manage peak events, such as a cold snap, but would also allow the assembly adequate drying capacity toward the interior.

    Any areas where the sheathing is plywood will still be susceptible. Sometimes plywood will be installed at the corners as structural shear panels when the rest is fiberboard, though with the correct faster spacing fiberboard can still serve as the structural sheathing. Is there any plywood sheathing facing (or touching) the stone clad areas?

  2. GBA Editor
    Martin Holladay | | #2

    As Dana pointed out, there is some risk if you leave the stone veneer in place. That said, I can understand your reluctance to remove the stone veneer.

    Installing closed-cell spray foam insulation on the interior side of your wall sheathing is the best approach if you insist on keeping the stone veneer. You can either use closed-cell spray foam as your only wall insulation, or you can follow the flash-and-batt approach.

    For more on flash-and-batt, see this article: Flash-and-Batt Insulation.

  3. Expert Member
    Dana Dorsett | | #3

    Flash & batt is cheaper than a full cavity fill of closed cell foam, and offers better drying paths for the studs, which will still be subjected to high moisture drives at their exterior edges.

    When the sheathing is moisture-tolerant asphalted fiberboard a cut'n'cobbled foam board approach isn't very risky, since moisture from minor wintertime air leakage from less than perfect seal doesn't damage the sheathing. It's riskier when the sheathing is OSB or plywood, which are far more moisture susceptible. An inch of closed cell is likely to be a bit more air tight, but at 1.5-2x the cost of rigid foam. (Or an even 5-10x more expensive if reclaimed foam board is being used rather than virgin-stock.)

    To bring the wall-R up to current code min performance, a flash inch of closed cell (or a cut'n'cobbled 1" rigid foam sealed with can-foam at the edges) with 1" thick strips of foam board on the interior facing edges of the framing works, and standard thickness batts could still be used. That requires a longer fasteners for the wallboard, but with only 1" of additional depth it's not very difficult.

  4. Edward Kramer | | #4

    Thanks for the advice.
    One change - wall is north facing not south - sorry.

    RE Dana - Yes, one of the corners is plywood and not particle board. I have not drilled holes in the plywood but from knocking and viewing the wall from the top side it seems that the stone masonry is touching the plywood in many areas.

    see the attached picture depicting most of the wall (not the plywood corner).

    Questions after reading
    1) would/could acoustic sealant be used with the canned foam to prevent air leaks from swelling of the studs.
    2) would EPS or Roxul ComfortBoard IS installation be better to use for the cut and cobble.
    3) should an air space be left on the exterior side of the ridged foam?

    Switching gears to spray closed cell foam
    1) any concerns or workmanship issues I should watch for?
    2) does the close cell foam have the same problems with air leaks as the studs swell?

    3 - I am getting quotes of $1000. for the closed cell foam. cut and cobble is about $300.
    There are so many issues with this house, I want to get this done and not have to think about it for a very long time. is closed cell foam worth the extra $700.00

    Lastly, according to the drawing of the walls and top of stone is exposed to the attic. are there any other concerns with fixing the wall.
    1) i am replacing the attic insulation. Should I insulate over the stone or treat it like a baffle?

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Would/could acoustic sealant be used with the canned foam to prevent air leaks from swelling of the studs?"

    A. Yes. Smaller cracks can be sealed with caulk or acoustical sealant, while larger cracks can be sealed with canned spray foam.

    Q. "Would EPS or Roxul ComfortBoard IS installation be better to use for the cut and cobble?"

    A. Definitely EPS, since EPS would be a moisture barrier as well as an air barrier. Mineral wool is inappropriate here because it is air-permeable.

    Q. "Should an air space be left on the exterior side of the rigid foam?"

    A. No, not in this case.

    Q. "Switching gears to spray closed-cell foam: Any concerns or workmanship issues I should watch for?"

    A. That's a vague question. Good workmanship is always preferable to sloppy workmanship.

    Q. "Does the closed-cell foam have the same problems with air leaks as the studs swell?"

    A. If the studs swell, the chance of air leaks increases, whether you install spray foam or use the cut-and-cobble approach. That said, there is some flexibility in either material, so air leaks aren't a certainty -- just a possibility.

    Q. "I am getting quotes of $1000 for the closed-cell foam. Cut and cobble is about $300. There are so many issues with this house, I want to get this done and not have to think about it for a very long time. Is closed-cell foam worth the extra $700.00?

    A. That's a tough call. I'm going to say that you are the only one who can make that judgment.

    Q. "According to the drawing of the walls and top of stone is exposed to the attic. Are there any other concerns with fixing the wall?"

    A. Can you be more specific? It's hard to know what kind of repairs are needed without a site visit.

    Q. "I am replacing the attic insulation. Should I insulate over the stone or treat it like a baffle?"

    A. You should insulate over the stone.

  6. D Dorsett | | #6

    How is the attic vented?

    Are there air handlers & ducts in the attic, or no?

  7. D Dorsett | | #7

    From the diagram attached in response #4 it appears as if the stone cladding is effectively being vented into your attic, which is probably contributing to the high moisture content there, both from exterior drive moisture in the stone, and any wintertime humid air leakage that finds it's way into the cavity above the stone.

    Fiber insulation should not be in contact with the stone, since it can wick moisture from the stone inward to the top plate and joists, and it would slow any miniscule amount of convection venting that you might get through the micro-cavity between the stone. Install a cut'n'cobbled foam board air & moisture barrier over the top of the stone with at least a half inch of clearance- use cut blocks of foam board or a similarly low wicking moisture tolerant material for spacers between the air barrier and ston to ensure the gap. Drilling a series of 1/2" (minimum) holes through the facia fiberboard every 8" to vent the established air gap to the air space behind the bottom course of vinyl siding is probably good enough- no need to poke it all the way through the siding. You may have to pop the bottom course of siding and drill from the exterior. If that's the case, going with 1" holes and 1" round vents with bug-screens would be even better. eg:

    (picked at random from a web search for illustration purposes- there are multiple products & vendors out there that will work.)

    It would still be useful to know how the attic is vented. On your other thread there is some description, but it may need modification in light of what's recommended here.

  8. Edward Kramer | | #8

    Thank you for all the help so far. A few more questions and some clarifications.
    RE Dana wall construction
    Yes, the stone cladding is vented into the attic.
    Only in the corners of the wall there is a 2x4 unsealed on top of the stone.
    Three feet of the walls in the corners are plywood rather than asphalted fiberboard.
    Also, i checked much of the wall fiberboard and plywood by knocking. It seems there is some size of airspace is about 75% of the wall.

    RE: Dana attic venting specs
    the attic is vented with 13 aluminum under eve soffit vents and 39 feet of ridge vent. The total free area of the soffits is 364 inches. I can not find the ridge vent online. It is a very cheap roll type. The average net free area from the products i saw was 468 inches.

    1. With those specifics does any advice change?

    2. With the closed cell foam in the wall cavity, should vent holes be added in the sheathing as well?

    3. should I add a higher R value foam like 3in R15 in the wall cavity so any winter or summer dewpoint is inside the foam?

    4. A contractor suggested building out the wall and spraying 5 inches R 20 open cell foam which would give a Class III vapor retarder at 4.25 inches. Good idea or bad?

    5. I am concerned about water condensation on between the foam and fiberboard or plywood in the summer and condensation on the inside facing surface of the foam in the winter.

    6. I am also concerned about condensation along the 2x4 studs since they are and will be touching the fiberboard.

    7. Also, please further explain your recommendation "with 1" thick strips of foam board on the interior facing edges of the framing works," What type of foam should be used? Seal or calk the foam to the stud? any links to suggestions on how to finish the window if there is a inch of foam around it?

    8. For the cut and cobble solution inside the wall cavity, any suggestion of type of foam insulation panel (xps esp ) or brand recommendation? I am interested in limiting the as much as possible off gasses.

    thanks in advance

  9. Edward Kramer | | #9

    Further specifications about the attic.

    The HVAC installer Plan.
    1. The air handler, dehumidifier, humidifier and ERV are in the attic. But they are enclosed in an insulated box special built around the equipment.
    2. ducts will be in the attic because the current in-slab ducts are not functioning
    3. blow in r49 insulation to bury the ducts.

  10. Expert Member
    Dana Dorsett | | #10

    If there is a way to increase the soffit air vent area to make it reliably bigger than the ridge vent's free area it would reduce the stack effect draw.

    Open cell foam would make it fairly air tight, but wouldn't necessarily fix the problem entirely. As stated previously, asphalted fiberboard can take a lot of abuse from moisture without damage- don't worry about condensation on the fiberboard. The plywood could be an issue if you make it too vapor tight toward the interior, but 5 perm foam behind 3 perm paint on wallboard is still over 2 perms.

    If you deepen the cavites with strips of rigid foam on the stud edges, foil faced polyisocyanurate would give you the best performance. Detailing around windows will vary with the type of trim & construction, and it may not always be "worth it".

    For a cut'n'cobbled foam tight to the fiberboard, use 1" EPS with facers (foil or plastic facers, one facer is fine if facing the interior of the house.) If you end up using 1" polyiso stud edge strips to increase the cavity depth, use 1" polyiso for the cut'n'cobble.

    Rigid foam with foil facers (both sides) would have the lowest possible outgassing characteristics, making polyiso the easy choice here.

    Burying ducts in attics with insulation will end up with wet insulation during the summer in a vented attic. ( Is there any way to go with a ductless heat pump solution here? ) Burying an ERV or HRV duct system in the insulation is fine.

    A humidifier is a universally bad idea. High wintertime indoor air moisture levels results in more moisture accumulation in the structural wood, aggravating any mold smell issues. Air sealing the house and dialing back the ventilation rate on the ERV/HRV when it's cold out will usually fix any wintertime dryness, even at 2x the IRC code-max air leakage. Your ideal target from mid-December to half-past March would be about 30-35% RH @ 68-70F. That's at the low end of the human-healthy range, and still quite comfortable from a chapped-lips, dry nose, and split fingernails point of view. In summer shoot for 50% (to mitigate dust mites), or at least under 60% (to mitigate fungus & mold.)

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