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What exterior foam insulation on retrofit?

kfree | Posted in Energy Efficiency and Durability on

My dilemma in my retrofit is this: many years ago, before I was better informed as to where the vapor forms in my exterior walls in my home in Maine, I installed 1 inch of ISO over my dense-packed cellulose walls. I have now been educated to the dramatic thermal inefficiencies in walls stemming from bridging.

I am planning on retrofitting the exterior. I keep coming back to the same prescription: never use two vapor retarders on opposite sides of the walls. The one-inch iso is .03 I believe, and I have been eyeing 2-inch XPS on the exterior, giving an R-10 bump with a perm of .55.

I have also read that there can be an applicable rule of, five times more perm on the exterior than the interior. Will this hold true for a home in Maine?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what you mean by ISO insulation, but I'm assuming that you mean polyiso (in other words, polyisocyanurate).

    There are two issues here:

    1. What is the minimum recommended R-value for exterior rigid foam in your climate? The answer can be found here: Calculating the Minimum Thickness of Rigid Foam Sheathing. In Maine (Climate Zone 6), a 2x4 wall calls for rigid foam with a minimum R-value of R-7.5, while a 2x6 wall calls for rigid foam with a minimum R-value of R-11.25.

    2. The second question is, "If my walls have an interior polyethylene vapor barrier" -- and I'm not sure whether your walls do, but it your question implies that they do -- "is there any danger to adding a layer of exterior rigid foam?"

    Here is my standard answer to the second question:

    Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly wasn't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

    That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

    If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

    If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

    To summarize, here are four caveats:

    1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter.

    2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

    3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

    4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

  2. Expert Member
    Dana Dorsett | | #2

    If I'm reading you correctly, you have foil-faced iso on the interior side(?).

    While 2" of XPS on the exterior would cut the drying rate of any moisture that got into the wall assembly, it wouldn't block it completely. But if you uses unfaced Type-II EPS instead of XPS you would have about 4x the vapor permeance. When it's 0F outside the EPS would have about same R-value as XPS, even though at the ASTM C518 center temp of 75F there's almost an R2 difference @ 2". (R8.2 vs. R10.) During the cooling season the difference in performance widens, but that shouldn't be much of an issue in a wall assembly in ME- it's the cold/very-cold outdoor temperatures that matter the most. (When the average temp through the EPS is 40F, it would already have climbed to R9.)

    Rules like "five times more perm on the exterior than on the interior" are without much basis in any climate. The stackup matters: With rainscreened siding over OSB sheathing it can be just fine to skate by with 2-3perm paint as the interior vapor retarder in the warmer parts of ME, (and all of US zone 5) even though the permeance of the OSB itself may be lower than 1 perm.

    And having 0.05 perm 3-mil poly on the exterior and 0.01 perm heavy foil facers on the interior is by definition a moisture trap in any climate, but would meet the 5/1 rule of thumb.

    Moisture accumulation in wall assemblies takes a fairly high permeance on the interior to reach problem levels, but it's the total drying rate in both directions that determines whether it can dry quickly enough over a season or year should any water manage to get in. While it's more resiliant if you can keep the interior + exterior permeance > 1perm, it's fine at 0.5 perms. It's even better if the temperature of the susceptible sheathing has a higher average winter temp than the interior air's dew point, for minimal accumulation via vapor diffusion, which is what exterior foam can buy you, if it's sufficient.

    With R6 iso on the interior and ~R10 on the exterior, it sort of matters from a sheathing temperature /dew-point perspective whether it's 2x6 vs. 2x4 framing. If it's 2x4 you have about R13 in the cavities + R6 on the interior, and with ~R10 on the exterior the sheathing is about 1/3 of the way through the center cavity R, and it's mid-winter average temp is warm enough to not hang onto wintertime moisture that get's in from the interior from minor air leaks, etc. If it's 2x6 you have about R20 in the cavity, R6 on the interior, and with only R10 on the exterior it wouldn't cut it strictly on dew point in colder parts of ME, which would makes the higher permeance of EPS and rainscreened siding preferable to an XPS solution, despite somewhat lower average R value.

    But with foil faced iso on the interior you have a true vapor barrier which is highly protective so long as it is air-tight, and the cellulose cavity fill will adsorb moisture more readily (without damage or loss of R unless it's HUGE amount of moisture) than plywood or OSB, which is also protective of the sheathing.

  3. user-901114 | | #3

    Kevin; have you considered exterior roxul? How much are you paying a month for heat spread over 12 months? What floor is the interior iso on and how many floors high is the house?

  4. heinblod | | #4

    The European experience:

    Hundreds of cases with rotten walls:

    Many insurers simply do not fork out for this type of damage, check your own.


    In English from page 13 onwards

  5. kfree | | #5

    when I installed the foil faced poly iso on the interior i was pretty meticulous with my air sealing,
    so im not as concerned with winter time vapor as i am with summer. I'm getting the sense that by having
    2 x 4 walls, dense packed cellulose, that even with some air movement through wall there is a higher drying potential because of relatively higher surface temp. wouldn't the r10 xps on the outside of the ply lower that dew point considerably? im wondering if there is info on average 'seasonal vapor transport' for regions, and a calculation to use that might make it less speculative? As a builder ive been partial to the use of xps and never really been a fan of eps. are there other insulation board options (reasonably priced)?

  6. GBA Editor
    Martin Holladay | | #6

    It looks like those documents refer to moisture problems in EIFS walls. The U.S. had its own clusters of EIFS failures, especially in North Carolina, 15 years ago or so. The details that led to these failures are no longer used in the U.S.

    I doubt if Kevin is considering EIFS, since residential EIFS is very rare in Maine.

  7. heinblod | | #7

    this is what the OP wrote:

    " I am planning on retrofitting the exterior. ................and I have been eyeing 2-inch XPS on the exterior, giving an R-10 bump with a perm of .55."

    On the exterior the OP plans the XPS.
    A known problem with moisture sensitive but loadbearing walls like timber boards.

  8. GBA Editor
    Martin Holladay | | #8

    The overwhelming evidence from building science researchers supports the conclusion that exterior foam does an excellent job keeping wall sheathing warm and dry, as long as the foam is thick enough to keep the sheathing above the dew point during the winter.

    In fact, there are far more moisture problems in leaky walls without exterior rigid foam than in walls with exterior rigid foam -- because the walls without exterior foam have cold sheathing.

    That said, it's always possible to screw up any installation. Bad flashing can introduce water into any type of wall.

    The EIFS failures were due to misguided attempts to build barrier cladding systems instead of drained cladding systems -- and were exacerbated by interior polyethylene vapor barriers. The rigid foam wasn't the problem. These days, experts always advise the installation of drainage cladding systems. They are used for 100% of EIFS installations these days. Moreover, GBA has always advocated rainscreen cladding installations.

  9. heinblod | | #9

    The OP :

    " im wondering if there is info on average 'seasonal vapor transport' for regions, and a calculation to use that might make it less speculative? "

    'Average seasonal vapour transport' doesn't matter to the individual construction element.
    The Glaser model might be helpfull. Check with your civil engineer or architect.

    To be on a sure path of wisdom use wufi. Very sophisticated but still no 100% guarantee.

  10. heinblod | | #10

    the dew point will always be IN the external insulant. Hence there will be condensation in the insulant.
    That's science, the laws of physics.

  11. GBA Editor
    Martin Holladay | | #11

    Condensation cannot occur inside rigid foam insulation. That's a fact. There is plenty of information on this web site and the Building Science Corporation website on the topic.

  12. heinblod | | #12

    XPS can hold up to 10% of water, by the volume.
    That's research work by Fraunhofer. And my experience as well.
    Contact a builder, a civil engineer.

    There is no vapour tight foam, that's a myth.

  13. heinblod | | #13

    To the OP's question:
    A dynamic dew point analysis is asked for.

    Therefore wufi.

    There is an American dynamic dew point analysis as well, maybe your advisor has it available. Check the search engine.

    The Glaser model is static, not dynamic.


    As far as I remember there are 2 American dynamic dew point analysis methods, check this out

  14. GBA Editor
    Martin Holladay | | #14

    For anyone intrigued by your comments, I'd like to provide a few more links to provide better context.

    You wrote, "The dew point will always be IN the external insulant. Hence there will be condensation in the insulant."

    GBA has long advocated that the thickness of exterior foam must be adequate to keep the sheathing above the dew point in winter. That means that the interior surface of the foam insulation will be warm and dry. You are mistaken about condensation. The usual U.S. method for avoiding problems is explained here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    You wrote, "To be on a sure path of wisdom use wufi." While WUFI is a valuable tool for researchers, it is very easy for builders to be misled by WUFI because of the difficulties inherent in its versatility. In other words, WUFI requires so many inputs that it is easy to get the inputs wrong. Anyone who doubts the usefulness of the table I linked to in my article ("Calculating the Minimum Thickness of Rigid Foam Sheathing") can perform a dew point analysis if they want -- although I don't recommend that they do. I explained the steps necessary for a dew point analysis here: Are Dew-Point Calculations Really Necessary?

    Finally, it's important for U.S. builders who install rigid exterior foam followed by furring strips and conventional siding (for example, fiber-cement siding) not to be misled by EIFS failure stories. For the reasons I listed before, EIFS failure stories are irrelevant to the type of installation contemplated by the original poster on this thread.

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