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My mistakes with rigid foam

JesiahClemons | Posted in General Questions on

I learned about this community after I had already started my whole home remodel, so now I’m going being and correcting some of my errors.

Zone 5- adding cellulose in walls

I have a balloon framed house with a large wrap around addition.
The original house had a hip roof with structural issues, so it was removed and replaced with a gable roof. During that process, the walls were extended 2′ taller. The new area was covered in 7/16″ OSB and Blockit drainage wrap and 2″ of EPS to line up with existing vinyl siding I thought I could keep it at the time. Later I found out the nightmare the last guy left me, and I removed the vinyl siding and old wood siding exposing the original cladding.

1st Question
I had to fur out the inside of the 2nd-floor affecting making it 2×6 walls. This requires more foam on the outside, so can I add 1″ over the 2″ that I have up-any problems with different thicknesses?  (I’ll stagger seams)
I’m worried about getting too thick for attaching vinyl siding and I have innies (yes warranty will be voided on siding)

2 Question
The soffit is already in place, will I need to take down and cut it down to allow the extra thickness. Or can I butt 2nd layer against the soffit?   I hope not 🙁   (WRB and 2″ go behind soffit)

3rd Question
On the 1st side I tackled, I only removed the wood siding, not the original 1×8 cladding, and it has some gaps due to its age. Will two layers of foam be enough for air sealing? I can tape the outer layer. I wish I would have replaced it but have it all house wrapped at a window installed already (I not going for perfect but I don’t want moisture problems)

By the last side, I was replacing all the cladding with 7/16 and taping the seams before the WRB.


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Q. "Can I add 1 inch [of rigid foam] over the 2 inches [of rigid foam] that I have up? Any problems with different thicknesses? (I’ll stagger seams.)"

    A. Yes, you can add 1 inch of rigid foam over the 2 inches of foam -- as long as you have a plan for attaching your siding.

    Q. "I’m worried about getting too thick for attaching vinyl siding."

    A. Three inches of rigid foam is a little much if you intend to install the rigid foam without furring strips. For more information on this issue, see these two resources:

    "Vinyl siding over rigid insulation"

    "Can Vinyl Siding Be Applied Over Furring Strips?"

    GBA details routinely recommend installing vinyl siding over rigid foam (without furring strips) when the rigid foam is 1.5 inch thick. I wouldn't do it with 3 inches of rigid foam.

    Q. "The soffit is already in place, will I need to take down and cut it down to allow the extra thickness [of rigid foam]. Or can I butt 2nd layer against the soffit?"

    A. Butting the rigid foam against the soffit won't cause any problems from a building science perspective. This is an aesthetic question, so the decision is yours.

    Q. "Will two layers of foam be enough for air sealing? I can tape the outer layer."

    A. The answer to this question depends entirely on your skill and attention to details. Remember that foil-faced rigid foam is easier to tape than unfaced foam. You'll need to pay close attention to penetrations and transitions if you want a low rate of air leakage -- and you'll need to verify your work with a blower door test.

    Some builders who attempt to use rigid foam as an air barrier achieve great results; others don't. Be meticulous.

  2. JesiahClemons | | #2

    Thanks Martin you're the best! My name is Jesiah (I updated that three times last night but it won't stick in settings) Maybe I will add furring strips and flush between with more foam. I think with siding trims I can hide the butting against. And my wife would say that meticulous is my middle name which is why I don't trust anyone in my area to do this for me. That being said the unfaced is tough to tape.

    So a followup question
    I have been using unfaced at first out of a lack of knowledge but I have been a bit worried about moving away from it due to losing permeability with facers. Is this unfounded with my assembly, as facers would let me air seal better? I have been thinking with imperfect cladding that it would be less risky, but it seems like a greyish area.
    Thanks for the help.

    1. Trevor_Lambert | | #3

      Go to your account settings and enter what you want to appear as your display name in the first name and last name fields. The screen name field isn't used for anything, contrary to what you might suppose.

      1. JesiahClemons | | #4

        It finally took-6th time is the charm. It must have been some issue with the servers.

  3. Expert Member
    Dana Dorsett | | #5

    In climate zone 5 you only need R7.5 for dew point control on 2x6 /cellulose construction. Even crummy Type-I EPS would deliver R7.8 (labeled R, more at outdoor temperatures that matter from a dew point perspective.) At 2" Type-II EPS would be delivering R8.4 (labeled, R9+ at temperatures that matter.)

    So you don't really NEED 3" of foam for a zone 5 location to meet either IRC code thermal performance or for dew point control. Whether you add the additional inch is up to you, and if it becomes too complicated & expensive the money / effort might be better spent elsewhere.

    Faced or unfaced foam is fine. At 3" Type-II unfaced EPS is a class-II vapor retarder, so the vast majority of the drying will already be toward the interior even if unfaced.

  4. JesiahClemons | | #6

    Thanks, I guess I shouldn't have asked the questions so late at night originally as I got myself confused. I have 1 1/2" currently so I will need to add. Shows R-6 plus been exposed for a little too long so want to stay a little higher to be safe.
    This is what I have currently.

    So I'm thinking to add 1" to be safe. A faced option would be way easier to tape. Unfaced just doesn't like tape, unless someone knows of one that works well.

  5. GBA Editor
    Martin Holladay | | #7

    Q. "I have been using unfaced [rigid foam] at first out of a lack of knowledge but I have been a bit worried about moving away from it due to losing permeability with facers."

    A. If you attach an adequately thick layer of rigid foam on the exterior side of your wall, you'll be creating a wall designed to dry to the interior, not the exterior. It's perfectly acceptable to install rigid foam with vapor-impermeable facers under these circumstances. The exterior rigid foam doesn't need to be vapor-permeable, as long as the rigid foam is thick enough to keep the wall sheathing above the dew point for most of the winter.

    For a more thorough discussion of these issues, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

  6. JesiahClemons | | #8

    Awesome, I will use a facer then and add strapping due to depth.

  7. Expert Member
    Dana Dorsett | | #9

    EPS with a foil facer it's easy to air seal with foil tape, and the gap provided by the strapping adds another ~R1.

    Type-I EPS will perform at R4.2/inch at temperatures that matter, so that 1.5" is really 6.3 from a dew point control perspective. Bumping that to 2" would deliver R8.4 at temperatures that matter, bumping it to R2.5" would be R10, nominal /labeled, R 10.5 actual, and with a foil facer next to 3/4" of air it would be north of R11.

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