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Community and Q&A

EPS on the inside of a solid Hemlock frame

Michael Oleksiw | Posted in Energy Efficiency and Durability on

I own a 1964 Duplex in Montreal. I have selectively gutted some key rooms for reno.
Once of which is my kitchen. I would like to improve the efficiency of the assemblies I have opened up. I have new windows.
I don’t know what my building structure type is …but it can it is SOLID – characterized by a series of nail,connected Hemlock (pruche) planks that are structurally notched for spans.
Here is the profile OUT to IN.

air space
asphalt impregnated fibre board “tent test”
asphalt paper #15
2 1/2 inch Hemlock planks
** #15 felt paper

My plan was and hopefully still is to use TYPE II EPS (ISOLOFOAM HD) 1 3/16″ directly nailed to the structure, followed by either some kraft, 15 felt or membrain or nothhing. The structure itself is quite drafty, so I caulked and sprayfoamed the joints between planks.

I am now struggling with the decision. Am i making a mistake by putting structure into the cold?? I cannot afford to build a cavity wall due to space constraints. What do y’all think / recommend?

What should I do and why? also..what is the framing I described called?
Considerations, ceiling joint spaces etc?

HELP! I need to get the job done for the holidays and this is a sticking point.

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  1. Riversong | | #1

    What do you mean by "a series of nail,connected Hemlock planks that are structurally notched for spans"?

  2. David Meiland | | #2

    Is drywall or plaster part of the interior finish?

  3. Michael Oleksiw | | #3

    -What do you mean by "a series of nail,connected Hemlock planks that are structurally notched for spans"?
    ANS: The wall looks like a juxtaposition of of large rough planks 2 1/2" thick (more like timber), of varying widths between 8 -18" nailed into each other. Some are horizontal, others at ends are vertical. Topmost horizontal members are vee'd into notches on supporting vertical members.
    I can email photos as required.

    The original finish was rocklathe (1st gen gyproc 2'x4') with a finishing coat of plaster. Total thickness around 5/8. New finish will be gyproc 1/2 or 5/8.

  4. Riversong | | #4

    I've never seen a structure quite like the one you describe. I once repaired a carriage barn that had been insulated and finished for living space, that had plank-on-plank walls composed of two layers of alternating vertical planks, which had large rectangles cut out for picture windows and then a second story built on top with insufficient structural support underneath.

    I would highly recommend that you have a structural engineer evaluate your building before doing anything. This does not sound like a structurally-sound building.

  5. GBA Editor
    Martin Holladay | | #5

    In this corner of Vermont, some of the oldest surviving houses (200+ years old) are so-called "plank houses." Basically these houses have solid wood walls make of very thick planks like you describe. The ones I heard about were basically assembled like a log cabin -- at least that's how I understand it. I wasn't aware that some plank houses had vertical planks -- that sounds like a hybrid timber-frame plus plank wall.

    And I've never heard of a plank house with brick veneer.

    I won't pass judgment on the structural issues involved. But I see no reason why you can't insulate your planks on the interior with EPS.

  6. Michael Oleksiw | | #6

    Thanks! I am amazed that I am still unable to label this house construction very widely used in a few Montreal neighbourhoods. I've heard some people call it" Quebec Framing".
    To me, it does seem like a hybrid, structure + additional filler planks as insulation.
    The subfloors are thin planks diagonally spanning the concrete foundation walls.

    Assuming structure is fine. (it is,verified) and we focus strictly on the moisture concerns..
    - the wall is well proteced an rarely gets hit with driving rain, flat roof is sound, with a good soffit overhang protecting the walll.
    -if we consider the profile of the wall with little "real" R value ,(est at 3.25 Brick,Sheathing,Felt and the planks), we then have a new R of 5 with the EPS on the inside + gyproc etc.
    - the wall leading up to the EPS on the inside will remain drafty.
    My concern is that the inner surface of the plank is now signifcantly colder than it was before and the EPS on the inside tight on the plank would limit inward drying.

    Remember this is a duplex, the majority of the home remains as-is, roughly 30% of the first of two above grade floors are subject to change.
    Am I moisture paranoid? I just feel I obligigated to add some efficiency to the asembly.

    What do you all think?
    - EPS on the inside ? one solid vote so far
    - Choice of vapor retarder - is the EPS enough or should it be faced with Kraft or something else?
    - Polyethelyne is a no , right?
    - drying potential? if the plank (aka WALL gets wet on the inside - how can it dry ?
    - some reading and intuition have me feeling I need to keep this house breathing.

    I hope my non-building background is not overy evident. :-)



  7. David Meiland | | #7

    Michael, it seems there are two concerns. One, will interior moisture migrate outward to the back side of the wood planks and condense there? And, two, has the brick veneer been dependent to any extent on drying to the inside?

    To a certain extent you can control interior humidity by ensuring proper ventilation, but you also need good air-sealing so that interior moisture isn't conveyed by air leakage around behind your foam and to your planks. It's hard to form a picture of all the building details you may have there--it's unusual to say the least. Is there wiring in the exterior walls? Plumbing? Anything else that's going to make air-sealing hard?

    I'd be more likely to worry about what the brick is doing, and what it will do if you reduce drying to the interior. How good is the airspace? Is it an inch or more, vented at the top and drained at the bottom? How are the windows and other openings flashed? Have you had any opportunity to remove any of the planks and see if they show signs of moisture on the exterior side?

    Seems to me you'd be well-advised to get a local expert to consult for you. There must be someone in the community there who has some experience with this type of construction.

  8. Riversong | | #8


    If this is an unfaced EPS foam, then it's relatively vapor-open (data sheet states Perm = 3.5), so there should not be an issue of drying to the inside. And, if you detail the drywall for good air sealing and control indoor humidity, there should not be a problem with condensation from the inside.

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