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How deep do I go? Replacing dormer siding on 1915 house.

Corvus_corax | Posted in Green Building Techniques on

Hello- I’ve been digging into this fantastic treasure trove of articles for a couple of days (and nights) as I plan a re-side project on my dormers (In St. Paul, MN).

The problem is that as I read more, I’m starting to add so much to the “might-as-well” list that it will easily spiral out of control in terms of time and budget.

So- where can I draw the line? What techniques should I use depending on where I draw the line? Here’s what I mean:

Siding: I want to add new shakes of engineered wood (LP SmartSide) or fiber cement. Currently has cedar that’s old, warping, poorly maintained trim, etc.

Rainscreen: I am convinced of the need to use a rainscreen.

WRB: I know I need a WRB (there’s probably real felt under there now).

Sheathing: As far as I’ve seen I have original shiplap for the sheathing. I don’t really want to update the sheathing.

Exterior foam boards?: having read the article about dew points and exterior insulation (How risky is cold OSB?), I’m not sure I can realistically handle the fattening of the dormers by the several layers of foam board (how to do tie-ins at roof lines and along lower level vertical walls- I’m not re-siding the whole house!).

So I am thinking that if I don’t do the exterior foam boards, I’ll demo down to the sheathing, wrap with tyvek or other WRB, and use Cor-A-Vent sturdie-strips behind the siding.

Am I crazy to be doing a partial re-side in MN and NOT add external insulation?


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  1. Expert Member
    Dana Dorsett | | #1

    Most houses can tolerate an inch of additional thickness. Adding an inch of rigid rock wool would cut the heat loss of the walls by about 1/3, maybe more (depending on the actual framing fraction) and wouldn't need more than 1/4" of rainscreen gap to offer HUGE drying capacity to the sheathing. Barring that 3/8" perforated (must be perforated) XPS siding underlayment would cut the heat loss by more than 1%, and wouln't need more than 1/4" of rainscreen gap, which is only about 5/8" thicker than the original stackup, maybe even less depending on the differences in thickness between the new & old siding.

    Now would be the time to look at the cavity insulation in the dormers too, unless it's had a recent gut-rehab on the interior. If it's the original plaster & lath and it was shot with cellulose back in 1982 there's a good chance it has settled. It can be topped off or packed out drilling from the exterior, before you install the WRB and kick-out flashing.

  2. JTyler | | #2

    My $.02: I agree that adding foam board to your dormers makes little sense if you have no set plan to do the whole house. If you do eventually add exterior insulation to the home, the dormers will be a minor detail of that project. I would think about adding a layer of plywood or osb with taped seams over your shiplap shearing. Unlike the foam board, this will have only a minor impact on your flashing and transition details. Without doing the rest of the walls, it will have a negligible impact...but if you're going to be in there you might as well create a good air barrier on these portions of the wall before re-siding.

    That's my vote anyway. Good luck with the project!

    Edit: scratch that - I agree with Dana. Rock wool is very permeable, so you can get away with a thinner layer and your assembly will still dry to the exterior.

  3. GBA Editor
    Martin Holladay | | #3

    There is no single answer to the question, as long as the details you choose are good ones. It's your house, so you can be as obsessive or casual as you want to be.

    As you realize, you can't add rigid foam to the exterior of your dormer cheeks unless the rigid foam meets the rules for minimum foam R-value. If you use mineral wool as Dana suggested, any thickness is acceptable.

    Whatever you do, pay attention to airtightness.

    I hope you have a good understanding of how step flashing is installed when the siding on your dormer cheeks is replaced.

  4. Corvus_corax | | #4

    Thanks everyone for the very prompt attention!

    Martin- If I may ask a follow-up question: what would be a preferred method for air-sealing the shiplap sheathing? I did an attic air-seal and insulation project a couple of winters ago so the stud bays don't leak upward, but I assume there are plenty gaps and holes in the sheathing. I've seen some down-talking of tapes, so are we talking sealant-caulk? Also- thanks for the flashing comment- if you could see my search history from yesterday afternoon, you'd know I've been tackling that very question.

    Dana- I hadn't even seen the posts about mineral wool, so this was a brand new idea for me. Thanks! There was certainly cellulose blown in at some point (saw the evidence when I was air-sealing and insulating the knee-walls)- I will add that to the list!

  5. GBA Editor
    Martin Holladay | | #5

    Q. "What would be a preferred method for air-sealing the shiplap sheathing?"

    A. There are lots of possible approaches, including flash-and batt insulation, new plywood or OSB sheathing with taped seams, a high-quality European WRB like Solitex Mento with taped seams, or the Airtight Drywall Approach.

  6. user-1072251 | | #6

    cover it with 3/8"sheathing, with the seams taped, and caulk the shiplap around the edges of the wall so the sheathing is tight. Not that expensive or time consuming.

  7. Corvus_corax | | #7

    Thought I'd return for a little update in case anyone ever has a similar question. I'm starting demo tomorrow.

    Along with the many articles discovered while browsing and searching around GBA, I found this building science document to be extremely helpful:

    Thanks everyone!

  8. GBA Editor
    Martin Holladay | | #8

    Thanks for the link to the Building Science Corporation document.

    Here is a better link to the acutal pdf document.

    The document discusses the Mark Yanowitz retrofit job in Massachusetts. Mark Yanotitz wrote an article for GBA on that project -- here is the link: Wrapping an Older House with Rock Wool Insulation.

  9. Corvus_corax | | #9

    Demo on hold due to rain, so I called to check in with the building inspector and....

    ...he is requiring that I have documentation from LP approving the use of furring strips for a rain screen gap, so my idea of using 1x4's is hitting a snag.

    I'd seen the discussion of this over here ( and here ( so I was planning to use ring shank nails and call it solid. Note that the new LP instructions allow for 1.5x3.5 furring strips instead of 1.5"x4":

    "For rigid foam sheathing up to 1 in. (25.4 mm) thick, siding
    may be nailed directly to the foam sheathing unless a drainage
    plane is required by the local building code. Nail length must
    be increased to ensure a minimum 1-1/2 in.(38.1 mm) fastener
    penetration into the structural framing.

    For rigid foam sheathing greater than 1 in. (25.4 mm), a minimum
    1-1/2 in. (38.1 mm) thick by 3-1/2 in. (88.9 mm) wide vertical
    strapping or furring strip must be installed over the sheathing
    to provide a solid, level nailing base for the siding. The strapping
    must be securely fastened to structural framing spaced no
    greater than 16 in. O.C. (406 mm) with a minimum nail penetration
    of 1-1/2 in. (38.1 mm) and a maximum nail spacing no greater
    than the width of the siding."

    So now I guess I'll have to go with 2x4 furring? What are the chances that LP will give me a different answer in an email?

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