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Zip R12 over vertical plank sheathing on my 1720 colonial

reiniken | Posted in General Questions on

Hello GBA,

Unfortunately, I don’t come across many examples of homes in blogs that are quite like mine and I’m hoping someone might be able to throw in their two cents and tell me if I’m heading in the right direction, or if I’m absolutely crazy.

I own a unique 1720 (year, not sqft) 2-story colonial (climate zone 5 – MA) that is going through an unplanned restoration after some gusty winds decided to send some large pine trees down onto my roof. The house is a timber frame structure (zero wall studs) sitting on top of the 300 yr old stone foundation (basement), which places the sill merely inches above grade. The frame is enclosed in vertical wide plank (18-29″ wide) 5/4 sheathing. The exterior walls consist of this plank sheathing, 30 lb tar paper, 1″ foil (both sides) polyiso rigid foam, strapping and cedar shingles. There is no stud framing at all on the interior side of the sheathing, meaning the only wall insulation is the 1″ polyiso that was installed in the late 70’s, in addition to 5″ of loose-fill mineral wool in the attic. Without any heating or cooling, the indoor temperature of the house is typically 10-20 degrees cooler when 55 degrees or warmer outside (max 79 degrees indoor temp recorded when 96 degrees outside), and a low indoor temperature recording of 31 degrees when the temperature was below freezing down to 0 degrees (monitored over two 24 months). The polyiso was not taped, only held on by strapping, and in many areas, was in full contact with the ground. Remarkably, the exterior side of the sheathing looks about as good as it does on the interior side, no rot whatsoever. When the house was occupied, I went through approx. 1,250 gallons of oil (Oct-Mar). The interior side of the sheathing only had 3/8″ lath and 1/2″ of horsehair attached directly to the sheathing.

Now, I am ditching the oil/baseboard heat in place of a mini-split system and need to rethink the wall system. My plan is to keep the original plank sheathing in place as-is and to cover it again with real 30 lb tar paper lapped, simply because it worked for the past four decades and I don’t see any reason why I should go with anything else. On top of that, I plan to install Zip R-12 sheathing (J roll lap tape the seams, liquid flash exposed penetrations, bent aluminum flashing at sill), purely for ease of installation with the foam attached. On top of that, 1×4 vertical strapping underneath the EWP clapboard siding. Rough window/door openings will be box framed with Advantech 3/4″ (sill at 5 degree slope) and completely liquid flashed, no stretchy tape. Inside of the plank sheathing, I am going to infill the timber frame with 2×6 studs, spaced 24″ oc, and fill the stud bays with Rockwool R23 Comfortbatts. The 2x’s will actually sit .5-1″ away from sheathing. Lastly, the 2x studs will be covered with 1/2 sheetrock. The roof is a cold roof and will have Rockwool R-23 Comfortbatts in the floor joist cavities, 3/4 plywood on top of it, and another 7″ of Rockwool R30 Comfortbatts sitting on top of the plywood flooring, for +/- R50-ish. For some reason, I just don’t like the idea of spray foam.

I’m a college educated pencil pusher, not a home builder, so most of my plans/ideas come from watching hours of videos (I sense eyes rolling) from Fine Homebuilding, Matt Risinger, etc. and reading articles from Martin Holladay. I apologize for the length of my first post, but I’m hoping someone with more experience than me could toss in their two cents (negative feedback preferred!) and let me know if I should put down my hammer and retreat back to my calculator. Thank you for taking the time to read this post. 


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  1. b_coplin | | #1

    Blown cellulose on the attic floor will be cheaper than mineral will batts, and still DIY-able (if you plan on doing this yourself).

    What's happening to the 1" polyiso? Maybe I misunderstood the new stack up. Is the Zip r going over the polyiso?

    1. reiniken | | #3

      Hi Bryan,
      Thanks for idea/tip. The 1" polyiso is going into the dumpster.

      1. b_coplin | | #6

        If you aren't going to reuse the 1" polyiso, consider an ad on your local Craigslist " free" listing. I listed some eps I had leftover from my four season room about a year ago and it was snapped up in about an hour. These were large pieces and not full sheets as well.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Ryan.

    I don't see any problems with your plans. Like Bryan, I think cellulose is a better option for both the walls (dense pack) and attic (blown in). It has a much smaller environmental footprint than mineral wool; it will be easier to fill the space between the sheathing and the stud wall too.

    I'm not sure why you are installing the plywood in the attic (for a walking surface perhaps), but in any case, before you install attic insulation, be sure the air seal the ceiling from above. This article will give you a good idea what is involved in that work: How to Insulate an Attic Floor

    1. reiniken | | #4

      Hi Brian,
      Thank you for your idea/tip about the cellulose, just as Bryan mentioned. I do have some experience using both in the past, however I'm behind schedule and the mineral wool batts are just a quicker install for me in this application. As far as the attic, there will be new roof decking & new soffits installed, so everything will be air sealed properly. Since there will be (3) mini-split air handlers (Mitsubishi Model: PEAD-A24AA7) in the attic, I thought it would be wise to have a solid flooring base to walk on instead of guessing if I were going to land on a joist. I thought the Comfortbatts would be easier to move around when I do a routine inspection of the attic in the future. Previously, the attic had 1x12x16 shiplap flooring laid over the joists, closing in the loose fill. Thank you for the link to the article. I will check it out right now!

  3. Expert Member
    Akos | | #5

    Air handlers in unconditioned attics are a bad idea for comfort and efficinecy. You are probably loosing 1 ton of your installed capacity by having them up there.

    If you must put them there, consider going for a hybrid insulation approach (see bellow) insulating the rafters and making that attic unvented.

    This would let you use the space for storage and you would not need to replace the existing plank flooring.

    You are doing a lot of insulation and air sealing work, I would re-run your numbers on heat load. There is no way your place after insulation upgrades needs 3x 24kbtu air handlers.

    1. reiniken | | #7

      Hi Akos,
      Thanks for your tips. You are correct, 3x 24kbtu air handlers would be too much. I only used that particular model to reference the type of cassette air handler that is going in the attic. The actual setup calls for (2) 9K units and (1) 14k unit. The existing plank floor in the attic is gone, nothing but exposed timber & open air from the ridge down to the 1st floor. As it is a timber frame (open floorplan), air handlers can only be in the basement for the 1st floor and attic for the 2nd floor. This eliminates any ducts inside the walls. I realize it may not be the ideal method, however I need the floor space.

      Thank you for the link to the hybrid assembly. My 1st plan was actually to cover the roof with SIPS panels, but then my mouth hit the floor when I saw the price installed. My 2nd plan was to stagger three 2" layers of Dow SuperTuff-R, but I wasn't really sold on it. My 3rd plan was to not rock the boat and just leave it alone as it has been for the past 300 yrs. The attic consists of 8x6 rafters spaced 6-8' apart with (5) 3x5 purlins across each rafter and wide (12-18") 5/4 vertical plank decking on top with no ridge whatsoever. I would consider spray foam all day long if this was a conventionally framed structure, but it just seems too risky for me to get on board in this application.

      What do you think about 6" rigid foam in-between the rafters with a 1-2" air gap between the rigid foam and the underside of the decking from the soffit up to the ridge? At least I could remove it in the future without it bonding to the timber. The exterior roof would be architectural shingles over Grace membrane covering the entire roof over 1/2" plywood nailed to the 300 yr old plank decking.

      1. Expert Member
        Akos | | #9

        The only time insulating the rafters is worth it if you want to use the attic for storage/living space or if you put the air handler/ducting up there.

        Blown in insulation on the attic floor is WAY cheaper, but you have to find a spot for the ducted unit bellow. These are thin enough that you can mount them the ceiling of a central hallway or the of the bathroom. Most mid static units can also be mounted vertically, so you can easy mount one in the back of a closet. Getting this figured out is not hard and is much less work than insulating the rafters.

        Having said all that, I'm never a fan of empty attic spaces (I'm typing this in the space that used to be the attic of my home). It seems such a huge waste of space. The challenge with your setup is you have purlins, which means there is no way to vent from soffit to ridge unless you install 2x4 on flat above the existing deck and put a layer of plywood over that. If you can do that, you can now go with fluffy insulation, which is pretty cheap. You might have issues if your spacing is non standard. You can also strap out your purlins to add extra depth or an interior layer of rigid over them if you want more R value. The interior rigid is actually a pretty good way to go as it provides a thermal break for your rafters and can also be your primary air barrier.

        Cut and cobble between rafters is generally a waste of time and money. Because of the thermal bridging of the rafters, the R value of the whole assembly is barely higher than batts.

        I understand the concern with SPF, but lot of times with older structures with add lumber size and uneven spacing, SPF is just simpler. If you can figure out how to vent the roof, going with open cell spray foam is a good option. It is much cheaper but unlike closed cell it can easily removed down the road.

  4. Expert Member
    Dana Dorsett | | #8

    >"My plan is to keep the original plank sheathing in place as-is and to cover it again with real 30 lb tar paper lapped, simply because it worked for the past four decades and I don’t see any reason why I should go with anything else. "

    Instead of #30 felt, a fully adhered WRB such as Vycor or Blueskin is a heluva lot cheaper than R12 ZIP-R, and probably tighter too, since detailing the air-tight interface on the top & bottom edges of the ZIP-R to the original structure will be difficult. (From an air sealing perspective, think of the fully-adhered WRB as a 4' wide roll of housewrap tape.)

    In MA there are multiple vendors of reclaimed roofing polyiso at pricing cheaper per R than mid-density fiberglass batts. (The Insulation Depot/Nationwide Foam in Framingham is the biggest, but Green Insulation Group in Worcester is competitive. Both sell factory-seconds new foam too, at a higher price point than reclaimed goods, but at a steep discount from factory-perfect goods from distributors. There are other, smaller foam reclaimers too.) Two inches of used fiber faced foam, seams taped strapped in place with the 1x4s would meet code min if you're keeping the interior polyiso but three would still be "worth it".

    R23 between the joists under the floor doesn't meet current code min (= R30), but you're allowed to go as low as R19 as long as it completely fills the depth of the joist bay. Air sealing the floor is going to be very important if you're not foam-sealing the foundation. A full cavity fill of 1.8lbs blown fiberglass or 3lbs cellulose would outperform cut & stuffed batts, since dense-packed fiber will follow the air flow during installation to clog all leakage paths.

    Dense packed cellulose is the best choice for your interior side wall insulation too, since it will protect the sheathing from excessive wintertime moisture accumulation by wicking and redistributing, safely "sharing" the moisture load without damage or loss of function. As long as there's at least 2" of roofing polyiso (R11.5-ish when new, R10-ish worst-case for used goods) on the exterior even in the parts of the wall where the cellulose depth might hit 7" there is sufficient dew point control at the sheathing layer to skip interior side vapor retarders.

    I agree with most that three 2 ton cassettes are likely to be extreme overkill for the "after building upgrades" condition of the house. Try to estimate the framing fraction and calculate U-factors, then run an I=B=R type load calculation to get a handle on the 99% design load: (Be aggressive rather than conservative on the air leakage estimates, especially if dense packing &/or using a fully adhered WRB.)

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