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Thermawrap R5.0 meets USA new wall – Wall assembly questions

JusH | Posted in Energy Efficiency and Durability on


I am doing a new build with a goal of a PGH. (Pretty Good House)
My planned wall assembly is unusual to everyone local to me. (Newfoundland, Canada)
Climate zone 6, almost 7
Very damp and cold for 9 months(Heating), with 3 months of with a mix of cold and warm humid weather(Heating/Cooling)

I have tried to read whatever I can find on high performance – low cost building envelopes and trying to get ideas and advice where i can. And plan as much out as I can. But it’s a work in progress.
I feel I need some ideas and suggestions from the GBA community.

In hopes to get a PGH I’m using something like a “USA New wall”, with the following:
interior gwb
vapor barrier – retarder . code requires it here
2×6 with cavity batts – planned for roxul
1/2″ plywood – Seams need sealing?
1.5-2″ of rigid mineral wool (roxul)
2×3 -2×4 strapping over rigid roxul.
Thermawrap R5.0
Vinyl siding

Questions I have: (Open to ideas, suggestions, etc. )
1)How do I do the most cost effective air barrier(s) to get air tightness down below 1 ACH?

2)I suspect I should seal the plywood seams, but what is a low cost method to do this myself? I was hoping not to purchase 1000’s of dollars worth of tape. (Wigluv /3m 8067 Flashing tapes are most commonly mentioned)
The plywood is sealed to the studs.

3) Do i need a 2nd layer of WRB directly on top of the plywood sheathing and then taped? The thermawrap install I don’t see as providing a good air tightness. I’ve read some posts about the 2 layers of housewrap, but mostly not in this type of application.
I feel I should use a 2nd layer but not sure if its helpful or hindrance.

4) Suggestions on proper mounting and flashing of flush mount “outie” windows. Plan is to install a small trim of XPS around windows then 2×4 to allow peel and stick flashing, then a piece of housewrap to go in over the sides to get tied into the thermawrap. this seems complicated to me.

5) is it worth using a “Smart vapor barrier”? (Membrain / intello / Majrex, other?) I have read mixed reviews. Is it needed in my climate?
Is the plywood permeable enough to have the assembly and cavity dry outward? I was trying to dry in both directions.

7) Any suggestions prior to roof going on for air tightness?

7) Are all my concerns with my walls getting wet and not being able to dry unwarranted?
I feel I am being a little paranoid.

Thank you for taking the time to read. Glad to have a conversation with someone by phone if easier.

I am hoping to complete a decently high performing durable home for near the cost of a regular build – So that others can begin to do the same here.


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  1. GBA Editor
    Martin Holladay | | #1

    I'm not going to tackle your questions right now. (You have way too many questions.)

    Instead, I'll note the following:

    1. Thermawrap R5.0 is a squishy WRB that strikes most builders as odd. It's an exterior pillow for your house, and when you attach siding to it, the siding will always have some "give," for the life of the house.

    If you install that product on top of exterior mineral wool -- itself a fairly squishy product (although nowhere near as squishy as Thermawrap R5.0) -- you're going to end up with a vertical trampoline.

    2. Most vinyl siding manufacturers do not permit their siding to be attached to furring strips. For more information on this issue, see Can Vinyl Siding be Applied Over Furring Strips?

  2. JusH | | #2

    Hi Martin,
    I'm sorry to dive in with boat load of questions. I wanted to get them all out at once as I felt some of them were related to each other. And now I wonder an alternative to allow drying of the sheathing. I appreciate your quick response.

    To omit the Thermawrap R5.0, my wall still be too squishy to support vinyl (The mineral wool would still require furring strips so no direct support behind the vinyl siding. Plus the 1.5" or 2" alone wont be thick enough to hit R-11 for my climate zone.
    I was thinking(but have not tested) that 16" oc would be close enough to have a pretty solid base for the vinyl.
    Not considering the compounding of "Squishy-ness" of both materials

    I feel I need the mineral wool for the ability to dry to the exterior and hopefully minimize insects. vs. going with Type 1 EPS foam with grooves for strapping which is readily available, but wont allow the sheathing to dry successfully. (R11 or more).

    The cost of 3" of Type1 is almost the same as 1.5" of Comfortboard 80. EPS is definitely higher R/$. The most importing question is my concern about not drying an actually concern?

    If anyone has any suggestions, I'm open to ideas.
    Are there applications where thermawrap R5.0 can be used in a new build with vinyl?
    Or method to use rigid roxul with vinyl?

    Thank you again, and appreciate all the articles and comments on the site.


  3. user-2310254 | | #3

    Could you substitute fiber cement or wood siding for the vinyl or is that out of your budget?

  4. JusH | | #4

    Sadly, It is out of budget.
    I have the vinyl and the thermawrap purchased already.

    I found the thermawrap for a very good discounted price. But not sure if it's realistic to utilize it.
    Plus need to have wind resistant walls to stand up to 150km/h winds and our other colourful weather.

  5. GBA Editor
    Martin Holladay | | #5

    I'm not sure why you purchased the Thermawrap R5.0 and the vinyl siding before you finalized the details of your wall assembly.

    Thermawrap R5.0 is designed to be installed over solid sheathing like OSB or plywood. If you follow that plan, vinyl siding manufacturers will allow the siding to be installed with long fasteners through the Thermawrap R5.0 to the OSB or plywood below. So if you decide to use the materials that you have already purchased, you can't install exterior mineral wool.

  6. JusH | | #6

    I do not need to use the thermawrap. It can be returned. R5 for the Thermawrap alone isn't enough to keep heating costs down. (Any electric based heating costs are expected to double in next few years here)

    Vinyl was always our intended cladding type which is why we went and ordered early so it would be in stock and ready for installation when the time came.
    I was trying to get a minimum of ~r11 on the exterior which the 1.5"of CB80 + TWR5.0 provided.
    With this not being a good choice what can I do now? 2-3in of CB80 can't be used with vinyl either because if only having support every 16 inches?
    Now I'm not sure what another option would be to have proper drying potential and insulation levels knowing the local codes want poly on inside. (Can use smart retarder like membrain to help)
    From everything I've read, 3 inches of eps with grooves for strapping (or any foam) won't allow proper drying to exterior. But type 1 eps is cheapest option for ext. Insulation here.

    Any suggestions?

    Thank you all (especially Mr. Holladay) for taking the time to read and respond.

    I am trying to make informed descisions. please try to have patience with me. I am not a builder. I'm just a average Joe trying to build a healthy, efficient home for my family to live in for the next 30+ years.

    Look forward to hearing any ideas people would like to share.

  7. GBA Editor
    Martin Holladay | | #7

    If you want to install vinyl siding in a way that complies with manufacturers' instructions, you'll need continuous OSB or plywood sheathing under the vinyl siding. If you want a continuous layer of exterior insulation, that means that you need to buy nailbase.

    For more information on nailbase, see Nailbase Panels for Walls.

  8. canadianexpy | | #8

    Which siding manufacture are you using, because I'm using vinyl over furring strips. and the manufacture mentions use of it in the install manual as an option. So no worries here.
    Also not sure you priced Plywood, but it's gone through the roof $$ with the BC wildfires, I'm using OSB to save money.

    Ontario .

  9. JusH | | #9

    Manufacturer is Kaycan.
    Did you use exterior mineral wool under the furring strips? (Or any ext. Insulation)
    Truewall is the EPS product here with groves for strapping.
    Plywood and osb have gone up about 50% here so far.

  10. GBA Editor
    Martin Holladay | | #10

    As GBA reported in one of our articles, "Can Vinyl Siding be Applied Over Furring Strips?", CertainTeed will not allow its vinyl siding to be installed over furring strips. What brand of vinyl siding are you using?

  11. canadianexpy | | #11

    Martin I will be using Mitten Board And Batten on the shop. On my house build I will be using Gentek. I could not find any info saying I could not use furring strips. Including talking to the reps. IN the install manuals they mention using furring and spacing so it should be fine.
    It states depth of fastener 3/4" min. and distance apart. Also that it should be a flat even surface where the fastener is attached.

  12. GBA Editor
    Martin Holladay | | #12

    Sounds good. I'm glad you found a brand of vinyl siding that can be installed over furring strips.

  13. Robert Opaluch | | #13


    You might be able to cut and install EPS or polyiso between the furring strips, using the same thickness as the furring strips. That would provide the flat surface that vinyl siding manufacturers want. Once glued, nailed and/or forced into place, there's nowhere for the lightweight foam board to go (once the siding is installed). It would provide some support behind the vinyl siding that spans the strapping. Or, you might consider adding another layer of plywood or OSB on top of the furring strips to provide a better flat surface.

    But doesn't it make more sense to abandon exterior insulation, and just use a 2x8 wall with Roxul R-30 batts, or a double 2x4 wall? You could use Roxul 3.5" batts in each of the double 2x4 walls, plus stuff Roxul batts into a 3.5" cavity between the walls? (Or use thicker batts to infill studs plus most of the double wall cavity, and fill smaller voids with EPS.) The vapor barrier could be inside the double wall to avoid penetrations. The total R-value would compare favorably with your planned assembly, and be a lot stiffer support for vinyl siding in those gale force winds. You just lose some interior space, but could expand your planned foundation footprint if interior space becomes an issue.

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