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Roof and wall design questions

pakrat1 | Posted in GBA Pro Help on


Need Help, studied practically every article on GBA and BCI and still have questions.

Here is my plan, what do you think of it?           Houston TX

Pros and cons of the various stack ups         currently framed in with ply decking and ply sheathing

1300+ SQ foot attached gar / sometimes man cave, will be used to store a motor home, 2 cars and a work shop.

Details: 24 feet wide by 62 feet long 16 foot plate height with 2×12 joists and 2×6 rafters at 3/12 pitch.

The 2×6 walls starting from the outside will have Hardy or boral siding, ¾ vent, 2” ISO or XPS, drainage plane, ½ ply sheathing, 2×6 studs with undetermined insulation to be installed later.

Roof starting from top down, .2+ reflective shingles / ??????????????????? / 5/8 decking, 5.5 to 8.0 oc-SPF Icynene or 1-2” cc-SPF + oc-SPF with airtight eves, sheetrock ceiling.

Car door will be 2” or 3” thick urethane 19’w x 14’h,   5) 2.5×5, e366 windows 10’ above floor for light and one 3.5 x 6.5 e366 over shop work bench. This will be just one big room with a free standing HVAC for humidity control and heating and cooling when working on cars etc. plus a dedicated dehumidifier and fresh air supply.


#1  What is your opinion for the ideal stack up above the primary deck  ( ??????????? )

     A: /WB / plywood or techshield-foil down /1×4 or 2x4s with ridge and soffit vents / 2” XPS or ISO / ice and water /

     B:  / WB / plywood or techshield-foil down / 1x4s or 2x4s with ridge and soffit vents / Ice and Water /

     C:  / Ice and Water /

     D: open for other sugestions

Option A and possible B, would this prevent enough solar heat to drive moisture out of 5/8 deck?

 Are A & B just a waste of money with minimal benefit.

#2  location of WRB in wall, foam type, best drain plane material “drain wrap” etc. , something with larger gap?

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  1. GBA Editor
    Martin Holladay | | #1

    Radiant barrier sheathing is a waste of money if you intend to install spray foam insulation on the interior side of the roof sheathing. (The radiant barrier does nothing unless it faces an air space.)

    You don't need Ice & Water Shield above your roof sheathing, since Houston buildings don't suffer from ice dams.

    The roof stack up that includes a continuous layer of exterior rigid foam provides a higher R-value than the other options, and reduces thermal bridging through the roof framing.

    Concerning the placement of the water-resistive barrier (WRB) on your wall, see this article: "Where Does the Housewrap Go?"

  2. pakrat1 | | #2

    Really appreciate your fast response, been dealing with indecision for 2 months due to so many opinions on the web. Must say my reading skills have really improved.
    As regards to ? #2 with outie windows
    What about JH or Boral siding / 1x4 vent channel / Tyvek drain wrap or hydrogap / 2” iso etc / plywood sheathing / OC foam / sheetrock
    How about the same config with regular Tyvek house wrap and a rain screen behind the 2” of foam as a backup
    Is water wicking between drain wrap or hydrogap and flashing tape a problem at vertical seams next to windows etc and horizontal seams?

    #1 potential stackups this is in a garage where humidity with door closed will be held at 55 – 60% there will be days where the door will be open for several hours
    A) 1] Shingles regular .1SR or .2+ SR or for another 1500$ (ouch) .37+ Solaris my concern is too much reduction in solar energy may not be enough to dry out any moisture under the deck.
    2] WRB was going to use I+W but what a pain if roof has to be replaced and useless if reused due to thousands of nail holes. Regular WRB probably fine with a vent channel under the nailer.
    3] Nailer .5” plywood would tech shield really be worth it with .75 - 1.5” vent channel
    4] 1 x 4 or 2 x 4 furring with ridge and soffit vents, is a 1.5” vent necessary just for drying the nailer or is .75” enough
    5] I+W would this be appropriate in this location or direct to deck, never have to worry about nail holes when replacing roof as long as roofers used short enough nails on original install
    6] 2” ISO or XPS or EPS Your opinion to get really crazy maybe a rain screen under the foam for insurance
    7] 5/8 ply deck
    8] 7 – 8” (this gets 1.5 – 2.5” under the rafters and gets foam around the joist tails for better sealing) OC-SPF less if I use the 2” ridged foam on top. Is the 7 – 8” oc spf too thick for adequate deck drying with or without the current above deck config
    pix 1 one possibility subject to change pix 2 shows soffit design

  3. pakrat1 | | #3

    pix failed to upload first try

  4. pakrat1 | | #4

    pix failed to upload

  5. GBA Editor
    Martin Holladay | | #5

    Pak Rat,
    Are you the same person as Michael Lind?

    Your writing style is rather dense and idiosyncratic, making it hard to read, especially if you ask 10 or 15 questions at once.

    Try asking one question at a time -- you are more likely to get an answer.

  6. pakrat1 | | #6


    I am the same person, pakrat is my user name, not sure why both are showing up.

    When I wrote this, I formatted it to make it clear. When I copied it to the blog, the whole thing got compressed together, all spacing was removed.

    I will re send in a more readable format, Mike Lind

  7. Expert Member
    Dana Dorsett | | #7

    IRC code min roof for climate zone 2 (that would be Houston) is R38. At 8.0" half-pound icynene is only R30-ish. With only open cell foam it may be subject to ping-pong humidity cycling over the course of a day. With a couple inches of EPS above the roof deck and 8" of open cell below you'd avoid that issue and it would meet IRC code minimum on an R-value basis. An alternative would be 10.5-11" of cellulose and no above-deck foam. Cellulose won't have the diurnal moisture cycling, and has substantially more thermal mass, which flattens and delays the peak cooling load contribution of the roof.

    On the wall foam use foil faced polyiso. The foil facer with a 3/4" ait gap adds ~R1 or so to the performance, and foil facers are dead-easy to air seal with tape. Polyiso is also blown with fairly benign hydrocarbon blowing agents. XPS is rated R5/inch in an LTTR test, but is only warranteed to R4.5/inch over the long haul, and may not even meet that after 20+ years of service. The additional performance compared to EPS if similar density is due to it's climate damaging HFC blowing agents, some of which are lost over the lifecycle of a building. XPS is the least-green commonly used insulation material in the US.

    Use a dumb text editor, not a word processor if pre-editing. Special formatting characters are not universal across platforms.

  8. pakrat1 | | #8

    Hi Dana
    thanks for your response
    Sticking to the roof for now, if I go with the 2" eps and 8" oc SPF should I assume that you are still calling for the 3/4 or 1.5" vent under a nailer as in my original config

    sh / nailer / vent / 2" eps / deck / 8" oc-spf

    note 2x12 joists with 2x6 rafters can 10+" wet sprayed cellulose be used with no support under it

    my other most pressing question is: in either case with a vent channel drying the nailer, would a VB peal and stick under the 2" foam be OK as a backup to the shingle system (to quote Doc Joe all roofs leak ) I don't trust typical WRB underlayments

    PS: Are you Dana1 on GBT

    Thanks for any help you can give me, I have stressed over this way too long
    Mike Lind "pakrat1"

  9. pakrat1 | | #9


    I keep trying to post a JPG but no luck. I was able to post pix with original post

  10. Expert Member
    Dana Dorsett | | #10

    Since it's not structural the nailer deck doesn't really NEED to be vented. If there's a chronic leak a sheet or three might need to be replaced when re-roofing, but venting it may save you from that additional expense.

    The only downside to a vented nailer deck is that it's more likely to peel off in a hurricane than an unvented nailer. (Do they even have hurricanes in Houston?) Using 2x furring through-screwed to the structural roof deck and an appropriate number & type of nails to secure the nailer deck can mitigate that risk.

    Putting down a self-healing membrane between the EPS and roof deck isn't really a "...backup for the shingle system...", but rather a WRB for the structural roof, the part that you really care about. The EPS and vented nailer above it can tolerate being wet. EPS is somewhat vapor open (unless it has a facer), and would allow drying into the vent channel above if gravity wasn't enough to move any minor leaks through the EPS seams along. Type-II EPS runs about 1.5 perms @ 2".

    I have no experience with wet sprayed cellulose on ceilings without additional support, but apparently that has been done with some products containing water activated adhesives. I'm skeptical about it's longevity without support though.

    Yes I've posted as Dana1 on GBT for about a decade (somebody got to "Dana" ahead of me on that forum, then never posted anything. )

  11. pakrat1 | | #11


    current plan

    shingles, .2+ SR

    I+W at eves then WB rest of way up

    1/2 in plywood Tech Shield foil into vent space optional

    2 x 4 furring laid flat with gaps every 2 - 4 feet for cross venting

    2 in foil faced ISO for R12 above condensing surface

    I + W

    5/8 decking

    7+ in OC SPF between 2 x 6 rafters, wall top plate to deck sealed

    This is in the garage described above, with 7 in OC foam (R 25+ ) and inside temps hovering in

    the 50s and outside temps in the low 30s in the winter, should I be worried about moisture

    under the deck.would the cool shingles and foil on the ISO prevent enough solar energy

    reaching the deck to dry it out. I will have a dehumidifier set to maintain a RH of 50 - 60%

    Thanks Mike

  12. pakrat1 | | #12

    PS: we do get hurricanes here but the pine trees and 2 story houses tend to reduce the (120 - 130 mph wind zone ) substantially, last big one here took out over 12 pines in my neighborhood.

  13. Expert Member
    Dana Dorsett | | #13

    In your climate wintertime moisture accumulation in the structural roof deck isn't a serious concern, especially when there is more than the IRC prescriptive R5 above the roof deck. The R12 polyiso is more than sufficient- it doesn't need the sun to dry out. See Table 3, column 1, top row in this document:

    Even with a highly reflective metal roof and NO foam layers the roof deck stays dry in locations as humid as Miami. (Unfortunately they didn't include simulations of the light colored metal roof for Houston, only tile vs. dark shingles.)

    Using a foil clad vent nailer deck would increase the shingle temp, high solar-reflective index (SRI) shingles would lower the shingle temp. The vent space does very little for cooling the shingles near the peak, since the air is being pre-heated with the heat drawn from the decking on the lower portion of the roof.

    Either vented or unvented it's unlikely to affect the longevity of the high SRI shingles, and only marginally extend the lifespan of more absorptive shingles. But venting does give the nailer deck a drying path.

  14. pakrat1 | | #14


    latest stackup plan

    going down: shingles, WRB, tech shield, 2x4 (1.5" vent ), 3 layers foil covered ISO (R36) - all seams taped, W & I entire deck 5/8 deck. no under deck ins except ccspf foam between top plate and deck for air sealing

    your opinion are high SRI shingles realy going to gain enough to compromise on the color scheme IE we are not crazy about the color choices.

    also thermally, will i gain enough with the tech shield to justify any of it's downsides

    what would be the best way to tape the ISO seams to allow for shrinkage of the foam. will use a strip of W & I at the ridge under the vent inlet. My preference is to have any water that makes its way to the foam to be shed to the eves

    pix of eve detail, dip in vent straping to lower facia height
    any comments of this design much apreciated

  15. pakrat1 | | #15


    latest stackup plan

    going down: shingles, WRB, tech shield, 2x4 (1.5" vent ), 3 layers foil covered ISO (R36) - all seams taped, W & I entire deck 5/8 deck. no under deck ins except ccspf foam between top plate and deck for air sealing

    your opinion are high SRI shingles realy going to gain enough to compromise on the color scheme IE we are not crazy about the color choices.

    also thermally, will i gain enough with the tech shield to justify any of it's downsides

    what would be the best way to tape the ISO seams to allow for shrinkage of the foam. will use a strip of W & I at the ridge under the vent inlet. My preference is to have any water that makes its way to the foam to be shed to the eves

    pix of eve detail, dip in vent strapping to lower fascia height
    any comments of this design much appreciated

    1. Expert Member
      BILL WICHERS | | #17

      You’d probably be better off with 2 layers of thicker polyiso instead of 3 layers of what I’m assuming is probably 2” material based on your R36 number. Two layers of 3” polyiso should get you R38, a layer of 3” and a layer of 2.5” would be R35. Two layers will likely be less labor (cheaper and faster), and will still allow you to overlap the seams to limit 3D airflow pathways. Polyiso can be ordered through suppliers in many thicknesses — I’ve seen up to 4.5” thick polyiso panels.


  16. pakrat1 | | #16


  17. GBA Editor
    Martin Holladay | | #18

    Q. "Latest stackup plan: going down: shingles, WRB, Tech Shield, 2x4 (1.5" vent ), 3 layers foil covered ISO (R36) - all seams taped, Ice & Water Shield entire deck, 5/8" deck. No under-deck insulation."

    A. In Houston, Texas, I assume that the code requires a minimum of R-38 ceiling insulation. On a U-factor basis, your R-36 roof will comply. So it will work.

  18. pakrat1 | | #19


    looking at the drawing, do you see any thing that you would change, have any suggestions

    Thanks Mike

    PS: the 5x7 piece just below the words "tech shield ?? " is a support nailed to the strapping

  19. user-6184358 | | #20

    Looking at the first picture you posted raises a Question- Where is the shearwall on the gable end?

  20. pakrat1 | | #21

    you are seeing the front which is 25 feet wide with a 19 foot door, 3 foot shear left side and 2 foot on right side. 5/8 + 1/2 glued and nailed outside surface and 1/4 or 3/8 inch steel plate inside surface

    any comments on the stackup design

    thanks Mike

  21. DSEGOOSE | | #22

    In the UK homes are now needing to be A1 compliant so non combustible. poly foam insulation will be banned. Highly combustible and gives of cyanate gas. No level of retardent will work. Yor fires in California we watch here with dismay and hope the replacements will be non combustible. Check out ou non combustible building material.

  22. pakrat1 | | #23

    another ?

    are there any people on this board that have experienced odor from peal and stick on ship lap and if so how bad was it. The application is a conditioned attic, ( no interior insulation under deck ) that communicates with the house interior

  23. pakrat1 | | #24

    Well the project got put on hold. Now I am back on it and wanted to run a few thing by you guys.

    This is on the original house with no way to ins under the deck.

    To save weight, here is the current stackup.

    2x6 rafters, original 1950s 1x6 plank deck, XXX, one 3" and one 2" layer foil faced ISO, top layer

    taped with a tape that can accommodate ISO shrinkage (suggestions ?), 1.5" vent channel,

    Standing seam metal (Kynar high SRI on top and bare galvalum on bottom .06 emissivity).

    The big question is XXX. vapor open or vapor closed peal and stick and what to do to limit any

    odor wafting through the ship lap plank decking

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