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Unvented 5/12 pitch slate roof built with Zip sheathing and closed-cell spray foam insulation

user-6053472 | Posted in Energy Efficiency and Durability on

I am Homeowner building house in Port Washington , NY, ( Long Island )
My Architect will try to go for passive house certification , so “She” needs to be air tight and super insulated .
“She ” will have ERV system and attic floor will not have any insulation , so temperature will be balanced throughout interior of the house
Exterior walls is constructed with 3/4″ zip sheathing over 5-1/2″ LSL Versa studs ,
Interior cavity of wall studs will be 3″ of closed cell spray foam and balance of 2-1/2″ will be open cell spray foam
“She” will also have Rainscreen system on exterior walls installed with 4″ Cascadia clip ( thermal spacers ) and will have 4″ of Roxul cavityrock insulation btw clips . Insulation will be covered with Solitex Mento 100 airtight , vapor open house wrap . Cascadia system have 1″ Z profile track that creates 1″ air space (btw rainscreen surface and Vapor barrier )
Upper portion of house will be 7/8″ IPE siding and lover portion will be stone veneer over 5/8″ Durock fastened to Z profile :
“She ” will be glazed with High efficient Passive house European Doors and Windows
About 70 % of the house roofing is 5/12 pitch roof with large 5′ overhangs and other 30% is flat roof .
Pitch portion of roof is getting build with 3/4″ zip sheathing over 11-7/8″ LVL rafters ,
We are planing to spray 3″ of closed cell foam insulation btw rafters & under zip sheathing and than balance (about 9″ )of rafter cavity will be filled with open cell foam ( to save on cost) .
As per manufacture Zip sheathing needs have Grace ice & water shield 2′ all around edges of overhangs and in valleys . Over the entire Zip sheathing we are planing to use vapor permeable roof underlayment (Delta-foxx ) from Cosella Dorken and than Vermont slate for finish
Unvented Flat roof :
32′ x 32′ surface is built with 9-1/2″ LVL framing covered with 3/4″ advantex subfloor .
Underneath Sub flooring , 9-1/2″ cavity will be filed up wit 3″ closed cell spray foam insulation and balance of 6″ will be open cell foam .
Entire surface of subflooring will be covered with Grace ice & Water shield membrane . Over the membrane we will have tapered insulation .First layer ( base ) will be 4″ throughout flat roof surface and than Tapered insulation will start as 1/2″ thick and will grow to almost 6″ maintaining 1/4′ per foot pitch over 32′
Primary waterproofing membrane will be Kemper system
We are in stage of roof framing completion and Roof sheeting will start end of this coming week
Please advice

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  1. user-6053472 | | #1

    Please see attached Roof view

  2. user-6053472 | | #2

    Wall & Roof section Colored hand SK

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    progress picture

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  8. Expert Member
    ARMANDO COBO | | #8

    IMO, the best way to avoid condensation and dew point issues, is to install 3" min. of polyiso rigid foam insulation (R20 min), 2-1 1/2" foam boards with staggered taped seams, above the 5/8" Zip Roof Sheathing (they don't have 3/4", unless you are using floor sheathing for roof sheathing) on both the pitch roof and the flat roof, then I would spray 8"-8.5" Open Cell Foam (R29) under the roof decking on both roofs. You need an R49 total MINIMUM!
    You then can install 2-2x4 perimeter roof nailers, at the overhangs, to wrap your WRB, fascia and flashing; and finally, you can install your slate over battens on the pitch roof and TPO for your flat roof.

  9. GBA Editor
    Martin Holladay | | #9

    I wouldn't design a roof this way. That doesn't mean your roof will fail -- just that it's strange.

    Sandwiching your roof sheathing between Grace Ice & Water Shield and closed-cell spray foam (as you plan to do, if I understand correctly, for the low-slope portion of your roof) means that the roof sheathing can't dry in either direction. That said, people build roofs like this all the time. If you go forward with the plan, make sure that the roof sheathing is very dry on the day that the spray foam contractor is encapsulating the foam -- because the moisture in the sheathing will stay there forever.

    A better approach to this low-slope roof (in my opinion) would be rigid insulation above the roof sheathing, combined with fluffy insulation (cellulose, mineral wool, or fiberglass) below the roof sheathing. In your climate zone (Zone 4), you would need a minimum of R-15 insulation above the roof sheathing to make this approach work. You mention that you are installing 4 inches of "tapered insulation" above the roof sheathing (without telling us what type of insulation it is). Assuming that there is a minimum of 4 inches of rigid foam insulation above the roof sheathing, you'll be OK -- and you can skip the spray foam under the roof sheathing, and instead choose a type of insulation than can dry to the interior.

    For more information on this approach, see these two articles:

    Combining Exterior Rigid Foam With Fluffy Insulation

    How to Install Rigid Foam On Top of Roof Sheathing

    When it comes to slate roofing, I'm more old school than many architects. It's true that 5/12 pitch can work, but I prefer 8/12 or steeper. In any case, I think that you should always install slate over skip sheathing, not just Zip sheathing. Slate is expensive roofing, so why cut costs where it counts? Install skip sheathing (1x4s or 2x4s, spaced according to the courses of slate) above the Zip -- you won't regret it. (The skip sheathing allows the slate to dry quickly, which will extend the lifespan of the slate.)

  10. user-6053472 | | #10

    Thank you Martin ,
    As I mention in my post , I will not use Grace Ice and Water shield over entire roof , Just where flat roof meets pitched roof and 24" in Valleys and over the edge of Eve as Zip Manufacturers recommends due to warranty .
    As I spoke to technical department @ Huber, Zip sheathing is design to breathe outside .
    Over the entire Zip sheathing we are planing to use vapor permeable roof underlayment (Delta-Foxx ) from Cosella Dorken which also breathe
    and than Slate will go on top of Delta Fox
    So roof sheathing will actually Breath toward outside ( this is what I think and It does not mean that I am right )
    The only reason I am using 3" of closed cell insulation on every wall and roof is to make sure that house is air tight and gain more true R value ( R18)
    I know that air tightens could be achieved by carefully paying attention when taping windows and connection btw roof and walls .. this work will be done by my Builder who is great guy but nobody can guarantee that will be done right

  11. GBA Editor
    Martin Holladay | | #11

    I may have misunderstood -- your use of the word "subflooring" instead of "roof sheathing" is confusing -- but it sounded to me like the low-slope portion of your roof would be 100% covered with Ice & Water Shield.

    Obviously, this is your roof. You can build it your way if you want to. However, by asking questions on this site, I thought you were looking for advice. You may have perfectly valid reasons to prefer your details to the details I suggest.

  12. user-6053472 | | #12

    You are right Martin ,
    On flat roof Zip Sheathing cannot be used , so in lieu we decide to use 3/4" Advantex sub flooring ( have better rain resistance than regular CBX or OSB )
    Flat roof will be fully covered with Ice and water shield as first layer before 4" Insulation

  13. Dana1 | | #13

    The sloped roof pitches & valleys dumping onto a flat roof and blocked on the drain end by an exterior chimney is a mistake. (What could go wrong? :-) ) Despite the generally temperate climate, this area can get over a half-meter of heavy wet snow in one go. After a big nor'easter you will occasionally get waist-deep or higher drifts in the valleys and on the flat roof.

    You don't need closed cell foam insulation for air tightness. Open cell foam at 3"+ on plywood or OSB sheathing is every bit as air tight as closed cell foam. When the thermal bridging of the framing is factored in the performance gain of the higher R/inch is almost vanishingly small. That 3" of closed cell foam buys you at best another R1.5 "whole wall R" in an advanced-framing/low bridging wall. (You could get the same thermal benefit by adding 1/2" of continuous EPS over the sheathing.) It's 4x the polymer of half-pound open cell foam, and the global warming footprint of the HFC245fa blowing agent is about 1000x CO2, compared to the effectively zero impact of the water used to blow half pound foam.

    In climate zone 4A you only need 30% of the total center-cavity R to be on the exterior of the roofing to be able to use class-III vapor retarders on the interior. You can get to code-min R with 3" of polyiso or 4" of EPS above the roof deck (both blown with pentane, at ~7x CO2, much of which is recaptured and burned at the factory for process heat rather than emitted into the atmosphere), and 10" of open cell foam on the interior. At 10" most open cell foam is 3-4 perms already a class-III vapor retarder on it's own, with no further treatment of the interior to lower the vapor permeance would be necessary for protecting the OSB. If you go higher-R than that it should be done proportionally.

    +1 on the skip-sheathing under slate recommendation.

  14. Expert Member
    Dana Dorsett | | #14

    On the snowfall front, I thought I might have been exaggerating (if only a bit), however there have been 6 snowstorms of roughly a half-meter or more in nearby Central Park over past 20 years:

    The fact that six of the ten all time deepest snow storms have been in the past 20 years may suggest a trend.

    As configured, that flat roof section is guaranteed to collect deep snow drifts no matter which direction the wind blows during those storms.

  15. user-6053472 | | #15

    Thank you Dana ,
    so what would be your hard recommendation for flat roof/valleys/chimney/snow accumulation issues
    At this stage I cannot afford to change construction or design of the house ...
    Should we install electric heated cables to melt snow ...if needed ?

  16. Expert Member
    Dana Dorsett | | #16

    That may be how you have to go, unless you think you can hire people to shovel it off when it gets too deep.

  17. user-6053472 | | #17

    Thank you Dana ,
    This would Work , and will be easy to do
    What I meant ask you earlier is to explain me simple how would you in my case insulate pitch roof and Flat roof since you understand How is house constructed
    Also If not to much to ask I would like to have your opinion/Advice on My exterior wall and rain screen system

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  21. user-4524083 | | #21

    Joe - I read your post last night and didn't have the heart to reply. I know that you are in the middle of this project and basic things cannot be changed, but the choice of this architect was not the best, I'm afraid. The first thing I saw were the things Dana pointed out: 3 roofs draining into a wide flat roof that slopes toward a chimney. I hope this architect knows more about energy efficiency than water management. The second thing I noticed is what Martin points out - I would never use slate on less than a 9/12 roof, though I have seen pictures of houses that do( I can guarantee that none of them were in Maine or Vermont!). Use a 4" headlap instead of the normal 3". If there is any way to increase the slope of that flat section even at this late stage, I would highly encourage it. A foot of wet snow, as Dana points out, will find even a pin hole to cause a leak. This is, IMHO, a much bigger potential problem over the life of this building than failure due to condensation ( not to minimize that potential). The flashing job on that chimney also better be completely water tight. You have a 3 sided swimming pool that is slowly draining water. You may want to run the design by an experienced architect or engineer as a precaution before going too much further ahead. I apologize for my negativity, but this design looks to me to be a problem waiting to happen.I wish you the very best of luck with this project.

  22. GBA Editor
    Martin Holladay | | #22

    I agree with Kevin and Dana that you (and your roofer) face some big challenges with the low-slope roof and the chimney. In my article on roof design (Martin’s Ten Rules of Roof Design), I wrote that "chimneys ... should penetrate a roof near the ridge rather than near the eave." But at this stage, this advice isn't helpful.

    When I worked as a roofer, we were occasionally called in to solve problems like this -- steep roofs that dumped snow, water, and ice onto a low-slope roof. The only dependable long-term solution is to install a soldered copper roof. The soldered copper roof should extend up the slopes of all the adjacent slate roofs, creating a 4-foot-wide band of copper at the eaves of the sloped roofs that dump onto the flat roof, and every single seam should be soldered. The flashing for the chimney also needs to be soldered copper; ideally, you would find a roofer who understands what "through-chimney flashing" means. (It's a type of chimney flashing that creates a pan that interrupts the bricks with copper -- that's much better than inserting the flashing into slots in the mortar.)

    There aren't many roofers who do this kind of work anymore, but in high-income neighborhoods with 100-year-old homes, you can sometimes find such roofers.


  23. user-6053472 | | #23

    Thank you All.. Martin/Dana/Kevin for very valuable comments , that's why I decide to come to this Site seeking for advice's !!
    Before is to late as you see stage of construction is going currently
    I will try to combine all of your advice's into one and keep current design approach
    As I see flat roof is one of the biggest issue here and slate vs 5/12 slope on pitched roof

    New approach on Roof from what I learn on this site :
    Unvented Pitched roof :
    If we swap Slate for Asphalt singles( I will need advice on this ) installed on same structure as follows
    -3/4 Zip sheathing ( material is purchased already )with Ice and water shield or equal (Must be installed 24" minimum all around perimeter of Pitched roof and in valleys as per HUBERT manufacturer of zip Sheathing direction)
    -Over the entire Zip sheathing to use vapor permeable roof underlayment (Delta-Foxx ) from Cosella Dorken which also breathe toward exterior same as Zip sheathing :see link :
    Below roof zip sheathing and Btw and over 12" Rafters fill up whole cavity with open cell insulation (total value of R 43 ?)
    On exterior and exterior portion ( overhung cavity ) where wall meets roof we will spray 3" of closed cell insulation to help airtight house ( I read that this area and around windows is most critical for air leakage)
    -Attic floor will be without insulation because , ERV supply/ return manifolds for 2nd floor and AC cassettes / duct's for 2nd floor will be in attic and interior temperature needs to be balanced throughout house

    Flat roof ( low pitched roof ) :

    -3/4" Advantech subfloring ( installed already as you see on earlier posted pictures ) over 9-1/2" LVL beam structure
    3/4" sheathing will be covered with Ice and Water shield membrane ( please advice if you have any better product in mind ) as first layer to protect interior while roof is being build
    Over this membrane we will install 4" of EPS insulation ( or any better ) throughout flat roof )
    next layer will be tapered insulation sheets starts from 1" at chimney and gradually increasing towards pitched roof over 30'
    -Increase pitch on low pitch roof from 1/4" per 1-0' to 1/2" per 1-0' or 3/4" per 1-0'
    On top of tapered insulation we need to install primary water membrane ..
    I like Martin's idea with Copper pan and I think that is correct approach for my problem but is there any other durable material , Plastic , Rubber ..etc black color ( Reddish color of Copper is not really what we had in mind designing this house )
    I will have gas fireplace with only 4" exhaust pipe and not generating much heat Chimney can be 100% wrapped in any suggested waterproofing behind stone veneer rain screen
    - Below flat pitched roof Whole cavity of 9-1/2" LVL's will be filed up with open cell insulation
    -We can use electric heating cables throughout of flat roof so it can be used to melt ice and snow if needed
    Please let me know your opinion/Advice on my new approach above

  24. Dana1 | | #24

    Again, 3" closed cell foam isn't any more air tight than 3" of open cell, is a seriously low-permeance vapor retarder, and has at best marginal thermal advantages over open cell when placed between framing. And it has a fairly hefty environmental hit.

    In zone 4A walls do not need vapor retarders other than standard latex paint, especially when there is vapor permeable exterior insulation and/or a rainscreen (and you have both!) With as little as R15 of rigid foam above the roof deck (cheaper than sprayed closed cell foam) you can install up to R34 )9" of open cell foam on the underside of the roof deck for air sealing to hit code-min.

    An all open cell R43-ish foam solution of fully filled 2x12 rafters under the roof deck has potential moisture issues at the roof deck and doesn't quite meet code min without foam above the roof deck. If you go more than R34 under the roof deck, the above-deck foam has to grow proportionally, since it's the ratio that determines the average temp (and moisture content) of the roof deck. With a full 2x12 cavity fill of half pound foam would require R19 mininum above the roof deck, so you're looking at 5" min on the EPS (or 4" if polyiso).

  25. GBA Editor
    Martin Holladay | | #25

    If you want a vapor-permeable roofing underlayment (and, on an unvented roof, that may be required, since most manufacturers of vapor-impermeable underlayment don't allow their products to be installed over unvented roofs), you can use asphalt felt. Asphalt felt has a variable permeance (and costs a lot less than the product you're thinking of using).

    Don't worry too much about outward drying under asphalt shingles. The asphalt shingles are vapor-impermeable, so you won't get any outward drying in any case.

    Like Dana, I think that it makes more sense to install rigid foam above the roof sheathing than spray foam between the rafters. You should think about this carefully; the use of open-cell spray foam on the underside of roof sheathing is associated with moisture problems. For more information, see Open-Cell Spray Foam and Damp Roof Sheathing.

    If you follow my advice and install a soldered copper roof on the low-slope portion of the roof, you shouldn't need any heating cables.

  26. GBA Editor
    Martin Holladay | | #26

    You wrote, "the reddish color of copper is not really what we had in mind designing this house."

    Two points:

    1. Copper doesn't stay shiny for long. The eventual color is a dull brown color.

    2. The only people who will ever see this low-slope roof are people in helicopters and roofers trying to address the leaks -- so you shouldn't worry about the color.


  27. GBA Editor
    Martin Holladay | | #27

    One more roofing issue I just noticed -- you are planning second-floor decks with living space below.

    I really hope (a) you have good details for these decks and the low-slope roofs under the decks, and (b) you choose a good roofer.

  28. user-6053472 | | #28

    Thank you Dana,
    Due to design of the house we want to keep size of Fascia about +/- 10" total and if we add some rigid EPS insulation at this stage on top of pitched roof , my roof fascia will grow on height .
    Rafters are cut already and 9-1/2" LVL fascia is installed adding height , we will have to extend eve.
    We really want to keep design intent / possible passive house certification and at the same time build long lasting roof which is even more important than any of above
    Dana I see that you are very knowledgeable with Green building , what do you overall think about our approach to meet Passive house standards( or come close to ) ..not just the roof but walls as

  29. user-6053472 | | #29

    Thank you Martin as well '
    I really want to make this design work , specialty in this stage of construction as you see on pictures posted earlier
    What is your opinion on substitution of slate tiles with asphalt shingles ...better approach in my application due to 5/12 pitch ?
    Now days they all kind of liquid resin poured products ( in lieu of copper ) what is your opinion regard this approach ?
    I know that you are old school and copper is definitely best solution in my case but as you mention yesterday I see potential problem with installation .. not many installers is left out there that know how install properly this large copper pan

  30. GBA Editor
    Martin Holladay | | #30

    I love the look of slate, but to me slate would look odd on a 5/12 roof. Everyone's idea of beauty is different, however.

    Asphalt shingles won't win any beauty contest, but they are affordable and they won't leak.

    None of my advice is useful if you can't find a competent roofer who can perform the work. If you don't think a roofing contractor is available to follow my advice, you'll have to talk to your roofing contractor and come up with a mutually agreeable way to go forward.

    I'm wondering who is on your team. I assume that you are the homeowner. Is that correct?

    Do you have an architect?

    Do you have a general contractor?

    Do you have a Passive House Consultant?

    If you have people in these three roles on your team, are they providing good advice and help?

  31. Expert Member
    Dana Dorsett | | #31

    You can probably hit PassiveHouse levels of air tightness in this house, but you'll need more insulation and a much more careful analysis of the energy flows to come anywhere near PassiveHouse standards on energy use.

    The only way to know for sure is to enter the design into the PassiveHouse tools and start tweaking the details until it works. Most of that work would normally have to happen before the house was even framed in order to have a decent shot at it. Looking at the roof view renderings and other view there is a lot of corners and additional complexity to the framing, which generally means more thermal bridging, which would then require higher-R to compensate in a PassiveHouse design.

  32. user-1072251 | | #32

    I have a suggestion - substitute Advantech for the Zip sheathing. Both made by Huber, but Advantech is high resin & pretty water resistant, while Zip is OSB with paint. OSB is probably the worst product for your application IMO. If you want to tape the seams, SIGA,Tescom Vana and 3M8067 work well.

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