GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Township Building Department & Unvented Roof Assembly

user-270695 | Posted in Green Building Techniques on

I am a few weeks away from beginning a roofing/insulation project on a cape. I went into my local code enforcement office to pull permits and the official told me that the code requires ventilation unless I am using spray foam.

The proposed roof section goes like this, from the top: shingles, 15 pound felt, Hunter Nail Base (2 inches of polyiso laminated to half-inch CDX) installed over old roof deck , old roof deck, 2 x 10 rafter bays dense packed, sheet rock. At the rafter/top plate intersection we will be installing CCSF to join ( as best as possible) the thermal and air pressure boundaries. The cellulose will be installed from the roof, not from the interior. We will also be adding overhangs to the entire house.

When I explained the proposed assembly, he seemed to understand and asked me to provide a roof section. Reading between the lines, it seemed as though he was prepared to accept this assembly with some documentation.

My goal here is to provide a meaningful, thorough, and repeatable energy upgrade to this 1950s Cape. There are thousands of these homes locally. I want it to be minimally invasive and will be doing as much of the work as possible from the topside on the roof. The small second floor attics will require gaining access from the interior knee walls. In these small attics we will be packing out the 2×8’s by 2 inches and installing foam blocking and sheet rock to contain the dense packed cellulose. The R-value of this assembly is 50 (not the Whole Roof R-value).


1. Can anyone direct me to a similar roof section that I could alter or use as is for the purposes of obtaining building permits? I will sketch one if needed, but providing one from a known source would carry more weight.

2. Does anyone have ideas on how to minimize the invasive nature of this work? It seems to me that the work behind the knee walls is the only reason we have to go inside. In a perfect world everything would be done from above. There is also the RRP rule consideration. If I could figure out how to retrofit these homes, as illustrated above, without subjecting myself to the RRP rule or having to go inside, I think I could keep the costs down and have a better chance at selling this approach.

3. Any comments on the effectiveness of this strategy, the assembly itself, or tactics would be greatly appreciated.

Many thanks

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. wjrobinson | | #1

    Vent over the foam. The layers from start to finish;

    -non nailbase foam taped, two layers better than one.
    -felt paper
    -ice and water shield all hazard areas, valleys, side walls, up skylights, and chimneys. etc
    -40 year shingles or whatever you prefer
    -the ridge vent sold in your area Riversong likes a certain one. Air Vent product I think, search it.

  2. wjrobinson | | #2

    Your picture is worth a million word ... post, Robert.

    See the snow.... ice and water shield is your friend there. low slope porch transition areas leak like the dickens, the heat from chimney pipes, valleys, sidewalls, and the worst thing I see is there are combinations of problem areas on top of each other.

    I just gave an estimate to repair a back entry. The design is the worst of all worlds. A valley ends above an exterior OUT opening wood door with a large PT wood landing. The water hits the deck and splashes onto the door and everything and people using the door. Ridiculous design. The door on this $700,000 dollar all cedar/Sikkens home cost $120.00 at Lowe's and had no water resistant features that a top end $450.00 Thermotru has. Two wrongs.. make a mess and a job for me after only 3-4 years since built new.

  3. user-270695 | | #3

    Reply to post #1

    Thanks AJ. The point is, however, to not vent. If you look at the picture and notice the valleys and dormers (on the rear as well), you'll realize that continuous, balanced ventilation is not possible. Additionally, the assembly I originally posted is more efficient in terms of labor and material cost.

    If I'm missing something here with your idea, please clarify it. I'm here to learn.

    Thanks again

  4. user-270695 | | #4

    Reply to post #2

    AJ- Sounds like you have your hands full with that poorly designed entryway. Good luck. No gutters?

    The house pictured has no history of any roof leaks. The roof is old and in need of replacement. It is the only opportunity this house will have (for the life of the new shingles) to insulate the roof plane in a way that doesn't put the sheathing at risk of rot.

  5. user-270695 | | #5

    Robert R.

    Perfect. Thank you.

  6. Riversong | | #6

    2007 Supplement to the IRC - three options for cathedralized roofs:

    1. Air-impermeable insulation only - air-impermeable insulation shall be applied in direct contact to the underside of the structural roof sheathing.
    2. Air-permeable insulation only - in addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.
    3. Air-impermeable and air-permeable insulation - air-impermeable insulation shall be applied in direct contact to the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

  7. wjrobinson | | #7

    Robert, Where I live in Upstate NY, we have had our share of failures when foam sheeting is anywhere in a roof assembly. A longterm friend who has built only foam sheeted exterior roof assemblies would never build without adding the vented section I described.

    Bruce Brownell is an engineer and only builds with foam sheeting. He is to me the number one person to work with to develop your system. Call him tomorrow and you will be doing yourself a huge favor.

    Also check out BSC and their thoughts.

    Here is BSC and doing your roof your way and in regard to Robert's post and the code.

    and read this "Understanding Attic Insulation" by BSC.

    This last document above talks about the trouble we have had up our way with unvented sheet foam roofs. The conservative approach is to vent above foam sheet and that's what my one builder friend does and depends on for no call backs.

  8. user-270695 | | #8


    BSC basically confirms the IRC referenced above by Robert R. These are taken from the 2nd and 3rd link you posted. I appreciate the referral to Mr. Brownell. How would I get in touch with him>


  9. Riversong | | #9


    Remind me what climate zone you're in.

    Ignore AJ. As usual, he doesn't have a clue. And forget Bruce Brownell. Just the fact that he's AJ's God of Foam Construction should be enough reason to keep him at a healthy distance.

    The roof you depict clearly cannot be ventilated. Follow the code, make sure there's a good air barrier below the cellulose and you'll be fine.

  10. user-270695 | | #10


    We are in zone 4, near the northern terminus. Again, planning on 2" polyiso. on roof deck.

  11. Riversong | | #11

    2" Hunter panels would be a little light to meet the IRC requirements. According to Hunter R-value specs, you would need at least 3" and would be safer with 3.5" panels - especially with 2x10 dense-packed rafters.

  12. wjrobinson | | #12

    Robert Post, as you noticed, BSC is very informative to do with the roof you desire. The climate you post is not extreme which is what BSC talks about when they say vent above foam sheating. I and the two builders I refer to build in extreme conditions deep in the Adirondacks. Roofs have failed that were not vented and or detailed as to not fail.

    Robert R. trashes myself and Bruce. Bruce has built and designed more homes than one would have to, to prove out a system. Not sure why Robert hates Bruce, like he states... because I bring him up. LOL Real adult behavior... LOL

    There are millions of homes worldwide. Robert R. has awesome ideas of what we should be doing for some of them if not all eventually. I like his natural green yap. Some times those you like hate your guts. And we never dated, go figure.

    I think a talk with Bruce would interest you. I also think the BSC unvented roof details will serve your need perfectly being that you are in climate zone 4.

    Adirondack Alternate Energy (Bruce Brownell)
    98 Northville Road
    Edinburg, NY 12134 Phone: (518) 863-4338
    Fax: (518) 863-4192
    E-Mail: [email protected]

  13. wjrobinson | | #13

    Since you are going with foam, you also can try Spider insulation instead of the cellulose . Chandler likes it. I am not against using it. BSC is the best source for your build.

    You should use thicker foam as per BSC pdfs I posted for you too.

  14. user-270695 | | #14

    Here is the info from the IRC reference. What is the difference between 4a, b, and c?

    Climate Zone Minimum rigid board or air-impermeable insulation R-Value
    2B and 3B with tile roof only 0
    1, 2A, 2B, 3A, 3B, 3C R-5
    4C R-10
    4A, 4B R-15
    5 R-20
    6 R-25
    7 R-30
    8 R-35

  15. Riversong | | #15

    Not sure why Robert hates Bruce

    I don't "hate" Bruce, since I don't know the chap. But I've seen his deceptive and ignorant website.

    I'm not sure why we need to go through this again, but for those who missed the last round, here's some quotes from Adirondack Alternative Energy (Bruce's website):

    What is passive solar?
    There are 3 major components to the "Low Energy Requirement Home" system:
    1. A six-sided performance R38 insulation envelope. It consists of 2 separate layers of 2" thick Thermax (a foil covered urethane) with joints staggered and foil taped. This results in a structure that needs less than one-seventh the energy of the typical new house.
    2. The long house wall orientated south with more glass south (not a huge amount) and less glass in the north wall.
    3. The excess winter sunny day input (which could result in over heating) justifies an air-integrated heavy mass storage system. A fan moves air from the higher house areas, down an airshaft through high capacity MERV 8 filters, and through an extensive grid of air pipes located in mass storage consisting of 12" of concrete under the lowest floor.

    This "passive" solar system requires a fan of up to 1,000 CFM running 24/7 and an underground remote storage medium, which makes it an active solar system with a high electrical demand (and a system that will fail during a power outage).

    Insulated and sealed on all six sides with 4" of polyisocyanurate (foam board) yielding a performance level of R38, 7 to 9 times higher than typical new homes.

    The best you can get from polyiso foam board is R-7.2/inch, and some sources give it R-6.8 with an aged R-value of as little as 5.5/inch. Brownell's claimed wall R-value would require R-9.5/inch.

    Even at R-7.2/inch, Brownell's 4" would offer no better than R-28.8, which is only 44% more than current zone 6 wall requirements and barely 60% of current ceiling/roof requirements, which makes his envelope underperforming by current code minimum standards - hardly "7 to 9 time better".

    An integral air handling system powered by a small fan gathers air at the peak of the house and pulls it down an airshaft to be delivered, through a grid work of pipes, buried in a heavy (70-100 ton) mass storage system under the lowest floor, and back to the house interior perimeter.

    This is a system that has proven problematic because of moisture and mold accumulation and adds considerable construction expense while being less useful than a simple 4" concrete thermal mass floor.

    Air Quality: Healthy high winter humidity levels of 45-50 % are maintained at all times. Filtration 3 times per hour of all indoor air eliminates most dusting. Steady temperatures changing only 4 - 6 F. in 24 hours because of mass tempering.

    In cold climates, indoor relative humidity above 40% is almost always problematic. And, as mentioned, his "small fan" would require an output in the neighborhood of 1,000 CFM to perform 3ACH in a typical house.

    Self-regulating system allows owners to leave for any winter months by simply turning the key, the house will never freeze. Excellent for a second home or ski lodge. Retired people enjoy the constant warmth, maintenance free operation and minimal fuel bills.

    No active system is maintenance-free, and a power outage or fan failure would result in a drastically overheated house when the sun shines (particularly given that Brownell doesn't believe in shading or interior mass).

    Why don't you follow the common design parameters of direct gain; shades and shutters, south over hangs, air-lock entries, etc.?

    Our superior six-sided performance R38 insulation envelope combined with modern windows negate that thinking- it is simple physics. You simply live in the house-passive solar works. We never have adhered to the old traditional rules in our designs.

    Simple physics, and thermodynamics, is what those "old traditional rules" of passive solar design were based on, and that's why they work for everyone else.

    Aren't your homes too tight?

    A key component of our energy efficiency is our sealed envelope which is many times tighter than a typical new homes and will blower door test at 0.2 air changes at 50 pascals. We have no interior combustion- no gas appliances allowed, airtight wood stoves are vented outside and a fan moves all interior house air 3 times an hour through high capacity MERV 8 filters. Most importantly, we are totally sealed from moisture with no dampness and not needing any humidifiers or dehumidifiers.

    I doubt that Brownells underinsulated homes are three times tighter than Passive House standards, and the fact that they are tight and don't include whole-house ventilation systems guarantees poor indoor air quality and a high probability of moisture problems.

    Do you need backup heat or cooling?

    You don't need a furnace to supply the small amount of heat needed as backup. Often a zone is taken from your domestic hot water supply to supply heat to a coil (like your car radiator) in the airshaft, heating the air going to the mass, then the house.

    Brownell claims that his under-insulated homes need no more supplemental heat than a Passive House, insulated to 2 or 3 times the R-value. And, if the heat goes first into the mass, it will have a very slow response time and will almost certainly lead to localized discomfort.

    As far as the information on Brownell's website indicates, this man is a complete fraud who understands little about hygro-thermal engineering or building science. Perhaps that's why AJ worships him - they are two of a kind.

  16. Riversong | | #16

    What is the difference between 4a, b, and c?

    Look at the IECC climate zone map at the top of the Q&A page (that I sent in to Dan). A is the moist region of all climate zones, B is the dry region and C is the marine region.

  17. user-270695 | | #17

    AHHHH. I get it. I see that R-15 is the requirement in my zone. Thanks.

    What type of vapor retarder would you place between the NB panel and old roof deck?

  18. Riversong | | #18

    I wouldn't use bituthene, but any of the new low-perm roofing underlayments should work fine.

  19. wjrobinson | | #19

    Robert, If I have to, I will get Bruce and you together at one of his homes. You make him out to be the biggest threat since the saber tooth tiger. If you met him and observed a build of his homes and checked out a built home, you just might think they are quite a bit better than what most builders are building nationwide.

    His R value by the way states "a performance level of R38" Bruce goes way back to the early days of superinsulation. I don't think he updates the site too often but he has changed his home designs over the years.

    Hey, I worship your Riversong Truss too Robert. What's that say about you? ha haa... ho ho... LOL

    You are so silly Robert.

    I suggest a visit to Bruce for anyone to see for themselves and then a visit to Robert at his amazing cabin after for a dip in his hot tub. You may not be wanting a copy of both homes. I will let you decide.

  20. wjrobinson | | #20

    Robert P, follow the BSC advice. The brain power at that company is not matched by many. They do what they do for a living and on a huge scale nationwide. Try to catch a seminar where Joe L speaks. No one is more entertaining or informative. I saw him recently and first met him when he was just a youngster. Great guy.

  21. Riversong | | #21


    You're quite right. Your Foam House God says "4" of polyisocyanurate (foam board) yielding a performance level of R38"

    So, in addition to displaying a profound ignorance of passive solar design principles and thermodynamics, Bruce Brownell also propagates the foam industry hype about less R-value performing like more.

  22. GBA Editor
    Martin Holladay | | #22

    Robert Post,
    I'm coming late to this thread, but I just wanted to say that Robert Riversong is giving you good advice.

    Robert Riversong -- thanks.

  23. user-270695 | | #23

    Once again, Thank you Robert.

    I believe I understand the assembly to be, from the top: shingles, felt, 3.5" Hunter NB, low perm vapor barrier, old roof deck.

    The Hunter NB comes with OSB or plywood. Based on what I've read here, I believe plywood would be the more durable choice because of its hygroscopic properties. Is this correct?

    I can see a few basic options for attaching the new overhangs. I can not return the overhangs with a flat or "closed soffit", they need to be "open soffits". This is because that window head on the left is right under the roof line.

    some options:

    1) Sister 2x4's to the existing rafters and layer the assembly on top resulting in a total thickness of close to 8-9 inches. Esthetically, I have reservations with this.

    2) Attach 2x4's on to the old roof deck as "outriggers" and then install the NB. We would need to notch the NB around the outriggers so it would sit flat on the old roof deck. A bit cumbersome install resulting in some thermal conductivity.

    3) Remove 34" of old roof deck and replace with plywood, overhanging by 12-14". then proceed with the NB and other layers. At the 14" overhang, I guess we would need to use adhesive to secure the NB to the plywood. Esthetically, I like the overall thickness of this approach (around 5"), but what are your thoughts of it's structural integrity?

    4) I'm even considering angle iron or aluminum sistered to old rafters to support the NB and plywood described in #3. This would only add 1.5" (after cladding) to the assembly; a smaller esthetic penalty resulting in ample structural integrity, I believe.

    Removing the plywood described in #3 would allow us to get into the knee wall attics, but I can not envision this being completely effective because in order to foam the rafter-top plate areas, the roof deck needs to be on. We could install most of the drywall under the rafters to contain the dense packed cellulose and even foam most of the rafter-top plate areas at the eaves, but we would need to "get out" of the attic and patch in at least 1 rafter bay. Maybe we can inject foam into this last bay to air seal and insulate the rafter-top plate area.

    I"m still trying to figure out how to do the whole retrofit from the top, with no need to go into the house. If I can figure this out, I'll be very happy. The weather will be the potential obstacle, but we deal with this on all types of exterior work.

  24. Riversong | | #24

    Plywood might be marginally more durable than OSB on top of the foam panels, but the difference would not be as important there as when used as a wall or roof membrane exposed to moisture flow from the inside. It's really just a nailbase in this case.

    I think the most practical strategy would be option #3. Assuming that the existing roof deck is 3/4" boards, replacing the bottom (and rake?) section with 3/4" CDX would be more than sufficient structurally. I would probably use construction adhesive in addition to nails to secure the plywood to the rafters.

    This option should allow most, if not all, of the attic access you need. Assuming there is very little HAP (height above plate) on the existing rafters, you should be able to drill holes in the plywood above the wall plate and inject foam to create an air seal. Depending on location of knee walls in relation to the outer walls, you may be able to install blocking to contain cellulose in the slant ceiling and blocking between joists under the kneewall for an interior air barrier.

    An alternative would be to use SIPS rather than nailbase foam panels, but if you can access the kneewall area from the outside by removing the bottom of the roof sheathing, that sounds like an excellent plan (as long as the weather cooperates).

  25. 2tePuaao2B | | #25

    I still can't visualize the overhang detail that will be added ~ how will you keep the window head from reading as chopped?
    I know that this is a little "out of the box", but as I suggested in earlier post, a carefully detailed eybrow balanced above the window could possibly kill two birds with 1 stone here(pun intended)~ especially if the removal of sheathing will take place. Here's a photo of eyebrow use to add light in third floor living space.
    A gentle curve to a house of angles sometimes compliments.

  26. user-270695 | | #26


    Eyebrows are a distinctive detail that I love. In the case of this project, the proximity of the valley is a problem. The eyebrow would be very close to, if not running into the valley and loose it's esthetic value AND there is the complexity of installation and flashing. A shed detail would be easier to install, but still be an issue adjacent to the valley. I'd love a solution for this, however, I have seen old homes where the open soffits do encroach a bit upon windows. Maybe I can figure out a way to over-frame some articulation without the risk of water issues

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |