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Community and Q&A

A low slope, cathedral roof in Zone A-4: builder says he can vent it

Michelle Paninopoulos | Posted in Green Building Techniques on

Hi! I’ve been following a variety of discussions about ventend/unvented assemblies in cathedral ceilings, in low slope roofs, in different climates, on FHB, JLC for close to two years. I’ve read Martin, I’ve read Lstiburek, I’ve talked to the architects (hopeless) and structural engineers. I thought I had this nailed down, and now the builder (who also happens to be a SE – long story), comes at me with a new idea.

So I’m finally going to “vent” my worries and seek some advice specific to our situation.

This is new construction with a big, simple shed roof, slope of 1.5″:12″ (7.13 degrees, or 12.5%) There will be 3 skylights, which cut across the slope/plane of the roof, so they will block/interrupt some 15-20% of the rafter bays. With overhangs, the roof is about 44 1/2’ long and about 47’ wide. I’m attaching a sketch which shows the shape and proposed framing.

Currently we are planning for the entire second floor to be cathedral ceiling, because there is little sense in framing in an attic floor due to the low slope of the roof. There is no need for ceiling penetrations except for bathroom exhaust fans and the skylights (one of which is in a shower with a cathedral ceiling, just so you know). We’ve found flush light fixtures we can live with, so no recessed cans.

We are just outside of Philadelphia, in climate zone A-4. Roof system will be modified bitumen. We are not using spray foam in the house, so don’t recommend that. Walls will be insulated with cavity batt with 1” continuous ISO exterior.

I thought that the best way to handle this roof would be an unvented, “hot” assembly with 3″ of ISO above the deck (code requires R-15 here), and fiberglass cavity insulation. No vapor barrier to the inside so that the assembly could dry to the interior. I assumed GWB with proper sealing around the few penetrations would be a sufficient air barrier (would it?)

Our builder/SE is now encouraging us to consider venting the roof assembly. I think most of his reasoning relates to the cost of the ISO. What he wants to do is to place 2x furring crosswise against the roof framing to create a ventilation channel above the (R-38 batt) insulation and under the roof decking. If I understand it properly, the idea is that those channels are not exactly continuous, so they create a pathway for air to circulate and – I guess we hope – move around enough to actually vent the roof. He’s trying to save us money (he is a construction manager as agent working for a flat fee, so the savings will be ours, not his).

The only thing I’ve seen that is close is Lstiburek’s vented-unvented hybrid assembly which adds a vented air space above the exterior rigid insulation (so still uses the ISO in any event) in high snow load areas to avoid dams. I think that system is prohibitively expensive for us.

We’re going to put this out to roofing contractors with both options to compare the prices. Our builder is sure that it will be far, far less expensive to do it the way he suggests, given the price of 3” of ISO. I’m not attached to one particular solution. I just want it to work and, yes, money is a definite concern. Can the system he is proposing work, ever? Is there any amount or configuration of above-rafter furring that can effectively vent a roof of this slope? Alternately, if we use the unvented assembly I’d settled on initially, what ABOUT ice dams in this climate?

Any advice is greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I like your unvented approach (with polyiso above the roof sheathing) better than your builder's suggestion.

    Since this is a low-slope roof, everything you need to know can be found in this article: Insulating Low-Slope Residential Roofs.

  2. Michelle Paninopoulos | | #2

    Thanks. I did realize you would prefer the unvented assembly. It has been your posts which have led me to fight on for two years with two separate architects (the first of which openly mocked the notion of a "hot roof" in an ugly break up e-mail to me) and two different structural engineers (who at least understand the reason for not wanting to vent, even if they think they can find a way around it)

    I am not sure I understand why there is so much resistance to the overdeck insulation. I realize ISO is not cheap, and requires time to install, and probably (maybe?) requires a cover board (still not sure if there are types of ISO that include a facing sheet which abnegates need for cover board), the cost of which I am unable to even come up with even a broad estimate for. But neither is the builder's proposal necessarily cheap: to use 2x12 framing rather than 2x10 for the roof, for the purpose of accommodating the additional batt, to use R-38 versus R-30 batt, and to construct 2x furring over the rafters, to find a way to locate sufficient openings for ventilation in the fascia/soffit that doesn't look horrible. I'm sure the unvented assembly will cost more to put in, but I'm not really sure how much more. And we get R-45 total, rather than R-38.

    More to the point, if I am reading the article correctly, the proposed vented assembly wouldn't meet code ventilation requirements without installing ventilation cupolas, a non starter for aesthetic reasons.

    One thing which is not addressed in the Insulating Low-Slope Residential Roofs in the article is the issue of ice dams. Any thoughts about that?

    Finally, should I be worried about the shower with the skylight set in a cathedral ceiling in an unvented roof? Is there anything in particular to be careful about given the additional warmth and moisture ...???

  3. GBA Editor
    Martin Holladay | | #3

    Q. "The proposed vented assembly wouldn't meet code ventilation requirements without installing ventilation cupolas, a non-starter for aesthetic reasons."

    A. Without the cupolas (or "doghouses") you won't have a vented assembly.

    Q. "One thing which is not addressed in the Insulating Low-Slope Residential Roofs in the article is the issue of ice dams. Any thoughts about that?"

    A. For more information on ice dams, see Prevent Ice Dams With Air Sealing and Insulation. The two most important measures for preventing ice dams are (a) creating an airtight ceiling, and (b) including insulation that at least meets minimum code requirements.

    Q. "Should I be worried about the shower with the skylight set in a cathedral ceiling in an unvented roof? Is there anything in particular to be careful about given the additional warmth and moisture?"

    A. The issues with bathroom skylights have nothing to do with whether the roof assembly is vented or unvented -- except for the fact that skylights interrupt ventilation channels, a factor that makes unvented roof assemblies a better choice than vented assemblies when skylights are installed. The main issues with bathroom skylights concern condensation, since bathrooms tend to be humid and skylight glazing tends to be cold. Problems will be minimized if the owners remember to use the exhaust fan, and if you specify a skylight with low-U-factor glazing -- ideally, a skylight with triple glazing.

  4. Michelle Paninopoulos | | #4

    Alright, I'm going to keep pushing back on this with the builder and architect and everyone who thinks I'm a PITA. I really think the 3 inches of ISO are more than worth the cost, and in the end won't even cost that much more than what the builder is proposing to do (his plan also will require 2x14 rim around the entire perimeter or that the framer taper each single rafter end).

    But this, I have to admit, I don't think I fully understand: "A. Without the cupolas (or "doghouses") you won't have a vented assembly." I think the builder intends to vent along the perimeter at all four sides, including the high end of the shed.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "I don't think I fully understand: ‘Without the cupolas (or "doghouses") you won't have a vented assembly.’ I think the builder intends to vent along the perimeter at all four sides, including the high end of the shed."

    A. The answer can be found in the article I linked to: Insulating Low-Slope Residential Roofs. In that article, I wrote:

    "The problem is that most of these roofs aren’t built correctly. ... The attic ventilation is inadequate. ... Here’s some advice from Joe Lstiburek, a principal at the Building Science Corporation: ‘If you have an airtight ceiling, and you have an air gap of at least 6 inches between the top of the insulation and the roof deck, and if you have perimeter air coming in at vents at the soffit or fascia above the insulation, and if you also have ventilation openings near the center of the roof through some kind of cupola or doghouse — not just a whirlybird turbine vent — there is nothing wrong with your roof assembly,’ Lstiburek told me recently. ‘You can build a 2 foot by 2 foot doghouse that sticks up a few feet, and put in some rectangular vents.’ "

    In case what I wrote wasn't clear, I'll elaborate. With a low-slope roof, a conventional venting strategy that depends on soffit vents and "ridge" vents (vents at the top of the shed roof) doesn't provide enough air flow to keep you out of trouble. Failed roofs have taught us that you need the doghouse in the center of the roof.

  6. Michelle Paninopoulos | | #6

    Thanks again. I had read all of those things, more than once. I totally get that the roof won't be adequately vented. I was curious about your claim that it won't be a vented roof, period. The subtext here, in case it wasn't coming through, is my working up an "air-tight" case to explain to my builder why I do not want to pursue the venting strategy at all. It will make life much simpler if we don't bid out alternatives, if the architect doesn't have to be thinking about whether to redraw with 2x12 rafter now or later, etc. So that is really why I followed up. If I say, "look, this isn't vented well enough," that's just my opinion against someone with years and years of experience. If I can say, look ,this plainly doesn't meet R806.2 for net free ventilation, period, that closes the book on it and we can move on. I have no idea how the calcs under R806.2 work but I may call the field inspector today - he should be around - and get his take on it. He'll be clear about whether it meets Code or not. That might be the way to go here.

    Thanks again for your responses. I hope this can help someone other than just us someday!

    Thanks again for your follow up.

  7. Expert Member
    Malcolm Taylor | | #7

    Michelle, Get an engineer or architect who has some commercial experience. Almost very store, office or public building you visit has an unvented flat roof assembly similar in composition to what you are suggesting. If they didn't work Costco and Walmart would be out of business.

  8. Michelle Paninopoulos | | #8

    Thanks, Malcolm. My builder is an engineer who actually has years of experience doing the structural design as well as building commercial and institutional buildings. He is very familiar with commercial engineering and building science. He is actually less familiar with residential, and may be resisting the idea of transplanting what he knows from commercial to residential (where decking and structure are lumber, not steel, etc.). Common wisdom says "vent the attic." Low slope cathedral roofs are quite uncommon here in the Philadelphia suburbs, to say the least. (As I alluded above, it is a long story why his company is building our house, but we are fortunate to have his expertise, even if I happen to disagree with his recommendation in this isolated instance.)

    In any event, I made an appointment to sit down with the (very knowledgeable) field inspector to review the requirements of R806.2, so that we can be clear about what we'd actually have to do to vent the roof to Code, as a first step. If it should turn out that he says we'd need more than 2x furring, as well as cupolas in the center of the roof, in order to meet the net free ventilation requirements, then it is all very simple and straightforward to stay with the unvented assembly. I would prefer to avoid arguing the merits of one versus another with building professionals, as I have found that no amount of citation to building science articles is ever sufficient to overcome the presumption that I can't possibly have the faintest idea what I am talking about. It is frustrating, but after two years of fighting these battles, I have finally learned it is best to keep it simple and find a way to rely on the Code to make the point whenever possible. There is a lot I don't know about buildings, to be sure, but this is one I've researched to death, and I cannot find any source that suggests that attemptint to vent the roof in this case would be anything other than foolishness.

  9. Carolyn Wood | | #9

    "I would prefer to avoid arguing the merits of one versus another with building professionals, as I have found that no amount of citation to building science articles is ever sufficient to overcome the presumption that I can't possibly have the faintest idea what I am talking about."
    I've been reading your discussion thread with interest. We have been building a new house with low slope roofs on the West Coast of BC for the past year, during which time I have been educating myself through reading FHB, GBA and the Building Science website. But, like you, no matter what I suggest or where I quote from, my ideas are dismissed because they (the male builders) have more experience and are used to doing things another way. We asked three roofers and many professional builders about the design of our low pitch roof and all agreed 2 inch venting was sufficient, and also that a vapour barrier was needed on the interior walls. I won the no vapour barrier argument, but have capitulated in other areas as I felt there were only so many arguments I can engage in. The whole process has been frustrating and confusing. I wish you success in your building and that your unvented roof works out well for you.

  10. Aj Builder, Upstate NY Zone 6a | | #10

    To ask a builder to build a roof he does not build is asking for problems.

    Both of you should use builders that build assemblies well of a type you desire period.

    You need stamped quality plans and qualified contractors.

    Building a home is not like trying a new pizza sauce recipe.

  11. GBA Editor
    Martin Holladay | | #11

    You have pinpointed a real problem, and I disagree with AJ's apparent dismissal of your points.

    It's entirely unacceptable for builders to treat women with condescension or to display an unwillingness to learn from authoritative sources (including researchers and published experts). The type of builder you describe is all too common; if AJ were able to view the world through your eyes, and be treated the way that many builders treat women, perhaps his response would be different.

    Don't give up and keep looking. There are builders out there who get it -- who have been educated by their mothers, girlfriends, spouses, and daughters that it's time to shut up and listen for a change. (There are also, of course, many excellent women in the trades.)

    Such builders may be in the minority, but they exist. Good luck.

  12. Michelle Paninopoulos | | #12

    Carolyn -- I'm glad this discussion is pertinent to your project as well.

    Update -- The inspector and I sat down. He agrees that venting this roof is a bad idea, as there isn't any known principle of air movement to would us confidence the air is actually moving and - - venting. He said he would not approve of the use of furring over rafters on the principle that the decking needs to be nailed directly to the rafter. I'm not sure that makes sense, and it would likely lead to an argument between the builder and the inspector.

    Here is another article I found that may be of interest to you:

    Although we are under the 2009 Code here, and it is clear enough that one isn't required to vent a low slope cathedral roof, the Code still doesn't seem to rule out the possibility of attempting it, and yet provides no guidance outside of "minimum net free ventilating area shall be 1/150 of the area of the vented space." That doesn't even make sense in this context, as the ventilating "area" is one and the same as the "vented space," isn't it?

    In any event, this is significant enough that I just told the builder we will stick with the previously-designed and drawn, Code-compliant unvented assembly because we know it works, and I am not concerned about the cost of the above deck ISO, which we are happy to pay for. There aren't that many issues I care enough about to argue about building methods: condensation is one. I have to live under this roof!

  13. Keith H | | #13

    Michelle, I'm a DIY and a homeowner, not a pro like many of your respondents but I thought I'd relay my remodel experience. FYI I'm in a dry climate in Zone 5. I have a 1970s low slope EPDM roof. Until some recent point, it was unvented, R-30 with an air gap above the batts. There is some limited and acceptable condensation damage on the bottom of the roof deck near the truss ends (likely exterior air leakage) and some limited sign of mold spotting from condensation falling from the roof deck (through the air gap) onto the insulation. At some recent point, a previous owner added a swamp cooler to the roof which effectively added two poorly installed dog houses to the roof (I'm in the process of removing it). I haven't been moisture monitoring so I have no idea if these faux vents helped or not. I do know that they made the assembly perform very poorly in terms of cooling comfort and air leakage. On paper, this house is significantly better than my last one (R-30 vs R-19, R-24 vs R-11) yet it doesn't perform as well. Unless someone smarter than me demonstrated to me that venting would work and venting would not wind wash my batt insulation (or did you have a vapor permeable air barrier planned as part of that assembly?), I would personally not try venting. We are moving towards retrofitting top of the roof deck mineral wool board as soon as the EPDM requires replacement with no venting and mineral wool batts inside the trusses. I do wonder: are you installing a vapor barrier under your roof assembly? I'd be concerned about vapor load from a bathroom without a vapor barrier or retarder. We are installing a smart vapor retarder. Did you consider having a WUFI model created? Good luck!

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