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Help deciding between vented vs unvented roof design

Tinybuilds4U | Posted in General Questions on

Hi , I’m working on some tiny house buildings and could use advice deciding between an unvented roof assembly vs a vented assembly. I’m needing a somewhat flat roof design. Between 1/12 – 3/12 slope. And it needs to be well insulated for Pacific Northwest zone 4 winters. kept as warm as possible in winter, without using too much electricity. However I have limitations on overall height. And the total R value I will be able to achieve. The tiny house is not mobile. It will be on a permanent foundation so I am planning to use a simple rolled roofing.  

 

My question is if I go with a 2/12 or 3/12 slope can I get away with an R-30 insulated ceiling that has a vented assembly, shed roof. The building is only 7 feet wide. Which is not very wide but I’m concerned the flat slope will not allow sufficient air flow for drying between roof sheathing and mineral wool insulation. It would have a two inch air gap for ventilation above the insulation, but it’s a very low slope.

 

It will have an interior vapor membrane on the inside of the building. Hoping this will be enough for a warm comfortable tiny home experience, that won’t have moisture problems inside the vented roof assembly. The building needs to be kept very warm in the winter.

 

If I go with an unvented roof, I will still need to use the same R-30 in the rafter cavity, just smaller rafters, but I can add r-10 rigid insulation on top of the roof sheathing. Would this be sufficient for zone 4? Do I need an air gap underneath the shingles?  I would only be able to gain r-10, but it would limit thermal bridging and possibly limit any moisture issues inside the roof assembly.  I am having trouble deciding between these two options. Will there be much difference? Is one clearly less vulnerable?  Will the unvented design be significantly warmer?  

 

Both designs will have intello vapor barrier inside the building. 

 

I’m hoping for some guidance choosing between these two options. 

 

Thanks,
Alex

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Replies

  1. andyfrog | | #1

    Some more knowledgeable people will probably chime in but in summarizing other discussions I've read, a flat roof won't vent properly anyways, so that might make the decision for you.

    Also, builders on here haven't experienced these problems, but I found this paper interesting anyways: https://www.rdh.com/wp-content/uploads/2019/10/TB-10-Attic-Mold-in-Vented-Wood-Frame-Roofs-2015-12-17-3.pdf

  2. Expert Member
    ARMANDO COBO | | #2

    FYI - The 2018 Code or older requires R49 for CZ4 insulation (see N1102.1.2) and R15 if using continuous insulation (ci) rigid foam for condensation control (See R806.5). You can use R38 if all rigid foam continuous insulation (ci) insulation is on top of the roof sheathing. If you apply the 2021 Code you need R60 total, R15 ci rigid foam and R49 all ci on top... and yes, low pitch roofs don't vent properly.

  3. jablo | | #3

    R value with rigid foam
    Is not comperable to R
    With lose insulation
    I would do just rigid styrofoam with aluminum skin 3or4 layers 2 inch 3/4 [email protected] plywood attached to joists wih long screws

  4. Tinybuilds4U | | #4

    Ok thank you for your input, Andyfrog this article is great, very helpful. thank you for sending.

    I will go with an unvented assembly. I’m not used to building this way, but looking forward to trying it out. Do I need two layers of roofing paper and two layers of plywood roof sheathing? Sandwiching the rigid insulation ?

    Roof sheathing above rafters, roofing paper, rigid foam, more plywood , roofing paper and shingles roll roofing , corrugated metal etc…whatever I decide for roofing material…

    So I need two layers of roofing paper correct ? And do I not need venting gap under the roofing? Above the rigid board ? Does this depend on which roofing I use. I image metal would need to be installed over furring or purlins to dry condensation above the rigid foam boards.

    Is this corrects ? Same for shingles ?

    Thanks so much,
    Alex

  5. Expert Member
    ARMANDO COBO | | #5

    You need to check your shingles manufacturer's instructions and warranties. Some won't cover low-slope roofs. I would recommend standing seam, the ridges will allow to clamp PV panels if you ever go that way. 🧐

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