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ventilation in irregular roof

Discussion in 'Canada' started by Mac Moonfire, Jul 23, 2020.

  1. Mac Moonfire

    Mac Moonfire Registered User

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    attached is a picture of the valley-rafters cavities that aren’t vented much. Although furring strips are in, ventilation isn’t adequate especially at the valley-beams. They’ve drilled holes at the blocking in the upper part to let air out, but the solid Valley beam LVL isn’t allowing air in. As a solution, although not perfect, would be to have holes drilled in the web of the I-joist rafters to allow a maximum of airflow. Provided the structural engineer is on board with drilling the web.

    Any suggestions or solutions?

    BCBC2018 requirements are :

    -1:150 ratio

    -min 25% air vented from the bottom.

    -min 25% air vented through the top.

    Thanks a bunch


    [​IMG]
     
  2. tmurray

    tmurray Registered User

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    Your photo did not come through.

    Purpose of venting is to ensure that the roof deck is at the same temperature as outside air to minimize ice damming. Other alternatives I have seen involved using sheet metal roofing and furring out the roof to provide a a second, vented roof sheathing.
     
  3. Ty J.

    Ty J. Sawhorse

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    Here is the picture that the link goes to...

    [​IMG]
     
  4. Ty J.

    Ty J. Sawhorse

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    IMHO, I don't see how this is ventilated. Are you sure that your not getting full depth insulation with no attic ventilation?
     
  5. Ty J.

    Ty J. Sawhorse

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    R806.5 Unvented Attic and Unvented Enclosed Rafter Assemblies

    Unvented attics and unvented enclosed roof framing assemblies created by ceilings that are applied directly to the underside of the roof framing members and structural roof sheathing applied directly to the top of the roof framing members/rafters, shall be permitted where all the following conditions are met:

    1. The unvented attic space is completely within the building thermal envelope.
    2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof framing assembly.
    3. Where wood shingles or shakes are used, a minimum 1/4-inch (6.4 mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.
    4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.
    5. Insulation shall be located in accordance with the following:
      1. Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
        1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.
        2. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Section 5.1.1. 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 in accordance with the R-values in Table R806.5 for condensation control.
        3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.
        4. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.
      2. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.


    upload_2020-7-28_11-6-22.png

    a. Contributes to but does not supersede the requirements in Section N1102.
    b. Alternatively, sufficient continuous insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.
     
  6. north star

    north star Sawhorse

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    # ~ #

    Shouldn't those rafter attachment clips be taller on each rafter ?

    # ~ #
     
  7. tmurray

    tmurray Registered User

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    I'd kick it back to them. How are THEY planning to provide the required ventilation.

    I usually have an idea of what I'm willing to accept.

    If they are using an unvented attic, they need to enlist an architect to design it.
     
  8. tmurray

    tmurray Registered User

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    For the architect designed unvented attic, you would need a statement that it is designed in compliance with the entirety of Part 5 to ensure all your bases are covered.
     
  9. Ty J.

    Ty J. Sawhorse

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    Why an architect, IRC R806.5 gives you a prescriptive method for unvented attic and encloses rafter assemblies.

     
  10. steveray

    steveray Sawhorse

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    Good catch!...If they are deeper than 2x10, Yes...or design....

    R802.8 Lateral support. Roof framing members and ceiling
    joists having a depth-to-thickness ratio exceeding 5 to 1
    based on nominal dimensions shall be provided with lateral
    support at points of bearing to prevent rotation.
     
  11. tmurray

    tmurray Registered User

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    Because there is no equivalent section in the British Columbia Building Code. If it is not addressed in Part 9 Small Buildings, it must be addressed by Part 5 Environmental Separation.

    Not necessarily by an architect, I would also accept a building science engineer.
     
    Mac Moonfire likes this.
  12. Ty J.

    Ty J. Sawhorse

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    I'm sorry... I totally missed that this was in the Canada forum section. My apologies.
     
  13. Mac Moonfire

    Mac Moonfire Registered User

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    Hi,
    Unfortunately, we don't require those details at planchecking. We can only address it at framing inspection when it is built and in your face. A wonderful way to practice my inner calmness.
    Yes, the roof is presented as a vented roof. The roof joists are 16" deep and the insulation batts are +/-12" (R-40), and 1.5" for the 2x furring.
    Tmurray is right, we ask an architect or envelope engineer to undertake unvented roofs as is it outside of part 9.
    I'm having conflicting information, the owner, the builder and the structural P.eng. are claiming this is how it's been done since the dawn of times, and the 1:150 ventilation ration is met from the soffits 60' on each side. Note the P.eng. isn't signing off on ventilation.
    Other professionals suggest drilling 1" holes along the top of web of the LPI roof rafter. I don't see how this could draw more air in those dead spaces.

    The roof rafters are blocked at the top and are fastened at the bottom with the hangers and furring/roof sheathing. I haven't encountered the 5 to 1 requirement and the manufacturer doesn't call for it. where's the text at? I'll take a peek at the very least.
     
  14. tmurray

    tmurray Registered User

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    I'm sure those involved in the leaky condo crisis also claimed "it has always been done this way"...In fact I literally just hung up the phone with someone using the same excuse. And that is just what it is. An excuse. A reason why they should not have to do it right.

    We can't always catch everything at plan review. Look at how you would need to modify your process to catch this kind of issue and evaluate how much time it adds to the process. Many times it does not make sense to check these items at plan review to catch the one-offs.
     
  15. Mac Moonfire

    Mac Moonfire Registered User

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    You speak wise tmurray, its good experience gain.
    thank you for your input yall! :D
     

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