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Community and Q&A

A-Frame Vented Roof Moisture Risk

derekr | Posted in General Questions on

New construction A frame house

Hoping someone in here can make me sleep better at night because I’ve been reading about roof moisture the past few days and getting worried

on a scale of 1-10 how worried should I be with this set up
Zone 3 A frame house, the roof is vented top to bottom with a 1.5 inch – 1 inch or so gap, the roof will have R38 rockwool. My lights are going to be the flat canless lights that clip to drywall with gasket on the back and I have outlets with flanges with gasket on those aswell, so I’m hoping both of those will be enough for air sealing

I want the house to be energy efficient but I’m more concerned about moisture than energy efficiency for now

if I did have a little air leaking into this roof would I still be fine from a moisture perspective since the roof is vented and in zone 3?

theres 3 possible places I’m concerned about for air leaking once the drywall is done.
in these 2 pictures, where the drywall meets the exposed collar ties and where the drywall meets at the peak below the ridge

and the 3rd place is on the 2nd floor, the deal with this 2nd floor is it’s connected to my roof so I could get air through my floor and into my roof, moisture rises though so I’m guessing this wouldn’t be much concern from moisture?
theres a gap where the drywall is going to meet the subfloor on the 2nd floor I think this needs to be sealed in some way or not major concern?

im expecting the drywall installers to seal these 3 locations but if they do not is it something i need to do since the roof is vented anyway? If I do how should I seal them? with caulk?

Trying to understand how this works, my rockwool insulation is vapor permeable, so if any hot air did leak through my drywall the vapor should pass through my rockwool then go up through my ridge vent right?

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  1. derekr | | #1

    Bumping for another day

  2. Expert Member
    Akos | | #2

    In zone 3 you are allowed unvented roofs with diffusion vent. I think this is best solution for you.

    The vents on the bottom should be sealed up and you need to add a code min sized diffusion vent at the ridge.

    The benefit of this setup is that you can detail the roof sheathing as your air barrier which is easier to get air tight than the collar ties and floor joist intersection. You can also fully fill the rafter space with insulation.

    You can also keep the assembly vented but you will need a continuous vent channel from soffit to ridge and to add some vents near the ridge.

    1. derekr | | #3

      It is a continuous vent from the bottom to the ridge, this is how the plans were done by the architect, I don’t want to change that now

      Also the roof is already shingled, it’s too late to make sure the outside is air sealed

      I thought having a vented roof in zone 3 would solve most moisture problems even if I did have some air leaking from inside, is that not the case?

      I mean my grandparents house is 20 years old and not airsealed inside and they dont have moisture problems that I’m aware of

  3. derekr | | #4

    The drywall is going to meet at the peak and touch the ridge beam, how should that be sealed? I’m wondering if there’s even enough room and the drywall on either side might not even be able to touch each other, I might take 2 small pieces up there to see if they can meet

    Is any insulation supposed to go around the ridge beam or no?

    Also my ridge beam is just tied together with a bunch of nails, if I ever did have a moisture problem
    Couldn’t all those nails just rust out and break and the ridge beam fall into the house?

    Edit: I think I’m going to put a 2x6 across on each rafter directly below the ridge beam for drywall to connect to and put insulation in between each 2x6, would something like this be ok? So I would have 2x6s and rockwool touching the bottom of the ridge beam then drywall on the 2x6s below that

    1. Expert Member
      Akos | | #5

      I think that bit of framing plus insulation is a good plan, the best way to go.

      Since you are building a vented assembly, make sure to add in the required vent area near the peak. Generally rule of thumb is 60% of your code required area would be in the soffits and 40% near the peak.

      A reasonably sealed roof with good venting works in any climate.

      Collar ties and floor joists are never easy to air seal. I would add blocking around all sides of them sealed with caulk. This will give you a flat surface for the drywall to sit against where you can seal it with caulk or foam. This will mean having to rip some odd angles but not a lot of extra work. Trying to air seal collar ties without this blocking is a loosing battle as the structure moves too much, any caulk between drywall and the collar tie will separate eventually.

      1. derekr | | #6

        Once the drywall is in I think I’m going to get some 3M flashing tape to seal that corner on the 2nd floor where the drywall meets the OSB subfloor, there won’t be any foot traffic or furniture in this deep corner anyway. have way too much wiring to put in block to seal in the joist itself

        Am I supposed to put insulation around the ridge beam? Or should I stop insulation right before I reach it

        Another thing is the hvac people had to run some of the duct work a few feet in the roof in order to reach the 2nd floor, the duct is insulated but I’m wondering if that will be enough in the winter time when the roof is cold and I’m running the heat

  4. derekr | | #7

    Wanted to bump this 1 more time

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