Cathedral Ceiling Insulation No ridge vent
Phanthony
| Posted in Energy Efficiency and Durability on
Hi all,
I am currently remodeling my home and wanted to know how I should tackle this job. My cathedral ceiling has no ridge vent along my roof ridge but there is soffit vents. I live in climate zone 3 (OC socal) next to a very big park with lots of fog sometimes in the early mornings. Should I just cover these soffit vents and insulate my ceiling/roof with r-38 rockwool?
My biggest concern is condensation building up as well as if my roof will be too hot in the summer time.
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Replies
If you're worried about condensation and roof being too hot, then you might consider applying a radiant barrier, then vent baffles, then insulation (Rockwool or whatever in the remaining cavity depth) then finish the ceiling.
Oh and install a ridge vent too. That should address all of your concerns.
Other options are possible in the CRC, but they mostly involve foam.
Consider a vapor diffusion vent. Look over at building science corp for a detailed guide.
Phanthony,
As Luke and freyr_design have said, if you want to stick with permeable insulation (batts) you need either a ridge vent, or a vapour-diffusion port. https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
Both by code and practically, you can't just use batts without one of those two ways of removing moisture.
My only issue with vapor diffusion port is that theres no code support for it, so it may be an AMMR hassle.
That is not true
https://codes.iccsafe.org/s/IBC2024P1/chapter-12-interior-environment/IBC2024P1-Ch12-Sec1202.3
Indeed
I'm thinking of that section exactly.
Note that section 5.2 only describes attics and not enclosed rafter assemblies.
The OP clearly does not have, or intend an attic, but rather an enclosed rafter roof assembly.
That is an interesting, and with the wording, valid interpretation of that code section but everything I have ever read on vapor diffusion ports makes me think that is not the correct interpretation. Here’s my reasoning:
The start of that section in which 5.2 is listed says:
Unvented attics and unvented enclosed roof framing assemblies created by ceilings applied directly to the underside of the roof framing members/rafters and the structural roof sheathing at the top of the roof framing members shall be permitted where all of the following conditions are met:
I think that is stating that the list applies to both, but perhaps this is wrong. To dig deeper I would go to Lstiburek who I believe was the main factor for writing the code language and having it included.
In his guide that describes how to build a roof assembly with a vapor diffusion port he has a section on dormers and shed roofs. See figure 12 & 14. Both of those show a sloped roof assembly that looks an awful lot like and enclosed rafter space.
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
In the green building advisor article about vapor diffusion ports Martin writes: “ What about hipped roofs and cathedral ceilings? Lstiburek advised, “Vapor Diffusion Vent for Hips: Just need the upper 4 feet of the hip to be diffusion vented. … Can this work for a cathedral ceiling? Yes.”
According to Lstiburek, this approach works with any air-permeable insulation materials: “Works with fiberglass batts. Works with netted cellulose. Works with adhesive spray-applied fiberglass. Works with fiberglass blown into a netted semi-permeable sheet.”
In this AIA continuing education course under the section “ Unvented Roof Assemblies – 2018 Updates”
They write: “ Based on the above, in climate zones 1, 2, and 3, section R806.5.5.2 of the 2018 IRC allows air-permeable insulations in unvented roof assemblies provided:
Vapor-diffusion ports are located within the highest 12 inches of the sloped roof.
The ports comprise an area equal to or greater than 1:600 of the ceiling area.
The vapor-permeable membrane over the ports has a vapor permeance equal to or greater than 20 perms as demonstrated by testing.
The vapor-diffusion port is part of a continuous air barrier between the attic and the exterior of the unvented roof assembly. This is particularly important since it means that the top surface of the sheathing material needs to be a continuous air barrier and the vapor-diffusion port needs to tie into that in a continuous manner.
The vapor-diffusion port still protects the roof from blowing rain and snow.
Framing members must not block the path of water vapor up toward the port.
The roof slope must be 3:12 or greater.
Where only air-permeable insulation is used, it must be installed directly below the roof sheathing.
Air-impermeable insulation may still be used either above or below the roof sheathing but is no longer needed for condensation control in specified thicknesses. If air-impermeable insulation is used in combination with air-permeable insulation, there shall be no space between the two.”
https://continuingeducation.bnpmedia.com/article_print.php?C=2022
And lastly, from a purely logical point of view, what is the difference between a conditioned attic and a second story vaulted ceiling?
freyr-design,
I'm still a bit hazy on the details of these assemblies - mostly because any interest is just out of curiosity as they don't work here.
In a conditioned, trussed attic with a vapour-diffusion port, you would need an interior air-barrier wouldn't you? Practicality, I wonder what that looks like?
I'm pretty Sure Freyr_Design is correct. My reading of the code is that diffusion vents are allowed for "roof framing assemblies created by ceilings applied directly to the underside of the roof framing members" that is cathedral ceilings.
Thinking about how these assemblies work, I can't see any reason why a diffusion vent won't work with a cathedral ceiling provided you meet the code requirements (minimum slope and diffusion vent area). The idea is that the moisture will move through the fluffy insulation to the highest spot (hot moist air rises) where it can be vented.
The hard part is air sealing the rafter bays which is not much more difficult than doing rim joist.
To the OP.
Block and soffit vents, air seal the rafter ends, install fluffy, cut a slot for a code min diffusion vent and install a vented ridge cap. Your roof will work.
Normally I would say limit the amount of pot lights, but it looks like that ship has sailed. At least get the air tight gasketed inserts for them if possible.
Thank you for your responses!
Is it possible for me to just close the soffit vents and then install rockwool insulation and close up the ceiling with drywall? Would I be running the risk of mold buildup or would the ceiling be too hot if I did that?
I think the answer is yes to both. You absolutely can do it unless there's something you haven't told us thats preventing you. Should you do it? Read above, NO
Yes its running a risk.
Cities in SoCal don’t seem to care about proper cathedral ceiling construction, but failures do happen. Your biggest risk is will be if you have a north facing roof.
If you have a tile roof with a vapor permeable underlayment you’re technically to code and can proceed with closing the soffit vents and just insulating with batts. Any other roof assembly requires ventilation, a vapor diffusion port, or rigid foam above or below the roof deck.