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Unconditioned Roof Cavity

B

Brant Blower

Registered User
Joined
Feb 4, 2019
Messages
14
Location
Bay Area
Is it required to ventilate an enclosed, uninsulated, roof covering an extrior porch? If so, is there a required ratio (free-air vent opening vs. roof area) for this type of roof?
 
No, the roof over the conditioned space, along the exterior wall line, will be sealed off with a barrier and have closed-cell spray foam insulation applied to the warm side.
 
R806.1 Ventilation required. Enclosed attics and enclosed
rafter spaces formed where ceilings are applied directly to the
underside of roof rafters shall have cross ventilation for each
separate space by ventilating openings protected against the
entrance of rain or snow. Ventilation openings shall have a
least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch
(6.4 mm) maximum. Ventilation openings having a least
dimension larger than 1/4 inch (6.4 mm) shall be provided
with corrosion-resistant wire cloth screening, hardware cloth
or similar material with openings having a least dimension of
1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum.
Openings in roof framing members shall conform to the
requirements of Section R802.7. Required ventilation openings
shall open directly to the outside air.
R806.2 Minimum vent area. The minimum net free ventilating
area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be
1/300 of the vented space provided one or more of the following
conditions are met:
 
Thanks Steveray. I did read through that code section. Was hoping that I may have missed something, somewhere, that applied more leniency in terms of required opening vs. S.F ratio. Getting 1/150, or even 1/300 is going to be extremely difficult just through eave vents, and littering a porch roof with a bnunch of dormer vents seems a bit overboard, and not something I'm used to seeing. Oh well, I guess I'm just venting at this point ;-)
 
As far as I know one could argue that these two terms, in this circumstance, are interchangeable. I tried to see if I could distinguish them in the code, but unless I missed it, I'm not finding anything in the Definitions chapter for either ceiling or soffit.
 
ADAguy said:
check Websters or other dictionaries
Click to expand...
is that ceiling is the surface that bounds the upper limit of a room while soffit is (architecture) the visible underside of an arch, balcony, beam, cornice, staircase, vault or any other architectural element.
 
Thanks. I suppose under our scenario, an open wrap-around porch would more appropriately fall under 'soffit' in this definition. I think I see where you're headed. If we can convince the plan checker or inspector that our condition is a soffit, as opposed to a ceiling, then we would not fall under the ventilation reequirement?
 
up to your AHJ, the definition seems to see it as not required if it is merely an architectural element.
 
Interesting. I had an ahj approve an unvented attic over a garage because there was no insulation. I questioned him on this and he affirmed it. 15 years ago. Does insulation (or lack of) have anything to do with the requirements?
 
For a moment. Throw the code away and lets talk condensation and dew point temperature.

In my understanding, ventilation is to keep water in vapor form and not allow it to condense on the underside of the cold roof deck. Water is held in vapor form because it is inside a warm house with warm surfaces. When the vapor diffuses through the ceiling (do to a gradient of relative humidity), the perm rating of the ceiling reduces the relative humidity inside the wall cavity thus lowering the dew point temperature. This is why a vapor retarder allows reduced ventilation. The air temperature in the assembly is also lower, which also lowers the dew point temp. The concern is the roof deck is still so cold that is is at or below dew point, so we ventilate.

Okay. So.... now lets have that discussion about an unconditioned porch roof and explain why it needs to be vented? I can't do it.

Would the temperature inside the assembly under the roof deck not be the same temperature as the underside of the ceiling finish? Heat is constantly moving to cold. What heat source would heat these differently. If any, I would say the sun would keep the roof WARMER than under the roof, which would make condensation even less likely within the assembly.

Just as air condenses directly on the blades of grass, so would it condense directly on the ceiling. Without a gradient in relative humidity from under the roof vs. inside the assembly, I also don't see why there would be any movement of vapor via diffusion. It moves due to an imbalance, but they are in the same environment, so little imbalance.

So... code aside. I really can't grasp why we would expect condensation inside a porch roof over unconditioned space, and then ask for ventilation to reduce the probability.

Of course... I'm no scientist... so what am I missing? Other than "the code says so".
 
Would the temperature inside the assembly under the roof deck not be the same temperature as the underside of the ceiling finish?
Click to expand...

Glenn, obviously you've never been in an unventilated attic space on a sunny day. Even at 70 degrees outside air temp, the attic temp can go to over 120 degrees. This shortens the life of the sheathing and the shingles.
 
We generally don't require for open porches/patio covers. IF they insulate (keep the NM heat out as best as possible) then would require ventilation.
 
rogerpa said:
Glenn, obviously you've never been in an unventilated attic space on a sunny day. Even at 70 degrees outside air temp, the attic temp can go to over 120 degrees. This shortens the life of the sheathing and the shingles.
Click to expand...
Yes, I was not thinking about heat damage to shingles, but I don't think the provisions for the ventilation in the code is either. Allowing half the area of openings due to a vapor retarder being installed on the warm in winter side doesn't equate to ventilation for heat removal for shingles. However, this would be the section referenced in the code for ventilation of a porch roof.
 
Rick18071 said:
Interesting subject. So what about an unconditioned sun room?
Click to expand...
Same, same.....The roofing manufacturer will likely require the ventilation for their product. So the failure might be:

R104.9 Approved materials and equipment. Materials,
equipment and devices approved by the building official shall
be constructed and installed in accordance with such
approval.

R106.1.2 Manufacturer’s installation instructions.
Manufacturer’s installation instructions, as required by
this code, shall be available on the job site at the time of
inspection.

R903.1 General. Roof decks shall be covered with approved
roof coverings secured to the building or structure in accordance
with the provisions of this chapter. Roof assemblies shall
be designed and installed in accordance with this code and the
approved manufacturer’s instructions such that the roof assembly
shall serve to protect the building or structure.

This would be a good one to clarify...
 
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