To vent or not to vent a cathedral ceiling?

Started by Jeff922, October 06, 2013, 11:14:07 AM

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Jeff922

The heated, detatched  barn/shop/studio I'm building has a full cathedral ceiling on the second floor.  I live in Maine.  When does one vent a cathedral ceiling?
"They don't grow trees so close together that you can't ski between them"

flyingvan

When, or where?  I can't imagine any situation where you wouldn't vent an enclosed space.  Living, cooking, burning stuff all creates lots of water that needs to get out somewhere....As to the where, around here people install ridge and soffit vents to allow air exchange in the rafter bays. 
Find what you love and let it kill you.


MountainDon

Quote from: Jeff922 on October 06, 2013, 11:14:07 AM
When does one vent a cathedral ceiling?

One vents such a cathedral ceiling when the insulation is NOT all located on top of the structural roof sheathing. That is if you install several layers of foam on top of the sheathing, then have the final roofing you do not need to ventilate below.

OR if you insulate in the rafter bays with spray in foam of many inches thickness, you may not need to ventilate. rwanders had the 2x12 rafter bays all completely filled with foam in his Alaska cabin.

Or a combination of some spray in place and some rigid sheets above.

Any use of fiberglass batts insulation below the roof with need venting if I have learned this right. The idea is that the foams are not vapor porous but fiberglass is. With fiberglass eventually some water vapor will migrate from below and may then condense on a cold roof underside.

Look at buildingscience.com.  Lots of info there on insulation and venting that is suitable for all climate zones. What is good in Maine can be terrible in GA.

Just because something has been done and has not failed, doesn't mean it is good design.

Jeff922

Thanks.  My understanding of roof venting is that the primary goal is to keep the roof deck cold and avoid ice daming.  I know in theory, moisture is supposed to escape through a roof, but it seems to me that the moisture should never get into the roof cavity to begin with.  I believe the caveat is that in climates where heat AND air-conditioning are used, water vapor must be free to travel in both directions through a wall or roof cavity.  In this case a vapor retarder (like coated kraft paper) would be used.  I am planing to use a vapor barrier (6mil poly) with taped seems over un-faced fiberglass batts which is what I did in my house.  I'm trying to understand the reason to vent a cathedral ceiling.  Is it really just the ice daming issue?  I need to research this more thoroughly...
"They don't grow trees so close together that you can't ski between them"

MountainDon

buildingscience.com    Joseph Lstiburek is the guru on insulation and venting.
Just because something has been done and has not failed, doesn't mean it is good design.


Squirl


http://publicecodes.cyberregs.com/st/ny/st/b400v10/st_ny_st_b400v10_8_sec005.htm

As don outlined skipping ventilation is dependent on many factors.  If you are going to not vent it, then:
QuoteNo interior vapor retarders (Class I or II) are installed on the ceiling side (attic floor) of the unvented attic assembly.


If you install a vapor retarder you would only need to vent 1 square foot for every 300 square feet of space.  On most buildings this would only be a few rafter bays.

If you breath, you release hot moist air.  This rises.  Vapor retarders (6 mil plastic) still let water vapor though, just not as much. Without venting this will condense when it hits the cold roof. Flyingvan was right. Many other appliances and daily activities, release hot moist air (showering, cooking, heating, doing the dishes).