Cathedral ceiling ventilation.

[Treehouse2339]

This is not a program issue.  I am looking for a good detail for venting a cathedral ceiling.  I get that we need 1.5" above the insulation in each bay or rafter space.  But how many sq. inches of opening at the soffit and at the ridge for the ridge vent?  and engineers hate this ridge venting in general because it destroys their roof diaphragm.  Anyone also have a detail to use on a plan?

[GeneDavis]

Look at the product literature from Cor-A-Vent.

[Electromen]

I prefer having two different pitches like 12/12 for the roof and 8/12 for the ceiling plane or 8/12 & 6/12.  That allows for plenty of insulation and venting.

[Alchemyjim]

In California it is 1sqft of vent per 150sqft of conditioned area. Same for roof as for crawlspace. It is important to note that the 150 sqft refers to floor plan area, not the surface of the roof planes. 

Venting should be about equal at top and bottom. 

If you have 1-1/2" gap at the blocking, multiply that by the length of the roof. (18sqin per foot) That is the lower number.

The cora-vent V-600 has 20sqin of vent area per liner foot of ridge. Multiply that by ridge length. 

Those numbers should be kind of close for it to work correctly. Keep in mind the ridge is venting two roof planes. 

The building department hardly ever looks at the ratio between upper and lower, but the combined vent area must be greater than 144sqin per 150 sqft of conditioned area. 

A better option is to use closed cell spray foam insulation.

It depends on the jurisdiction and climate but I have successfully specified it in Humboldt and Monterey counties. Sometimes they require that you can prove it's efficacy. So I send them the technical data (like attached) and they approve. 

It sounds crazy not to vent a roof. But look into the "first condensing barrier" and see how closed cell eliminates that. Fascinating building nerd fodder.

Essentially it causes the heat to slowly cool as it moves through the foam instead of hitting a hard cold surface immediately. Thus not creating condensation.

Also batt insulation loses A LOT of it's R-value when air passes over the unfaced side. Venting the cavity creates an air current that pulls the entrained air from the fiberglass batt. That entrained air is how batt insulation works.  

Closed cell will cost more but it eliminates the need for vented blocking, baffles and ridge vent. The framers and engineer will like that. 

I have been building and designing roofs like that for years and never had a call back. But they have all been in California. I am really not sure if the same methods would apply in Louisiana (humid)  or Minnesota (frigid). 

Heatlok HFO High Lift TDS_19.pdf

[Treehouse2339]

Thank you for that.  But with closed cell foam don’t you have to fill the whole bay to the face of rafter??  Or can it be half filled?

[Alchemyjim]

Closed cell is R-7 per inch. The min. roof insulation in our climate zone is R-30. 

So you only need about 4 1/2" thick from the underside of the roof ply.

[Ed_Orum]

Closed cell foam @ R7 per inch will allow you to meet the energy code with no venting, however you now need to consider that area volume into the requirements for the HRV/ERV system.
 If you use fiberglass or cellulose, you may not have enough R-value because of the venting requirements, but the Energy Code allows a lower R-value in ceilings without attic space.
In fifty years of doing construction drawings, I have never seen one building department kick a plan back if "Continuous Ridge Vent" and "Ventilated Soffit" is specified.  But, around here we can always tell when the building inspectors have gone to a continuing education seminar, because for the next few months they are all looking for the same thing.

[mborean]

Alcehmy Jim.  You missed one small detail.  If half of the venting is at the eve line, the required venting is 1/300.  Fairly easy to achieve with eve and continuous ridge venting.

I agree that closed cell insulation solves the venting issue, but it is 4x the cost.  In my area anyway.