Insulation of Vaulted Ceiling

[Ferrino]

I have a vaulted ceiling in my double-garage which is unfinished and which I would like to drywall for aesthetics. You can see the underside of the plywood sheathing and then there is a series of 2x6 rafters. On the roof above there are curved tiles, common to Southern California.

I was thinking that, although this is a shallow cavity, it might be a good idea to pack some insulation in there before drywalling, to reduce some of the intense heat that gathers in the garage in the Summer. Would that be a problem if the cavity is not ventilated? There are no soffit vents at the bottom of the rafters (and no vents at the top/ridge either). There is another similar vaulted ceiling above our living room and that is finished with drywall and I assume has insulation in it, yet it does not have any soffit ventilation either.

Is the need to ventilate such vaulted ceilings a regional thing? Or is there some special arrangement of insulation required when the roof deck is not ventilated by soffits? I really an new to insulation and don't want to seal up the cavities only for the sheathing to rot out from condensation or something.

Many thanks for any advice!

 

[Flange]

Is the need to ventilate such vaulted ceilings a regional thing?

Many thanks for any advice!

In a word, no. Its to do with basic science. If you don't ventilate then your roof will be at risk of rotting.

In the UK there are several "standard" designs for the type of situation that you describe where the design allows for both insulation and ventilation. You should consult someone who knows your local building codes to see what the permisable arrangement is for you in your area.

 

[LX-Markham]

Looking forward to some solutions in this thread. Dealing with a similar situation right now. I just created a vaulted ceiling in my garage. Was going to install a ridge vent for the exhaust, and on the south side there is a vented soffit for the intake. My problem is the north wall is a shared/common wall (so no soffit vent). Not sure where to draw intake air on the north side.

The other option I've investigated is spraying the ceiling with a closed-cell foam insulation. Maybe that would work for you Ferrino?

 

[Ferrino]

I was reading an article somewhere that said soffit vents were not used in California due to the wildfire risk (embers flying into the roof through vents). Also, surely the milder, more arid climates are less susceptible to the problems associated with unvented roofs? All houses in our area are from 1980 and are unvented.

 

[theoldwizard1]

My son had is roof re-done recently. Vaulted ceiling with 2x8 rafters. Do to other issues, the roof deck need to be replaced so the old insulation (4") was replaced. Baffles were installed on top of the new insulation to allow ventilation from the soffit up to the ridge vent.

 

[theoldwizard1]

... On the roof above there are curved tiles, ...

Interesting foot note. Tile roofs are a big part of the fire resistant houses in California. Also stucco exterior and no exterior wood or plastic.

I know if I lived in any of those fire prone areas I would either have a pool or a very large cistern plumbed to an external sprinkler system with a pump running off of a buried LP tank.

 

[Motofixxer]

I would only spray foam a vault ceiling like that nowadays. Best R value per inch and no venting. Little pricey to purchase but it's a one time deal.