Why vent the roof above a room on the top floor?

NECHater

I've been curious about something. I'm aware of the proper way to vent the roof when making a room on the top floor (attic) and how you should use baffles to keep the insulation away from the roof to allow air flow from the soffit to the ridge. My question is why do this for the roof and not the walls?

When making up walls you have an outer sheathing, a hollow space filled with insulation, then an inner sheathing.

When making the ceiling of a room on the top floor, you have the outer sheathing (the roof), an open air space for air ventilation, then insulation, then the inner sheathing.

So why do you ventilate the roof and not the exterior walls?

gobug

This topic interests me, thanks for posting it. Just don't call it pre-vent shun.

What are you venting?

I see 5 things needing to be vented: plumbing, gas appliances (furnace, clothes drier, water heater), fireplaces, radon, and living space air.
IIRC codes require all, except living space air and the drier, to vented out the roof.

In the attic, I recall vents in the soffit which supply the air in and then a larger vent out. The soffit vents should not be covered up with insulation. My climate is dry, but I suspect condensation and summer attic heat are both mitigated with the venting.

I do not see what venting in the walls would do. That might make the house into an envelope house. I don't think that is a retrofit task, but an initial design and construction project.

NECHater

I'm not venting anything, just curious why you vent the roof but not the outside walls.

Tham

You want cold air in your attic so your roof doesn't make ice. Through melting and freezing everyday/night. Also you don't want to trap superheated air in the summer.

Walls don't have a foot of snow sitting on them,
Tham

concretemasonry

Heat builds up in the attic if it cannot be vented out. This heat causes higher AC usage and ice dams at the eaves. The goal is to make the attic air space as close as possible to the outside air temperature. - If you try to insulate the underside of the roof itself, you have created a different situation and have what described as a "hot roof"

Walls are exposed are exposed to the outside air and they function just well enough without adding ventilation.

In both cases, the vapor barrier is applied to the inside of the insulation in heating climates.

Dick

hand drive

it also helps with the life of the shingles when properly vented, especially with black shingles. I've seen plenty of premature cooked roofs from not having attic ventilation and cooked plywood as well that is de-laminating and falling apart from the heat exposure...