Why are walls NOT ventilated?

[Inventive1]

When buildings with rafters and vaulted ceilings are constucted, drywall is attached to the bottom side of the rafter, insulation is installed above the drywall, and then baffles are installed betweent the insulation and roofing to allow air flow. Where I'm confused is that walls are built in a similar manner. In other words, there is drywall, insulation, and then siding, instead of roofing, on the outside. So, why are walls NOT constructed with a ventilation space between the insulation and siding?

 

[Colonial Cobra]

Moisture is the enemy with Fiberglass insulation. Any humidity at all dramatically cuts the r-valus of fiberglass insulation. The older technique of an airspace between the underside of the insulation and the roofing is to keep the underside of the shingles from overheating and curling. You still need to seal the baffles to the rafters to keep air from infultrating the insulation. Many are now opting for spray foam directly onto the underside of the roof sheeting and a sealed attick space. Fiberglass isulation is fine but each cavity must be airtight to maintain the r-value.

 

[rwhite692]

Not only that...but under a fire scenario, you don't want the wall cavity to act like a chimney.

 

[Shott8283]

also. for the sheathing for your roof, if its not ventilated that moisture will also contribute to accelerated rot for the OSB used for the roof. heat and moisture will wreck that sheathing quick fast and in a hurry

most shingle companies will not warranty there product unless there is a vent channel when insulating the roof instead of the ceiling.

rwhite raises a good point. balloon frame construction (firefighter killers) is illegal now for that exact same reason.

 

[sberry]

Isnt any ice to form on the sidewalls like on a roof, on roof, snow melts, re-freezes makes ice, becomes problem.

 

[bigdav160]

So, why are walls NOT constructed with a ventilation space between the insulation and siding?

Some places do

It's called "skin vent construction". It works well here, near the gulf coast, but is an expensive way to build. I have a rent house that is skin vented.

 

[jumpingryan]

The thing to realize, is that where hot meets cold, there is moisture....... the attic is subject to some of the greatest swings in temperature possible every day. Air movement clears this moisture to a degree, and allows it to escape.

An attic should ideally be of the same temperature as the outside air.... however in real world situations it is not. It is heated from the outside by the sun. Hot air moves upwards from the inside of the garage and through some of the insulation, regardless of R factor.... insulation just slows the movement of heat by resistance. It does not stop it.

Air movement is important in the attic to prevent the buildup of moisture. Air movement flows over top of the insulation.... but not through the insulation..... (from the bottom soffits, to the top vents). Air flowing through the insulation would displace the trapped air pockets that are holding your heat.

The reason walls generally have no space in between them, is because air flow would be hard/impractical to achieve. Venting at the bottom would let in snow for example. Venting would take up floor space & increase the size of lumber. It would also allow air movement through the insulation to the vapour barrier, clogging the small spots in between insulation that trap heat with dirt. It would work against you.

The last thing to remember, is that water generally flows sideways, and then down. With moisture buildup inside the attic, the water will drop down into the insulation, and into the ceiling. With moisture on the exterior..... it generally wicks its way downwards..... moving to the sill plate and likely into the foundation. The process is fairly rapid.

A far better way is to allow moisture to escape using a building wrap on the outside, and limit (yet still slightly allow) some air movement.

On the inside you stop both air and moisture movement. This technique applies to situations with batts insulation.

With closed cell foam, you technically do not need a building wrap..... the foam stops air movement..... however I still recommend it.... The building wrap is a layer to protect the wood from the outside in case there is a failure of the exterior finish. For brick finishes, it is generally a must & possibly a code in your area to avoid issues with two porous materials (and one being a biological decaying material: wood) wicking moisture into each other.

Anyways, I guess some of those technical points will assist in the reasoning for todays stick framing & modern building techniques.

R

 

[tcianci]

The type of roof construction you are referring to is called "cold roof" construction. The purpose of this type of construction is to keep the back side of the roof sheathing "cold".
Cold roof construction is implemented to prevent leaks due to ice dams that form when snow, typically melted a short distance up the roof from the drip edge due to the proximity of the interior heated space to the back side of the sheathing flows down the shingles and re-freezes on the edge of the roof. This re-frozen melt forms a dam that hold back other melted snow. This water building up behind the dam eventually finds its way through the shingles and underlayment, resulting in a leak. The cold roof construction keeps the backside of the roof sheathing as cold as possible to prevent melting and backing up of water. This technique along with an ice and water barrier typically installed to a point 3 feet up the roof sheathing does a good job of keeping the water at bay and out of the building.

There is a vented wall type of construction that consists of a venting layer applied over the housewrap and under the siding. There are air intake vents near the bottom of the wall and outlets near the top of the wall. The purpose of this system is to keep the wall sheathing dry (funny, I thought that's what siding used to do). Do a little web searching on cold roof construction and products like "Cedar Breather"