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Last modification: Wednesday August 28 2001

The principle: Heat and cold control by air circulation in the attic.

In winter, the intent is to keep the heat inside and the cold outside. In summer, it is to keep the cold inside and the heat outside. Before the availability of modern insulation, attics where sealed as much as possible and there was no air circulation.

The results: Heat accumulation in the attics.

With the accumulation of heat in the attic in summer, there was a rapid deterioration of the roofs and the top floor of buildings was extremely warm.
Condensation and frost developed in the attic in winter and the humidity froze at the perimeter walls at the attic level. Sometimes, the humidity descended behind the brick walls turning them white with efflorescence and then, there was a deterioration of the bricks themselves and of the mortar joints would pop-out.
Brick joints had to be re-pointed with mortar every 10 to 15 years.

The overheating of the attic and roof was worse if there was not a good thickness of crushed stone laid on the impervious membrane to dissipate the sun heat. This situation often leaded to have to change the roof.

The improvements: Availability of modern insulation.

With the invention and manufacturing of modern insulation, it gave the possibility to insulate the attics from the cold in winter and from the heat in summer.

The result: Possibility to have air circulation in the attic

Now it was possible to vent properly the attics to dissipate the heat in summer and in winter, to circulate air in the attic to vent out the humidity. This humidity freezing behind the brick was the cause of the brick joints to pop out and of the whitening of brick walls called "Efflorescence".

This section describes the vents needed to permit proper air circulation in the attics to dissipate heat and humidity.

Section 1 - The Ridge Outlet Vents.

A - The goose-necks
B - The turbines
C - The louvered towers
D - The continuous ridge vents
E - The flat individual vents
F - The hay barn ridge vents
G - The architectural vents
H - The ridge dormers with a triangular louvered vent.
I - The top of the vertical wall grate outlet vents
J - No ridge air outlet vents

Section 2. - The Soffits Inlet Vents.

A - Soffits with air inlet grates
B - Soffits with perforated aluminum as air inlet
C - Soffits with insufficient grates as air inlets
D - Soffits with no air inlets
E - The plain wall with no soffit or air inlet

Section 3. - The Cornice Inlet Vents

A - The cornice with grate inlets
B - The cornice with continuous slot and screen
C - The cornice with perforated aluminum used as air inlet
D - The cornice with no air inlet grates
E - The cornice of decorative granite
F - The cornice of decorative metal
G - The cornice of decorative wood without air inlets

Section 4 - The Wall air Inlet Vents.

A - The hooded inlets
B - The grate inlets
C - The one brick replacement inlet
D - The big grates where there is also decorative granite cornice

Click photos for an enlarged view, then back.

Section 1 - The Ridge Outlet Vents.

A - The Gooseneck outlet Vent.

a) The Goose Neck is the outlet vent normally used on flat roofs.

Different views of Goose-necks on flat roofs.
Besides the gooseneck, the vertical protruding pipe seen on the roof is the air stack or the vent pipe feeding air to the top of sewers to avoid vacuum when water is evacuated from fixtures inside the building. The lower grate normally called the basket is a grate to stop leaves and balls from blocking the roof drain used to evacuate rain water from the roof.	There is enough heat coming out of the gooseneck to make the snow melt on the roof. This building has no air inlet soffit, cornice or wall inlet. The attic has only outlets. Because the building was built in the 1950's, there is also a lack of insulation in the attic. In the enlarged picture, it can be seen that the snow is also melting on the roof because of the heat lost.	There is much snow on the roof. There is a depression in the snow under the neck where is located the outlet grate. The heat coming out melts the snow and causes a deeper depression in the snow at that point.

B - The turbine outlet vent.

a) The wind spins the turbine to keep rain and snow out by centrifugal force.
It happens that if there is no wind or if freezing rain freeze-up the turbine; by not spinning, snows or rain may come through the turbine openings that are quite large.

The turbine became very popular in the 1970's and 80's and is now more and more replaced by other kind of ridge vents.

It is to be noticed that there is no formation of ice at the edge of the roof. The soffits are opened and the air circulates properly through the attic.

C - The circular louvered outlet vent replacing turbines.

a) This louvered circular outlet vent is static as it has no part in movement.

This vent is replacing the turbines. It is interesting that the shape and size are the same as the bottom part through the roof that was used by the turbines.
Turbines eventually also make noise when the bearings wear out.
Turbines have been found to freeze during ice storms. The only reason for the turbine to turn was to use the centrifugal force generated by the wind to eject rain, snow and ice.

b) - A second type of circular ridge vent.

In the clicked enlargement, it can be seen that this outlet alone is insufficient to circulate the heat lost.
The soffit is blocked. There are no air inlet grates or perforated soffit. In winter, the heat lost entrapped in the attic melts the snow on the roof and creates icicles.
It is the same principle as closing the damper on a 30 foot chimney. Air cannot go in and out by the same opening.

c) - The more modern square louvered ridge vent.

It has been found that there is never too much ventilation of attics. These louvered square vent chimneys are very efficient and are static.
On single family homes, two units are installed at the ridge. Individual units installed 20' feet apart are sufficient in number.

D - The continuous ridge vents

These are continuous ridge vents.
a) - The continuous ridge vent is an outlet often as long as the length of the ridge of the roof. It is used on metal roofs or homes with cathedral ceilings that do not have inside attics.This is a new lower model for tile roofs.	b) - In this picture, it is slightly higher and is shorter than the whole ridge.

Here are patented continuous ridge vents.
This is a Patented ridge vent of very good design	This one also.	And a third type of continuoust ridge vent.

E - The flat individual vents

a) - The flat outlet vents where one of the first type used to vent attics.

There is a tube of eight (8") to ten (10") inch diameter under a cap opened on the 2 sides and bottom. Normally one used every 10 to 15 feet apart at the ridge.

b) - In this photo, used as ridge vents and as vents for side triangular attics on the second floor.

It is one of the only kind of vent that is low and strong enough to be installed on the middle or bottom slopes of roofs and not be thorn out by packs of snow and ice slipping down the roof.

c) - Here used as ridge vents on a church roof.

On major size roofs, it is common to have these outlet vents installed on each side of the roofs venting the same attic.

F - The hay barn vents

a) - The tower louvered outlets where huge. They had to circulate enough air to dry and keep dry the hay.

G - The architectural vents.

a) - These tower louvered outlets often used on major buildings are almost a copy of the barn tower ridge vents.

H - The ridge dormer vents.

a) - Instead of having a window in the dormer, there is a triangular louvered vent with a back screen. It is sometimes used on roofs of churches as ridge vents. The pointed ridge of the dormer vent resists better to ice sliding down sloped metal roofs.

One dormer vent on each slope would have seem sufficient to vent the attic, sometimes some components become architectural design or esthetics.

b) - Here is an enlargement of the right one. Materials used are strong and rigid as ridge vents.

If a pack of ice slides down the metal roof, these vents would not be ripped off.

c) - Finally, the dormer vent used on a church as ridge vents.

The proportion of air vent to the size of roof is very different. The same kind of vent is used on the other slope of the roof which is sufficient as outlet at ridge of the attic for a good air circulation.

I - The vertical wall ridge vents

a) - They where used originally as the only vents at the top of attics. One at each end of the attic. If there was no wind, there was no air circulation in the attic.

This is a gable roof. The attic vent can be seen at the right of the chimney. In most of these design outlets, air does not come in by the lower soffits.
The solution is to add ridge tower vent at the top of the back slope of the house as outlet vents and these rectangular vents become the intake to the attic. Then there is a good air circulation in the attic.

J - No ridge vents

a) - Here what happens when there are no ridge vents. It is worse if there are no soffit and a lack of insulation.

As it can be seen, there are no ridge vents. In other photos where two neighboring properties can be seen, the snow melts first on this roof at temperature well below the freezing point.
There are no ridge nor soffit vents in this property. When this property was built, insulation materials did not exist except 3/4" inch thick cardboard.

Section 2. - The soffit is the horizontal part under the roof edges.

A - Soffits with grate Inlets

a) In the past, grates of all kind where installed in the wooden soffits.

Grates in wooden soffits should not be more than 10 feet apart. It is often not the case.
In this case, the grate is installed in a perforated Tentest.	In this building, there is one grate per side of building.

B - Soffits with perforated aluminum Inlet.

a) Now, there are aluminum soffits.

This property has a good wide soffit all around the house.

It would be an error to completely block the wide soffit by laying insulation from inside the attic on the inside side of the soffit. It would block the ventilation and keep heat and humidity inside the attic. A soffit is like a damper on a fireplace. It needs to be opened.

b) - This property has also Aluminum perforated soffits on the whole length of the building. The roof above the balconies has the edge of the ceiling opened as soffit inlets.

When the top of brick walls, right under the soffit, is in perfect condition, it is because the soffit is functional and there are gooseneck vents on the roof.

C - Soffits with insufficient grates as air inlets

a) Some attics have soffit inlet and ridge outlets. It has been found that some grate inlets are too small to let sufficient air in the attic.

The inlet grates in the soffit are too small and snow on the roof still melts and produces icicles. The heat lost in the attic is not dissipated.

b) There is a lack of insulation plus a bad communication between the soffit air inlets and the 2 ridge square vented towers.

The solution is to add more grates or to make a four (4") inches wide slot all along the wooden soffit and cover with a brown aluminum soffit with the bigger size holes normally used on the underside of balconies.

c) - Some vents are much too small and limited in number.

Now, full perforated soffits are used. If there are icicles, the 3 only causes are a lack of inlets, a lack of outlets, a lack of insulation. One, two or all of these situations are enough to melt the snow on a roof.

D - Soffits with no air inlets

a) It was often thought that this kind of soffit extension where simply architectural. The architects designed them originally to install air inlets into. Contractors who built the houses sometimes did not understand the function of the roof extension or it may have been up to the owner to add the grates.

These older properties have no air inlet grates or slots to let the air in their attics.
There are no inlet grates installed in the soffit to let air in the attic. There is then very little air circulation in the attic even if there is goose-necks on the roof. Some hot air will still be replaced by cold air replacement to avoid vacuum when the heat lost will rise out of the attic by the goose-necks. It is the same principle as leaving the damper of a fireplace opened in winter.	The entrapped humidity in the attic makes the paint scale and eventually the wood rots where there should have been a grate inlet installed. Older buildings had sealed attics as there was no insulation available.	This is the most important photo of all for information. The wind has blown off part of the soffit, the metal facia and some of the shingles. It is taught that the moment there is a metal perforated soffit installed, there are air inlets to the attic! To the contrary, at least in this case there is no continuous slot of four (4") inches cut out of the plywood before installing the metal soffit.

c) There is a soffit, but without air inlets, there is no air circulation. It is worse when there are no ridge vent and insufficient insulation in the attic as in this case.

This is the same property at different times.
Water freezes even on the wall and sill	and on the window and frame.	Efflorescence is appearing under the sill.

d) The soffit has a slight slope toward the wall.

The same property at different times. The icicles are getting longer and more ice has developed on the left side of the window.

e) Interesting to see the size and length of the icicles in relation to the window

Again at different times. There are no ridge vents, no soffit vents and no insulation in the attic.

E - The plain wall with no soffit or air inlet

a) When there are no architectural features to install air inlets.

Every 15 to 20 years, the joints need to be redone if there is no ventilation of the attic.

b) Around 1928, there was a roof edge that was very popular. It had an overhanging roof with no horizontal roof edge soffit for air inlet.

If the underside of the roof was followed, eventually the brick wall would be encountered with no opening vent inlet to the attic.

Modern insulation did not exist. Most attics where sealed to create a thermos as much as possible. A floor made of tongues and grooves boards would partially add to the thickness of the ceiling inside. Some had saw dust between the beams.

Note: See the section on walls:

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Section 3 - The in wall cornices used when there is no soffit.

A - Cornice with grate inlet vents.

B - The cornice with continuous screen

a) The cornice with continuous screen wall air inlet vent.

Full length in wall cornice used as air inlet to the attic replacing the soffit architectural design.
This double slotted cornice had a screen to stop birds, bees, squirrels from entering the attic. This roof will never overheat in summer at the condition there are also gooseneck vents to let the air out.

C - The cornice with perforated aluminum inlet

a) Full length in wall cornice used as air inlet to the attic replacing the soffit architectural design.

Here is a good design for a continuous wall cornice to let the air in the attic.

b) Some are very big.

The air does go in the soffit of this cornice.

D - The cornice with no air inlets

a) Partial wall cornice supposed to be used as air inlet to the attic.

Again these cornices may have been simply decorative in the past. Because of the height of the attic and the sloped roof that gives more height at the four corners it is possible to insulate the attic and add grate inlets to the cornices.

E - The cornice of decorative granite

a) A granite cornice used in the architectural design. The attics of buildings of this age where not vented.

On the enlargement, it can be seen that there is the formation of efflorescence on the bricks. This is the indication of a lack of ventilation. The humidity is trapped in the attic and eventually lies on the inside of the brick wall. Eventually, the mortar between the bricks pop's out pushed by frost.

b) An other granite cornice used in architectural design.

No air inlets in the facade. On the side past the chimney, there is one hooded wall inlet to the attic. There are still not enough air inlets. That is why the mortar between the bricks above the cornice had to be re-pointed.

c) Some designs may be considered more extreme having both huge grate inlets and architectural granite cornices.

There are big air inlet grates to the attic. There are also two granite decorative cornices in this design. Huge grate inlets are intakes for circulating air in the attic in replacement of having the cornice designed with air inlets.

F - The cornice of decorative metal

a) Some cornices where made of metal.

Decorative metal cornices.
Many decorative soffits or cornices where made of galvanized or copper metal. In this case there is no air inlets toward the attic hidden behind the cornice .	This one was made of thin sheet metal. Some times, behind the hallow metal cornice, there is a series of openings leading to the attic. This property was built at a time attics where sealed as insulation did not exist.

b) This is a higher attic with a metal decorative cornice.

The white paint is scaling off the metal as the humidity behind the cornice freeze in winter. The cornice being decorative and not functional, the brick joints had to be redone as the trapped humidity pushed out the mortar at the top of the wall.
In the enlarged photo it can be seen that it is a double brick wall. A double brick wall normally has the bricks seen on their end every six or seven row.
On that particular row, it can be seen that at every next brick, there is one seen on it's end.

G - The cornice of decorative wood

a) There are no inlet grates installed in the white wood.

The darken granite can simply be washed with soap and a good floor brush. The dirt is not washed off by the rain as it is under the cornice.

b) During the first part of the century, there was more detailing in the designs of wood works. More precisely before the depression of 1930.

This is a good architectural design for a cornice, but there is no inlet vents to the attic.
c) Many older buildings had extensive soffits called cornices which could extend many feet down the front walls.

This cornice has the horizontal soffit perforated with small holes used as air inlets to the attic.

Section 4. - The vertical wall inlets used when there is no soffit.

A - The hooded inlets

a) The single wall air inlet vent to the attic.

Hooded wall inlets should not be more than 10' to 15' feet apart.
These hooded grate inlets are the best. They are the most common and popular wall inlets.	They seldom let snow in and even less rain as it is intercepted at the horizontal grate inlet.

b) Here is the perfect air inlet ventilation for a vertical wall. Normally used on the side walls of homes where there is no soffit or cornice. This wall is in perfect condition.

With sufficient wall inlets, the attic is properly vented.

B - The grate inlets

Smaller homes and bigger building grate wall inlets.
The most simple wall inlet. This is the old style grate. In major storms the wind sometimes blows in the attic rain or snow. It is recommended to lay a loose plastic 6' feet by 6' feet on top of the insulation near the grate inlet. Snow or rain will eventually evaporate without causing damage to the underlying ceilings.	These following grates have wider blade louvers. Properly designed grates are very efficient air inlets to attics. Again, bigger grates left rain or snow blow in the attics. In the worst cases, a baffle was installed behind the grates that had the shape of a half funnel or an inverted hood.	Bigger size buildings without inlet grates always have trouble with the top of the brick walls and the overheating of the roofs.

b) Here is a very big grate used as air inlet to the attic. There is also not one, but two decorative granite cornices.

When these kind of huge grates are used on four walls, the flow of air through the attic may be so important that rain and snow may be pulled in the attic. An inverted hood is then installed as a baffle on the inside of the attic.

C - The one brick replacement inlet.

a) These vents are the dimension of one brick and are set every 10 to 15 feet apart at the level of the attic.

The One (1) brick wall inlet to the attics.
There is grates having the dimension of one brick that is used to vent triangular attics at the level of the bedrooms. These may be used in conjunction with flat vents on the roofs.	A brick is replaced by a grate inlet where there has been a problem of humidity discoloring the walls and creating efflorescence in brick walls. They should be installed not more than 10' to 15' feet apart.	In many buildings, this grate is added when there is a need to add ventilation to an attic when the attic is very small or has almost no height. In this wall, as the attic is high, a hooded grate inlet could have been used.

Note: See also the section on the construction and maintenance of flat roof with crushed stone (temporary in French) and how to vent properly an attic of a property with sloped roof with shingles. ( in French)

In Conclusion

The most important part of this section is: Is there a slot cut out hidden behind an aluminum perforated soffit?

It is something to check in our own home using a wire or a tool with a fine point.

If there is bubbles of tar protruding from the crushed stones on the roof, there is great chances that there is not any slot behind the perforated metal soffit.

What is to be done is explained during a building inspection.

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