Soffit venting needed for 3/12 roof?


  #1  
Old 02-28-06, 10:48 PM
M
Member
Thread Starter
Join Date: Feb 2006
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Question Soffit venting needed for 3/12 roof?

Does anyone know if I should install soffit vents with this type of application?

I have a 3/12 gable style roof over a ranch style home, approx. 1000sqft. located in the Chicago area suburbs.
House was built in 1963 with no soffit venting. I currently have 6 roof type vents (static) across the span of the roof and 2 gable end vents. I recently purchased the home to find out that loose fill blown insulation was installed w/no baffles in eaves. No soffit vents.
Do I need soffit venting with this pitch roof? Inside eave access is to tight to install air baffles properly. Or can they be installed from outside in, if a larger soffit vent area is cut-out? Insulation in eave area is all the way upto under sheathing and between rafters.
If soffit vents are recommended, should I then remove the gable fan and close off the gables?
Also has anyone heard if these newer style dormer vents are any better than the standard square static vents?
Thanks to any replies.

Mbweeks
 
  #2  
Old 03-01-06, 04:44 PM
Lowongas
Visiting Guest
Posts: n/a
If your not having attic moisture and condensation problems in cold weather I wouldn't worry about it.
 
  #3  
Old 03-01-06, 05:00 PM
P
Member
Join Date: Nov 2003
Location: Montreal, Canada
Posts: 105
Upvotes: 0
Received 0 Upvotes on 0 Posts
Soffit Vents

You appear to have exhaust vents, with the gable vents providing the air intake. This configuration does not vent the area of the attic below the level of the gable vents. If your vapour barrier is excellent, then there will be no entry of moist air into the attic, and the venting situation is not dire. However, most vapour barriers have leaks (tears or rips, cuts made for pot lights and other electrical gear that runs on top of the ceiling), and therefore warm, moist air from the living space enters the attic. Since the attic is colder, the moisture condenses out, usually on the roof sheathing. Besides wetting the sheathing, the condensation will also run downhill, on the underside of the sheathing towards the eaves. If there are no soffit vents, then there is no circulation of air in the lower portions of the roof to dry out the condensation.

Since the air in the attic tends to be warmer than the outside air (partly due to heat loss from the house through the insulation, and partly due to heating by the sun), it will rise and escape by the exhaust vents. This creates a small vacuum, which is then filled by air coming from any roof opening. In your case, this would be the gable vents, and the lower portions of the attic are not ventilated. If, however, you installed soffit vents, you would also need to ensure that there is enough space between the insulation and the sheathing to allow air to move without great resistance. An air channel of about 3-4 inches of clear space will usually allow air movement.

To calculate how much venting you need, the usual method is to determine the square foot area of your attic and divide this by 300 to get the minimum amount of ventilation required (1:150 is a better ratio) in terms of square feet. Half of this should be at the exhaust, and half at the intake. Venting capacity is usually given in terms of inches of ventilation per unit or per lineal foot (of soffit). For example, assume that your attic has an area of 900 sq.ft. The minimum ventilation requirement will be 3 sq.ft, and the desired would be 6 sq. ft. Dividing this in two, you will get a minimum of 1.5 sq.ft. of exhaust and 1.5 sq.ft. of intake. Depending on the model of vent you have, each vent may have an area of 0.5 to 1.0 sq.ft. of net free air. If the vent is rated at 0.5 sq. ft., then you will need 3 such exhaust vents to satisfy the exhaust requirement. At the soffits, soffits are rated by net sq. inches of ventilation for each lineal foot of soffit. Let's say your soffit has a rating of 6 inches per lineal foot of soffit. You will need 1.5 sq.ft x 144 sq.in/sq.ft. divided by 6 inches giving you 36 lineal feet of ventilated soffit. Of course your particular situation will have different numbers, but you get the idea.

With low slopes, it is much easier to insert the baffles from the outside than try to position yourself inside the attic.

Again, the issue for you is whether the existing situation is causing a problem. If your attic is bone-dry even with the current situation, then you don't have a problem to solve, and if it ain't broke, don't fix it. I have seen a number of older homes with no effective ventilation, but the reality was that the attic was dry and no condensation or rot was visible. I've also seen homes where we had to resheath the roof because condensation caused the plywood to delaminate and rot all over. So, it depends on your specific circumstance.
 
  #4  
Old 03-01-06, 09:57 PM
M
Member
Thread Starter
Join Date: Feb 2006
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Soffit venting needed for 3/12 roof?

Thanks to both of you, for your suggestions.
It puts my mind a little at ease, in regards to the fiberfil blown insulation everywhere.
I'll just moniter the attic from here on and see what happens. It could also be a blessing that the home is older and a little drafty, possibly adequately airing it out, unlike the airtight homes we have of today.
Thanks again.
Mbweeks
 
  #5  
Old 03-03-06, 04:50 PM
B
Banned. Rule And/Or Policy Violation
Join Date: Mar 2006
Location: Bend
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
Disagree with Pgriz

Box vents with gable vents generally provide adequate airflow. It is an absolute myth that air below the level of the gable vent will not circulate. It does circulate, just not at the same rate it would if there were vents installed at the eaves instead of the gables. Ridge to eave is optimal but not required.
-Best
 
  #6  
Old 03-03-06, 05:37 PM
P
Member
Join Date: Nov 2003
Location: Montreal, Canada
Posts: 105
Upvotes: 0
Received 0 Upvotes on 0 Posts
Disagreements...

Bestmetalroof, you are entitled to your opinion. The "myth" you refer to has resulted in two of my customers in the past three years having to have resheathing done. In both cases there were gable vents with ridge outlets and above a certain level the wood was in good shape, whereas the lower portions showed increasing amount of condensation damage and mold. The sheathing near the eaves, where the soffits were blocked by insulation, had the most damage. One of the roofs had two levels, and the upper level had functioning soffit vents, whereas the lower level did not.

Of course, the amount of damage condensation can cause is also dependent on the climate. I live in Montreal, where very cold conditions are frequent in winter, and water damage from ice damming contributes to problems if the roof is not well constructed. Condensation problems are often seen in older homes (without vapour barriers built in) where the home has changed hands and the new owners have a different life-style than the previous owners, with much more humidity in the air compared with before. The new owners often upgrade the windows and doors to eliminate drafts, and suddenly there is much more humidity finding its way into the attic. The new owners think the roof is leaking, but checking for either leaks or entry of vapour often shows that the reason is due to house vapour condensing in the attic.

If the above situation exists and the venting is good, then usually there is enough air circulation to dry out most of the new humidity before it causes serious problems. But if venting is poor, then we do find the lower portions of the attic suffering the most.
 
  #7  
Old 03-05-06, 07:21 AM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 9,500
Received 68 Upvotes on 62 Posts
Climatic differences

I would like to add that when offering advice it must be kept in mind that local outdoor temperature and humidity levels determine construction methods.
The region the poster lives in may require techniques that are different than what experts are familiar with in their area.
The theory behind this is the same but some details need to be fine tuned to local conditions to prevent problems.

To give you some idea, what do y'all see when you look out your windows?
This morning I am looking at a snow squall with about three feet of snow in the back of my property.
Today's temperature is quite mild at -6 degC (20 degF) and there is about two feet of snow piled on my 7/12 roof.
http://weather.ec.gc.ca/city/pages/mb-30_metric_e.html

I can well imagine one or two of you see something quite different.
 
  #8  
Old 03-05-06, 03:57 PM
B
Banned. Rule And/Or Policy Violation
Join Date: Mar 2006
Location: Bend
Posts: 8
Upvotes: 0
Received 0 Upvotes on 0 Posts
Whoops!

Forgive me, I assumed that in talking about roofs we were discussing roofs that adhere to the Interantional Building Code--not roofs that lacked vapor barriors. Regardless of climate, if a roof is not installed as it should be, one can assume there will be problems. The lack of a vapor barrior has noting to do with box vents and gable vents not working together. It sounds to me like the problem your client experienced was unrelated to the venting, although better venting may have helped compensate for an improper installation.
-Best
 
  #9  
Old 03-05-06, 07:07 PM
P
Member
Join Date: Nov 2003
Location: Montreal, Canada
Posts: 105
Upvotes: 0
Received 0 Upvotes on 0 Posts
Vapour barriers...

Although the vapour barrier was invented around 1938, it was not until the 1970's that it became a common feature in home building. In Canada, the vapour barrier did not enter the building code until 1986. So many homes built before the 1980's did NOT have vapour barriers explicitly included in the construction. Homes built at a certain time must adhere to the building codes in force at the time of their construction, and as building codes evolve, new construction must meet the codes, but older homes are not required to be retrofited with every change of the code. Therefore, it is very common, when doing renovation or reroofing on an older home, to have to make a decision whether to bring the structure to the new level. Most municipalities do not require this, and since such remedial work can be very expensive, it is often not done UNLESS there is a problem developing.

Prior to the 1970's homes tended to be rather drafty, (with up to four changes of air per hour). The move to make the homes more airtight after the energy crisis of the 1970's lead to efforts to reduce the air leakage, by upgrading doors, windows, and various other openings to the outside. However, the construction of the walls, basements and ceilings was not (usually) changed, so while there was now less air loss due to drafts, there was an increase in interior humidity. Since there was no effective vapour barrier in the homes built before the 1970's, the increase in humidity started causing condensation in attics and walls.

We were involved last year in a project where we had to reroof a home built at least 150 years ago. At that time, sawdust was used as insulation, and wood shakes were common roofing materials. We had a major job in retrofitting modern roofing materials and techniques on a house that was build before building codes existed. At the same time, we had to preserve the "historical" aspects of the house. It was not an easy task to make it look "authentic" while still incorporating modern knowledge and understanding.

So the lack of a vapour barrier in many homes is not due to "improper installation" but to the prevailing practices of the time the homes were built. These older homes often have a lot of charm and appeal, but the new owners face challenges in ensuring that their improvements take into account the house as a system. Failure to do so gives rise to the "law of unintended consequences".
 

Last edited by pgriz; 03-06-06 at 04:46 AM.
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: