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  1. #1
    Join Date
    Jul 2011
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    14

    Default Roofing ventilation

    I would like to know when having a new roof installed, is it better to have a ridge vent or is it better to install attic fans?

  2. #2
    Join Date
    Aug 2007
    Location
    Portland, Oregon, formerly of Chicago
    Posts
    1,808

    Default Re: Roofing ventilation

    My preference would be for passive rather than active venting.Passive would be venting at the highest point of the roof with soffit venting around the perimeter of the house. This allows cooler outside air to wash the underside of the roof, exiting through the top via natural convection currents.

    The problem with electic venting, especially with air-conditioning, is that the average house leaks like a sieve. If you create a negative pressure in the attic, you will be sucking conditioned air into the attic and to the outside. If you do go the attive route, don't fall for "more is better". Just do enough to gently exchange the air without creating a low pressure in the attic.

    Every ceiling light fixture, juncture of walls with ceiling, pull-down attic staircase, etc is potentially leaking air to the attic. Don't aid in sucking the conditioned air out of your house!

  3. #3
    Join Date
    Jul 2011
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    14

    Default Re: Roofing ventilation

    Good point, I didn't think of it that way. It would be better to have the area ventilated from peak to peak, rather than the air being drawn out through a hole in the roof. Thanks for your advise.

  4. #4
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,969

    Default Re: Roofing ventilation

    Peak to peak?

    I trust you meant eave to peak.

  5. #5
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    6,758

    Default Re: Roofing ventilation

    Yes, peak to peak is probably a mistake.

    I back ordjen's idea.

    The volume size of your attic, should determine the size of your vents. The cooling of your attic should be done by those vents.

  6. #6
    Join Date
    May 2011
    Posts
    251

    Default Re: Roofing ventilation

    If you have no attic venting currently or a limited amount of venting, you could consider sealing hte attic and insulating the roof deck with spray foam. Why vent an attic and still have it over 110F, when you can insulate it and keep it around 80-90F with no vents? Just a thought.

    I have my roof deck spray foamed for about the same amount it would have cost me to add soffit and ridge vents without havingot cut holes in the house.

    After a month of near record temps in Iowa, my 3200sqft 1925 2 story home with 42 single pane windows and two 17 year old air conditioners, only cost $240 to cool with electric rates at $0.14/kw-hr. I was pleasently suprised. I was prepared for a $350 cooling bill.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  7. #7
    Join Date
    Jul 2011
    Posts
    14

    Default Re: Roofing ventilation

    My mistake, I ment to say eave to peak. Thanks for the info.

  8. #8
    Join Date
    Aug 2007
    Location
    Portland, Oregon, formerly of Chicago
    Posts
    1,808

    Default Re: Roofing ventilation

    Motoguy,

    Wally's premise was if he should increase venting at the time of re-roofing. At that time, venting is relatively inexpensive additional cost.

    I can believe that a 1920's house might greatly benefit from encapsulating the attic. Homes of this era were really prone to air leakage. It is possible that there are not even firebreaks in the walls preventing air from traveling from the basement to the attic! I remember my father once trying to add pour insulation to his old farm house walls. When, after pouring in several bags of insulation into the walls from the attic, he decided to go down to the basement and see what was happening. It should have been filled, he thought. There in the basement he found a large pile of insulation on the basement floor! His house had been built with balloon frame construction with narry a firestop in that wall!

    One of the benefits of sealing the attic with foam is the sealing of air leakage from the house into the attic,as the attic becomes part of the house envelope. However, you are still going to be conditoning additonal volumn of air.

    One other possible solution to air sealing is to remove the existing attic insulation, spray foam the attic side of the ceiling, thus ceiling all points of leakage, and then replacing the blown in insulation. This is, however, not an inexpensive option. Whether the roof is sprayed or the attic floor, sufficient foam needs to be applied so that water cannot condense on a cold surface. This is also true when spray foaming wall cavities. A thin foam coat is insufficient.

    One thing that would bother me about spraying the underside of a roof is what happens if a leak developes? Water is then held somewhat captive in the above wooden structure with the source of the leak difficult to locate.

    Just food for thought. This topic is greatly debated on the building blogs.

  9. #9
    Join Date
    May 2011
    Posts
    251

    Default Re: Roofing ventilation

    I have a furnace in the attic, so not really an option to insulate the attic floor. I still have quite a bit of humidity in the attic so I think there were some leaks along the rim joists that got missed. I'll have to completely remove the old cellulose and have the insulation company come back and spray a little more.

    I used open cell, so in theory, a major leak would work it's way through the foam to the ceiling.
    1925 Two-Story Stucco Beaux Arts Neoclassical

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