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Attic ventilation


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  • Attic ventilation

    Not sure if this is the right forum, but...

    I'm a first-time homeowner. My house is like a number of the 2-story homes in the neighborhood. Attached is a crude cross-section of the second story, attic space in red. The house technically has 3 attics - 1 on top, 2 on each side.

    Top attic: 6' high, 12'wide, 30' long
    Side attics: (approx) 4' high, 4' wide, 30' long (access doors are blocked with carpet installed by former resident)

    I need to get the attics ventilated, and I'm looking for opinions on the best course to take. One idea is to ventilate all attics separately - a combination of undereave and soffit vents for the side attics and something else for the upper. The other idea is to install ventilation shafts on the roof to connect the side and upper attics, so all attics are ventilated together. Any thoughts?

  • #2
    There should be pathways for air to travel from the lower area up over the knee wall of the room and into the upper attic area, this area is between the rafters, the insulation should NEVER stop this airway. If it does for any reason it's a simple fix by buying rafter baffels (about a $1/piece) and installing these between the rafters to hold back the insulation.
    Correct ventilation means air entrance vents in the soffits and an exit vent/s at the peak (ridge vent) or can/pop vents or powered roof vent. Another option would again be soffit vents with gable end vents.
    You need to have 1 sq. foot of intake area and 1 sq. foot of exit area for every 150 sq. foot of attic area.

    Ventilation is an very important factor in homes, not only does it keep mold etc under control correct ventilation will make your home much more cost effective in both heating and cooling.
    Last edited by pushkins; 06-27-2011, 07:54 PM.
    Little about a lot and a lot about a little.
    Every day is a learning day.


    • #3
      I'm not sure yet how much space there is in that diagonal section connecting the lower and upper attics. I'm hoping it's wide enough to insert some PVC pipes or something like that to create air channels. Will try to find the time in the next few days.


      • #4
        The rafters will sit on top of the knee wall so there should be a space between each rafter. I often see where the insulation is installed and stuffed into the rafter space, if so the baffels installed into this area will solve your problem easily and cheaply.
        Little about a lot and a lot about a little.
        Every day is a learning day.


        • #5
          I got up there today. It seems the only thing blocking air flow between n the lower and upper attics is the placement of the insulation on the floor of the upper attic:

          ...../ --------------
          ..../ /
          .../ /
          ../ /
          ./ .| Why monospace fonts aren't obsolete!
          / ..|

          I'm thinking that even after shortening the attic insulation slightly the ends meeting the diagonal pathway may need to be taped down or otherwise secured to ensure an adequate opening - since the insulation is 4 inches thick.

          That diagonal pathway separating the roof from diagonal wall sections appears to be 2-3 inches wide.

          UPDATE: Forum doesn't insert leading spaces - fixed that with periods and white foreground color.
          Last edited by AKH; 07-02-2011, 08:46 PM.


          • #6
            There is no need to cut or worse tape down the insulation, I've mentioned before use insulation baffels, they are available at all big hardware's in the insulation section and cost about $1.00 each. They simply slide in between the rafters between the insulation and roof deck, they are kind of corrugated and allow the insulation to not be compressed (making it worthless) and making airway paths. As well the baffels allow the insulation to cover all the way to the corners of the room below and not leave sections with reduced or no insulation value.
            The baffels are code required in situations like yours, on standard roof line to ceiling meet points (near soffits) or cathedral ceilings where you simply put them end to end (slightly overlapping) for the entire distance required to keep the insulation off the roof deck.
            Little about a lot and a lot about a little.
            Every day is a learning day.


            • #7
              I've attached a somewhat more accurate image of the cross section of the upper story - attic space in grey, baffles in red.

              Lowes sells a baffle (DUROVENT 48" x 22" Extruded Foam Rafter Vent) that sounds like what you're describing.. It's foam so it can be trimmed. I would think that it needs to go only high enough to clear the insulation, with a few inches to spare. And short enough that the top of it is lower than the lowest part of the exhaust ventilation. I'm considering undereave vents.

              Is it necessary for intake and exhaust ventilation to be about the same size?

              I still haven't had a chance to directly observe the diagonal channels for any possible blockage other than the upper attic insulation. I may be able to lower a light source from the upper attic to the lower (I've got the remains of a lamp, essentially an electrical cord with a light socket and switch at one end), and observe the channels that way.


              • #8
                Yes a "durovent" is one of the baffles I've been talking about.
                Yes it only needs to be long enough to clear the obstructing insulation.
                Yes if you have 1 square foot of exit air you need to have 1 square foot of entrance air, anything less creates a negative void for the difference.
                Little about a lot and a lot about a little.
                Every day is a learning day.


                • #9
                  We have a nearly identical situation to this - except we have batting insulation in the knee and blow in insulation in the top attic.

                  Do we need to have baffles between every rafter, or should they be staggered or should the number of them be determined by the airflow of the vent/intake?

                  It looks to me like we will need 5-6' of baffle to make it through the knee and the blow-in insulation - how would you suggest installing these?

                  Thanks in advance for your help.


                  • #10
                    Most importantly you need a baffle between rafters that have a vent opening in the soffit to allow air movement from that intake vent up into the attic space, ideally every rafter space should have a baffle to allow complete air movement.
                    You can run the baffles as far as you need to clear any insulation obstructions, just keep adding one to the other with a 2"-4" overlap.
                    (cathedral ceilings have baffles running from soffit all the way up to the ridge vent)
                    Little about a lot and a lot about a little.
                    Every day is a learning day.