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Frost in attic / roof vent condensation

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  • Frost in attic / roof vent condensation

    Issue : I noticed that I have frost in my attic in the winter and that one of my 2 roof vents is forming condensation which is dripping onto the insulation.

    Background info : cold weather climate (winter lows of -25C). I have a 1000sqft attic floor area with two 301 maximum static roof vents (https://www.ventilation-maximum.com/...f-ventilators/). The vents are spaced about L/3, L being total roof length). I have continuous perforated soffits along the front and back side of the roof. There isn’t a perfect vapor barrier, we have some sort 1/8 inch or so fiber board on top of drywall which isn’t sealed between boards.

    Frost issue. I realize this is likely a vapour barrier + ventilation issue. What has me perplexed is where I’m finding concentrations of frost. The frost seems to be concentrating along the lower end of the gables and I’ve even noticed some frost forming between my air barrier and stucco cladding. Any idea why the frost is accumulating in this area? See attached pictures. Would adding baffles along soffits improve ventilation? Insulation isn’t blocking soffits currently.

    Vent issue
    i don’t understand why I have condensation on inside of roof vent..I thought that the airflow would keep this area dry. Any idea what’s happening here? Could it be that the hole cut in sheathing is too small? Normally holes are 12x12 but in this case they cut it to about 9x11.

    i really appreciate any help
    i can get on this. Im a structural engineer and will gladly return the favour by contributing to the forum in whatever way I can.
    Profile of envelope along gables Condensation in roof vent Example of frost issue

  • #2
    Hey there. I'm a little confused by your first pic. "Stucco, air barrier, fiberboard..." which part is the air barrier? It looks like daylight?

    Anyways, it's hard to accurately troubleshoot without being on site and tracking temperatures, but my seat of the pants answers are:

    You're seeing frost where cold air is entering your attic space and cooling nearby surfaces to below the dew point. Air (thus heat and water vapor) leaking through the ceiling assembly will make this worse, as will inadequate ventilation. Seeing as the frost is forming down low where the cold is entering, and not in roof sheathing in general, that would indicate that the attic space is being kept warm. And as that warm(er than outside) air hits the vent hood up high, it condenses and drips back down.

    I would check the math on your vent area and make sure its adequate, and perhaps more importantly, try to spend some time airsealing your attic floor/ living space ceiling as well as possible. Spray foam any pipe or electric penetrations in top plates of walls, bury ceiling light boxes in foam or build air tight boxes around them.

    Exercise caution air sealing around recessed can lights, if you have those. If they're equipped with high wattage bulbs and/or arent rated for insulation contact, enclosing them can cause temperatures high enough to damage the wiring and create a dangerous situation.

    Hope this helps.
    -Dan

    Comment


    • #3
      I ignored your baffle question. Whoops! I'm not sure that the baffles would improve your frost situation, and if the insulation isnt blocking the vents, that's good news. The baffles can prevent air movement from reducing the efficiency of the insulation next to the vents, though. "Wind washing"

      What's the insulation situation in there, anyways? Looks like its pulled out of the one joist bay in the photo.

      Comment


      • #4
        My thought is that the problem is more related to lack of air barrier measures than that of lack of a vapor barrier. The warm/conditioned/humid air in the house is escaping into the attic, reaching the dew point and condensing and in some instances freezing. Think of a glass of iced tea on your porch in the summer time. There's plenty of ventilation around the glass. It's the warm/humid air that contacts the cold surface , condenses and runs down the side of the glass.
        In 1982 I built a house a few miles below the Arctic Circle. Building Science was not nearly as advanced or on the minds of builders as it is today. While we installed Tyvek on the exterior we didn't do anything to stop air infiltration into the attic. I stayed in touch with the owner who told me that by mid winter there was appx an inch of ice covering all the framing members and the sheathing. He literally banged it loose, collected and removed it.
        The house I live in now has no vented soffits and a ridge vent the attic is bone dry. Granted I'm in upstate SC but my last house in Ps. was the same situation. I'd look to seal up your attic ceiling, pipes, lights, partition intersections with walls & ceilings as a first measure.

        Comment


        • #5
          Thank you! I really appreciate these recommendations.
          The insulation is about 10-12 inch thick fibreglass insulation. I see black strips along the bottom of it where air is coming in from house, where “vapour barrier” is discontinuous.

          I did some airsealing two summers ago but I guess it would be a good idea to take another look. If the air sealing around pipes and light fixtures etc is done, would my next course of action be improving my vapour barrier? I was thinking of adding a vapour barrier paint to the ceiling, minimally the bathroom ceilings. Do you you have any tips as to how to improve/add a vapour barrier in an existing home?

          My second course of action might be in regards to the ventilation. My two static vents are maybe two close together. Manufacturer recommends an even distribution of vents ( so if two vents, space Lroof/4 away from edges): In my case they are spaced significantly farther away from the ends (see attached picture). I see that before previous owners had roof redone, there were 4 square louvre vents, well distributed along length (I still see holes cut in sheathing). These louvre type vents were replaced by two static vents. The contractor used the two most center louvre vent positions to install the new vents. So I’m wondering if now the two more powerful vents aren’t providing enough “pull” near the gables. I also read something about how placing static vents too close to each other could have a negative neutralizing effect on airflow in attic. Do you think this could be an issue worth tackling? Does the theory make sense to you?

          Comment


          • #6
            Originally posted by brassaw View Post
            Hey there. I'm a little confused by your first pic. "Stucco, air barrier, fiberboard..." which part is the air barrier? It looks like daylight?

            Anyways, it's hard to accurately troubleshoot without being on site and tracking temperatures, but my seat of the pants answers are:

            You're seeing frost where cold air is entering your attic space and cooling nearby surfaces to below the dew point. Air (thus heat and water vapor) leaking through the ceiling assembly will make this worse, as will inadequate ventilation. Seeing as the frost is forming down low where the cold is entering, and not in roof sheathing in general, that would indicate that the attic space is being kept warm. And as that warm(er than outside) air hits the vent hood up high, it condenses and drips back down.

            I would check the math on your vent area and make sure its adequate, and perhaps more importantly, try to spend some time airsealing your attic floor/ living space ceiling as well as possible. Spray foam any pipe or electric penetrations in top plates of walls, bury ceiling light boxes in foam or build air tight boxes around them.

            Exercise caution air sealing around recessed can lights, if you have those. If they're equipped with high wattage bulbs and/or arent rated for insulation contact, enclosing them can cause temperatures high enough to damage the wiring and create a dangerous situation.

            Hope this helps.
            -Dan
            Thank you! I really appreciate these recommendations.
            The insulation is about 10-12 inch thick fibreglass insulation. I see black strips along the bottom of it where air is coming in from house, where “vapour barrier” is discontinuous.

            I did some airsealing two summers ago but I guess it would be a good idea to take another look. If the air sealing around pipes and light fixtures etc is done, would my next course of action be improving my vapour barrier? I was thinking of adding a vapour barrier paint to the ceiling, minimally the bathroom ceilings. Do you you have any tips as to how to improve/add a vapour barrier in an existing home?

            My second course of action might be in regards to the ventilation. My two static vents are maybe two close together. Manufacturer recommends an even distribution of vents ( so if two vents, space Lroof/4 away from edges): In my case they are spaced significantly farther away from the ends (see attached picture). I see that before previous owners had roof redone, there were 4 square louvre vents, well distributed along length (I still see holes cut in sheathing). These louvre type vents were replaced by two static vents. The contractor used the two most center louvre vent positions to install the new vents. So I’m wondering if now the two more powerful vents aren’t providing enough “pull” near the gables. I also read something about how placing static vents too close to each other could have a negative neutralizing effect on airflow in attic. Do you think this could be an issue worth tackling? Does the theory make sense to you?

            Comment


            • #7
              Bob,
              Where are you located, what is your climate zone?
              You have 1000 SF of roof and only two vents.
              How big are the vents?
              what is the soffit ventilation SF?

              Have you done any calculations as to how much ventilation is recommended for your roof?
              Mark Parlee
              BESI(building envelope science institute) Envelope Inspector
              EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
              EDI Seminar Instructor
              Level one thermographer (Snell)
              www.thebuildingconsultant.com
              You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

              Comment


              • #8
                Originally posted by Mark Parlee View Post
                Bob,
                Where are you located, what is your climate zone?
                You have 1000 SF of roof and only two vents.
                How big are the vents?
                what is the soffit ventilation SF?

                Have you done any calculations as to how much ventilation is recommended for your roof?
                Hi Mark, I’m located in climate zone 7. I have two vents which are of this model https://www.ventilation-maximum.com/...f-ventilators/.

                I have perforated soffits that run continuously, along front and back edge of roof. See picture for an example of the soffit I have.

                i did not do any elaborate calculations. I saw some rule of thumb calculations where the total sqft area is divided by 150. Then half of that is for soffits and the other half for the vents. Wasn’t sure what to consider in terms of area for my soffits. And for the vents, well the manufacturer claims one of these can vent 1200sqft. Any thoughts on this? Do you have any tips or references I could use?
                thanks again!

                Comment


                • #9
                  Yes, there is a lot of information here
                  https://www.lomanco.com/

                  Measure your soffits and get the area. Then do some searches as to what the effective area of ventilation is for aluminum soffit such as yours.
                  http://www.emcobuildingproducts.com/...-Soffit-Panels
                  This one shows the full vented soffit to be 13.inches per lin ft.
                  I would go with 1 to 150 instead of 1 to 300.
                  I m betting you are under vented at the roof.

                  Mark Parlee
                  BESI(building envelope science institute) Envelope Inspector
                  EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                  EDI Seminar Instructor
                  Level one thermographer (Snell)
                  www.thebuildingconsultant.com
                  You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

                  Comment


                  • #10
                    Bob,
                    You don't have enough ventilation and it looks like what you do have is improperly placed. At 1000 ft.sq. you need a code minimum of 2.64 sq. ft.(40% of 6.66 sq.ft) within 3 ft. of the ridge. You said your openings are only 9"x11" which equals 1.375 sq. ft. or 1/2 enough. (R806.2 https://codes.iccsafe.org/content/IR...3_Ch08_SecR806)
                    I would block off the existing upper vents and replace with continuous ridge vent.

                    Comment


                    • #11
                      In my climate, significant undersizing of vents is common and usually fine.
                      Based on that experience, I would consider focusing on sources of moisture entering the attic.
                      Obviously you have some vapour transmission through the ceilings, but my guess is you have two much larger issues: air leaks from the house, and water leaks in your gable cladding. Either of these can put much more moisture into the attic than vapour diffusion.
                      Looking at the photo you posted from outside the gable, there appears to be an exterior water issue, the horizontal board looks to be soaked and damaged, if i'm seeing it right there may have been a wood cap on the horizontal board but it's mostly deteriorated--am i seeing little bits of it sticking out? These horizontal joints are notorious for letting rain water enter unless constructed in a fastidiously correct manner. Stucco is also known for transmitting water inward, and the vertical joints between stucco and trim boards are also common leak spots. All in all, a fair amount of water could be coming in from the gable for days after a rain--how's the weather been up there?
                      The black lines you reference in your insulation batts are what we see where batts cross unsealed top plates. Top plates shrink a bit and let air leak out of the house into the attic, due to "stack effect" these small leak spots on attic floors leak a lot over time, the air carries dust from inside which the fiberglass filters out of the air--that's what makes the stripes. Anyway, air sealing all the joints on both sides of all the top plates is the way to solve this. A decent resource is the EPA DIY Guide to Home Sealing, linked here.
                      Hope that's not all too basic, but in my climate (4, obvs much milder than yours) 1/300 venting is more than adequate provided the house is well air sealed and there are no rain water issues, I would start by addressing both of those as no amount of venting will solve significant-enough moisture entry, and nominally inadequate venting will solve an attic that's pretty dry to begin with.
                      Another important question...did anything change this year that would make the attic more wet? Add a humidifier inside the house, more people showering than previous years, more rain...?
                      Hope that's helpful
                      Doug

                      Favorite tool this week: Makita double-battery "worm drive" framing saw
                      http://www.jlconline.com/author/doug-horgan

                      www.bowa.com

                      Comment


                      • #12
                        Very good points Doug

                        https://www.energystar.gov/campaign/...yourself_guide
                        Mark Parlee
                        BESI(building envelope science institute) Envelope Inspector
                        EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                        EDI Seminar Instructor
                        Level one thermographer (Snell)
                        www.thebuildingconsultant.com
                        You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

                        Comment


                        • #13
                          I would improve the air barrier between the house and the attic by sealing all of the penetrations, fixtures/devices, recessed can lights, tops of partitions, etc., then greatly improve the attic ventilation.
                          When you've got them by the balls, their hearts and minds will follow.

                          Theodore Roosevelt

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