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Community and Q&A

Adding Vents to a House With No Soffits

victorianhouse | Posted in Energy Efficiency and Durability on

I could use some advice on adding vents to my house which has a flat or almost flat roof on the very top and no soffits.  I have a Second Empire Victorian House with a Mansard roof that is in Western Massachusetts.    The top roof is a Firestone 60 mil TPO fully adhered roof which was put on top of 1″ polyisocyanurate insulation over modified bitumen roof.  When we installed this roof I eliminated some of the vents which probably was a mistake.

I now plan to add insulation to the attic but am told I need vents in the roof.  One roofer suggested two Lomanco Whirlybirds but also said it might not work because there were no lower intake vents.  A Lomanco engineer recommended against his own vent because there are no lower vents and suggested a gravity vent made by Greenheck.   But a Greenheck technical sales rep recommended against installing a gravity vents because I would be
expecting the air to flow in and flow out voluntarily.

You can see from these two picture that there is no place for soffits and that the roof is basically flat.

So what kind of vents should I install?

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  1. Expert Member
    Michael Maines | | #1

    VictorianHouse, you won't hear me saying this very often because I avoid foam whenever possible due to climate and health concerns, but this is a situation where closed cell foam is likely the best way to ensure your structure remains safe from moisture issues. I specify HFO-blown foam, which has a blowing agent that is not as terrible a greenhouse gas as conventional HFC-blown foam. If your installer doesn't know what HFO-blown foam is, find a different installer.

  2. victorianhouse | | #2

    Thanks for responding to my post and questions Michael. A couple vent manufactures recommended I go with gravity vents. One has recommended two intake vents at low points at both ends of the roof, and two exhaust events at the highest point on the roof. Active Ventilation Products Inc is recommending:

    >AV-16-C8 or C12 (with an 8" or 12" collar height) for exhaust

    >PV-16-C8 or C12 for intake.

    Does this make sense to you?

  3. Lindaloowho | | #3

    Love that house!!!

    I’m not an expert, but I’ve been researching some options for my soffit-less 100 year old cottage.

    If you did want to look at adding soffit vents, two things I’ve found are:

    1. Vented Drip Edge

    2. Shingle Over Intake Vent

    Look them up and see if either of these would work on your beauty!

    1. victorianhouse | | #4

      Thanks for your response Lindaloowho,
      I think replacing the current drip edge which was custom made with new vented drip edge would be very very expensive and is not really an option. But maybe vented crown molding could be something to look into. Shingle Over Intake Vent would also not work given that the shingles are the exterior walls for the 3rd floor. I'm hoping gravity vents will work and now I'm waiting for experts to weigh in and tell me if these will work...

  4. Expert Member
    Akos | | #5

    With living space under a roof like yours, any type of spinning vents, powered or not, are a bad idea. It is very easy for these to depressurize the attic space and exacerbate moisture problems by drawing in more moist air from the house through leaks in the ceiling/walls.

    Stick to simple vent openings.

    Around me (Zone 5) lot of low slope roofs are vented with a combination of edge vents (just a gap behind the drip edge) and the occasional flat roof vent.

    If you can get a vented crown installed and there is enough opening behind to feed the attic, it is probably your best bet. You can also strip back the top two rows of shingles in areas, cut an opening near the top to the attic space and install shingle over vents. This might be cheaper as it can be done by the roofer and no carpenter is needed.

    Our code requires about 2x the vent area for low slope roof vs pitched roof, so make sure to size your vents accordingly.

    Some exhaust vents near the top of the ridge might also be needed. You can try just installing the edge vents first and install these if you see issues.

  5. victorianhouse | | #6

    Thanks for weighing in Akos,
    The exhaust vents I'm thinking of getting do not spin. The Active Ventilation Products Inc. says "the wind spins around in the head to cause the drawing out of the air beneath it."

    Does this make sense and have any impact on your thinking?

    I will look into vented crown molding and stripping back the top two rows of shingles. But that seems like it would be very expensive.

    What if I had gravity vents like Greenheck’s GRSI and place them at the lowest point on the top roof and at the highest location of the top roof? Would that work? Here is a picture of what the Greenheck gravity vent looks like. There is nothing spinning here.

  6. igrigos | | #7

    I'm curious as to where your thermal boundary is currently. You mentioned polyiso under the membrane, which seems like a waste as it won't be doing much over a vented attic. Do you have an attic space with insulation on the floor?

  7. victorianhouse | | #8

    There is insulation over the ceiling of the 3rd floor. Our plan is to add additional insulation in the attic. There is a trap door on the ceiling of the 3rd floor, leading to a hatch to the roof. These photos are of the space between the roof and 3rd floor attic. And yes, the insulation under the TPO is a bit of a waste with respect to insulation. I can't remember why we added that. It's possible the roofer thought it would be a good surface upon which to lay down the Firestone TPO.

  8. Expert Member
    Akos | | #9

    Those vents are for commercial application where there is fan pushing air out, closer to a range hood cap than a roof vent. You want simple passive vents, essentially a big hole protected from rain.

    Depending on how your crown is installed and finished, you might be able to drill holes in it and protect with color matched covers. In your case it is also much easier to add vent area to the peak of the roof, our code allows only 25% of the total vent area to be down low, which would reduce your install costs a fair bit.

  9. victorianhouse | | #10

    Thanks for this comment Akos,
    What if I had two whirlybird style turbine vents at the highest points on the roof, and then two gravity vents like Greenheck’s GRSI to allow the air to flow in on the lowest points of the roof?. Would that work?

  10. Jon_R | | #11

    You can power vent an attic, routing ducts wherever convenient. For moisture control (vs attic cooling), very little CFM is needed. So it will use little electricity. The common concern about depressurizing a) isn't significant at very low flow rates or b) can be avoided completely with push/pull ventilation.

  11. victorianhouse | | #12

    Thanks for this comment Jon R.
    The house has no ducts. The house is heated with hot water. Can you explain what you mean. Are you saying I should have an exhaust fan on the very top of the roof and an intake vent also installed on the lowest section of the roof?

  12. SecondEmpire | | #13

    Great question and situation. We have a simular situation but more of a temperature control issue due to heat in our attic. With 4 vents on the mansford roof and our soffits containing the old historical gutters, I am searching for a solution to control radiant heat. I was thinking of adding a low cfm fan/vents with thermostatic monitoring to two vents at the front of the house to pull air in from the back vents. When researching this topic I found this post. I certainly do not want to cause moisture issues. What did you finally decide? Our place is a second empire 1875 brick home.

  13. victorianhouse | | #14

    Hi Second Empire,
    Great to hear from you and compare notes. I decided to not install any vents in the roof. Instead I installed a Monnit humidity and temperature sensor in the attic that transmits data via wifi to monitor the environment there. I had Mass Save dense pack cellulose in the walls of the mansard roof which are the exterior walls of the 3rd floor, and spray cellulose into the attic.

    I connected with a lot of people to help me figure out what to do. The building scientists each recommended something different.

    In the end one insulation company recommended that I not add the vents, and that I spray in the cellulose and monitor the attic. He said there was a lot of ventilation in the attic given the cracks along the crown molding, and that the attic was not so well sealed from the outside to begin with. He said I probably don’t need vents.

    The roofer who would have installed the vents and profited from the job also recommended against vents, telling me I didn’t need them give that the attic had air leaks.

    So this is a work in progress. The Monnit device is great. I can check the humidity and temp on my iphone. I also have some Proteus sensors on the 3rd floor, 1st floor and basement.

    I might have to eventually install vents but at this point I don’t think I have to.

    As far as I can tell there has not been condensation in the attic. Although I might have to install another sensor, one that really detects the presence of water, to really be sure there is no condensation.

    It does get humid in that attic, but it does that when it’s humid outside. Right now the humidity is 77.02% and it’s 67 degrees in the attic. But outside its 68 degrees with 87% humidity. I can’t expect to humidity to remain below 60% when outside the humidity is above 60%.

    So I will have to see how things go this winter when there is low humidity outside. What will the humidity be in the attic? Hopefully it won’t be too high.

    What is the problem with radiant heat in your wonderful house? Is the 2nd floor really hot from all the heat in the attic? What kind of roof do you have and what kind of insulation do you have in your attic? My house is in Western Massachusetts. Where is your house? Also, do you really have vents in the mansard part of the roof or are the vents on the very top of the house?

  14. SecondEmpire | | #15

    Excellent update and I look forward to hearing how this goes with your situation. So yes, I am using radiant heat as a term. The roof if metal with a slight pitch. There are 4 vents on this with insulation which looks simular to your initial picture of your house. A low crawl space is the attic with perhpas 30" space between the ceiling and the roof.
    I have been exploring upstairs bedroom temps vs air conditioning effectiveness. The Nest thermostat is downstairs and we get the cooler locations downstairs and good airflow from the upstairs vents with a 10 degree difference. When I open the upstairs closet I notice a 15 degree temp difference in the closet. So this makes me think it is not an air-conditioned issue but perhaps looking at attic ventilation would help with keeping the attic a bit cooler to help with impact of upstairs temps. Right now I set up a high capcity fan downstairs and circulate air from downstairs through the front staircase to the back staircase and I notice a great temp control with the air on.
    So your thoughts on humidity definitely could increase with forced circulation in roof vents and perhaps looking at increasing insulation via blown in might be a better option to explore.
    We are located in Lancaster Ohio. Thanks for the feedback and update!

  15. victorianhouse | | #16

    But is the only problem is that your closets on the 2nd floor are 10 degrees hotter?

    1. Expert Member
      Akos | | #17

      Generally increasing attic ventilation barely drops the attic temperature, so unless you have mold and condensation issues in the attic, it is not worth it.

      With 1/2 story construction, hot upper floor is almost always from air leaks. Since it is colder inside, in the summer time stack pressure pulls in outside air through all the air leaks in the upper floor. To make the problem worse, lot of AC systems don't have a large return air register near the ceiling of the 2nd floor, so all that hot air stratifies overheating the upper floor. Lot of times comfort can be improved by adding a high return.

      This won't solve your air infiltration issues and associated heat loss, tightening up the place and stopping the air leaks will significantly improve comfort and reduce energy use.

      The first step would be to air seal around your knee walls, there are many articles out there on how to do this properly, this is a good start:

      Keep in mind that insulating is usually not the same as air sealing. Stuffing some batts into gaps or filling the attic with a foot of loose fill will do almost nothing for air leaks. To deal with air leaks, you need to plug the holes with a combination of rigid insulation, wood blocking and spray foam.

      With older construction and complicated roof line, there will also be a lot of other major air leaks paths, the best is to do blower door directed air sealing. There are companies that specialize in this type of work.

      1. SecondEmpire | | #19

        Thank you and I appreciate your reply. Great information!

  16. SecondEmpire | | #18

    No, I was referring to the thought the top floor tempature is increaswd due to the hotter tempature in the attic. The closets are just an indication that the radiant heat in the closets-attic could be affecting hotter temperatures upstairs and pehaps control of the attic temps could affect the overall temperature upstairs. This is where I was thinking additional forced low cfm vents could circulate the air in the attic causing a cooler attic and this would reduce the impact of upstairs temps.

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