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Roof ventilation solution

Post in 'DIY and General non-hearth advice' started by Vic99, Jul 5, 2013.

  1. Vic99

    Vic99 Minister of Fire

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    My house is a Dutch gambrel, so the roof is quite large. My second floor is really hot without ac and I wanted a realistic solution. Opening windows is not always an option because of oppressive humidity for several weeks and I don't have many windows up there.

    Last year old roof was striped off and new asphalt shingles were put on. Ridge vent was put on. In theory air flow happens up soffits and out of ridge vent. Solar panels sit on top of 1/2 of roof and were professionally installed with railing for ventilation, etc.

    Attic is unfinished and has ~12 inches of fiberglass batts and 3-4 inches of cellulose on top of that.

    Should I also install an attic fan? . . Could even get a solar one.

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  2. woodgeek

    woodgeek Minister of Fire

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    Your attic insulation is rated to R-50. The heat from a 140°F attic that leaks through that to the second floor should be around the same BTUs as comes off a 100W light bulb. Since your upper floors are hot, the only conclusion is that your attic floor is not airsealed, and superheated air is being pulled down from the attic by a reverse stack effect (when your house is cooler than outside) and getting dumped into the second floor.

    An attic fan will only pull cool air out of your house through the attic. MIght help with the balance issue re the second floor, but will def increase your AC bill, by drawing in hot humid air into your house through (non-attic) openings.

    IOW, if your attic is not airsealed, an attic fan increases your AC bill. If it is airsealed, using one won't do anything for you at all.

    So, you need to find some way to airseal that attic under that insulation...there might be some big/easy bypasses like the plumbing stack, kneewalls and open stud cavities. Sealing the interior top plates will also help. DIY, a cheap IR thermo at the ceiling wall interface will show where the hot air is leaking in. Or you could get pro help, prob with rebates from MA.
  3. bmblank

    bmblank Minister of Fire

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    One of the easiest things is to make sure your soffets are venting properly. A ton of houses have the problem of the insulation plugging of the soffet vents. With no cool air to take its place, the hot air can't go anywhere.
  4. heat seeker

    heat seeker Minister of Fire

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    One house I bought had exactly that issue. Someone had stuffed the attic insulation into the soffit area, thus no ventilation to speak of. They make channels to let the air pass:

    http://www.lowes.com/pd_135598-75227-UDV2248_0__?productId=3012019&Ntt=attic insulation vent

    An attic vent fan would be an excellent idea, IMO.
  5. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Just went through this drill while having my new roof installed. I had the continuous perforated soffit unplugged, ridge vents installed and blocked off the gable vents and removed the gable fans. If you have ridge vent and the soffit vents are open and have adequate net free area at the soffits you need to block off any gable vents you have. They short circuit the air flow from the soffits up through the ridge vent and can actually turn the ridge vent into an intake instead of exhaust. A gable vent fan would just make it worse.

    I had lots of doubts but with the blast furnace sun we have right now I am watching the temp/humidity monitor I have up there and the stuff is actually working.

    Search the web for attic ventilation and you will find tons of information.
  6. Vic99

    Vic99 Minister of Fire

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    BB even though your situation sees better airflow, do you see a noticeable difference in terms of comfort, or less ac cost, or something else. Gonna be a sucky job with all that heat and fiberglass. I wonder if temp will be under 100 F if I go up as the sun rises.

    If you think its worth it, I'll go through and see if all the soffits are unplugged. I have an old half round gable window in the attic that I'll caulk seal. Will report back within a few days.
  7. jharkin

    jharkin Minister of Fire

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    Get a free energy audit from MassSave. They do air sealing for free.
    seige101 likes this.
  8. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    New roof and ridge vent is only a couple of weeks old and I just closed off the gable vents day before yesterday. With a large 4/12 roof with dark shingles soaking up sun the attic is 134 degrees at the moment with 87 degrees and super humid outside at high noon. Before with the solar gable fans I had it would be hanging out at 150 about then. The big noticeable thing is how quickly it drops back to close to ambient when the sun head/goes down. A big thing with ridge vents is they are more efficient when there is some breeze blowing over them. Earlier today we had a steady breeze and the attic stayed 20 degrees cooler at the same outside temp until the breeze died. Then it started moving up.

    As to clearing the soffits, only as much air can go up and out as can come in at the soffits.
  9. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    A little update. At ten pm the difference in temp outside and in the attic is six degrees. The setup is shucking heat pretty well given the heavy dark architectural shingles, low slope and the wood holding heat. A steeper slope would move more air.
  10. Hogwildz

    Hogwildz Minister of Fire

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    Vic, first thing to do is make sure the soffit area is clear, then make sure the ridge vent is also clear. I have seen half assed jobs where the roof decking under the ridge vent was never cut back or cut back enough to allow air to flow out of. Adding a fan, roof vent or gable vents only does as BB stated, they short circuit the soffit to ridge vent flow and you loose that flow in the soffit up and out the ridge vent. As far as air sealing, I highly doubt any small air leaks at the ceiling level is enough to be any cause of your issue. Aint no hot air going to flow downward, surely not enough to turn your upstairs into an oven.
    Check the soffit and ridge vents and make sure they are clear, and have enough space for airflow. I think you are going to find that the f/g batts &/or the blown in is blocking the eave edges where the soffit is. Another reason to keep this clear is lack of air flow will cause mold on the underside of the decking.

    What many folks forget is with a 2 story, you have the heat from the first floor rising to the second along with the heat on the second floor. And the tighter the ceiling in the second floor is to the attic, the less that hot air can escape. So much heat is trapped upstairs, esp when windows are kept closed due to outside humidity and attempting to keep it out of the house. I do the same here. That R50 in your attic is also doing its job and keeping the heat down in the 2nd story also.
    Is there much for attic space? Is the second story ceilings following the roof line? I hope they didn't pack the roof rafter cavities tight to the decking with insulation, that will bring serious problems.
    Butcher likes this.
  11. bmblank

    bmblank Minister of Fire

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    You could put in a fan and use the forced air movement if you block of the ridge vent. That way there is a path from the bottom to the top, no short circuit.
    But (as repeated ad nauseum) with proper soffit and ridge venting you shouldn't need forced venting.
  12. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    For sure. I discovered that when this barn was built in 1985 that the full perforated soffit was rendered useless because they stuffed batt insulation between the rafters behind the the rafter tails. I had the roofing gang pull those plugs out. Last winter because I had to raise the humidity in the joint for a health problem a big air leak around the A/C ductwork going up and the lack of ventilation caused a bad mold problem on the decking over the house. Crawl around in the attic scrubbing stuff, pay somebody a few grand to do it or pay $1,500 to have all of the decking ripped off and replaced while replacing a 28 year old roof.

    A no brainer. After they spudded the shingles off the guy hollered down "The wood is good.". I yelled back up "Rip the top off of this sucker!".
  13. woodgeek

    woodgeek Minister of Fire

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    R-50 times 750 sq ft * 70 degree temp difference = 750*70/50 = 1050 BTU/hr = 307W, or about 4% of the capacity of a small 2 ton central AC.

    If your second story is an 'oven' the heat is not coming through conduction in R-50 attic insulation.

    I am assuming that you have correlated the oven effect with attic temp/midday sun, and that you don't just have an AC balance issue.

    If the attic has not been pro airsealed, there could be as much free area between the house and the attic than the ridge and soffit vents combined. In my house I previously had ~3 sq ft of gable vents, no soffit vents and ~8 sq ft of opening between the house and attic. :eek:

    If it has been airsealed, then if the whole house is very leaky, the usual winter pattern of leaks is reversed in the summer...hot outdoor air leaks in the upstairs windows and attic, and cool air escapes downstairs windows and rim/sill. Could also cause the problem.

    Central AC (in attic)? or window shakers?
  14. Vic99

    Vic99 Minister of Fire

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    I will go up in the attic this morning and clear the soffits.

    The roof is roughly 55L x 30 T. It's a Dutch Gambrel with a ~56 degree pitch for ~25ft and and ~28 degree pitch and the top for about 5 ft. I've attached my quick attempt at a paintbrush sketch.

    Upstairs is a 2/3 story with 2 dormers. It has a 20 x 15 master bedroom with an 11 ft ceiling at its peak. There is a Mitsubishi ductless AC there that performs well for that room. Across the hall is the kids bedroom, 16 x 12, 8 ft ceiling. They have an 8,000 BTU energy star window AC, maybe 5-6 years old. There is also a small bathroom on that story.

    First floor is larger, but a well placed 8,000 BTU AC or ceiling fans handle most days well or adequately.

    I have done an enormous amount of air sealing over the years in the basement and living space. I don't think the whole house is overly leaky. I would say I've upgraded it from poor to average. All windows but one have been replaced professionally. Almost whole first floor has 3 inches of closed cell spray foam in the walls. Same with sill plate around entire house perimeter. Second floor is a mixture of cellulose, FB batts, and 1 inch polystyrene.

    I've done some air sealing in the attic as well, but I suspect I am missing some areas like around the chimney. Will by some fire rated spray foam for that.

    Mass Save will come out and see me at the end of the month for an audit and hopefully any air sealing that I missed.
    woodgeek likes this.
  15. Vic99

    Vic99 Minister of Fire

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    Roof.jpg
  16. woodgeek

    woodgeek Minister of Fire

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    That clears things up....are the dormers on the second floor or in the attic? Is the attic floor the same level as the soffits/eaves, or are the soffits/eaves way below the attic floor?
  17. Vic99

    Vic99 Minister of Fire

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    Dormers on 2nd floor.

    I believe the soffits are at the same level as floor. Going up now.
  18. Vic99

    Vic99 Minister of Fire

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    Went into attic and removed batts from soffits.

    You could see black on fb where air tried to flow through. Made sure I could see daylight and there was airflow at every bay I corrected. Could see cobwebs sway. A few bays I couldn't clear yet due polystyrene right up against the sloped wall on the 2nd floor leading into the attic. A few others had fb batts stapled right up against the sloped wall. I can get to those in the near future s they are in dead space that I have access to. I'd say that 80% of the bays are clear.

    Also found a 2 ft by 1.5 ft hole where a vent pipe passes up. Huge amounts of air passes through that. The effect was really magnified by how hot I was being in the attic. Shoved some extra fb in as a temp solution, but will probably rough cut polystyrene and spray foam the heck out of it to cover it properly in the near future.

    Didn't get to check chimney area. Had to come out of the heat.

    Will see if this makes some difference.
    woodgeek likes this.
  19. semipro

    semipro Minister of Fire

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    Just thinking out loud here: would it be helpful to introduce some smoke into the soffitt vents on a hot day to track air flow. I guess you'd want to see it exit your ridge vents and not see it in the house.
    Probably ought to let the neighbors know what you're up to before trying something like this though.
  20. Vic99

    Vic99 Minister of Fire

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    Didn't take temp when I went up to clear soffits, but was sure it was over 100 F.

    Hour after sun went down temp was 80. Next morning temp was 70 (67 outside on this cloudy morning)

    Will still do some more air sealing.

    Thanks for all the help.
  21. mrjohneel

    mrjohneel Burning Hunk

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    Vic99: as other have mentioned, look into MassSave. It's for real. And it's funded through a surcharge on your energy bill so in a sense you've already paid for it. No reason not to take advantage of it.
  22. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    That close between the attic and ambient says you are getting good ventilation now. It takes a while after sundown for the shingles, decking and framing to give up stored heat.
  23. Hogwildz

    Hogwildz Minister of Fire

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    You will prolly see a huge difference now that you cleared soffit areas out. This is usually the majority of the problems like yours that I saw when i was roofing. Good catch.
  24. woodgeek

    woodgeek Minister of Fire

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    If you have MA doing some airsealing for you, make sure they do the top plates...where the interior walls meet attic floor. I my case the little 1/16" crack on both sides between the drywall and 2x4, times the length of the plates, worked out to be a few sq ft. In the end a bigger leak than the ones near the plumbing/vent stack and chimney chase. I caulked mine and it made a huge difference in summer AC loads.

    Pros will prob just hit it with a swath of spray foam 4" wide. Quick and effective.
  25. Vic99

    Vic99 Minister of Fire

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    Thanks, all.

    Mass Save will come out Aug 1.

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