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1-year-old home with a low-slope shed-style roof: Lots of condensation is leaking in during cold snaps

LeakyRoofOntario | Posted in General Questions on

Hello all, what a great website this is. So much useful information. We had our 2 story about 3000 sf home built in Ontario. Moved in Jan 2018. We have a low slope (4/12 i think) shed style roof. Its a simple roof its just one big low sloping roof with 2×12 roof rafters, plywood sheating, water and Ice membrane, greyish/bluish shingles. Interior 2nd floors ceilings start at about 7 feet at one side of the house and end up at about 14 feet at the other end of the house. We have an HRV system. 2018 no condensation issues or at least none that I could see in the ceilings. this winter Jan 2019, after a recent cold snap (ie it was -30 c or so outside then went down to above 0 over a day or 2), in multiple areas I could water leaking through the ceiling. About 5 different areas of the 2nd floor ceiling. 2 of the areas where around pot lights in the 2nd floor ceiling. (yes My builder put leaking non airtight pot lights in the 2nd floor ceiling just below the attic). 99% sure its not a leaky roof its condensation. Leading up to this, I had the humidifier cranked so high, all my windows had ALOT of condensation on them. My mistake Fast forward to the next cold snap, again temps went down to -30 c then above 0 again and again, there were leaks in the same spots as well as a few new ones. And this time, it sounded like it was raining all over my ceilings. I thought it was raining outside, I looked out and saw NO RAIN and stepped out and there was no rain. The attic space has R31 batt insulation, and then maybe 2 -3 inches of baffles from soffit to soffit for “ventilation”. And thats it. So now, my builder is prosposing we rip off the entire 2nd floor ceiling, get rid of everything (ie drywall, insulation and baffles) and make it an unvented attic with closed cell spray. Ive done a lot of research and now I see that low slope/crammed attic style roof are notoriously difficult to insulate and ventilate. i just want to know if theres anything majorly wrong with unventilated attics. I know shingles will cook off faster but I mean low slope roofs barely ventilate properly anyway. Pls advise!!

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  1. user-723121 | | #1

    I would get a second opinion and not rely on this builder for any additional work. He clearly does not understand cold climate building.

  2. Expert Member
    Peter Engle | | #2


    Not so fast. While the use of non-airtight cans is a red flag, I wouldn't rule out some other possibilities for the condensation problems. The OP admits to running the humidifier cranked high enough to get a LOT of condensation on the windows, and we should assume that these are energy-efficient double-pane windows that meet local codes. It takes a lot of moisture to do this and there is a good chance that the OP did this to himself, or that he's at least part of the problem.

    If he was running the humidity very high all winter, that introduces a huge moisture load into all of the hygric materials in the house, and it takes more than a week or two to wash that moisture out of the house. That kind of moisture load can overwhelm any wall or roof system.

    The builder's willingness to rip everything out and take a totally different approach does smack of ignorance, but it may also be an honest attempt to fix things. There's nothing inherently wrong about a sealed, unvented attic in this cold climate, but it does require doing things properly.

    To the OP, FWIW, a 4/12 pitch is enough to get a decent amount of ventilation, especially if this is a south-facing roof. North facing in Canada, not quite so much.

    I agree with Doug that you want a good second opinion before tearing everything apart. You should find a local building performance contractor to act as your consultant, and to make sure the builder doesn't mess everything up in an attempt to fix things. It seems like he is making an honest attempt to fix it, and that's rare enough for any builder. If he's willing to spend the money to rip and replace, then he might be willing to do the work properly. He might even learn something in the process.

    1. LeakyRoofOntario | | #3

      House faces South and roof rafters are north/south facing. and the higher end of the roof faces south (lower end faces north).

      Other than the non air tight pot lights, I'm betting there is no caulking or barrier with regards to any plumbing pipes or electrical wires going through the attic.

      Also there is no polyetherne barrier between the ceiling drywall and rood deck. Its just drywall, Batt insulation, vent baffles, roof deck - all within the 11.25 inch deep roof rafters. I cant see how this is well ventilated OR well insulated, especially with a super air leaky ceiling.

      Ceilings also have exhaust fans, I wonder how much warm moist air those allow up into the attic as well.

      I just read somewhere that unvented attics could cause ice damming if there's say 10 inches of snow on the roof due to snow's insulating effect + the roof deck would not be the same cold temp as outside bc it is not vented and thus could cause the snow at the bottom to melt and Ice dam.

      Also heard that there could still be thermal bridging issues with regards to the 2x12 roof rafters.

      I just want to hear some opinions here and educate myself before getting 2nd opinions!

      1. Expert Member
        MALCOLM TAYLOR | | #4

        Leaky RO,

        There is a bit of confusion in your description because you use the term Attic when you don't have one, just rafters. I'm pointing it out not to be pedantic, but because you are also looking at advice on ice damming etc. related to attics. They are different animals with different problems and solutions to cathedral ceilings.

        1. LeakyRoofOntario | | #5

          My apologies. Some websites still call it attic/crammed attic?

          But yes its roof rafters 2x12 and then roof deck right on top, then ice and water then shingles.

          I guess technically there is no attic.

          1. Expert Member
            MALCOLM TAYLOR | | #6

            Leafy RO,

            I sure didn't want to solicit an apology, just to make sure we were all on the same page. It must be an annoying and stressful situation. If it's any consolation I also think it is something that either your builders solution, or one that preserves the ventilation channels and air-seals the rafter bays, will solve.

            Good luck!

  3. keithhoffman22 | | #7

    I'm not an expert on your climate (just a DIYer) nor your roof assembly but I wonder if a tear off is really the most efficient remediation.

    I'm wondering what the experts here would think of this plan for modifying the structure:
    - Permanently seal and block ventilation (perhaps filling the ends of the vents with canned foam and then capping with some detail.
    - Rip off the shingles.
    - Elevate and reinforce roof edges as required to install foam
    - Install R-30+ worth of foam or mineral wool board and new top layer roof deck
    - Install new shingles.
    - This plan seems like it would give you R value much closer to your climate zone requirements (you only have R-31 right now??) and significantly reduce condensation at the roof deck.

    I don't see the point in destroying the drywall layer, existing batt insulation, or roof deck.

    Other stuff:
    User behavior modifications:
    - Humidifiers in winter need to be metered/monitored. Get a more sophisticated humidifier, humidity meter, humidistat, etc. so you can maintain <50% RH. You might find that the humidity isn't as bad as you think.
    - Get timers or humidity sensors for your bathroom fans.

    Air sealing:
    - Use a caulk or acoustical sealant around the can bodies and exhaust fans to seal any sloppy drywall cuts. I like QuietRock's acoustical sealant as a seemingly inflammable and low toxicity product. Other things would work too (including 5 minute drywall mud)
    - Get and install some relatively tight led can lamps that have built in trims that seal against the drywall. These are cheap relative to your problems. Replace bulbs and trims with these pre trimmed cans. Consider caulking them on.
    - Use something to determine whether your exhaust fans are functioning. Whether that's the cover the fan body with individual pieces of toilet paper or colored smoke or an actual ventilation measurement tool

    - Do I understand that you have both exhaust fans and an HRV? Or are your exhaust fans part of your HRV?
    - Do your HRV ducts run in this low slope atticless roof?
    - You should check that any condensate line from your HRV, if included, is not obstructed etc?
    - Where is your HRV installed?

    Those are my ideas other than total destruction.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      That might be easier, less disruptive and yield a more resilient assembly than working from the inside.

      The only problem might be the building code. I'm not that familiar with Ontario's code, but I seem to remember reading that if rigid insulation in used on the roof deck, there can't be any insulation in the cavity. Again - I may be wrong.

    2. LeakyRoofOntario | | #9

      Thanks this is great, different way of thinking. I did read about that - insulating above the roof rather than under the roof.

      So we're not sure yet but we're assuming alot of the batt insulation is ruined which needs replacing and also, alot of the ceiling drwyal is probably rotting.
      Lastly, the only way to. Properly check the roof deck to make sure it's not destroyed with mold etc is to remove the ceiling drwyal. There's not hatch to open and look up in there.

      The hrv is nothing special. It has one intake vent located upstairs in the stairs wall area. I'm not sure how effective it is in removing say moisture from Our showers which are not near the one hrv intake vent. Only ducts that run in the low slope roof are exhaust fans, ducts are insulated and about a 7-8 foot run to the outside.

      1. keithhoffman22 | | #13


        Again, I'm just an DIYer.

        1. Fiberglass batt insulation certainly can be ruined by condensation but what you need isn't a tear off, it's a prairie dog hole through the drywall into the wettest roof rafter bay to inspect the situation. You need this regardless. Then you can assess if you have lost 10% of your height and R or all of it. You can assess whether the roof deck has badly molded or just been wetted (plywood can tolerate a lot of wetting, osb not so much). You can assess whether the drywall has been wetted (discolored) or molded.

        Endoscopic inspection camera:
        2a. They make little endoscopic inspection cameras with led lights. They aren't that expensive to buy (given the scope of work you are considering). Sometimes you can rent them. Get one and get an electrician to see if you can non destructively disassemble your can bodies in the ceiling (many can bodies can be serviced). Then you can use these holes to inspect in many different places. Or drill several little tiny holes right next to a few different ceiling joists and insert the endoscopic camera into the hole and try to see what the insulation and roof deck look like. It takes some practice to use these cameras but you can do it. If you can't gauge the depth of the insulation by the length of camera in the hole, you could make one bigger hole and insert a yardstick to look at. Or if not potentially destroy a couple can bodies in the ceiling (snip snip snip snip snip snip snip ...) and install remodel fixture boxes and a new fixture that is low profile (phillips lightolier?).

        3. The HRV requires diagnosis. I think you got better advice below than I can give.

        MOLD SAFETY:
        If you really think your roof deck and drywall is molded, the thing you should care the most about is your health, not your roof deck. I strongly recommend if you do proceed with full demo that you use an abatement company. Even if you just cut a hatch in, I'd use precautions like air scrubbing, plastic walls, and negative pressure. No way would I let a general demo company come into my living space and do that work. Seems like you will have to move out to do the tear off anyways but what about your things, your ducts, the walls, the flooring, etc. It's possible you shouldn't be living there right now if your drywall has mold or your HRV is obstructed etc.

        Bear in mind that while your problem seems to be the roof, if you over humidified the building long enough to saturate the roof surfaces, you may have wall sheathing condensation problems. The same mold inspector can use a camera and some wall outlets as well as a moisture meter (2 prong is good but something with a scope might be better) to investigate % moisture content of your drywall etc. It's probably more important to get moisture readings ASAP than anything else so you can start tracking whether the building is drying or remaining wet.

        Professional help:
        Mold safety:
        I strongly recommend consulting a mold safety expert in your area (your builder is not a mold expert). You might even find one to subcontract out the inspection camera work to or one with some expertise in malfunctioning ventilation. Their inspection camera can also be used to investigate the exhaust ducting run.

        I'd think about molding your current roof system and the proposed system to see if it even addresses your peak moisture problem.

        The more I read about this the more I feel like there is due diligence investigation to do before you let anyone tear off your roof structure. But time is of the essence.

        Good luck!

  4. PAUL KUENN | | #10

    Were those exhaust fans to deal with hot summer evenings to pull in lower cool air? How many? For now I'd pull the fan out of ceiling and set it in rafter bay to start drying. Fiberglass insulation will settle a bit but not ruined. Opening holes at lower portion of rafter bays (each) should allow stack effect to help dry a little. These are things we do after tornado, rain storms mess with homes.

  5. tommay | | #11

    It sounded like it was raining, I assume from the inside of the roof to your ceiling. I would guess that maybe one of your vent hoses is leaking or perhaps some birds made a nest in the exhaust blocking flow, or stuck damper on the outside. Check to see it opens while in use, if closed and still sucking from the inside then there is a break somewhere. (Rather than running it while you shower, open the bathroom door instead and give the humidifier a break)
    If water is condensing on your windows while running your humidifier, then time to turn it off since the air is reaching a saturation point, aka high relative humidity. The excess moisture has to go somewhere.
    Others have made some good suggestions also. Except for maybe that contractor.

  6. PAUL KUENN | | #12

    I've also seen coil tubing over 8 feet long with kinks from vent fans in poorly insulated space hold up to 5 gallons of condensation water. When they spring a leak it's like a dam breaking.

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